Speed control for induction motor having improved sensing of motor operative conditions
The present application is a continuation-in-part of U.S. patent application Ser. No. 08/065,604, filed May 21, 1993, now U.S. Pat. No. 5,540,219. 
     The present invention relates to a control for an AC induction motor that provides improved speed regulation and avoids regenerative operation of the motor. 
    
    
     BACKGROUND OF THE INVENTION 
     In many applications where a load is driven by an electric motor, it is desirable to vary the speed of the motor to provide optimum performance of the driven load. 
     DC motors have long been widely used for this purpose, since the speed of a DC motor can be easily controlled by changing the voltage applied to its terminals. A good measure of the DC motor speed is the armature voltage, and this voltage is used in many control systems for feedback purposes to regulate the motor speed. Similarly, armature current is a good indication of load torque. 
     However, a DC motor is expensive because of the complex windings on its armature, and because a commutator is required. In addition, a DC motor has brushes to conduct current to the armature. These brushes have a limited life, making periodic maintenance necessary. 
     A &#34;squirrel cage&#34; AC induction motor, on the other hand, is relatively inexpensive, since the rotor windings consist of metal bars which are cast into steel laminations that make up the remainder of the rotor and the stator windings can easily be inserted in slots in stator laminations. An induction motor, at least in a polyphase variety, has no brushes, no moving parts other than the rotor, and virtually no maintenance. 
     While the speed of an induction motor can be varied to a certain extent by varying the voltage applied to the stator winding, and holding the applied frequency constant, only a limited speed range is available. And the electrical losses in the motor can be excessive, if the load is too great, with a serious potential for burning out the motor. 
     It is, however, possible to efficiently control the speed of an induction motor by varying the frequency as well as the magnitude of the AC voltage applied to the motor. 
     Solid state inverters are often used for this purpose. The inverter generates variable frequency multi-phase AC voltages to feed the induction motor stator windings and varies the magnitude of these voltages to be essentially proportional to the frequency. The waveform of the applied voltage may be a square wave; a multi-stepped rectangular waveform that approximates a sine wave; or a true sine wave, commonly generated by pulse-width modulation. For any of these waveforms, AC power in AC power mains is rectified to provide a DC voltage on a DC bus, and solid state switches, connected between the bus and the motor windings, operate sequentially to provide the AC voltages to the motor stator windings. 
     When operating at no load, the induction motor rotor operates at a speed that is essentially proportional to the applied frequency, known as the &#34;synchronous speed&#34;, since it is the same speed that a synchronous AC motor will run at the same applied frequency. When a load is applied to an AC induction motor, the rotor speed changes in an amount known as &#34;slip&#34;, with the rotor speed being slower than synchronous speed for loads that absorb power, and higher than synchronous speed for loads that attempt to drive the motor at a speed greater than unloaded speed. The difference between synchronous speed and rotor speed is &#34;slip speed&#34;. 
     When the rotor speed is greater than synchronous speed, the induction motor becomes a generator and returns power to the source, which in this case is the DC bus, derived from the AC mains. If no mechanism is available for the DC bus to absorb this &#34;regenerative&#34; energy, the bus voltage rises, potentially to a destructive level. 
     Regeneration may result from as simple an action as turning the speed control of an inverter to a lower level or attempting to connect a coasting motor to an inverter set to a lower frequency. In the prior art, many methods have been devised to absorb any regenerative energy that may appear, or to shut down the inverter if regeneration is attempted. 
     Whatever means is used to absorb regenerative energy, it adds significantly to the cost of the inverter, since instantaneous power may be quite substantial. If the inverter is programmed to disable itself by &#34;tripping&#34; or shutting down on regeneration, there may be many nuisance shut downs. When the application does not require regeneration, it is therefore desirable for an inverter to be able to detect when regeneration is imminent and to prevent it at a signal level where the cost is negligible, and without shut down or other disruption. 
     One object of this present invention therefore is to provide an AC induction motor control that senses when the rotor is running faster than desired, and prevents regeneration, thereby saving the circuitry cost and complexity required to absorb this regenerative energy, and avoiding the inconvenience of resetting an inverter that has tripped out because regeneration has occurred. 
     In prior art, the speed of the induction motor has been controlled by applying variable frequency voltages, the frequency of which approximates the desired speed and the magnitude of which is proportional to the frequency. At very low speeds, the voltage is usually &#34;boosted&#34; to account for the impedance change of the motor at very low frequencies. At any given speed setting, the frequency remains constant, and therefore the rotor speed will vary with changing loads according to the slip of the rotor. 
     However, if at any given setting, the load on the motor becomes excessive, the slip will increase substantially beyond the rated level, and the motor current will become correspondingly excessive or even destructive. For this reason, most prior art inverters are designed to &#34;trip&#34; out during overload conditions, requiring resetting and restarting. In most cases, the motor must coast to a stop before restarting. 
