23 replies to this topic

[easyser]

We want to convert a 440 V, 60 Hz "normal" induction motor to an "inverter grade" motor by rewinding.

What are the things that need to be changed, like upgrading the insulation to Class F insulation etc., in the existing motor?

Also is it a good idea to consult the motor manufacturer before running a "normal " motor from an "inverter" / AC Drive.

Thanks in advance

easyser

[jraef]

I'm not so sure that a motor can be "converted" to inverter duty too well. There is more to it than just changing the insulation temperature rating. A few years ago some motor mfrs attempted to just derate larger motors and call them "inverter duty" but that was a disaster for them. There is a lot more to it, and most (if not all) inverter duty motors were redesigned from the frame up. Cooling at low speed is a big issue, as is the voltage rating of the insulation, bearing grounding, bearing speed ratings, balancing against mechanical harmonics etc. etc. etc.

It's always prudent to consult the motor mfr on using a non-inverter rated motor with an inverter, especially at 380V and above.

[Tua]

Hello.
Please, explaine me following:
What does it mean "inverter grade"?
It means that you want to use your "normal" motor as a generator or as motor which is controled by a driver?

[marke]

Hello Tua

The inverter grade motor is a motor that is rated to be used with an inverter (VSD). It will usually offer improved cooling to allow for operation at lower speeds and will have muc sbetter insulation in the windings to allow for the high voltage peaks that can come form an inverter. Additionally, it can have conductive bearings to overcome the EDM problems associated with some drives.

Best regards,

[Subhashish]

In inverter duty motors the windings during motor run are subjected to PDIV and measures have to be taken to
proof the winding against the same. In absencence of such measure motor may fail due to insulation failure .

[easyser]

What is PDIV ? Peak ..... Inverse Voltage ?

easyser

[jraef]

Good question. I know what you mean, but I have never seen that acronym used before. Partial Discharge Inverse Voltage? Still not clear even then.

[jOmega]

easyser & jraef:

Would you believe....... Partial Discharge Inception Voltage (PDIV)


Do a Google with the words enclosed in quotes. There's a potpouri of info on the subject available.

Also, if you are fortunate enough to be able to locate Volume 42, Number 1 (Jan/Feb 2006) of the IEEE Transactions on Industry Applications, there are several excellent papers therein; including the following:

-Michael J. Melfi's paper on "Low-Voltage PWM Inverter-Fed Motor Insulation Issues"

- Dennis Bogh, Jeff Coffee, Greg Stone, Jim Custodio joint paper on "Partial-Discharge-Inception Testing on Low-Voltage Motors"


Kind regards,

jΩ

[easyser]

JOmega

Thanks for the clarification.


easyser

[mariomaggi]

Dear all,
First of all I would suggest to divide this discussion in two parts:
- converting motors from "normal" to "inverter grade" (many people and end users are interested)

and create a new one:
- PDIV, a not-so-know argument, only for specialized people, not for massive end-users.
-------------------
Regarding conversion to "inverter grade" I would remember that in many cases it is not convenient to modify existing motors. Alternatively, it is possible to act on inverter parameters (i.e.: reducing carrier frequency) or installing a dV/dt filter between inverter and cable to the motor, or calculating the proper cable lenght to avoid undesired voltage reflections on the cable.
Inverter-grade motors could have special lamination, special bearings, special windings, special rotor, special insulation, sensors, too many special things not available on normal AC motors. Using the proper motor for that application means to have the maximum performances using the minimum quantity of energy, having also the maximum reliability and safety.

Regards
Mario

[Tua]

...Ok.

Please, answer me.
What happen if we'll mount a driver between the motor and the power line and we'll do automatic identification the motor by a driver? After that we start the motor.
I think no bad things happen.

[marke]

Hello Tua

You can use a standard motor on a variable speed drive, but there is a higher posibility of the motor failing early due to insulation issues and bearing issues.So to answer your question, in the short term, in most cases there will not be any problem.

Best regards,

[Tua]

Hello Marke!
Thank for your answer.
I have read this topic one again, but I cann't understand one thing.
Why have you decided that is especial motor or this device is very hardworking?
See

The inverter grade motor is a motor that is rated to be used with an inverter (VSD).

It is your reply Marke.
Ok. Please, show me, why this motor is especial?!
From this topic I don't undersatnd that it is especial or particular or peculiar.
Sorry me I repeated myself.

[marke]

Hello Tua

A standard induction motor is designed to operate of a three phase sinusoidal voltage at rated frequency.
An inverter based variable speed drive does not produce a sinusoidal output voltage. The inverter uses PWM techniques with very fast switching devices to synthesize a sinusoidal current in the motor, but the voltage applied is a high frequency switching waveform.
This places the winding insulation under much higher stress. Additionally, the high frequency fast switching waveforms can result in high earth currents flowing from the rotor of the motor to the frame of the motor. This can cause serious pitting of the bearings. If the motor operates for extended periods at less than rated speed, there can be issues with cooling, shortening the life of the motor.

To sum up, the speed controller places much higher electrical stress on the motor than a standard supply.

Best regards,

[Tua]

Hello Marke
I have known about these problems.
I newer asked myself about "Can or cann't any motor be connected to a driver".
Could you show any documentation or any quote of documentation with a mark(note), which explains that "Can or cann't any motor be connected to a driver"?
Please.

[easyser]

Hello Tua

A standard induction motor is designed to operate of a three phase sinusoidal voltage at rated frequency.
An inverter based variable speed drive does not produce a sinusoidal output voltage. The inverter uses PWM techniques with very fast switching devices to synthesize a sinusoidal current in the motor, but the voltage applied is a high frequency switching waveform.
This places the winding insulation under much higher stress. Additionally, the high frequency fast switching waveforms can result in high earth currents flowing from the rotor of the motor to the frame of the motor. This can cause serious pitting of the bearings. If the motor operates for extended periods at less than rated speed, there can be issues with cooling, shortening the life of the motor.

To sum up, the speed controller places much higher electrical stress on the motor than a standard supply.

Best regards,

[easyser]

I have been told by some ac drives vendors that a "normal" induction motor can be supplied from an inverter with a sine filter connected at the inverter's output. The sine wave filter ensures that the high frequency waveforms are filtered suitably from the output voltage and only sine wave voltage is applied to the motor terminals.

Any comments / views ?


Thanks,
easyser

[marke]

Hello easyser

Yes, you can connect a sinusoidal filter to the output of a drive to eliminate the high frequency component of the voltage waveform applied to the motor. This is sometimes used with very long cable runs to eliminate the cable capacitance problems on the inverter output. The filter will eliminate the high voltage and transient problems in the motor as well as the standing wave problems in the cable.

The sinusoidal filter would be used more if it were not for the high cost of such a filter.

Best regards,

[Tua]

Hello!

Ok..
I am understanding all, but I didn't see an answer.
We should use an intermediate filter, we can use an especial motor, but how have you done this conclusion?
May be from your experience, may be from your manufacture's explaination for the motor?

[marke]

Hello Tua

This is a pretty well accepted fact in the industry and many of us who have been around for a while have bothe experienced the problems, sorted the problems and read lots of information on the problems.
If you talk to any motor suppliers, or check and motor suppliers web sites, you will find reference to inverter grade motors. For example, at the Weg site, there is a brochure on their inverter grade motors and this lists some of the differences.
http://www.weglibrar...index.asp?ID=4#

You can find plenty more. If you do a goole search, you will finds lots of infomation on this subject.

Best regards,