This invention relates to starting circuits for electric motors having a main winding and a start winding and, more particularly, to a circuit which continuously monitors an average value of the magnitude of current in the main winding for switching electric power to the start winding.
A common form of electric motor is the induction motor which is manufactured in fractional horsepower capacities typically in the range of from one-quarter horsepower to one horsepower. Such motors are provided with two windings, one winding being the main winding and the other winding being a start winding, also known as a phase or auxilliary winding. The main winding continuously draws alternating current from a source of electric power throughout the operation of the motor. The start winding is required in addition to the main winding for generating a rotating torque field in order to initiate rotation of the motor shaft after which the start winding is disconnected from the source of power. In some motors, a capacitor may be included in series with the start winding, such motor being known as a split-phase motor or capacitor-start motor. In either case, the start winding must be energized for activation of the motor in the event of a stall or when initiating operation of the motor. The start winding is not to be energized during continuous operation of the motor to avoid burnout of the start winding. Also, there is no need for the start winding during continuous operation of the motor become rotation of the rotor in cooperation with the magnetic fields of the main winding induces the necessary rotating torque field for maintaining rotation of the rotor under load.
In order to disconnect the start winding from the source of power after rotation of the motor has commenced, various forms of circuitry have been employed. Some of these circuits take advantage of the relationship between main winding current and motor speed. The main winding current has a maximum value when the rotor is stationary, this value of current being referred to sometimes as a locked-rotor current. As the rotor begins to rotate, the current in the main winding begins to drop such that, at 75% of full speed of rotor rotation, the main winding current has dropped approximately 20% in magnitude. At full speed, the main winding current drops to a magnitude in the range of one-sixth to one-quarter of the locked-rotor current. One form of motor control circuit makes use of this relationship and operates with a threshold based on a testing of a specific model and size of motor. A relay responsive to the motor current is set to operate at the threshold to disconnect the start winding. Alternatively, an electronic switch set to the predetermined threshold has also been employed to disconnect the start winding. Methods of measuring the main winding current include the use of a sense resistor in series with the winding, as well as computerized circuitry which calculates peak value of the current based on measurements of the slope of the current waveform.
A problem arises in that the use of a fixed value of threshold is disadvantageous in that such threshold must be preset for each size and type of motor. In addition, such setting of the threshold presupposes that the operating voltage provided by a power supply will have its stated value. However, as is well known, the voltages of electric power sources, such as an incoming residential power line, may vary considerably depending on the loading of the line. Also, the characteristics of the motor itself may change in the event that the motor becomes heated. Also, computerized circuitry tends to be excessively costly.
A further disadvantage of the foregoing control circuitry lies in the fact that some of these circuits are designed to operate with initial values of current established when the motor is first activated. In the event of a stall due to a momentary overload, an operator must activate a switch to restart the motor. This is also true of starting circuits employing a timer. It is also noted that the circuitry employing a fixed value of threshold suffers from the disadvantage that it is not interchangeable among the various sizes and types of motors because of different requirements for specific settings of th values of threshold for the different motors.