     Unlike the DC motor, the operating speed of the AC induction motor is not easily discernible by observing voltages and currents. For example, the voltages and currents of the induction motor are AC signals, and as such, if used directly, require rectification and filtering to determine the absolute values for feedback purposes. At lower speeds, however, with frequencies of only a few Hertz, the filtering required to obtain a DC signal has such a long time constant as to render any control action very sluggish. 
     And, while the torque of a DC motor is almost exactly proportional to the armature current, this is not true in an induction motor, where the relationship between torque and input current is very non linear. Some AC induction motors require almost as much current at no load as they do at full load. 
     In the prior art, speed measuring devices, such as tachometer generators, have been used to improve speed regulation by providing a speed feedback signal to the motor control, and the input frequency is adjusted accordingly to compensate for the slip. Various methods, known generally as &#34;vector control&#34;, are used to adjust the input voltage or current to a level appropriate for the existing slip. 
     The use of speed measuring devices, while effective, requires access to the motor shaft, and additional connections between the motor and the control, both of which add to the cost of a system. When it is desired to add speed control to an existing motor installation, it may not be practical or even possible to add the speed measuring device. 
     It is, therefore, a second object of the present invention to employ electronic measurement and signal conditioning to interpret the information available at the terminals of an AC induction motor to determine motor load and speed conditions without excessive filtering, and to use this information as feedback in a closed loop speed control for a power supply, such as an inverter. 
     More specifically, it is an object of this invention to use the voltage and current of the induction motor stator windings, with improved signal conditioning, to determine speed and load and, using this information as feedback in the motor control, to accurately regulate the speed of the motor under varying conditions of load. 
     A third object of the invention is to use the load information so obtained to provide safe operation during overload conditions in the motor by reducing speed, and to allow starting a motor that is rotating prior to the time it is started without creating excessive currents or torques. 
     SUMMARY OF THE INVENTION 
     The apparatus and method of the present invention regulates the speed of an AC induction motor by providing current command and frequency command signals to an AC inverter supplying an AC motor. The command signals control the magnitude and frequency, respectively, of the current supplied to the stator winding of the motor. 
     A signal proportional to the counter EMF of the motor is obtained. This CEMF signal is employed as a frequency command signal to the inverter so that the frequency of the stator winding current is proportional to the CEMF of the motor. This maintains the magnitude of the magnetizing current component of the stator winding current at the same value for all operating frequencies of the motor. 
     A CEMF regulator receives a CEMF reference signal which approximates the desired speed. The CEMF signal from the motor is employed as the feedback signal to the regulator and the output of the regulator comprises the current command signal to the inverter. 
     The load current component of the stator winding current is determined. For small values of slip, the load current is proportional to slip, and the determined load current can be used as a measure of slip. 
     The CEMF reference signal is boosted by a signal proportional to the load current component of the stator winding current. The boosted CEMF reference signal, acting through the CEMF regulator, causes the CEMF of the motor to increase. This, in turn, increases the frequency command and the frequency of the stator winding current. The frequency of the power supplied to the motor is increased by an amount which compensates for the slip of the motor so that the speed of the rotor remains at the desired level. The CEMF reference signal thus serves as a speed controlling signal for the motor. 
     The magnitude of the output current of the AC supply is limited. If the load increases beyond the limited magnitude, the CEMF and the frequency both decrease, reducing the speed while maintaining rated magnetizing current, to a level at which the load is no longer potentially damaging to the stator or the rotor. 
     Regenerative operation of the motor is avoided as follows. The voltage of the internal DC bus of the AC inverter is monitored and compared to a reference value that is representative of the normal resting bus voltage. If the bus voltage is less than the reference value, a motoring condition is indicated, and the frequency of the inverter is determined solely by the CEMF signal as described above. If, however, the bus voltage rises above this reference value, regeneration is occurring, and the frequency of the inverter is increased by an amount sufficient to prevent regenerative operation of the motor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The present invention will be further understood by reference to the following detailed description and accompanying drawings in which: 
     FIG. 1A is a simplified equivalent circuit of an AC induction motor; 
     FIG. 1B is a vector diagram of voltages and currents in the motor when driving a load; 
     FIG. 2 is a schematic diagram of a speed control system for an AC induction motor of the present invention; 
     FIGS. 3A, 3B and 3C are detailed drawings of a technique that may be used to determine load current magnitude; and 
     FIG. 4 is a detailed diagram of a circuit that may be used to determine whether the motor is motoring or regenerating. 
     FIG. 5 is a diagram showing a modification of the circuitry used to generate the current command for the speed control system. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An AC induction motor consists of two parts. The first part is stationary. It is thus known as the &#34;stator&#34; and contains electrical windings for establishing a magnetic field in the motor, mounting feet or a flange for holding the motor in place, and bearings for supporting the shaft of the second part of the motor, known as the &#34;rotor&#34;, which rotates and transmits power to or from the motor load. 
