Electricity: motive power systems – Induction motor systems – Braking
Patent
1989-03-13
1991-12-03
Smith, Jr., David
Electricity: motive power systems
Induction motor systems
Braking
187109, H02P 326
Patent
active
050702904
DESCRIPTION:
BRIEF SUMMARY
REFERENCE TO RELATED APPLICATION
This application discloses subject matter which may be disclosed and claimed in co-pending U.S. application Ser. No. 07/196,199, now U.S. Pat. No. 4,902,954, which issued on Feb. 20, 1990.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a control system for an alternating current motor, such as an induction motor. More specifically, the invention relates to a control system including an inverter circuit, for driving an induction motor of an elevator hoist. Further particularly, the invention relates to an inverter circuit for an elevator driving induction motor which assumes an emergency control mode upon power failure, such as service interruption and so forth.
2. Description of the Background Art
When an inverter is employed in a driving system for an elevator, driving power which is variable of voltage and frequency is applied for accelerating and decelerating an induction motor which drives the elevator cage. The amount of driving power to be applied to the induction motor is controlled by pulse width modulation (PWM) and by maintaining the current level to be applied to the inverter constant.
In modern elevator drive systems, particularly the drive systems for large size elevators, high power efficiency has been achieved by recirculating regenerated power to the inverter circuit.
In such elevator drive systems, the inverters are disabled in response to service interruption or phase interruption. Mechanical brakes are then applied for stopping the elevator cage. Since the inverter in such conventional drive systems is not in operation during emergency control operation triggered by service interruptions or phase interruption, the inertia energy of the elevator is absorbed solely by the mechanical brake. As a consequence, the mechanical brake employed in the emergency control of the elevator drive system must be a heavy duty device which is rather bulky.
Especially, in the case of the large size and high speed elevators which are now available, it is difficult to obtain satisfactory braking torque through the use of mechanical brakes. Furthermore, when the induction motor is in a regenerative mode, the regenerated energy cannot be recirculated to the inverter during emergency control initiated by service interruption or phase interruption. In such cases, a difficulty is encountered in absorption of the regenerative energy.
SUMMARY OF THE INVENTION
Therefore, it is an object of the present invention to provide a drive system for an elevator driving induction motor, which can solve the aforementioned drawbacks encountered in the prior art emergency control systems.
Another object of the invention is to provide a drive system for an elevator driving an induction in a regenerative mode motor which can reduce the load on the mechanical brake that stops the elevator and thus can allow the mechanical brake to be made compact.
In the above-mentioned co-pending application, now U.S. Pat. No. 4,902,954, a system was disclosed in which upon detecting a power failure, the control circuit prevents the flow of power through the converter to the fixed frequency AC power source and reduces 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. When the power failure continues for a predetermined time, an electric resistor is connected between the relatively positive and negative conductors to provide a dynamic braking of the motor. If the power failure occurs during a regenerative condition, an overvoltage detector will produce an indication thereof to start dynamic braking before the predetermined time elapses. But it is possible that even though a power failure has occurred it will be undetected because the control can produce AC onto the grid that appears the same as that normally provided by the AC source.
A further object of the invention is to provide a drive system for a regenerative induction motor which can prote
REFERENCES:
patent: 3829757 (1974-08-01), Frank et al.
patent: 4445167 (1984-04-01), Okado
patent: 4661757 (1987-04-01), Hokari et al.
patent: 4678063 (1987-07-01), Kitaoka et al.
Iwasa Masao
Mori Masayuki
Otis Elevator Company
Smith Jr. David
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