Temperature measuring system for rotating machines
BACKGROUND OF THE INVENTION 
     The present invention relates to a device for a contact free transmission of signals between stationary and movable portions of a machine or the like. 
     It is often desirable to be able to transmit signals between, for example, two machine components having different conditions of movement. One example of this is where the temperature in the rotor of a rotating electrical machine is to be measured. When transmitting the measurement signal between the rotating portion and the stationary portion, it is desirable for known reasons to use a non-contact, i.e., contact free, connection between the two machine components. Furthermore, it is desirable to avoid having to arrange a separate supply current source for the part of the measurement device which is arranged in the rotating portion. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a contact free signal transmission system for transmitting signals between stationary and movable portions of a machine. 
     Another object of the present invention is to provide a system for transmitting signals between stationary and movable portions of a machine in which it is possible to avoid having to use a separate supply current source in the movable portion. 
     According to the present invention both of these desires are fulfilled by connecting the supply source for the entire system as well as the indicating device to the stationary member of an inductive coupling, whereas a pulse generating device arranged in the movable portion of the machine is connected to the movable member of the coupling. The auxiliary supply voltage from the source to the pulse generating device as well as the signals from the pulse generating device are transmitted by way of the inductive coupling. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The accompanying drawing shows a schematic circuit diagram of a contact free signal transmission system in accordance with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As an exemplary embodiment of the present invention, a system for providing a measurement of the temperature in the rotor of a rotary machine will be described below. The machine includes a rotor shaft 1 and a stationary U-shaped magnetic core 2. A magnetic coupling is used for transmitting an auxiliary voltage and signal pulses between the stator, the stationary core, and rotor of the machine. The coupling has a stationary member and a rotating member, the latter being arranged on the rotor shaft. The stationary coupling member is formed by a winding 3 on the U-shaped magnetic core 2. The rotating coupling portion includes a winding 4 on the shaft. 
     The above description presupposes that the shaft is formed of magnetic material, which is usually the case. If the shaft is formed of a non-magnetic material, the winding can be wound on a magnetically conducting bobbin, in which the flux can be formed. 
     An oscillator 5 for generating a sinusoidal alternating voltage is connected to winding 3 for generating a sinusoidal magnetic field in the shaft or the bobbin. The frequency of the oscillator is chosen with regard to the size of the magnetic circuit, the air gap and the highest limit of the frequency of the signal source. Furthermore, an even multiple of the power frequency should not be chosen. Thereby the frequency of the oscillator will not be able to resonate with the power frequency. 
     The magnetic field generated in the rotor shaft generates a corresponding alternating voltage in winding 4, and this voltage is supplied to a pulse generating device 6 in the rotor. The pulse generating device is arranged to generate one pulse during each period of the transmitted alternating voltage. This means that the pulse can be given a large energy content, which is desirable because of the relatively great energy losses occurring in the air gap of the magnetic coupling. The voltage required for driving the pulse generating device is obtained by rectifying and smoothing the positive half-wave of the transmitted alternating voltage. Since the frequency of the alternating voltage is relatively high--on the order of magnitude of a few thousand Hz--the ripple becomes negligible and a pure direct voltage in the rotor portion may be counted on. 
     A temperature-dependent resistor 7 arranged in the rotor portion is connected to the pulse generating device. Resistor 7 controls the pulse generating device in such a manner that the phase position of the delivered pulses in relation to the zero passage of the alternating voltage becomes dependent upon the resistance of resistor 7. Thus, the phase position of the pulses in relation to the zero passage of the alternating voltage is a measure of the temperature at the location in the rotor where the resistor is placed. 
     The pulses generated in the pulse generating device are returned to the stator through the magnetic coupling so that practically the same pulses will occur in winding 3 as those generated in the rotor. What is essential is that the pulses maintain their original phase position in relation to the zero passage of the alternating voltage. The pulses in the stator are fed to a pulse detector 8, to which there is also supplied the sinusoidal signal from oscillator 5. In the pulse detector, the sinusoidal signal and the pulses from the rotor are compared and the time between the zero passage of the sine wave and the pulse is determined. This time differential forms a measure of the temperature prevailing in the rotor, which temperature is indicated on instrument 9. 
     It is noted that the above description and the accompanying drawing are provided merely to present an exemplary embodiment of the present invention and that additional modifications of such embodiment are possible within the scope of this invention without deviating from the spirit thereof.