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Stepper Drive for Three Phase Squirrel Cage Motor

IP.com Disclosure Number: IPCOM000051747D
Original Publication Date: 1981-Mar-01
Included in the Prior Art Database: 2005-Feb-10
Document File: 3 page(s) / 51K

Publishing Venue

IBM

Related People

Pivnichny, JR: AUTHOR

Abstract

A circuit for control of AC induction motors is described that provide a stepping action similar to that provided by conventional stepping motors.

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Stepper Drive for Three Phase Squirrel Cage Motor

A circuit for control of AC induction motors is described that provide a stepping action similar to that provided by conventional stepping motors.

The circuit involves the operation of the induction motor stator windings in a transient mode. This yields a low-speed stepping torque in excess of the normal torque rating of the induction motor when operated in a conventional steady-state manner (i.e., rotating field with slip)/ The transient drive is never allowed to reach steady state by the control circuit.

A phase control is also used in this circuit to provide for a wide stepping speed range, similar to that achieved with conventional (nonstepping) induction motor speed controls.

Motor Control

At least one phase connection, e.g., phase a, of a three-phase motor M (Fig.
1) is interrupted with a silicon-controlled rectifier SCR. Current in phase a is interrupted while a single phase current is allowed to flow between the phase b and c terminals. Torque is negligible, no work is being done, and the current magnitude is low, consisting primarily of magnetizing current. However, if the SCR gate is energized, then a more complex transient current starts to flow in all three phases. A significant positive current impulse occurs, producing a positive torque. This current transient exceeds the steady-state three-phase current in magnitude and thereby produces a torque which exceeds the motor's rated torque.

The transient in the stator windings induces a rotor current transient which is allowed to "die-out" as a function of the rotor time constant before the SCR gate is again energized. In this manner the induction motor never reaches a steady state.

Stepping Drive

In the stepping drive control (Fig. 2), the target address At (desired position) is stored as a digital word in register 10 and is compared in subtractor 11 to the actual address Aa provided by a digital encoder (not shown) coupled to the motor shaft of motor M (Fig. 1). The digital difference signal generated by digital subtractor 11 is then used to generate and control the SCR gate (Fig. 1) via digital-to-analog (D/A) converter 12 and phase angle control
13. The encoder also provides a single digital pulse signal P each time the position reaches any address target, i.e., one pulse per step when a step is completed. Signal P is used to reset S-R latch
14. The latch 14 is set, initiating a motor step by a pulse rate generator 15 that is controlled by by the digital difference signals of subtractor 11.

It can be shown that the characteristics of the pulse rate generator 15 are dependent...