The present invention relates generally to the control of a polyphase alternating current (ac) motor and more particularly to the control of the torque and speed of an ac motor, particularly in the starting mode and low rotating speeds.
A large number of systems are known for controlling the operation of a polyphase (typically, three-phase) electric motor. One commonly employed system is the socalled load commutated inverter. In its customary threephase embodiment, a load commutated inverter comprises a source side converter connected to a suitable source of power such as three-phase power lines and a load side converter connected to the source side converter by way of a direct current (dc) link circuit. Each of the converters is comprised of six legs of controlled semiconductor devices (e.g., thyristors). The source side converter converts the ac power into dc power which is supplied via the link circuit to the load side converter which changes the dc power to variable frequency ac power to be supplied to the load, for example, a motor. As is well known in the art, the magnitude of the voltage (and current) supplied to the motor is fundamentally a function of phase controlling the source side converter while the frequency of the power supplied to the load is the basic function of the load side converter - often referred to as an inverter.
A prime advantage of the load commutated inverter is its simplicity. That is, the load commutated inverter depends upon the load to which it is connected to develop the requisite reactive volt-amperes (VARs) to commutate (turn off) the control devices or thyristors of the load side converter. This is opposed to other types of converters, generally classified as self commutating inverters, such as controlled current inverters or pulse width modulated inverters which require some additional commutating scheme, such as commutating capacitors or commutating networks, to effect a commutation of the thyristors of the bridge network.
It is known that a major problem in load commutated inverters is to develop sufficient reactive volt-amperes (VARs) to commutate the thyristors of the bridge. This is particularly true at low speed operation. The load commutated inverter or, LCI, as it is more commonly known, was first used with synchronous ac machines and utilized the voltage at the stator terminals of that machine to commutate the inverter. At startup and at low speeds, however, the terminal voltage of a synchronous machine is not sufficient to commutate the thyristors of the inverter and thus some other technique is necessary to effect thyristor commutation. The most commonly used method for startup and low speed operation is to force the current in the dc link circuit to zero by controlling the action of the source side converter and to change the gating of the load side converter prior to re-establishing current to the stator. Since the load side inverter firing must be changed every 60 electrical degrees, the dc link current must be brought to zero six times for each cycle of the load voltage. This current pulsing action can create power pulsations which result in undesirable shaft torque pulsations in many applications.
One example of a LCI controlling a synchronous type motor may be found in U.S. Pat. No. 4,443,747 "Transitioning Between Multiple Modes of Inverter Control in a Load Commutated Inverter Motor Drive" by B. P. Chausse et al., which patent includes, inter alia, a discussion of the zero current mode of operation just discussed. Another example of such a load commutated inverter may be found in U.S. Pat. No. 4,449,087 "Flux Feedback Firing Control for a Load Commutated Inverter" by D. L. Lippitt et al. Both of these patents are assigned to the assignee of the present invention and specifically incorporated hereinto by reference.
Subsequent to the development of the use of the LCI with synchronous motors, this same type of system was employed in an alternating current induction motor drive by connecting capacitors in parallel with the induction motor to supply the lagging VARs required by that motor and the load side inverter. An example of this type of application of the LCI may be found in U.S. Pat. No. 4,602,198 "Induction Motor Drive Using Load Commutated Inverter Circuit" by L. H. Walker et al., issued July 22, 1986 which patent is also assigned to the assignee of the present invention and is specifically incorporated hereinto by reference. As in the case of the use of the LCI with the synchronous motor, the application of the shunt capacitors to the induction motor to supply the lagging VARs is only practical at higher voltage and frequency of the adjustable speed motor drive system where the capacitor current is relatively high. Depending upon the system design and power requirements, the capacitor circuit isnot normally capable of supplying the VAR requirements to the system below a certain speed. This, typically, is approximately 50 percent of rated speed. As such, some strategy must be used to start and accelerate this type of drive to about this point.
A second problem involved in the startup control strategy of the LCI induction type system is one of resonance between the shunt capacitors and the motor leakage inductance. One criterion in the selection of the capacitors is that the resonant frequency is not within the normal operating speed range of the motor. With a 50 percent operating speed range, the resonance frequency would normally be selected to be approximately 135 Hertz which is the fifth harmonic of the fundamental motor frequency at 27 Hertz corresponding to approximately 45 percent speed. If the startup control is a six step inverter, which is typical in a three phase system, there will be harmonic orders of 5, 7, 11, 13, etc., of the fundamental which will excite resonance at drive speeds of 45, 32, 20, 17, etc, percent. Since the resonance is undamped, it is not advisable to operate at these speeds with six step wave forms.
A further application of the load commutated inverter is in what is known as a twelve pulse ac motor drive system. This type of system includes two parallel, substantially identical paths each including a load commutated inverter system. The paths are, however, operated phase displaced with respect to one another and, additionally, the outputs of the inverter portions of the LCIs are connected to separate sets of windings within the motor which are also phase displaced from one another. The result is that the voltages and the currents in the respective sets of windings are typically separated by approximately 30 degrees. An example of such a system is found in U. S. Pat. No. 4,426,611, "Twelve Pulse Load Commutated Motor Drive System" by P. M. Espelage et al. issued Jan. 17, 1984. This patent is assigned to the assignee of the present invention and is specifically incorporated hereinto by reference. The twelve pulse system, while generally providing a smoother operation than available with a standard six pulse, three phase motor, is still subject to the power pulsations and resultant undesirable shaft torque pulsations as described above.