Electricity: motive power systems – Induction motor systems – Primary circuit control
Patent
1994-08-18
1995-10-17
Ro, Bentsu
Electricity: motive power systems
Induction motor systems
Primary circuit control
318805, 318810, 318811, H02P 7622, H02P 7628
Patent
active
054593875
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to control devices for electric motors according to the preamble of claim 1. The control device is intended to control the output of the motor depending on the applied load. The device is arranged to achieve this control in an electrical way by means of an electronic circuit.
STATE OF THE ART
There are a number of types of electric motor which operate according to varying principles and which have different constructions and control means. A current important type of motor is intended to function with current from a direct current source; this often concerning vehicle equipment or motors in mobile equipment such as battery-operated tools.
For direct operation with direct current, commutator motors having brush-commutation or electronic type commutation can be used. These motors are however relatively expensive to produce since they have both stator windings as well as rotor windings, added to which comes the commutator circuit if no brush commutator is used which in turn gives a limited operation time between servicing. Thus a different type of motor driven via an alternating current transformer has been used increasingly, namely synchronous motors often with permanent magnets. Even these motors are however relatively expensive to produce and, for certain purposes, do not allow adequate regulation.
Asynchronous motors with short-circuited rotors have achieved very widespread use for alternating current operation. This has occurred because the type of motor is robust and, above all, cheap to produce since the rotor with its single short-circuited windings requires very little work.
With an asynchronous motor the field is produced in the short-circuited rotor by means of the rotating field of the stator which is formed by using three-phase power or which respectively can be produced by using single-phase along with the provision of auxiliary circuits. In order for this to occur, a certain slip of the rotor with respect to its synchronism with the stator field is required, so that this passes the short-circuited windings of the rotor. A greater produced torque requires a greater induced field in the rotor, which arises since the larger the load, the larger slip. With very large slip the torque development starts to decrease with the risk that the motor will fall out and stop. Such low torque development can also occur at the start, the asynchronous motor having a low starting torque from rest. These characteristics and limitations have resulted in the fact that the motor has achieved only limited use for said direct current applications even though the use of the necessary alternating current transformer for the operation of the motor is today not regarded as any appreciable disadvantage.
OBJECT OF THE INVENTION AND ITS SOLUTION
The object of the invention is to assign a control circuit for operation of asynchronous motors and in particular for operation of such motors from a direct current source. Thus the device should overcome said disadvantages and achieve a motor operation which is suited to the torque load en the motor and which thus even allows a favourable starting cycle.
This is achieved by the features defined in the characterising portion of claim 1. More particularly, use is made of sensing an arising electromotive force caused by the slip when a rotor winding current becomes zero. The size and the duration of this EMF increases with increasing slip and consequently also with increased loading. This can be seen when operating using square pulse, whilst when operating with sine waves the relationship is not as easily detectable.
Using electronic means, the size and the endurance of the change in the EMF can be measured and used for controlling the motor. The change has a certain proportionality to the slip and thus to the torque load. If a control device is arranged to control the value of the EMF through change of the operating frequency, a motor is obtained which works with relatively constant developed torque but varying rotational speed. This
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patent: 3529223 (1970-09-01), Vergez, Jr.
patent: 3648138 (1972-03-01), Kalis
patent: 4057228 (1977-11-01), Volker et al.
patent: 4071900 (1978-01-01), Jensen
patent: 4145647 (1979-03-01), Nielsen et al.
patent: 5194797 (1993-03-01), Kahkipuro
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