Soft-starter for AC motors
Soft-starter for AC motors
(OP)
Hello,
I was wondering what is the behaviour of an ac motor soft-starter (variable frequency) in regards of:
-Short circuits
-Power factor
Thanks.
I was wondering what is the behaviour of an ac motor soft-starter (variable frequency) in regards of:
-Short circuits
-Power factor
Thanks.






RE: Soft-starter for AC motors
Keith Cress
kcress - http://www.flaminsystems.com
RE: Soft-starter for AC motors
RE: Soft-starter for AC motors
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RE: Soft-starter for AC motors
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.
RE: Soft-starter for AC motors
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.
RE: Soft-starter for AC motors
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.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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RE: Soft-starter for AC motors
RE: Soft-starter for AC motors
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies