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Why Thyristors in Soft starters burn. I am using a 400 KW AC3 rated soft starter to start a 300KW motor. Power supplied by a 750KVA Generator set. So far four soft starters are burnt during testing?
Quazi
Did you measure the currents?
Look also to the voltage (peaks). Thyristors can damage due to voltage peaks form the grid (in your case the generator). Are there more loads connected to the generator which can cause voltage peaks?
Dear Sir,
One of the possible reasons for voltage over-shoot is your Generator which is not rated much beyond your load. Please note that any AC Motor will draw heavy current at start-up depending upon load characteristics which may even be four times than your rated current. Under such circumstances, your generator will be overloaded and frequency and voltage may take a momentary dip. ( Yes, Dispite using a soft starter!). This will create a complex situation which is will make the governor of Generator feed more fuel for increasing the speed and exitation of Alternator to inject more current to increase voltage.
This may make the voltage to overshoot momentorily, which might probably destroy your thyristors. If more details are provided, like when do thyristors fail, at startup or while running, may help analyse this better.
Hope this helps.
Mahesh Vyas.
mail@archana.co.in
There may also be a possible ground fault within the motor or the wires to the motor. The ground fault occurs when the motor is conducting through an alternate path than the phase wires. Some systems use a current transformer around all the phase wires to determine if there is a ground fault as it will detect any time the currents don't sum to zero in a three phase system.
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∙ 13y ago
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