Electricity: motive power systems – Induction motor systems – Primary circuit control
Patent
1987-03-27
1988-12-13
Shoop, Jr., William M.
Electricity: motive power systems
Induction motor systems
Primary circuit control
318767, 318807, 318794, H20P 528
Patent
active
047913401
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to induction motor drive arrangements and particularly to such arrangements employing a motor convertor supplied from a controllable D.C. source.
2. Description of the Related Art
Such arrangements are known in which the D.C. source is provided by a three-phase thyristor rectifier bridge, the `supply convertor`, which can be phase controlled to determine the D.C. link current between the supply convertor and the motor convertor. One or more D.C. reactors in the D.C. link provide current inertia to maintain the motor convertor current during switching operations.
In addition, it is known to provide a capacitor bank in parallel with an induction motor to provide external excitation, since an induction motor, unlike a synchronous motor for example, will not generate voltage and provide its own excitation.
While an induction motor can be made to operate with excitation capacitors in this way, there is no direct control of the speed.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an induction motor drive which offers speed control over a wide range and constant or controllable torque over this range.
According to the present invention an induction motor drive arrangement comprises a controllable D.C. source, reactance means providing D.C. current inertia, a motor convertor circuit comprising a thyristor-bridge whose input is connected to the D.C. source and whose A.C. output is connected to motor supply terminals for connection to an induction motor, capacitive excitation means connected to the motor supply terminals for maintaining induction motor excitation, and a commutation circuit comprising commutating capacitance connected between each of the motor supply terminals and a commutation neutral point, and a bypass path comprising two thyristor arms in series between input terminals of the thyristor bridge, the junction of the two thyristor arms being connected to the neutral point, the thyristors of said bypass arms being fired to bypass current from said motor convertor thyristors to said commutating capacitance, the thyristors of said motor convertor bridge being fired cyclically in dependence upon the voltage across said commutating capacitance, and said D.C. source current being controlled in dependence upon required motor speed and torque.
The controllable D.C. source is preferably a thyristor bridge convertor having a phase-control firing circuit for controlling the current supply to the motor convertor.
The capacitive excitation means preferably comprises a bank of capacitors connected symmetrically to the motor supply terminals.
A firing circuit for the motor convertor bridge thyristors and the bypass arm thyristors is preferably initiated by voltages between neutral points of the commutation and excitation capacitors.
BRIEF DESCRIPTION OF THE DRAWINGS
An induction motor drive arrangement will now be described, by way of example, with reference to the accompanying drawings, of which:
FIG. 1 is a diagram of an induction motor drive circuit fed from an A.C. mains;
FIG. 2 is a waveform diagram for a particular operating condition of the circuit of FIG. 1;
FIG 3 is a motor/convertor operating characteristic for the circuit of FIG. 1; and
FIG. 4 is an overall block diagram of the drive circuit including various control features.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 1 of the drawings, a controllable D.C. source is provided by a thyristor bridge supply convertor 11 connected to a 3-phase A.C. supply 13. The supply convertor is connected to a motor convertor 15 which also comprises a 3-phase thyristor bridge. The two convertors are connected by a D.C. link 17, in one or both of which a D.C. link reactor 19 is connected. The inertial effect of this reactor tends to stabilise the current and maintain conduction in the motor convertor during commutation. Control of the current level in the D.C. link is largely effected by the supply convertor, as will be explained.
The
REFERENCES:
patent: 4100469 (1978-07-01), Nelson et al.
patent: 4443747 (1984-04-01), Chausse et al.
patent: 4446414 (1984-05-01), Tupper
patent: 4633158 (1986-12-01), Hirata et al.
Japanese Abstract 55-37839, Mitsubishi Denki K.K., "Eciting Current Controller for Induction Motor" 3/80.
Jap. Abs. 55-139093, Tokyo Shibaura Denki K.K., "Device for Controlling Phase of Induction Motor" 10/80.
Shoop Jr. William M.
The General Electric Company p.l.c.
Young Brian K.
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