7 replies to this topic

[dougc]

have been having a few problems with motors burning out when used in conjunction with vsd drives. this particular installation has 3# 5.5 kw 4 pole motors driving fans, all connected to one drive. on dissassembly of motors it was found that the motor closest to the drive had compleatly burnt out one phase, on the next closest motor only 3 coils of one phase had burnt out and the last motor had one coil completly burnt out ond the second coil shown signs of over heat.
the other two phases of all the motors are in perfect condition.
all the motors are delta connected and all have the same phase rotation. each motor has its own thermal overload.
this is now the third time these motors have failed with the same fault.
has any one got any clues or had a simular problem.
kind regds

[marke]

Hello Doug

Thinking about your problem, I suspect that this could be an insulation breakdown due to the high dv/dt output from the drive. This puts more stress on the winding insulation and under some circumstances, the cable between the drive ant the first motor can become resonant and further amplify the problem.

One clue is that only some coils on one phase are damaged with the second motor, another is that the problem seems worst closest to the drive. I would suggest that if you are going to get the motors rewound, that you use higher insulation voltage wire, and that you look at fitting a dv/dt limiting filter at the output of the drive. Also investigate the length and type of cable on the output of the drive and refer to the drive manufacturers to see if there is a potential resonance problem there. You caould also talk to Murray Porteous or John Penny at Vectek in regard to a filter or suggestions.

Have a good day,

[Harry Dampers]

With respect , the Vectek filter will cost you a pile of bucks .............Have you got a Motor reactor on the output of the drive?


If not , buy a 22kW(typical current), 3%, three phase reactor from a reputable supplier and fit it in the Motor supply cable (at the output of the drive)...........it will cost you almost nothing and will sort out any dvdt problems.

Just remember that although the individual cable runs to your motors may be relatively short, the drive sees the TOTAL cable length .

Let us know how you get on.

[theDOG]

Another alternative would be to use a "motor terminator" or "reflected wave reduction device". These units connect at the motor terminals, and reduce the impedence mismatch of the cable and the motor. A 3% impedence reactor will reduce the reflected voltage, but a motor terminator is more effective. They also do not introduce voltage drops as a reactor does.

[marke]

I believe that this problem was resolved by using a different brand of drive. (including dv/dt filters??)

[theDOG]

marke,

Do you know if the drive has a dv/dt filter? Modern drive manufacturers are actually "slowing" down the turn on time (NOT switching frequency) of IGBT's to decrease the reflected wave effect. Technology has come full circle!

[marke]

Yes, you can use a integral dv/dt filter to limit the risetime of the voltage applied to the output, or you can slow down the switching time of the devices, or you can use a combination of both.

Slowing the switching speed of the power devices increases the switching power loss and requires more heatsinking to prevent the junction temperature from getting too high. It also does not necessarily correct the dv/dt as the load is inductive, and the voltage will swing from one rail to the other the moment that the switching device begins to turn off. This is well before the other half begins to turn on. With an inductive load, it is more the turn off time than the turn on time that has an effect.
As usual, there are many theories and even more explanations. A simple high frequency dv/dt filter works well.

Best regards,

[GGOSS]

Hello theDOG,

Are you able to provide any further detail regarding the 'motor terminator' described in your post of 1/7/02. Not looking for brand names as such, just a description of operation.

Look forward to hearing from you.

Regards,
GGOSS