Inverter-Duty Motor Repair 3

[zlatkodo]

Every day more and more motors operate through a VFD. It would be good to see what the practice of rewinding such motors. In addition to using special wires, which are important differences in rewind "ordinary" and "inverter-duty" motors?
Zlatkodo

 

[petronila]

Hello Zlatktodo

Standard motor drived by VHF = Premature Failure

The inverter duty motors most whitstand the fast rise time pulses and capacitive voltage potencial,the insulation and wire most be special in order to avoid P.D at lower voltages.

ID Winding features are:

Wire coated with a high dielectric strenght film: To avoid the fast rise time pulses caused by the VHF operation.

Phase and slot Insulation increase: To avoid an insulation breakdown by PD at lower voltages

Thermal calass increased(H): To whistand the overheat when the motor operates below rated frequency.

Insulated Bearings: To avoid shaft currents

VPI treatment and lacing reinforcement : To increases the mechanical strenght,due to the winding will be experiment more vibrations.Some manufacturers use 2 or 3 VPI cycles.

If motors operating below 30 Hz you will find external cooling like in DC motors.

Attached you will find a good article.

Regards

Carlos

 

[rhatcher]

Zlatkodo, from the view of a motor rewinder the most important consideration is the winding insulation. This is because of the peak voltages and the increased heating caused by the square wave voltage of a pulse width modulated VFD.

- The magnet wire should have 1600V peak insulation rating instead of the standard 1000V peak rating.
- All winding materials should have minimum Class F insulation rating (magnet wire, slot liners, wedges, varnish, ties...everything). Class H may be required for some types of VFD specific motors. Check the nameplate for 'insulation class' and/or 'temperature rise' information.
- Phase paper should always be used.
- Slot liners and top/bottom coil-slot separators should always be used.

The best practice for rewinding random wound motors is to wind all motors using Class F, 1600V, VFD rated insulation materials.

If you try to keep both types of materials on hand (ie, Class B/1000V and Class F/1600V), then the cost for material inventory will be twice as much.

The other problem with having both types of materials on hand is the risk of accidentally substituting the wrong materials into a VFD winding.

Overall, it is less expensive and more reliable to use the same high quality materials and winding practices on all windings.

 

[zlatkodo]

Thanks for both answers.
What is the opinion about the statements:
- that the concentric winding must be avoided,
- phase insulation between each coil on the end turns (not just between phases) should be used,
- medium or hi-voltage motors must have additional turn insulation on the first and last turns of each phase.
Zlatkodo

 

[zlatkodo]

One more thing.
If the motor is running above the input frequency (for example between 100% and 150% of the input frequency), whether the output voltage will be increased by the same ratio, ie, whether the winding insulation is subjected to increased voltage?
Zlatkodo

 

[rhatcher]

The VFD will maintain a constant volts/hertz ratio up to the rated voltage and frequency, such as 380V/50hz. The motor produces constant torque during this part of the speed range.

Above the rated voltage/frequency, the VFD maintains rated voltage while increasing the frequency, such as 380V/75hz. The voltz/hz ratio decreases as speed increases. The motor produces constant power in this part of the speed range. For most motors, operation above base speed is limited to 150%.

 

[starkopete]

Hi,
I am attaching some information from a motor manufacturer that might help illustrate what rhatcher and others are talking about. It must be a little dated, REA bought out Phelps Dodge's magnet wire division a few years ago and unfortunately discontinued the Thermaleze Quantumshield wire.

Thanks

 

[starkopete]

I'll try again.

 

[starkopete]

Here's the link:

http://www.toshont.com/ag/mtrldesign/AG37EM.pdf

 

[rhatcher]

Nice link starkopete.

Many EASA (Electrical Apparatus and Service Association) member shops use the latest and
best materials available.

The standard practice for my employer, an EASA shop, is to wind all low voltage, random wound motors using procedures and materials that are similar to those described in the link.

It is much more cost effective and much more reliable to use the improved, VFD rated, insulation systems on all rewinds. This reduces the inventory costs for keeping both types of materials on hand and it eliminates the chance of using that wrong materials on a VFD motor.

Finally, VFD power supplies are very common now and are often used on standard motors that do not have special VFD rating infomation on the nameplate. For this reason, a rewinder often cannot know for certain whether a VFD is being used on a particular motor.

The safest and most cost effective way to handle this is to treat all motors as if they are supplied by a VFD.

 

[starkopete]

rhatcher,
We do exactly the same thing here. Another thing we do that isn't mentioned in the article is we VPI all rewinds. As you say, it is cost effective in the long run, it gives us confidence in our work and we are giving our customers a better product.

Thanks

 

[rhatcher]

starkopete,

VPI of all windings is a good practice. I would add a suggestion. For windings in open motor (ODP) and in an abrasive environment, an over-dip of regular varnish can be helpful. Specifically, the best practice is a complete VPI cycle with curing followed by a regular dip cycle with curing.

This is because VPI varnish treatments have a lower build thickness. Although the electrical characteristics of VPI are better, the abrasion resistance, due to reduced thickness, is less. A varnish sequence of VPI then regular overdip gives the best qualities of both.