Continuous Duty motor on VFD?

[sjohns4]

I'm looking at a submittal for a blower which has a 200 HP continuous duty motor. The motor will be connected to a VFD and will operate most of its life less than 60 hz. I was under the impression you needed an inverter duty motor for VFD operation, but I'm no expert in this department.

Also, the starting method listed on the cut sheet is DOL. I'm thinking this enough is reason to reject the motor for the application?

Can someone lend advice?

Thanks,

Mike

 

[Skogsgurra]

There is a set of possible problems when running an existing motor off a VFD. Like these:

1. Extra heating because of VFD waveform. This used to be a problem when VFDs had a very crude output waveform. But not a big problems with the newer VFDs.

2. Insulation problems. Partial Discharge in windings and between winding and stator wall. Modern motors can usually handle this, but older motors may have their insulation destroyed in a year or two. Need to know age of your motor, and insulation type, before saying anything definitive. If your grid is 480 V and above, you need to be careful. 400 V grid is usually not so risky.

3. Bearing EDM. Modern PWM inverters can kill bearings in months. Extra precautions may be needed. Test bearing voltage after you have installed the VFD. Install filters or grounding brushes/rings if peak voltages are 2 - 3 V or above.

4. Cooling at low speeds can be a problem. But a fan, with its reduced torque at lower speeds, usually doesn't mind.

Then, there is the other bunch of EMC, RCD and harmonics problems to deal with. But they do not kill the motor.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[sjohns4]

The motor & drive will both be brand new. It's a 480 Volt application. TEFC

How do you test bearing voltage?

 

[Skogsgurra]

OK, brand new motor. No probs with motor life as such.

Bearing voltage can be read with an oscilloscope with gnd connected to motor frame and input to motor shaft. Set it to 1 or 2 microseconds/division and triggered mode. Increase trigger level from zero and up. Last peak voltage before it doesn't trigger any more is your peak bearing voltage.

There are other devices that can do it also. Plus sophisticated scopes with automatic measurement. The procedure described is for simple hand-held Flukemeters, Tek THS scopes and such.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

DOL start. The most rugged motor. Suitable for all other starting methods. Some motors and motor load combinations are not suitable for DOL starting and reduced current starting methods must be employed.
Suitable for DOL start is not a reason to reject a motor.
UNSUITABLE FOR DOL START may be a reason to reject a motor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

So Gunnar, at what voltage will you start seeing an issue?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

In Europe, we don't worry much below 400 V. But some inveters are nastier than other. Have had severe damage on small fan motors also at 400 V. So, I shouldn't really say that 400 V is OK.

Doesn't hurt to check bearing voltage there, too.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[itsmoked]

No No.. I mean with your scope setup as described. 5V? 12V? 40V? When do bearing start eroding?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

OK. It depends. You are always safe below 2 - 3 volts. Problems can start as early as 5 V and it is definitely dangerous at 10+ volts.

Motor speed and bearing temperature also play a role.

Sometimes, at low RPM, you may have steel/steel and then even 2 V is a problem. Usually, you see no voltage at all at very low speeds because the oil film doesn't exist.



Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[jraef]

The submittal data stating the starting method as DOL is just because when you ask for Ttorque-speed curves for a motor, they are based on the worst case scenario on starting; DOL (Across the Line for us North Americans). That means nothing with regards to the motor's suitability. Starting with a VFD is far far less stressful on the motor compared to DOL.

If you are the one writing the specifications, I recommend being VERY clear about what you want, rather than rely on relatively undefined marketing terms such as "inverter duty". Unfortunately there is no universally accepted standard for what that means, so even if they submitted on one that said it was, that would not mean it will adequately address the issues brought up by Gunnar.

What I tell people to ask for is at least the following 3 features as a way to ensure getting a motor capable of addressing these issues:
1) Minimum Class F or even H winding insulation with 1600V minimum rating, sometimes referred to as "spike resistant magnet wire" (but again, an undefined marketing term). Another way recommended by the DOE is to use a factor of 3.1 times the applied voltage.
2) Shaft Grounding bushings. I happen to like Inpro Seals as well, and Inpro makes a nice combo package that motor mfrs can use.
3) 1000:1 speed range. Not that people need to always use this, but it sets a definitive design criteria that needs to be met with regards to cooling.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[itsmoked]

Thanks for that info Gunnar.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[electricpete]

Good idea to slowly bump the trigger voltage to make sure you catch the highest peak.

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