     The stator is a hollow cylindrically shaped magnetic steel structure. The solid cylindrically shaped rotor is placed in the stator&#39;s hollow center, with an air gap between the outer surface of the rotor and the inner surface of the stator. A multitude of stator windings are arranged in slots around the inside periphery of the stator and grouped into poles. Electrical excitation of a stator winding causes that winding to establish a magnetic field in the air gap between the stator and the rotor. 
     The multiple windings of the stator are arranged so that when they are energized by a multiphase AC voltage or current, the windings are sequentially energized with magnitudes of voltage or current that result in a magnetic field in the air gap which rotates at a speed determined by the frequency of the AC voltage or current and the number of poles in the stator winding. The magnitude of the magnetic field is determined by the vector sum of the currents in the windings. For constant excitation current, the magnetic field is a constant value. 
     The rotor has bars of copper or aluminum imbedded just below its surface, parallel to the motor shaft, all of which are connected together at each end of the rotor by a conductive ring of copper or aluminum. When the rotor is physically rotating at a different rotational velocity than the rotating magnetic field of the stator, the rotor is said to &#34;slip&#34;, and the rotor bars will experience magnetic flux changes, which induce voltages in the bars, and cause currents to flow through the bars and the end rings. These currents, in turn, develop a magnetic field that interacts with the rotating field of the stator winding to produce torque in the motor. 
     Since the stator windings are also magnetically coupled to the rotating field of the stator, they also experience periodic flux changes, and a voltage is induced in them which is known as the &#34;counter emf&#34;, or CEMF, since it opposes the applied voltage. This CEMF voltage is analogous to the CEMF induced in a transformer winding when it is energized. 
     Another similarity to a transformer exists. In an induction motor, the total stator current consists of two components; the component that establishes the rotating magnetic field, hereinafter referred to as &#34;magnetizing&#34; current, and a component that results from current flowing in the rotor, hereinafter referred to as &#34;load&#34; current. This is analogous to the transformer which also has a magnetizing current and a load current. 
     In the same fashion as a DC motor, the torque developed by an AC induction motor is proportional to the strength of the stator magnetic field, and also proportional to the rotor magnetic strength, determined by the rotor or load current. With constant stator field strength, rotor current, at least for small values of slip, is proportional to the difference in angular velocities of the rotating field and the rotor speed, and therefore the torque delivered by an induction motor is proportional to slip. This very nearly linear relationship holds true for values of slip velocity of several percent of full rated motor speed. 
     If the frequency of the current establishing the rotating magnetic field of the stator is known, and if the rotor current can be measured, because slip is proportional to rotor current, the motor speed can be ascertained because the motor speed will be equal to the algebraic sum of the rotational speed of the stator field and the slip speed. 
     FIG. 1A shows a conventional equivalent circuit of an AC induction motor, which is simplified for conditions of small slip speeds, where the frequency of the voltages induced in the rotor is small, and any rotor inductance can be neglected. Only one phase of the motor is shown, since in a balanced construction the other phases are the same, except for a 120° phase displacement. This equivalent circuit is referenced to the stator of the motor, since this is the only part that is readily accessible for application of power and for measurement of currents and voltages existing in the motor. 
     Equivalent circuit 100 shows an applied AC voltage 102 connected to terminals 104 and 106 of one phase of the AC motor. Total stator current I S  identified with the numeral 108 flows through stator resistance 110 and stator leakage inductance 112. Stator impedance voltage drop 114 results from current 108 flowing through the combined stator impedance of resistance 110 and inductance 112. 
     At node 116, the stator current divides into its two components, the magnetizing current I M  identified with the numeral 118, flowing through magnetizing inductance 120 of the stator windings, and load current I L  identified with the numeral 122, which is a result of current flowing in the motor rotor. It is proportional to rotor current and hence to slip speed. The magnetizing current 118 flowing through magnetizing inductance 120 induces CEMF 124 across inductance 120. 
     FIG. 1B is a phaser or vector representation of the voltages and currents of induction motor represented in FIG. 1A for conditions where the slip speed is negative, meaning that the rotor speed is less than that of the stator magnetic field so that the motor is being subjected to a load. 
