PMSM unknown voltage withstand rating and using it at higher voltage (600V vs 480V)

[patm72]

How do motor manufacturers do to obtain a given class of insulation? I am mostly interested in knowing if a motor that I know runs very well with a 480V rated VFD (supplied with the same line voltage), can run well with 600V as the line voltage at the VFD. Obviously, the VFD would be rated for 600V in that case.

The motor is a high-speed PMSM (18krpm +) with an unknown dielectric withstand rating. It currently runs very well with a line voltage of 480V to the VFD, and we use a sine wave filter between VFD and motor, resulting in a voltage measured at the motor leads of some 300-350V, depending on the power size. Bearings are not the typical ball/roller bearings, they are air foil. Pitting is not an issue. Insulation class is H, temp rise is B.

My concern has only to do with the pulses magnitude and their possible stresses on the insulation at this point.

The manufacturer of the motor would not give me much in terms of answering my question, in case you intend to make me ask. It is as if they don't know themselves what they are doing, but it is indeed a reliable motor, and specifically adapted.

What could I ask for about their manufacturing process that might help reveal what voltage insulation they are getting?

Cheers!

 

[waross]

Insulation: You would probably have to ask the manufacturer of the winding wire, rather than the motor builder.
The motor designer will send a spec to purchasing and purchasing will find a product to fill the spec.
How the wire manufacturer meats that spec is the wire manufacturers problem.
BUT.
This problem is common in Canada, where 600 VAC is the standard voltage but special equipment may not be available for 600V.
The tried and true solution is a pair of auto-transformers in open delta to adjust the voltage.
480:120 Volt transformers are used.
The 120 Volt windings must have a current rating of at least equal to the load current.
Case 1; A plant with a mix of 600 Volt equipment and 480 Volt equipment.
The 600 Volts supply fed a 600 Volt MCC and a 600:480 Volt auto-transformer that fed a 480 Volt MCC.
Some 480 Volt motors were changed for 600 Volt motors. It would be difficult to change the supply to the 600V MCC so a second open delta auto-transformer set was installed next to the motors to transform back up to 600 Volts.
Case 2; A 600V plant needed temporary power for a turnaround (maintenance shutdown). 480 Volt standby generators were rented along with auto-transformers to supply temporary 600 Volt power.
Some rental ventilation fans for confined spaces were rated 480 Volts. A second set of auto-transformers stepped the 600V back down to 480 Volts.

Another solution that is appropriate for long runs from the VFD to the motor:
A full transformer stepping the 480 Volt VFD output to 600 Volts for the motor. The transformer acts as a filter and reduces the high voltage transients at the motor.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[patm72]

Thanks Waross for responding.
I indeed possibly need to push for the specs the motor designer has given to the wire supplier. The answer probably lies there.
On the other hand, the idea behind my post is exactly to avoid using autotransformers. This is the solution we have currently, but it would simplify things if we could just change the VFD and SF and be done with it. If I can be sure the motor can handle the higher stress from a higher voltage on a VFD DC Bus, I am good.

 

[waross]

You may ask if the same insulation is used for 575 Volt motors as for 460 Volt motors.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Gr8blu]

Some key points in the original post:
1) The insulation "class" refers to the insulation SYSTEM's ability withstand thermal stress, not voltage-induced (i.e., dielectric) stress. Note that this is somewhat independent of the capabilities of individual insulating materials (strand insulation on the conductor itself, possibly dedicated turn and/or phase insulation, and possibly ground wall insulation). In most cases, they match up pretty well, but there is always a "weaker" link in the chain that sets the limit.
2) Does the motor nameplate specify "inverter duty"? Or does it say "inverter capable"? Or does it say something else entirely? This is where the allowable voltage stress comes into play. Something with "inverter duty" stamped on it is designed to withstand the transient overvoltage conditions associated with a drive that outputs the motor's RATED NP VOLTAGE. It may survive operating at the new-and-improved higher terminal voltage level on a continuous basis, but may NOT be able to withstand the higher transient amplitudes associated with the higher terminal voltage.
3) As others pointed out - ask if the SAME insulation is used for 575 (and even 600) V motors. Chances are, unless the manufacturer does a LOT of business with the Canadian market, the answer is "no".

In the opening post, the way I read it is the "old" drive is rated for a 480 Vac input, and the desired output is in the 300-350 Vac range. The question is whether a "new" drive can be fed from a higher voltage supply (600 Vac for example). The answer to this part is YES - provided the drive is rated for a 600 Vac input condition. Does not matter what the drive output voltage is, as long as it's at or below the approximate input level. This means a 600 Vac (in) can produce a 300 or 350 Vac (out) condition just as easily - with no apparent difference to the motor at all if the both the peak transient and common mode voltages of the new drive is no more than that of the old drive.

