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Using a Slipring Motor for VFD Operation

03/05/2020 11:52 AM

We are having a Medium voltage(3.3KV) slip ring induction motor.The specifications are given below.This motor is used for Preheater fan in a cement plant.We want to use this motor with variable speed.We want to connect it to a VVVFDrive(Variable voltage,variable frequency drive) and run it at various speeds.We want to operate Preheater fan at different speeds.Request your guidance to know whether I can proceed to do so and what is the care I should take. Thanks.

Motor Specifications.

3 Phase AC Induction motor, Make: Crompton Greaves

KW : 500

RPM:988

Full load Current : 118A Amp

Voltage:3300

Duty ; S1

Frame Size : BTPR450E

Insulation ; Class F

Weight : 6250 KG

Bearing No.

DE : NU226C3

NDE : NU221 C3+6221C3

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Guru
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#1

Re: USING A SLIPRING MOTOR FOr VFD OPERATION

03/05/2020 11:55 AM

These people can help over the telephone.

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Guru

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#2

Re: Using a Slipring Motor for VFD Operation

03/05/2020 1:31 PM
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Guru

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#3

Re: Using a Slipring Motor for VFD Operation

03/05/2020 10:34 PM

Another name for your motor is wound rotor. You can dynamically adjust the effective rotor circuit resistance and use it at any speed you want, considering the load torque of the fan at different speeds and temperature.

There is a nameplate for your secondary winding, usually separate from the motor nameplate.

No need for 3300volt phase controls, these solid state controls work in the rotor circuit

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Guru

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#4

Re: Using a Slipring Motor for VFD Operation

03/05/2020 11:43 PM

Short the rings.

The motor behaves similar then to a standard squirrel cage motor.

Check with your drive supplier and the motor manufacturer - it is too expensive to just play with.

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Guru

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#6
In reply to #4

Re: Using a Slipring Motor for VFD Operation

03/06/2020 10:25 PM

There is some testing to be done before shorting the slip rings, including verifying that your motor runs with some rotor resistance at full speed or not.

Rather than experiment with expensive high voltage inverters, it makes much more sense to use the motor the way it was designed, with a variable resistance connected to the rotor winding. This provides the most starting torque with very little effort.

ABB last held the patent on slip power recovery system, which allows variable speed operation of wound rotor motors. This may have been a G.E. innovation, back in the '70s or '80s. The SPRS system is described in the link:

https://library.e.abb.com/public/a871dbcb1293adc948257d080018579f/EN_ACS800_IGBT_SPRS_Brochure_A.pdf

You may find this a cheaper inverter than the 3.3kV model, and gives you the ability to recover the energy normally dissipated in the resistors during part speed operation. Plus, if the VFD fails, you can always operate it with the resistors. If the 3.3kV VFD fails, you will have to put the resistors back, as the wound rotor motor will likely not be able to be started across the line with shorted sliprings.

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Guru
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#5

Re: Using a Slipring Motor for VFD Operation

03/06/2020 9:33 PM

Find a trusted supplier of medium voltage drives in your area and explain to them what you want to do. Then yes, you would short the slip rings and use it as a standard induction motor.

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#7

Re: Using a Slipring Motor for VFD Operation

03/06/2020 10:59 PM

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

-------------------------------

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

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