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pskaran
Active Contributor

Join Date: Nov 2009
Posts: 11

Making a Motor VFD-Driven

11/20/2009 3:01 PM

I have a 175 KW motor running with a DOL starter.Can I convert the motor into a VFD driven?

The motor is old and The motor OEM says you can't but the VFD OEM says you can beacuse the new type of VFD's are having less effect of transient voltages.

Another motor rewinding Company says they can modify the winding and replace the non drive end bearing with insulated bearing and the motor becomes fully compatible.

Please post your valuable opinion.
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tcmtech
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Join Date: Jun 2009
Location: South of Minot North Dakota
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#1

Re: Making a Motor VFD-Driven

11/20/2009 4:20 PM

Yes.

However you will need to confirm that the VFD unit is capable of working with that specific motor. Otherwise you may need to install a filter system between the motor and VFD unit.
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jayasankar
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Join Date: Nov 2009
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#2

Re: Making a Motor VFD-Driven

11/21/2009 1:20 AM

If you need to control the speed of the motor then u can use that. Already you are using DOL, So i think its not required.
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MalcolmK
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Location: UK
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#3

Re: Making a Motor VFD-Driven

11/21/2009 5:31 AM

If you can fit the VFD quite close to the motor (<10m motor cable) it is hard to see why you would have any problems with the insulation.

If you have serious doubts, add a motor output choke.

'because the new type of VFD's are having less effect of transient voltages'

Volt doubling effects are normally associated with the interaction of the motor and cable, less cable means little problem. strategies have not changed much in the past 20 years but you can set the switching frequency to a lowish level, say, 2 to 3 kHz, which will reduce transmission line effects and earth leakage. The only strategy that is particularly problematic in some applications is a bipolar which can lead to volt quadrupling rather than just doubling at the motor terminals (some ABB/Danfoss designs I think?).

Bearing currents are mostly a product of supply voltage, switching frequency and strategy, cable capacitance and bearing size (therefore motor size). At 400V levels and minimum cable run with normal size motor its unlikely to happen, in my opinion and experience, however, if critical, fit the motor choke.

If considering rewind and rebuild, why not give it a try and monitor vibration etc. If bearing damage occurs, replace before failure.

Generally, I think nearly all of these applications with standard motors, even older motors, are OK but beware of a long motor cable with no output choke / filter to prevent potential damage and failure of very old motor insulation.

What supply voltage do you have and what is the motor rated insulation class?

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pskaran
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Join Date: Nov 2009
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#4
In reply to #3

Re: Making a Motor VFD-Driven

11/21/2009 6:53 AM

Dear malcolmk,

Thank you very much for your excellent reply.The motor class is F and the distance between motor and VFD is 40meter.
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MalcolmK
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#5
In reply to #4

Re: Making a Motor VFD-Driven

11/22/2009 12:07 PM

So if you are at 400V supply I would estimate no problems with insulation or bearing currents. If 460/480V consider an output choke.

If production is absolutely critical, fit a motor choke as insurance against insulation damage.

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pskaran
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#6
In reply to #5

Re: Making a Motor VFD-Driven

11/23/2009 12:31 PM

Again thank you for the reply and people like you can make this site a wounderful paltform for Elecrtical Engineers.

Regards,

pskaran@eim.ae
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giri
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Join Date: May 2009
Location: Some where on Earth !!!.
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#7
In reply to #3

Re: Making a Motor VFD-Driven

11/23/2009 12:51 PM

good one..thank u

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rajeswari
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Join Date: May 2011
Location: Chennai, TAMIL NADU, INDIA
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#8

Re: Making a Motor VFD-Driven

07/31/2011 8:09 AM

Dear Mr.pskaran,

The Straight Answer is NO.

But, YOU CAN PUT THE SAME MOTOR with VFD for your system, PROVIDED,THE FOLLOWING IS TAKEN CARE

1. The cooling of the motor is to be taken care by providing an external blower of suitable size which should be electrically inter-locked with motor in such a way the blower will start first and UNLESS BLOWER starts, the main motor should NOT start. If BLOWER trips the main motor SHOULD TRIP.

When your old motor will run at full or rated speed the cooling of the motor is taken care. Suppose you run at 40% of rated speed with VFD, quantity of air blowing for cooling will come down and Motor winding will get more heated up. Hence external air blower is required and the capacity of the blower is to be properly calculaed.

2. Already as pointed out by other CR4 MEMBERS and your VFD SUPPLIER, you have to provide INSULATED BEARINGS or provide insulation to the existing bearing by suitable modification, in the motor. It is to be seen whether this modification is possible or not.

3. The earthing of the Motor and VFD PANEL is to be taken care and it should be 3 times more for CROSS SECTION of EARTHING required for normal motor application.

4. The Cable distance between the VFD PANEL and the Motor should NOT EXCEED 15 to 20 Metres.

5. You have NOT mentioned whether your VFD SUPPLIER is giving 6 pulse system or 12 pulse system. In your case 12 PULSE VFD is COMPULSORY,in order to control or minimise the over-all Hormonic Effect and Impact.

Thanks.

RAJESWARI.
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Users who posted comments:

giri (1); jayasankar (1); MalcolmK (2); pskaran (2); rajeswari (1); tcmtech (1)

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