     In operation of the AC motor, it is very desirable for the magnetizing current I M  to be at its rated magnitude at all times, in order to develop full rated torque. For this reason magnetizing current I M  vector 118 is chosen as the reference vector for the diagram. CEMF 124 is determined by the following relationship: 
     
         CEMF=L.sub.M dI.sub.M /dt                                  (1) 
    
     where L M  is the magnetizing inductance 120 and I M  is the magnetizing current 118. The magnetizing current 118 is sinusoidal and can be represented by: 
     
         I.sub.M =I.sub.M sin wt                                    (2) 
    
     where w is the angular velocity, or frequency of the magnetizing current. If equations (1) and (2) are combined, the CEMF becomes: 
     
         CEMF=L.sub.M w I.sub.M cos wt                              (3) 
    
     The cosine term in this expression indicates that the vector for CEMF 124 is leading the magnetizing current I M  vector 118 by exactly 90 electrical degrees, regardless of frequency. Equation (3) also shows that if the magnitude of the magnetizing current I M  is maintained at constant magnitude for all frequencies, the magnitude of the CEMF is directly proportional to the frequency w. Conversely, if the frequency w is maintained at a magnitude which bears a fixed ratio to the CEMF, the magnetizing current will be constant at all frequencies. 
     For small values of slip, where rotor frequency is small, rotor inductance has no significant effect, and the rotor current can be considered to be purely resistive, and therefore in phase with the CEMF. Load current I L  vector 122 is shown adding vectorially to magnetizing current I M  vector 118, with the vector sum being the stator current I S  vector 108. Since this vector diagram applies in principle to all frequencies and at all loads, it is shown at some arbitrary angle. 
     Stator current flowing through stator resistance 10 and stator inductance 112 causes a voltage drop 114, whose angle is dependent upon frequency, but will typically be slightly leading stator current I S  vector 108 due to stator inductance 112. Voltage drop 114 is added vectorially to CEMF vector 124 to show the applied voltage vector 102. 
     The CEMF voltage can be determined by subtracting the stator impedance drop voltage 114 (as determined by multiplying the measured stator current I S  by the known stator impedance) from the terminal voltage. Once the CEMF is determined, the relationships described by equation (3) will provide the magnetizing current I M , since the frequency of the stator and magnetizing currents is known and the magnetizing inductance L M  is a constant motor characteristic that can be measured or determined from the design of the motor. 
     In FIG. 1B when the magnetizing current I M  118 has been ascertained, the load current I L  122 can be derived from the values of the total stator current I S  108 and the magnetizing current I M , since the vector diagram of FIG. 1B shows a right triangle relationship between the three currents, and load current 122 is related to the other two as follows: ##EQU1## where I S  is the stator current 108, I M  is the magnetizing current 118, and I L  is the load current 122. 
     Since for normal values of load, slip is proportional to load, or rotor, current, the rotor speed can be determined by subtracting a signal proportional to the load current from a signal proportional to the frequency at which the stator winding is energized. 
     With the speed of the rotor known, the speed of the motor can be controlled by varying the stator winding frequency and current to establish motor speed at the desired value. 
     FIG. 2 shows a control system that uses the principles described above to ascertain the induction motor speed at various loads and to regulate the motor speed to a desired value independent of load and using only terminal voltages and currents for feedback. 
     In FIG. 2, speed control 200 contains a sine wave output variable frequency inverter 202, which has two independent control inputs, a frequency command signal at conductor 206, which determines the frequency of the inverter output to AC induction motor 201 through conductors 210A, 210B, and 210C, and a current command signal at conductor 208, which causes the inverter output current to be proportional to the current command signal, at whatever frequency has resulted from the frequency command. Conductors 210A, 210B, and 210C contain phases A, B, and C, respectively, and are connected to the stator winding of motor 201. 
     Conductor 263 is connected to the internal DC bus voltage of the AC inverter 202, and is provided as an additional output signal from the AC inverter, representative of the DC bus voltage during operation of the inverter. 
     Numerous commercially available inverters may be employed as inverter 202 and any conventional inverter design that meets the above requirements may be used. 
     The basic principle of operation of control system 200 is to regulate the CEMF of the induction motor to a value proportional to frequency, thereby maintaining constant magnetic excitation for the motor, and additionally to vary the CEMF of the motor and frequency of inverter 202 in accordance with the load, so that the speed of the motor remains independent of load. This is accomplished as follows: 
     Regulating amplifier 244 receives a CEMF command or reference signal, representing the synchronous speed of the motor, from speed adjusting potentiometer 242, through scaling resistor 240 and summing junction 241. 
     A feedback signal proportional to the motor CEMF is obtained as follows and provided to summing junction 241. Summing amplifier 226A senses the Phase A output voltage of the inverter in conductor 210A through conductor 224A, and scaling resistor 225A. This voltage corresponds to voltage 102 shown in FIG. 1. The stator winding current in Phase A is sensed by current transformer 212A and applied to impedance calibration potentiometer 218A through conductor 216A. An adjustable portion of this stator winding phase current signal is selected by potentiometer 218A, and connected through conductor 220A to a resistor and capacitor network 221A and 222A. This network converts the voltage from potentiometer 218A to a current proportional to the stator impedance voltage drop, shown as voltage 114 in FIG. 1. The resistive portion of the current, through resistor 221A represents the resistive portion 110 of the stator impedance voltage drop 114, while the capacitive current through capacitor 222A represents the inductive portion 122 of the stator voltage impedance drop. The combined current from resistor 221A and capacitor 222A flows through conductor 227A and is subtracted by the summing amplifier 226A from the phase voltage signal through resistor 225A so that the output of the summing amplifier 226A becomes an AC signal that represents the phase output voltage 102 of inverter 202 less the stator impedance voltage 114 drop due to stator current, and is thus proportional to the CEMF 124 in phase A. 