If, on the other hand, the intent is to ALSO increase the terminal voltage at the motor (presumably by the same proportion as 600/480) then more details on the STRAND and PHASE insulating materials (and methods) is necessary. The ground wall insulation should be good for either condition - the typical minimum for a machine of this type is in the range of 1000 V between conductor and ground.

Converting energy to motion for more than half a century

 

[waross]

With 600 Volts, the RMS voltage may be the same but the DC bus voltage is related to the input voltage the pulse peaks will may be higher.
(4 x 115V = 460V. 5 x 115V = 575V
600 Volt rated motors are as rare as 480 Volt rated motors.
They may be available from offshore manufactures who are not familiar with North American standards.)

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[iop95]

Depending of VFD/motor power level may looking for an AC/DC converter 3x600Vac/650Vdc (standard rectifier + buck converter or controlled rectifier with good DC filter).
Need to check if VFD has DCbus external connections and it's ok with DC input only.

 

[waross]

iop95:
I believe that 600 Volts supply is standard in patm72's location and that he is using auto-transformers to drop the 600 Volts to 480 Volts.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[iop95]

waross:
Yes, so to avoid auto-transformer may use a converter from 3x600Vac to VFD DCbus that must be around 650Vdc (480x1.35).

 

[edison123]

If you are not changing the motor operating voltage, I don't see how the VFD input voltage change affects the motor.

OEM's won't give out proprietary winding design data.

Muthu

http://www.edison.co.in

 

[waross]

Assuming that the VFD uses Pulse Width Magnitude to control the frequency and the effective voltage:
The transients and harmonics are generated by the leading edge of the square wave pulses.
Control is affected by the width the pulses.
Each pulse is created by switching between zero volts and the DC bus voltage.
So with a 600 Volt supply, the pulse magnitude will be higher than with a 480 Volt supply, regardless of the effective output voltage.
Two transformers in open delta auto-transformer configuration is probably the cheaper solution compared to trying to convert the DC bus voltage.
Even at very low effective voltages and frequencies, the pulse magnitude is bus voltage and the rise time (square wave) is the same with the same risk of exciting ringing and higher voltage harmonic transients.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[LionelHutz]

Well, the VFD output will be a varying duty square wave that is about 650V peak to peak on 480VAC or 850V peak to peak on 600VAC. Then, there is switching transients that ring the edges even higher. However, using a sine wave filter theoretically turns that back into a sine wave at the RMS output voltage the drive is simulating, meaning that the change in input voltage would have no effect on the voltage the motor sees.

In my experience, motor manufacturers use the same insulation for all voltages of motors.

 

[edison123]

Yup, there is no special ground insulation for 600 V. The magnet wires must be VFD duty rated and they serve well up to 690 V.

Muthu

http://www.edison.co.in

 

[patm72]

Thank you all for your inputs.

The application is in Canada, so 600VAC is the norm here. The idea is to simplify the design and hopefully be able to get rid of the autotransformer, so any use of other hardware like a converter is not ideal nor the goal here, I would just have as much complexity to my machine, which has limited space inside.

I intend to feed my motor with the same voltage, and if by adapting it with the right sine wave filter I am effectively shedding it from the inherent higher voltage pulses and transients resulting from the 600VAC input, I will concentrate on that.

 

[waross]

After comparing costs of a 600 Volt VFD and a sine wave filter with the costs of a 480 Volt VFD and autotransformers, you may opt to continue with the 480 Volt setup.
If space is limited, don't forget to compare the physical size of the auto-transformers with the size of the filter.
Another factor: The auto-transformers may be installed anywhere between the supply and the VFD.
The filter must be installed between the VFD and the motor.
Don't forget to check the price for a 600 Volt VFD and 600 Volt motor.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[patm72]

Waross:
The cost is obviously going to have some influence on the final decision, but not much. As mentioned initially, the motor is specific, can work on 480V (at VFD input), but can questionably be used with a 600V VFD line feed.

Sine wave filter is there already with 480V systems, to be adapted if used with resulting higher DC bus voltage from 600V feed. But aside from its upgrade to withstand higher voltages, it is not really a burden costwise - yet and we'll see. Believe me, when Sales want something, they can disregard costs quickly!

We have a finite footprint inside the equipment enclosure and currently, it is in fact necessary to install the autotransformer outside. Getting rid of that to supply the equipment 600V directly, in its finite enclosure, would be advantageous.