Precautions for start and stop control of AC drives and problems that might arise
STOP/START control of the AC drive
The protection of the mains supply side of the converter requires a short-circuit protection. This can be, either in the form of a set of adequately rated fuses, usually as part of a switch-fuse unit, or a main circuit breaker.
The stop/start control of the AC drive can be achieved in a number of ways, mainly:
- Controlling the start/stop input of the converter control circuit
- Breaking the power circuit by means of a contactor.
The first is the recommended method, of controlling the stop/start of an AC converter. This may be achieved, by the stop and start pushbuttons being wired directly to the control terminals of the converter, as shown in Figure below.
Alternatively, if the control is from a remote device such as a PLC, it can be wired from the PLC directly to the terminals of the AC converter, as shown in Figure below.
The second method is, the one most commonly used, for the DOL starting of normal fixed-speed AC motors. Following from the previous DOL ‘standard’ practice, this method is also quite commonly used in the industry for the control of VSDs, particularly for conveyors.
It is usually a safety requirement, to provide an emergency stop or pull-wire switch, to interrupt the power circuit when operated. While this method satisfies the safety requirements by breaking the power supply to the motor, there are a number of potential hazards with this method of control.
The main problems are:
1. Contactor on supply side of the AC converter VSD
Opening/closing the supply side of the AC converter for stop/start control should be avoided because most modern converters take their power from the DC Bus.
Every time the power is removed, the following takes place:
- Power to the control circuit is lost
- Control display goes off
- Diagnostic information disappears
- DC capacitors become discharged
- Serial communication is lost.
When the AC VSD needs to be restarted, there is a time delay (typically 2 seconds), while the DC Bus charging system completes its sequence to recharge the DC capacitor. This stresses the charging resistors, the DC capacitor, and other components.
ATTENTION:
The drive is intended to be controlled by, control input signals that will start and stop the motor. A device that routinely disconnects and then reapplies line power to the drive, for the purpose of starting and stopping the motor, is not recommended.
If this type of circuit is used, a maximum of 3 stop/start cycles, in any 5-min period (with a minimum period of 1-min rest between each cycle) is required. These 5-min periods must be separated, by 10-min rest cycles, to allow the drive precharge resistors to cool.
Refer to codes and standards applicable, to your particular system, for specific requirements and additional information.
2. Contactor on motor side of the AC converter
Opening/closing the three-phase power circuit, on the motor side of the AC converter, for stop/start control, should also be avoided, while the AC drive is running. Breaking the inductive circuit to the motor produces transient over-voltages, which can damage the IGBTs and other components.
The following is an extract from the manual of one of the leading manufacturers of AC converters:
ATTENTION:
Any disconnecting method, wired to the drive output terminals U, V and W must be capable of disabling the drive, if operated during the drive operation.
If opened, during the operation, the drive will continue to produce an output voltage between U, V and W. An auxiliary contact must be used, to simultaneously disable the drive, or else output component damage may occur.
The objective is to ensure, that the AC Converter is OFF before the contacts between the converter and the motor are opened. This will avoid IGBT damage due to transient over-voltages.
In addition, closing the motor side contactor, when converter output voltage is present, can result in a motor inrush current, similar to DOL starting. Apart from the stress this places on the converter, the drive will trip on over-current. Repeated attempts at closing the motor contactor after the converter has started, may eventually lead to IGBT failure.
Source : electrical-engineering-portal.com