     Impedance calibration potentiometer 218A provides adjustment to meet specific characteristics of the motor being used, since stator impedance varies over a wide range, even with motors of the same horsepower and speed. 
     Phases B and C have similar sensing circuits, numbered the same as Phase A, except for the A, B, or C designations. The three potentiometers 218A, 218B, and 218C may be mechanically coupled in tandem as indicated in FIG. 2, so that all three can be adjusted simultaneously, since the motor impedance is the same for all three phases. 
     While a current transformer is shown for sensing the currents in conductor 210, it will be appreciated that other means of measuring phase currents may be used to develop the signal proportional to the stator impedance voltage drop, such as a series resistor or a combination of a series resistor and series inductor. If the inverter 202 uses internally generated individual phase AC current command signals, these signals are also equivalent to the actual phase current, and can be used instead of directly sensing phase currents. 
     The signal outputs of summing amplifiers 226A, 226B, and 226C, representing motor CEMF, are AC in nature, and thus cannot be used directly for feedback to summing junction 241 and regulating amplifier 244. While a single phase CEMF signal could be rectified for feedback purposes, the ripple voltage of the rectified DC would be excessive, and would require filtering to obtain a reasonably smoother DC signal. At high frequencies, this is effective, but since the inverter may operate at frequencies as low as a few Hertz, the filter time constants to provide a smooth signal at these low frequencies would be too long to allow fast response to changes in operating conditions. 
     However, when three phase full wave rectification of the three AC CEMF signals is accomplished, the ripple magnitude becomes only a few percent of the total AC magnitude, and the frequency of the ripple is increased to six times the inverter output frequency, making filtering unnecessary or minimal. Three phase, full wave rectifier 232 takes the AC outputs from the three CEMF summing amplifiers in conductors 230A, 230B, and 230C and converts the combined outputs to a DC CEMF signal in conductor 234. This signal becomes the feedback for regulating amplifier 244 through scaling resistor 238 and summing junction 241. 
     The full wave three phase rectifier 232 is shown functionally in FIG. 2 as a conventional six diode bridge. At low levels of CEMF signals, the forward diode conduction voltage drop may be excessive, causing non-linear feedback. Where it is desirable to eliminate the diode drop, electronic rectification by standard precision &#34;absolute value&#34; operational amplifier circuitry is preferred. Examples of this circuitry can be found in various operational amplifier application manuals, such as &#34;OPERATIONAL AMPLIFIERS&#34;, published by Burr-Brown Research Corporation, Library of Congress No. 74-163297. See page 249. 
     The output of regulating amplifier 244 operates to increase or decrease the current command signal in conductor 208, thereby increasing or decreasing the stator current and causing the CEMF feedback signal, and the actual CEMF, to become proportional to the reference signal from potentiometer 242. 
     As noted in equation (3), the CEMF is related to the magnetizing current I M  of the motor. A further function of speed control 200 is to provide constant magnetizing current I M  in motor 201, independent of frequency, by causing the frequency of the current applied to the motor to become proportional to the CEMF, also as described in equation (3). To accomplish this, the DC CEMF output of rectifier 232 is provided by conductor 234 through scaling resistor 236 to summing amplifier 204. Summing amplifier 204 also receives another signal through resistor 270, but only when regenerative operation of the motor is attempted. During normal motoring operation, the only signal to summing amplifier is the signal proportional to CEMF. The output of summing amplifier 204 becomes the frequency command signal in conductor 206 to inverter 202. The frequency of operation of motor 201 is thus proportional to CEMF, resulting in constant magnetization current I M  at all frequencies. The resistance of resistor 236 is chosen to establish the correct proportionality so that the magnetizing current is that magnitude which provides full rated field excitation for the motor at all frequencies. 
     In the operation of motor 201, an increased load torque causes the rotor to slow until its slip speed, i.e. the difference between the synchronous speed and the rotor speed, develops enough load current and thus enough torque to balance the increased load. If constant output speed is to be maintained, this slip must be compensated for by increasing the frequency of the stator current and the synchronous speed of the motor. A corresponding increase in CEMF is needed to maintain the magnetizing current constant and to maintain full excitation of the motor. 
     Since slip speed is proportional to the rotor or load current represented by vector 122 in FIG. 1, determination of this current will provide an indication of slip speed. With a squirrel cage induction motor, the rotor current cannot be measured directly, but equation (4) provides a means of calculating it, since two of the three variables to the equation are known. The stator winding current can be ascertained by either direct measurement or by using the current command signal to the inverter 202. Due to the proportionality between CEMF and frequency established by the frequency command signal in conductor 206 to inverter 202 described above, the magnetizing current will be a constant value as defined by equation (3). 
