An alternating current machine drive system, comprised of an alternating current machine and an inverter configured of a plurality of pairs of transistors with the transistors of each pair coupled in series-aiding fashion and each of the pairs of serially coupled transistors connected across a direct current source, is controlled in accordance with inverter current. The transistors of each inverter transistor pair are alternately rendered conductive to supply alternating current at the inverter output in accordance with a current error signal, proportional to the difference in magnitude between actual inverter phase current and a sinusoidal reference signal, with the conduction state of the transistors of each pair reversing each time the current error signal exceeds an upper and lower hysteresis limit, which limits vary in accordance with actual inverter phase current. Conditioning of inverter output voltage to control machine speed and torque is achieved by varying the sinusoidal reference signal frequency and amplitude, respectively, in accordance with operator commands. Optimum inverter machine drive performance is achieved by regulating the sinusoidal reference signal amplitude and frequency in accordance with a feedback machine torque signal and a feedback machine flux signal, respectively.

The rotation of a motor is sensed and a signal indicative of any error in desired motor speed or phase is multiplied by the angular position of the motor shaft. The resulting product defines a sinusoidal drive signal which is amplified and introduced to the motor windings. As the error signal varies, the sinusoidal drive signal amplitude varies therewith to slow down or speed up the motor accordingly. Thus the invention provides both drive and servo control to a motor from a multiplier device disposed within the servo circuit. The multiplication process provides the ideal drive signal since the motor windings are driven with continuously varying drive signals, which provides accordingly a constant motor torque output inherently free from velocity variations.

A typical alternating current two phase servomotor has two windings displaced 90.degree.; these windings are normally identified as the reference winding and the control winding. Rotational direction and speed of the motor may be controlled by applying AC voltages of the proper waveform amplitude and phase relationship to these windings. When controlling the direction of rotation and speed of such alternating current servomotors in accordance with a DC error signal, pulse width modulation is utilized. The DC error signal is compared with a periodic varying function and its inverse to generate a series of pulses that are logically combined with a square wave and its complement during an enable period to generate a first and second series of pulses each having a width related to the magnitude of the error signal and coupled to energize the control winding of the alternating current servomotor. Similarly, a second periodic signal and its inverse are used to generate pulses for the reference winding. The second periodic signal is displaced 180.degree. in phase from the first periodic signal used for the control winding above. The DC error signal is compared with the second periodic function and its inverse to generate a second series of pulses each having a width related to the magnitude of the error signal. The second series of pulses is logically combined with a square wave and its complement during an enable period to produce output pulses to the reference winding. The motor energizing signals control a power switching amplifier connected to the respective windings of the motor to be controlled and responsive to the appropriate pulse width modulated signals.

Thermal overload of the power tool is sensed by comparing a continually updated thermal model of the tool with a predetermined limit. The thermal model is updated in accordance with operating parameters such as motor speed and power delivered (conduction angle) and implemented using numerical integration based on a look-up table matrix. When overload is detected, power to the tool is gradually reduced, with attendant reduction in speed, followed by a cyclic or warbling increase and decrease in power to provide fluctuating low speed behavior, which may be readily recognized by the tool operator as an overload warning.

A pulse-width modulated (PWM) circuit applies a voltage across a load in accordance with a PWM command signal. The command signal is converted into first and second trains of pulses, by electronically comparing the command signal with a triangular dither signal and also with the dither signal whcih has been shifted by half its period. These two pulse trains, and two more pulse trains obtained by taking the complements of the original pulse trains, are applied to a pair of bridge circuits. The load is connected to both bridge circuits. Both bridge circuits include electronic switching devices actuated by the pulses. The bridge circuits are configured such that when a pulse is present, from either the first or the second pulse train, no net voltage appears across the load. At all other times, substantially the entire power supply voltage is applied across the load. In another embodiment, one or more additional pairs of bridge circuits are connected across the load. The pulse trains controlling the switches in the bridge circuits are generated using dither signals which are shifted in time. In general, there can be n pairs of bridge circuits, the pulse trains of one pair being shifted from the next pair by t/2n, where t is the period of the dither signal. The greater the number of pairs of bridge circuits, the less the effective ripple current in the load. The invention is especially suitable for use in controlling the gradient coils in a magnetic resonance imaging system.