Electricity: motive power systems – Braking – Dynamic braking
Patent
1996-12-23
1998-09-29
Masih, Karen
Electricity: motive power systems
Braking
Dynamic braking
364483, 307 68, 318783, 318471, 318490, H02P 314
Patent
active
058149546
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to an inverter for controlling an operation of a servomotor, and more particularly, to a method of and an apparatus for protecting a regenerative resistor connected to an inverter.
BACKGROUND ART
FIG. 12 shows a conventionally known inverter-type control device for a servomotor. In FIG. 12, a DC link 10 connects a power supply unit 30 for rectifying an alternating current from an AC power source and for supplying DC power, and an inverter 20 for inverting a DC voltage from the power supply unit 30 and for supplying it to a motor M. The inverter 20 is controlled by, for example, a transistor PWM control circuit 21.
In a control of a servomotor by the inverter 20, a smoothing capacitor 4 is connected to the DC link 10. A voltage applied to the capacitor 4 changes depending on the operation modes of the motor, including acceleration, constant-speed, and deceleration modes. In the acceleration mode, for example, electric power is supplied from the power supply unit to the motor, so that the voltage drops. In the deceleration mode, in contrast with this, energy is returned from the motor to the power supply unit, so that the voltage rises.
While a field capacitor is used as the smoothing capacitor 4, for example, the capacitor used has a predetermined dielectric strength level. If a voltage higher than the dielectric strength level is applied to such a capacitor, the capacitor will get out of order.
Thus, the capacitor 4 is protected by connecting a regenerative resistor 5 in parallel with the capacitor 4. In FIG. 12, a level detection circuit 7 turns on a transistor 14 to connect the regenerative resistor 5 with the DC link 10 when it detects that the voltage of the DC link 10 has become equal to or higher than a set voltage. By doing so, electric current is made to flow through the regenerative resistor 5 so that the voltage applied to the capacitor 4 is lowered. The regenerative resistor 5 generates heat corresponding to the applied power, thereby consuming an excess power due to excess voltage.
If the regenerative resistor 5 consumes the excess power due to excess voltage until over-regeneration occurs, the regenerative resistor 5 itself is thermally broken to be fused, and becomes unable to protect the capacitor. It is necessary, therefore, to protect the regenerative resistor and cut off a power supply circuit from the servomotor, thereby protecting the motor and the supply circuit, by detecting the over-regeneration before the regenerative resistor is fused.
The voltage of the DC link and its fluctuation can be satisfactorily coped with by using a resistor with an adequately large capacity (wattage) as the regenerative resistor 5. In order for the regenerative resistor to be increased in capacity, however, its volume needs to be increased to withstand a large amount of heat resulting therefrom. In view of the packaging space and cost of the regenerative resistor, however, it is to be desired that the regenerative resistor for the inverter should have as small a capacity as possible. In general, a regenerative resistor has a fusing characteristic such as the one shown in FIG. 13, and will be fused in a short time if an excessive electric power is applied to it.
Conventionally, therefore, the regenerative resistor is protected by the following method, for example.
(1) Protecting method using a thermostat: FIG. 14 is a diagram for illustrating protection of the regenerative resistor by means of a conventional thermostat. In FIG. 14, a thermostat 2 is attached to the regenerative resistor 5 which is connected with the DC link 10. The thermostat 2 is opened and closed in accordance with heat generated by the regenerative resistor 5. A detection circuit 3 detects the opening and closing the thermostat 2. In the case of over-regeneration, the circuit 3 gives an alarm to stop the voltage application to the regenerative resistor, and cuts off the DC power supply unit, thereby protecting the regenerative resistor 6 and the servomotor.
(2) Protecting method using an
REFERENCES:
patent: 3584241 (1971-06-01), Nakamura
patent: 4695961 (1987-09-01), Arinobu
Horikoshi Shinichi
Suzuki Naoyuki
Fanuc Ltd.
Masih Karen
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