Alternate arrangement of VFD for chilling compressor application

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Thread Starter

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We are having a 100 HP slip ring induction motor for chilling compressor application which naturally needs a higher starting torque.. We are facing frequent problems in the rotating rotor starter. Is there any alternative arrangement of using a VFD for this arrangement. If so VFD has to be installed in stator or rotor side. Some say that there is a magnetic core available which reactance changes with rotor speed (which is max at start and min at full speed) and has no moving parts which is desirable for a fail safe automated environment. Can anyone suggest me suitable ideas regarding this. With thanks, Veda, India, [email protected]
 
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Hakan Ozevin

> We are facing frequent problems in the rotating rotor starter. I assume that you are using resistor banks. For this power range, they are out-of-date. > Is there any alternative arrangement of using a VFD for this arrangement. If so VFD has to be installed in stator or rotor side. You can use a softstarter instead of a VSD. You have to short circuit the rotor side and connect the soft starter to the stator side. In that case slip-ring motor is like a squirrel cage motor. You have to be careful about the starting current and *time* (this is usually neglected) when choosing the softstarter. There are programs to choose it. Ask your vendor or download it from internet. > Some say that there is a magnetic core available which reactance changes with rotor speed (which is max at start and min at full speed) and has no moving parts which is desirable for a fail safe automated environment. I have never heard about this, but maybe someone from the list can argue about it. It seems like a self saturated auto transformer, which is expected to have high losses. But I am not sure what it really is. Best regards, Hakan 0zevin
 
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Fintan Lawlor

I have not seen the original email but may be of some assistance, having spent over 20 years working with motor starting & control.

You must NOT short the sliprings when using a softstarter because you will have no torque & extremely high current. A resistor calculated to provide between 10 & 20% torque must be fitted. This will be shorted out with a contactor when the motor attains full load speed.

Alternatively, it may be worth ascertaining the reason for your problem.

Feel free to contact me if you want more help.

Regards
Fintan Lawlor
 
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