Electricity: motive power systems – Constant motor current – load and/or torque control
Patent
1988-06-06
1989-09-26
Smith, Jr., David
Electricity: motive power systems
Constant motor current, load and/or torque control
318811, 318806, 187119, H02P 742
Patent
active
048703346
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to an apparatus for controlling an adjustable-speed electric motor.
Electric power converters or inverters have been employed for the application of adjustable-speed drives using alternating current motors. A typical converter includes a direct current (DC) rectifier for rectifying three-phase AC input voltage and for supplying the resulting direct current (DC) bus potential to an inverter. The inverter comprises a plurality of pairs of series-connected switching elements to generate an adjustable frequency output. In many applications, such a frequency adjustment is effected through a control circuit which employs a pulse width modulation (PWM) control technique in producing variable frequency gating pulses to periodically switch the respective switching elements so as to operate the motor at a variable speed. The motor can be propelled (motoring mode) or retarded (braking mode) as desired by appropriately varying the frequency and the amplitude of the excitation that the inverter applies to the motor.
The actual motor speed is sensed and compared with a commanded motor speed. An speed error signal, which depends on the difference between the actual and desired values of motor speed, is derived and applied to a proportional-plus-integral control circuit which converts it into a torque command signal. The control circuit responds to the torque command signal by controlling the operation of the inverter so as to vary, as a function of the torque command signal, the amplitude of the voltages supplied from the inverter to the motor.
One problem occurs where the motor is coupled to drive a mechanical load having elastic elements. For example, an elevator has an elevator rope, springs, rubber cushions and other elastic elements for providing improved passenger's comfort and vibration insulation. In such a case, the control circuit tends to resonate with the mechanical load. This tendency neutralize any attempt to increase the response time of the control circuit with respect to commanded motor speed changes.
SUMMARY OF THE INVENTION
Therefore, one main object of the invention is to provide an improved motor control apparatus having a rapid response rate with respect to commanded motor speed changes without any influence of its resonance with the mechanical load driven by the motor.
There is provided, in accordance with the invention, an apparatus for controlling an adjustable speed electric motor. The apparatus includes a motor drive circuit for applying power to drive the motor, and a control circuit responsive to a torque command signal for controlling the operation of the motor drive circuit so as to vary the power as a function of the torque command signal. The control circuit includes a speed sensor sensitive to motor rotation speed for producing a speed feedback signal indicative of a sensed speed of rotation of the motor, a source for producing a speed command signal indicative of a desired motor speed, a first summing circuit for summing the speed feedback and command signals to provide a speed error signal indicative of the difference between the sensed and desired motor speeds, a proportional-plus-integral amplifier connected to the first summing circuit for converting the speed error signal into a corresponding first torque command signal, a second torque command signal generator connected to the source for converting the speed command signal into a corresponding second torque command signal proportional to the rate of change of the speed command signal, and a second summing circuit for summing the first and second torque command signals to provide the torque command signal.
BRIEF DESCRIPTION OF THE DRAWINGS
This invention will be described in greater detail by reference to the following description taken in connection with the accompanying drawings, in which:
FIG. 1 is a schematic block diagram showing one embodiment of a motor control apparatus made in accordance with the invention; and
FIG. 2 shows signal waveforms obtained at various point
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patent: 4625834 (1986-12-01), Tanahashi
Iwasa Masao
Kito Yasutami
Mori Masayuki
Greenstein Robert E.
Maguire Francis J.
Otis Elevator Company
Smith Jr. David
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