Electricity: motive power systems – Induction motor systems – Primary circuit control
Patent
1990-08-30
1992-09-08
Shoop, Jr., William M.
Electricity: motive power systems
Induction motor systems
Primary circuit control
318807, 318 34, 388814, H02P 540
Patent
active
051461480
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a process devised for changing the effective speed of a polyphase asynchronous motor until it reaches a setpoint speed, the polyphase stator current of the motor having a variable frequency and its effective value being determined by a current control variable, and a device for the application of the process plus a suitable motor system for the application of the process.
2. Discussion of the Prior Art
The control of speed and torque of a polyphase asynchronous motor is per se well-known and may for example be implemented by means of an inverter.
For instance, it is known from U.S. Pat. No. 4,825,131, by Nozakietal, to compare the stator current of the motor with a current control variable in order to infer from it the stator current to be supplied by the inverter to the motor and its frequency. In this way, the speed of the motor may be brought to a desired value and be kept there, while the torque of the motor may be controlled according to the current control variable. This state of the art is a typical current limiting control for the continuous running duty of the motor whose purpose it is to provide optimum control of the motor in continuous running duty (i.e. in essentially steady conditions).
For optimum motor control in the transient running duty, i.e in conditions which change relatively fast (dynamic control of desired speed changes in the case of changing load) no provisions were made.
SUMMARY OF THE INVENTION
It is the object of this invention to provide a process of the kind mentioned at the outset, a device for the application of this process and a motor system suited for the application of the process with which the maximum utilizable speed range of the motor used is permanently available in the transient running duty and by means of which a maximum of speeds may be covered by a minimum of motor definitions, in particular because speed control requires no limitation of the stator current and no reduction of the stator voltage below the maximum tolerated values of the motor.
The invention is based on the knowledge that (1) the static characteristics of a polyphase asynchronous motor are sufficiently defined if a characteristic torque/speed-curve of the motor is specified which in turn is determined by the magnetic saturation of the iron parts in the motor, and that (2) the dynamic characteristics of the motor are determined by an electrical and a mechanical time constant each, causing the conversion of electrical energy into kinetic energy (or vice versa for generator operation) to be performed in a finite time which is determined by the motor.
The interaction of these static and dynamic motor characteristics results in the motor characteristics being sufficiently defined if a characteristic operating voltage/operating frequency value is specified in combination with the torque. This in turn renders it possible to control the stator current of a specific polyphase asynchronous motor with steady stator voltage/frequency-characteristic curve via its frequency in such manner that the stator current maintains a value predetermined by the current control variable, if the operating conditions (load, torque, etc.) are changing or to be changed. With this process, this predetermined value of the stator current may, in principle, be freely selected.
However, in order to preserve optimum active power of the motor the predetermined value of the stator current needs to be selected from within the range in which the torque of the motor is approximately proportional to the stator current. As a consequence, it is recommended to select the current control variable, which determines the stator current, in such manner that a torque between zero and the breakdown torque of the motor is obtained. For this reason, the current control variable is to be selected from the range between zero and the effective value (depending on the intensity of magnetization) of the stator current with which the breakdown torque is reached.
With the
REFERENCES:
patent: 4418308 (1983-11-01), Bose
patent: 4437051 (1984-03-01), Muto et al.
patent: 4575667 (1986-03-01), Kurakake
patent: 4767976 (1988-08-01), Mutoh et al.
patent: 4825131 (1989-04-01), Nozaki et al.
patent: 4965897 (1990-10-01), Jurkawski et al.
Cabeca John W.
Europe Patent Ltd.
Shoop Jr. William M.
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