Soft-starter for AC motors

[radug]

Hello,

I was wondering what is the behaviour of an ac motor soft-starter (variable frequency) in regards of:
-Short circuits
-Power factor


Thanks.

 

[itsmoked]

Soft starters are not variable frequency. Do you mean a VFD used as a soft starter?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LionelHutz]

For the items you list, a soft-starter is very similar to a full-voltage starter.

 

[jraef]

First get the correct usage of your terms, then we can discuss the issue. The differences between soft starters and VFDs make a a big difference in those two issues.


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[radug]

Sorry,

I meant a VFD used as soft-starter. I would like to know what happens with power factor and above all with short-circuit. I know that a directly connected ac motor contributes to short-circuits and I wanted to know what happens with VFD connected ac motors.


Thanks.

 

[homoly]

Probably the VFD will issue alarms like f.e. short circuit or thermal overload ( i2t ) in the case of fault on the VFD output.

I do not get exactly what do you mean by PFC and VFD relation. Typically the PFC is done by external capacitor bank, and VFD used for AC motors can provide mostly only the break resistor connection and class I or class II EMC filter but not the power factor correction. But maybe I am wrong it is some time since I have solved VFD issues.

 

[jraef]

The VFD will present a good displacement power factor (typically .95) to the line, but there is a variable amount of distortion power factor to contend with depending on load. You can get VFDs with 18 pulse and/or what are called "active front ends" which can present cleaner power and better distortion pf to the line.

As far as short circuit contribution, it depends on the VFD topology. I'm not sure if there is any "official" determination as to whether a motor can theoretically contribute through a 6 pulse inverter, but technically no, it does not. Any energy left on the motor during a fault somewhere else would be blocked by the rectifier from contributing to the fault. But if you consider the very remote possibility of a complete short of the drive's power structure, i.e. line to load, then it is theoretically possible. Also, "regenerative" drives, i.e. those which pump motor energy back into the line for braking purposes, or the aforementioned active front end drives, are capable of putting energy back on the system.


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[davidbeach]

But if there are bypass contactors, very common with conventional soft starts and more common in the earlier days of VFDs, the motor will contribute to the fault if the bypass is closed.

 

[jraef]

Good point. Especially if a VFD is used just as a soft starter, it's likely that it is bypassed when at full speed.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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