Using a Slipring Motor for VFD Operation

Dear Mr. SRINIVAS,

Conventional Motor can be put on to work with VFD and pl. do the following.

1. Since your motor is 500 KW, pl. provide a separate air blower (independently to operate, and by chance this blower trips, the main motor should trip) since at low speed the the quantity of air blowing will come down and the winding temp. will increase beyond permissible limit.

2. Pl. arrange a dedicated separate earthing arrangement with 2.5 times more cross sectional area of the normal earth flat needed.

3. Provide one side INSULATED BEARING or BEEARING INSULATION to prevent the shaft current circulation.

4. The motor speed to be min. 10% and above the rated speed.

5. This is optional - you may have to provide conventional starter with by-pass arrangement for VFD since the motor cannot be started against load, since VFD has got a low- starting character and cannot start against load. If you cannot have this arrangement, you have to cut down isolate the the load and start and then load.

DHAYANANDHAN.S

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In this regard pl. read below my reply on this topic posted some time back and the link is also given.

https://cr4.globalspec.com/thread/131529/Ordinary-Motor-on-VFD#newcomments

Technically we should not use ordinary motor with VFD combination for the following reasons.

1. Ordinary motor runs at constant speed with small to blow air to cool the motor. When the speed is reduced by VFD the fan speed also will come down and hence air flow will be reduced leading to rise in winding temperature and lead to premature failure.

2. The ordinary motor uses un-insulated bearings. The motor for VFD application should be fitted with insulated bearing on one side or provide bearing insulation to avoid the shaft current circulation.

3. Normally the ordinary motor speed will be (synchronus speed – slip) and slip may be 4 %, where as the VFD motor speed may require to run it above the synchronous speed, and vibration severity will increase to the square of the speed. If you consider your speed required is 1.2 times than synchronous speed, the vibratin amplitude wi,, be (1.20) ^2 = 1.44 times and the rotor and bearings may not with stand.

4. The earthing may not be dedicated one for ordinary motor an earthing flat will be alf of the current capacity where as for VFD application, it should have dedicated earthing, and earthing should be 2.5 times more in cross section if G.I flats are used.

This topic is discussed in this forum 4 or 5 times. Pl. search in this forum and you will find many members have shared their experience and knowledge.

DHAYANANDHAN.S