Changing the rated voltage of a VFD driven motor 2

[edison123]

A 220 V, 150 HP, 50 Hz, 375 RPM motor is being driven by a 440 V VFD. The motor has failed and I want to redesign and rewind the stator winding for 440 V operation. My reasoning is that the old arrangement, you would be seeing lot of harmonics (440 V VFD delivering 220 V). With the new voltage of 440 V, the output will see much less harmonics.

Do you, drive experts, see any problem with this idea ? I somehow got a nagging feeling about V/Hz issue but just could not pinpoint it.

 

[jraef]

You are on the right track. If you think about it, when you have a 440V VFD (fed from a 440V source), the bus voltage is 622VDC. This means that no matter what the PWM pattern is to achieve a 220V RMS output (assuming full speed), the pulses are still going to be made from 622V. If it were a 220V drive, the bus/pulse voltage would be 312V, much easier on your winding insulation.

 

[edison123]

Thanks jraef. So, do you agree rewinding the motor to 440 V is better ?

 

[LionelHutz]

Are you running the motor above the rated speed? If so, you may need the drive to have a higher voltage so it can keep the V/Hz ratio constant. Otherwise, rewind for the higher voltage to reduce the stress on the insulation.

A good shop will also take care with the coil-coil insulation and the coil winding distribution in the slots to avoid applying the whole coil voltage across the wire insulation or applying the phase-phase voltage across the wire insulation.

 

[edison123]

Thanks Lionel. The client doesn't run the motor above the base speed and mostly runs it below the base speed (which makes the existing set-up still worse).

I run a rewind company and will take care of the insulation aspects for the VFD application.

 

[jraef]

I agree, although I would just add that you should rewind even if you were to need to run at over base speed, because if your motor was never wound originally for a higher voltage, the insulation they used is likely rated much much lower that what you need now in terms of withstanding the stresses of VFDs. Updating your insulation for VFD operation is always a good idea if practical.

When people ask me if it's OK to use old motors on VFDs, my usual response is to say try it, it will work for a while, but keep an eye on it and have a spare at the ready. Then when it does start releasing the magic smoke, get it re-wound for inverter duty.

JRaef.com
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[edison123]

Thanks jraef.

To clarify a little more, I am just not upgrading the insulation but actually changing the winding design itself from 220 V to 440 V (twice the no. for turns and half the wire area).

Does it matter whether the winding is wye or delta ?

 

[jraef]

No, not really. The issue is mainly the insulation.

 

[edison123]

In addition to the 220 V to 440 V change of the motor, the client wants the speed also to be reduced from 375 rpm to 200 rpm.

Below is some additional info about the drive

Drive make – Telemecanique

Model – Altivar 71 ATV71HC1 3N4( for 132KW/200HP) motor

no. of pulses – (This is available for 690V drives only, This drive has built in EMC filter and DC inductance and external input line choke)

carrier frequency- (Switching frequency)- 2.5kHz without derating from 37kW, adjustable from 1 to 8kHz

input voltage range – 380V-15% to 480V+10%

output voltage range - Maximum 3 phase output voltage equal to the mains voltage

frequency range – 0-500Hz

maximum ampere rating – 259A, 388A for 60 sec, 427A for 2 sec

cable distance from the drive to the motor – 15 meters

present operating speed range – 150 to 185 rpm


I do not understand the point about the pulses. Why would he say that the drive doesn't have pulses ?

 

[edison123]

The motor is arriving at my shop tomorrow. Then I will post more info about it.

 

[ozmosis]

I don't know what is meant by pulses within the sentence used either.
If there is something that is 'only used for 690Vac drives' then it is possible this is an output LC filter, but using the term 'pulses' is an odd one.
Is the final speed referred to from a gearbox or other method of speed reduction from the motor? seems very low if it's direct from the motor.

 

[CJCPE]

The number of pulses probably refers to the input rectifier. The standard would be a six-pulse design with larger drives having the option of going to 12 pulses or more to reduce harmonic content in the input line current.

 

[jraef]

edison123,
I know it's probably a bad time to mention this (since the drive is due to arrive), but I happen to know that ATV71 drives are somewhat unique in that they can accept up to 50% of rated voltage input (even though they don't show that on the data sheet, it should show in the manual). This means that you could feed it with 220V and run your 220V motor, no risk of motor damage from voltage pulses too high. It does also mean however, you would need a VFD twice as big as what you have coming in now because at 220V your 200HP motor current would be in the area of 480A.

So maybe the route you are taking is fine, especially if you are the rewinder. Just thought I'd mention it.

 

[edison123]

The motor details

GE make, 220 V, 75 HP, 350 RPM and.... 60 Hz (I didn't see that coming).

It is a custom-built unit with an odd frame and is used for machining glass (saint gobain). I will be talking to their engineer shortly about the set-up.

ozmosis

Thanks. I agree that the speed is too low for a direct motor application. My client imported the whole unit as a package and doesn't know why a gear box was not in this case.

CJCPE

Thanks. So you mean the pulses refer to the output of the rectifier unit ?

jraef
The supply voltage here is 415 V, 50 Hz. So, I can't use the 220 V configuration. Now in addition to the voltage change, I gotta deal with frequency change (60 to 50) and pole change (350 rpm to ~ 200 RPM). First day, first look says the pole change option may not be available.

 

[itsmoked]

There is a VFD in the picture why care(much) about the frequency? i.e. change it?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[edison123]

Keith

The pole change is to lower the base speed so that VFD output voltage is nearer to maximum to reduce harmonics. But as I said, lowering the base speed may not be viable for the existing winding configuration.

 

[LionelHutz]

You really won't be helping any harmonics by changing the base voltage or frequency of the motor so quit worrying about it.

The main concern when operating with a VFD is the insulation. This is the only reason changing the operating voltage is a good idea.

 

[edison123]

Here is the feedback.

We converted the original 60 Hz, 360 RPM winding to two speed 415 V, 150/300 RPM, 37.5 / 75 HP, 50 Hz with single winding (Dahlander winding).

The no-load currents in the shop floor (without VFD) were 33/66 Amps at 150/300 RPM.

The motor was installed today at site and was run at 150 RPM with VFD. The 132 KW VFD initially refused accept the motor saying that the KW is too low. It wanted 45 KW minimum. So the client false-fed 45 KW and the motor ran up to full speed in 10 seconds with 33 A no-load current.

Why would the drive want a minimum KW requirement ?

* I would go green if only I were not yellow *

 

[jraef]

Too smart for its own good?

More likely it was a load loss detection circuit that was enabled unknowingly. Either that, or the drive did an auto-tune function and determined, through resistance values or some other method, that the motor was smaller than you said it was, so it was insisting it knew better than you did. It is a French drive after all!

 

[edison123]

Thanks Jeff.

How do I make these French understand that we know better ?

I am asking the client if the drive has the facility to program the winding resistance values (instead of auto tuning). Do you think inputting these values will make this problem go away ?

* I would go green if only I were not yellow *