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Vsd Failure


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#1 anu_rags

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Posted 19 August 2008 - 06:59 AM

Dear All,

We had installed a VFD for a rack and pinion hoist (passenger and material elevator in construction industry). The drive had inbuilt brake chopper and brake resistor was connected to it. Brake resistor had been mounted on top of the elevator. The drive was operating satisfactory for 2 days and on the 3rd day there was some smoke coming out from the resistor and the main breaker tripped. Once the supply was regiven customer tried to start drive twice but it didnt started. Later the drive was completely broken and there was no display. These events have been described by customer, so Im not sure if the drive was broken the first time itself when breaker tripped or it broke later when they tried to restart it.

The opened the resistor and found nothing burnt inside. They also checked the resistance and found OK. Im not sure if they properly did the continuity test. Later it was replaced by new spare VSD by the customer and as soon as they connected the brake resistor cable to the VSD, the main breaker tripped. That confirmed to them, that there is something wrong in resistor only. Later when our service engineer reached the site and opened the resistor, he found that one of the cable sleeve inside the resistor panel was cut and the cable was touching the body. Resistor manufacturer is explaining that the cable had not come with cut when delivered as the VSD has run without any problems for 2 days. Since there is vibration in the elevator, the inside panel acted like knife and cut the cable.

When new VSD was connected, the main breaker was tripping on earth fault as it was detecting cable touching with the body. During running when the cable touched body, the breaker tripped. Why did the 1st VFD got broken? It should have tripped on overcurrent or earth fault.

Regards,
Anurag


#2 marke

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Posted 19 August 2008 - 07:19 PM

Hello Anurag

Without a detailed analysis of the failure, it is very difficult to make much of a comment, however if there was arcing between the cable and the case, there can be very high voltages developed and these voltages can cause components to fail.

I suspect that the arcing formed a "boost" circuit with the DC Bus and elevated the voltage on the bus and caused the switchmode supply to fail.

Best regards,
Mark.

#3 mariomaggi

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Posted 21 August 2008 - 07:29 AM

Dear anu_rags,
in an inverter drive, the most critical part to protect is the braking resistor.
There are no sure and cheap solutions, if there is a short circuit between resistor wires and earth.

You have only to realize all wirings in a very professional way, considering also vibration problems.

Regards
Mario

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#4 marke

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Posted 22 August 2008 - 09:06 AM

Hi Mario

It is also common to undersize the cable to the brake resistors.
The current into the brake resistor is pulsed, so the losses in the cable are much higher for a given average current, than with the same average current from DC.
This can result in the softening of the insulation and consequential insulation brake down.

Best regards,
Mark.




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