     Function generator 258 electronically calculates equation (4) from one input proportional to stator current I S  from regulating amplifier 244, in conductor 260 and connected to input terminal 252, and from a second input, from potentiometer 256 which provides a signal representative of magnetizing current to input conductor 154 of function generator 258. 
     The output of function generator 258 in conductor 250 thus is a signal proportional to rotor or load current, vector 122 in FIG. 1, and can be used to boost the frequency and CEMF to compensate for slip, and therefore to keep the rotor at constant speed. 
     Slip compensation potentiometer 248 is connected to the output of function generator 258 by conductor 250, and a portion of this output is adjusted by potentiometer 248 and added to regulating amplifier 244 through scaling resistor 246, connected to summing junction 241. This signal through resistor 246 adds to the reference signal from potentiometer 242, and increases the CEMF, which in turn, through summing amplifier 204, increases the inverter output frequency to provide the slip compensation necessary to maintain rotor speed. Potentiometer 242 thus becomes a speed adjusting means for control 200. 
     Potentiometer 248 is adjusted to provide the desired amount of slip compensation, so that the motor speed either does not vary at all with load, or varies by an amount deemed appropriate for the application. 
     In contrast to prior art inverters that will disconnect or &#34;trip&#34; when an excessive overload condition appears, it is more desirable to merely have the motor slow down and operate safely until the overload is removed. Speed control 200 provides this feature in the following manner. Regulating amplifier 244 is connected to zener diode 245, which serves to limit the output of amplifier 244 to a definite level. This limits the current command to the inverter and therefore the output current of the inverter 202. If the slip of the motor and the load current increases to a magnitude where the limited output of the inverter is no longer capable of supplying the total required magnetizing and load currents, the CEMF will drop. The limited output of amplifier 244 cannot command any additional current to restore the CEMF to the proper level. The reduced CEMF, acting through summing amplifier 204, accordingly reduces the frequency of the stator winding current, the synchronous speed of the motor, and the speed of the rotor. This action progresses until the rotor speed has reduced to a level at which the load is no longer excessive, and the control can establish a stable operation, albeit less than the set speed. If the load is large enough, the motor will actually stop rotation completely, but, since the CEMF and the frequency will be very low under this condition, excessive rotor losses and motor burnout will not occur. After the load is removed, the speed control will restore the motor to its original speed. 
     This same regulating action allows speed control 200 to safely start a large inertia load without the need for timed accelerating circuits to prevent excessive currents during acceleration. 
     As mentioned earlier, it is desirable to avoid the need to dissipate regenerative energy that results from operation at speeds greater than the set speed, either when the speed adjustment potentiometer is turned to a lower speed, or when the load tends to overdrive the motor. This feature is accomplished as follows: 
     DC bus voltage detector 262 is a circuit that utilizes the DC bus voltage of the AC inverter in conductor 263 to determine when the motor is regenerating. When the bus voltage is equal to a reference signal, the magnitude of which is that which exists when the motor is operating at a no load condition, the bus voltage detector provides no output at conductor 264. 
     If the bus voltage is less than this reference signal, indicating a motoring condition, the bus voltage detector 262 provides a positive output signal at conductor 264. 
     If the bus voltage is greater than this reference signal, indicating a regenerative condition, the bus voltage detector 262 provides a negative output signal at conductor 264. 
     Diode 266 blocks a positive output signal from the bus voltage detector 262, but allows a negative signal to pass through conductor 268 and scaling resistor 270 to the summing amplifier 204 that provides the frequency command to inverter 202. Thus, when the motor is acting as a load, a positive signal from the bus voltage detector will not reach summing amplifier 204, and the frequency of the inverter is determined solely by the CEMF signal in conductor 234 to the summing amplifier 204. 
     When regeneration is attempted, with the rotational speed of the rotor exceeding that of the rotating magnetic field, the negative output from bus voltage detector 262 is applied through conductor 264, diode 266, conductor 268, and scaling resistor 270 to summing amplifier 204, and adds to the signal from the CEMF, increasing the frequency command from summing amplifier 204 to the inverter. The result is that the frequency increases until the rotational speed of the rotating field is equal to the rotor speed, at which time the output of bus voltage detector 262 drops to a level just sufficient to maintain the proper frequency. 
     When this condition exists, there is no generation of power by the motor, nor is there any power consumed by the motor, since the rotor speed is matched by the speed of the rotating magnetic field. 
     The ability to prevent the motor from acting as a generator has several useful functions. First, when the speed adjusting potentiometer setting is reduced from a given speed to a lesser setting, the control allows the motor to coast to the new setting without the need to absorb the energy that would result if braking were attempted. 
