Electricity: motive power systems – Induction motor systems – Braking
Patent
1988-06-06
1990-02-20
Smith, Jr., David
Electricity: motive power systems
Induction motor systems
Braking
318798, 318806, H02P 318, H02P 540
Patent
active
049029541
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to an apparatus for controlling an adjustable-speed alternating current (AC) motor coupled to drive an elevator cage.
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 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 AC motor at a variable speed. The AC 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 AC motor. The AC motor operation is changed frequency between the motoring and braking modes particularly when the AC motor is used to drive an elevator. It is necessary to flow power from the AC motor back through the inverter to the converter during the braking mode or other regenerative conditions in order to operate the AC motor with high efficiency.
It is the conventional practice to bring the elevator to a stop by applying a mechanical braking while stopping the inverter operation when a power failure occurs. However, this requires a large-sized mechanical braking system which can absorb the whole inertia energy of the elevator. This is true particularly for large-sized and high-speed elevators. In addition, there is no way to handle the regenerated power when the regenerative function is inhibited in response to the power failure.
SUMMARY OF THE INVENTION
There is provided, in accordance with the invention, an apparatus for controlling an adjustable-speed alternating current motor coupled to drive an elevator cage. The apparatus includes a converter connected to a source of AC power for converting the AC power into DC power, and an inverter connected through relatively positive and negative conductors to the converter for inverting the DC power into AC output power having adjustable frequency, voltage magnitude and current magnitude to drive the motor. The converter includes regeneration means controlled to permit flow of power from the inverter to the power source during regenerative conditions. The apparatus also includes a control circuit for controlling the inverter to drive the motor at an adjustable speed. The control circuit includes means responsive to a power failure for disabling the regeneration means to prevent flow of power to the power source and reducing the current magnitude of the inverter output power to a predetermined minimum limit enough to maintain a synchronous relationship between the inverter and the motor. The control circuit includes means for connecting an electric resistor between the relatively positive and negative conductors to provide a dynamic braking of the motor after the power failure continues for a predetermined time.
BRIEF DESCRIPTION OF THE DRAWINGS
This invention will be described in greater detail by reference to the following description taken in connection with the drawings, in which:
FIG. 1 is a schematic block diagram showing one embodiment of an alternating current motor control apparatus made in accordance with the invention; and
FIG. 2A-2H are graphic illustrations used in explaining the operation of the invention.
DETAILED DESCRIPTION OF THE INVENTION
With reference now to the drawings, and particularly to FIG. 1, a converter system 20 includes input conductors 11, 12 and 13 for receiving electric power from a suitable three-phase AC power source 10 The input conductors 11, 12 and 13 carry this power to a three-phase power rectifier 21 which is operative to c
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Kito Yasutami
Oshima Kenji
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