US 4,281,276 AGranted

Dual mode AC motor drive system

AssigneeGeneral Electric Company

InventorsJohn H. Cutler, Loren H. Walker

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Abstract

Disclosed is an AC motor drive system having a variable frequency, variable magnitude AC current supplied from a controlled inverter coupled to a variable DC current source in the form of a converter by way of a DC link including an inductor. The AC motor load is controlled so as to have dual modes of operation in order to optimize torque at any speed which can be produced within the voltage and current limitations of the variable frequency power source. The first mode comprises a constant slip or constant angle mode for operating speeds up to the corner point speed which is defined as the speed at which the motor voltage at full torque matches the maximum voltage the variable frequency inverter coupled thereto can provide. Thereafter, the system enters the second mode by automatically shifting to a constant horsepower variable slip or changing angle mode for operating speeds beyond the corner point speed.

Claims18

Dependent claims

A system for providing control of an AC electric motor having a stator and a rotor spaced from the stator by a gap across which flux is produced in response to an electrical motor current of variable magnitude and frequency supplied from a controlled source, said control being a function of both torque command and motor speed, and comprising:

(a) means to determine with respect to the motor the instantaneous motor current and the instantaneous angle between the gap flux and said motor current and to provide signals proportional thereto;
(b) means to establish a torque reference signal proportional to a desired level of electrical torque;
(c) means to compare said torque reference signal with said signal proportional to the motor current and to generate a first error signal representative of any differences therebetween, said first error signal being coupled to said controlled source to vary the magnitude of the motor current supplied from said source;
(d) means responsive to motor speed and said torque reference signal to derive an angle reference signal representative of the desired angle between the gap flux and motor current for first and second operational modes, said first operational mode being at speeds less than a predetermined reference speed wherein said angle reference signal is of a substantially constant angle value, and said second operational mode being at speeds greater than said predetermined reference speed wherein said angle reference signal is of a variable angle value;
and
(e) means to compare said angle reference signal with said signal proportional to the instantaneous angle between the gap flux and the motor current to generate a second error signal representative of a difference therebetween, said second error signal being coupled to said controlled source to vary the frequency of the motor current supplied thereto from said source.

3 more elements

A method of driving an AC motor from a controlled power source supplying a current of variable magnitude and frequency to said motor, comprising the steps of:

(a) determining with respect to the motor the instantaneous motor current and the instantaneous angle between the gap flux and the instantaneous motor current and providing signals proportional thereto;
(b) establishing a torque reference signal;
(c) comparing said torque reference signal with the signal proportional to the instantaneous motor current and generating therefrom a first error signal representative of the difference therebetween;
(d) applying said first error signal to said power source to vary the magnitude of the motor current;
(e) deriving angle reference signals of first and second types in response to actual motor speed and the torque reference signal, said first type angle reference signals being derived at speeds less than a predetermined reference speed and said second type angle reference signals being derived for speeds greater than said predetermined reference speed;
(f) comparing the derived one of said first and second type angle reference signals with the signal proportional to the instantaneous angle between the gap flux and the motor current and generating a second error signal representing any difference therebetween;
and
(g) applying said second error signal to said controlled power source for varying the frequency of the motor current, said second error signal of said first type being adapted to provide a constant angle mode of operation and said second type of angle reference signal being operable to produce a variable angle mode of operation.

5 more elements

Dependencies are inferred from the claim text.