     Second, if the load overdrives the motor, the control merely allows the motor speed to increase, again without any need to absorb the braking energy which would accompany any attempt to keep the speed from increasing. Additional means may be provided to prevent the speed from increasing indefinitely in the event that the load continues to overdrive the motor beyond safe limits. 
     The third benefit of not attempting regeneration is when the motor must be started when it is already running at some speed, either because it is coasting after removal of power, or if it is connected to some load that causes it to rotate, such as a fan or pump that has air or fluid passing through it. 
     With most prior art inverter controls, if an attempt is made to start a rotating motor, excitation will be applied at the set frequency, or if acceleration control is used, at a very low frequency. In either case, it is unlikely that the frequency of the rotating field will match the speed of the rotor, and excessive current and torque will result. Tripping out of the inserter usually occurs. 
     Elaborate schemes have been devised to sweep the excitation frequency over its range at a low magnetizing level to determine the rotor speed before applying full power to the motor. Most of these methods take several seconds to accomplish a safe startup, with lost time when fast startup is critical. 
     With the present invention, startup under these conditions is automatic, rapid, and free from undue stress. When the control is started, regulating amplifier 244 provides a current command to the inverter 202 to attempt to establish the CEMF which has been set by potentiometer 242. If the motor is running at a speed less than the set speed, regulating amplifier 244 quickly reaches its limiting condition, keeping the CEMF and the frequency low until the motor can accelerate at a controlled torque to the desired operating speed. 
     If, on the other hand, the motor is operating at a speed higher than the setting on the speed adjusting potentiometer 242, regulating amplifier 244, operating through the current command to inverter 202, can easily establish the commanded CEMF. With the rotor operating in an overspeed condition, however, the bus voltage detector outputs a negative signal to summing amplifier 204, causing its output frequency command to increase the inverter frequency to a level that matches the speed of the rotating magnetic field to the speed of the rotor. The motor can then coast down to the desired speed. 
     A function generator, shown as block 258 in FIG. 2, that can be used to determine the load portion of the stator current is shown in FIG. 3C. 
     FIG. 3A, accompanying FIG. 3C, shows a family of curves that represent the relationships between the stator current I S , the load current I L  and several levels of magnetizing current I M  as defined by equation (4), restated in FIG. 3A. To remove the dimensions from this relationship, three new variables A, B, and C are defined for the three currents, each as a fraction of the maximum stator current allowed by the limiting action of amplifier 244 of FIG. 2. 
     Since boundaries exist for these variables, the range of possible values for each variable are shown. Note that the stator current can never be less than the magnetizing current, since it is the vector sum of the magnetizing current and the load current. Equation (5) restates equation (4) in the dimensionless variables. 
     It is possible to use multipliers and square root circuits to calculate equation (5) exactly. The results of such calculations are shown in FIG. 3A as a family of curves for various magnetizing currents, each expressed as a fraction of the maximum stator current. However, the maximum value for C, which represents the load current, can be seen in FIG. 3A to be somewhat dependent on the variable B, which represents the magnetizing current of the stator winding. For a practical control to be adjustable to work with many motors that have widely different magnetizing currents, it is desirable that the maximum value of C be unrelated to the magnetizing current. This avoids an extra calibrating adjustment, which may be difficult for a non-technical user of the control to understand and make. 
     FIG. 3B shows a linearized family of curves that closely approximate those of FIG. 3A. These curves 20 show the maximum value of C, or load current as independent of the magnetizing current represented by B and as converging at a single, common maximum value of C. Use of the linearized curves of FIG. 3B also has the advantage of being realizable by means of simple operational amplifiers, rather than complex multipliers and the like. 
     Equation (6) defines the value of C for the set of linearized curves shown in FIG. 3B. It is re-arranged in equation (7) for ease of implementation by operational amplifiers. 
     FIG. 3C is an operational amplifier circuit that may be used as the function generator 258 in FIG. 2. Several resistors R are shown, and each is the same value of resistance, for example 10k ohms, since equation (7) does not have any scaling constants, except for the adjustable constant B related to the magnetizing current. 
     The input at conductor 252 corresponds to the stator current I S  and is represented by variable A. The output of amplifier 304 in conductor 308 is the variable C, which corresponds to the load current I L . Variable B is adjusted by potentiometer 310. Diode 334 allows only a positive output at conductor 308 to reach output conductor 250, satisfying the range for variable C, being from 0 to 1, as shown in FIG. 3A. Conductor 250 is connected to slip compensation potentiometer 248 and to summing junction 241. 
     Operational amplifier 302 is connected in a standard differential configuration, and receives a noninverting input from conductor 308, which is variable C, through conductor 322. Amplifier 302 receives an inverting input from variable A at conductor 252, through conductor 324. The output of differential amplifier 302 is thus equal to (C-A), the left hand term of equation (4). This output appears in conductor 318. 
     Operational amplifier 306 is also connected in a differential mode, and also receives its non-inverting input from conductor 308, variable C, through conductor 328. An inverting input is provided at a constant value of 1 at input 314, and through conductor 326. The output of amplifier 306 is thus (C-1) of equation (7), and is connected to potentiometer 310 by conductor 312. Potentiometer 310 selects a fraction of this intermediate variable (C-1), adjustable from 0 to 1, which is the range specified in FIG. 3A for variable B. The selected fraction is equal to the right hand term of equation (7), namely B(C-1). This output appears in conductor 320. 
     Operational amplifier 304 acts in a feedback mode, with negative feedback from operational amplifier 302 through conductor 318, and positive feedback from that portion of the output of operational amplifier 306 which is selected by magnetizing current calibration potentiometer 310, and connected to amplifier 304 through conductor 320. Operational amplifier 304 regulates the voltage difference at its input terminals to zero, making the signal at conductors 318 and 320 equal. Since the signal at conductor 318 is (C-A) and the signal at conductor 320 is B(C-1), equation (7) is satisfied, and the value for C is that specified in equation (6). 
     Even though amplifier 304 has positive feedback, the amount of negative feedback from amplifier 302 is always greater than the positive feedback, since only a fraction of the output of amplifier 306 is used for the positive feedback, and stability is therefore guaranteed to be unconditional. 
     The values of resistors 330 and 332 are chosen to provide the proper scaling and to reintroduce the dimensions lost when equation (4) was converted to equation (5). The output of the function generator at conductor 250 is connected to the slip compensation adjustment potentiometer 248 shown in FIG. 2. 
     It is recognized that other means of generating the function defined by block 258 are possible, including but not limited to analog or digital multipliers and dividers, or microprocessor signal processors, without changing the intent or usefulness of this feature of the invention. 
     A circuit that may act as the DC bus voltage detector 262 in FIG. 2, is shown in FIG. 4. Inverting operational amplifier 402 receives two input signals. A fixed reference voltage of negative polarity, shown for exemplary purposes as obtained from battery 268, is applied to conductor 265, through scaling resistor 404, to the inverting input of amplifier 402. 
     Additionally, a positive polarity voltage proportional to the DC bus voltage internal to AC inverter 202 of FIG. 2 in conductor 263 is connected through scaling resistor 408 also to the inverting input of amplifier 402. The other input to amplifier 402 is connected to ground. 
     The relative sizes of scaling resistors 404 and 408 are selected so that with the DC bus voltage at a magnitude representative of a no load condition on the motor, the two input signals exactly cancel, and the amplifier receives no net input. 
     Resistor 406 determines the gain of the amplifier 402 when the input signals are not equal and opposite, so that a net input produces a specific desired output level. 
     At no load on the motor, as mentioned above, there is no net input to the amplifier, and its output at conductor 264 is zero. When the motor is loaded in a motoring manner, the DC bus voltage drops, the negative reference signal in conductor 265 is now larger than the positive bus voltage signal in conductor 263, resulting in a net negative input, and the output of inverting amplifier 402 becomes positive in polarity. 
     Similarly, when the motor operates in a regenerative manner, the DC bus voltage rises to a level greater than the no load magnitude, and the positive bus voltage signal in conductor 263 is now of greater magnitude than the negative reference signal in conductor 265, providing a net positive input to inverting amplifier 402, and a negative polarity output. 
     As previously described, when the output at conductor 264 is of negative polarity, this output is utilized for increasing the frequency of the inverter to prevent regeneration. Any positive output is blocked by diode 266 of FIG. 2. 
     FIG. 5 shows an alternate structure for generating the current command signal to the inverter 202 described in FIG. 2. At very low speeds and during transient conditions, the output of amplifier 244 in FIG. 2 may drop to a magnitude that commands a motor current less than the rated magnetizing current of the motor, and torque capabilities may suffer. 
     To insure that the rated magnetizing current is maintained at all times, a signal representing the desired magnitude of the magnetizing current for the motor is selected by potentiometer 504 as a portion of a fixed voltage, shown as battery 502. This signal is provided to summing amplifier 244A through resistor 506 and added to the output of CEMF regulating amplifier 244, connected to summing amplifier 244A through resistor 507 and diode 510. The output of amplifier 244A in conductor 208A is the total current command for the AC inverter. 
     With this circuit, the magnitude of the motor magnetizing current is determined by the setting of potentiometer 504, and only the additional current command required to support a motor load, and to maintain constant magnetizing current for the motor when loaded, is supplied by amplifier 244. Diode 510 prevents the output of amplifier 244 from reducing the excitation level to the motor below the magnitude established by potentiometer 504. 
     The function generator of FIG. 5 utilizes the combined current command as its input in conductor 260, and provides the same output signal for blip compensation as in FIG. 2.