motor space heater restoration practices 4

[electricpete]

If a large motor (>500hp) space heater is deenergized during a maintenance period (with motor secured also), should we require a minimum time for space heaters to be energized prior to starting the motor?

In some cases, the maintenance period may have included insulation resistance test, although it may have been very early in the maintenance period. In other cases, there may have been no insulation resistance test (the space heaters may have been unavailable due to maintenance which deenergized the power supply to the space heaters).

Motors in question are in all types of environments: outdoors, enclosed in non-air-conditioned building, and enclosed in air-conditioned building.

The duration of space heater deenergization in question may be 1 day to 1 month.


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[TheBlacksmith]

I would check the insulation resistance at the conclusion of the maintenance and if it is less than your nominal specifications, energize the heaters until it rises above that number or at least 12-24 hours, whichever is more. In some cases, supplementary drying may be needed. Any of these environments, depending on the weather and time of year, may have caused condensation in the windings, which is what the heaters are to prevent.

Blacksmith

 

[Mendit]

The space heaters usually have only enough wattage to keep the dewpoint above ambient(prevent condensation forming). After a lengthy shut down it may be required to dry the windings if they are located in a very humid environment. This can only be determined by insulation test as stated by blacksmith. If the test values are low drying would be required to bring the values back up. Either Low AC accross the windings or use a welding machine. The space heater watts are not enough to heat the mass of the iron core and windings no matter how long you leave it on.

 

[petronila]

Hello Pete, Interesting question.

I think you most have to take account the age of the motors, and the seasons change because is not the same start the maintenance in winter and finalize in summer or start in summer and finalize in winter, the ambient temperature is different.

I think is a risk if you don´t have the chance to test the insulation, for this is very important you count with all maintenance records including PI, and Insulation test prior the maintenance.I know by experience some large and old motors running with 70 MOhm and stops for maintenance, 1 mounth later with space heaters connected the corrected Insulation is only 5 MOhms.

Is very important to test the Insulation before and after perform the maintenance. The space heaters are used to prevent condensation on windings and other internal parts when the motor is not runing, the idea is to keep the windings warmed 5 or 10 C above the ambient temperature.

The less you can do after maintenance is to keep warm the winding closing the end belts and using some lamps inside the stator.

regards

Petronila

 

[aolalde]

Hello Electricpete.

I agree with the opinions expressed above.
The space heaters will never dry a winding if it catches moisture. The power they provide is just enough to avoid condensation and will keep the stator only a few degrees above ambient.
After servicing and before energizing a motor, always measure the IR (insulation resistance) and PI (Polarization Index), it does not matter the age or time it has been idle, it could catch moisture in mater of hours.

 

[electricpete]

I think IEEE43 suggests that megger readings for purposes of condition determination should be taken when the motor is still warm so the least temperature correction is required.

Logic would dictate to do the test at the beginning of a maintenance period so if there is an actual problem (not just moisture) you have time to do something.

If we do it at the front end of the maintenance period to get that good data, it sounds like you guys are saying that we also need to do it at the back end. Lots of work. Particularly in some cases where we have to determ the motor to get any good reading since the cable sometimes is in the neighborhood 50-200 megaohms while the motor is typically 500-5000 megaohms plus for our 4kv and 13.8kv machines. Or do you think 100 megaohms temperature corrected and 2.0 PI is enough to energize it immediately? Regardless If I just turned the heaters on?

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[electricpete]

I think it's worthwhile to compare the motor repair shop enviroment and plant environment.

In a motor repair shop, it is very difficult to energize space heaters and very easy to do a megger. The logical choice is to never energize space heaters and always do a megger immediately before energizing any motor in a repair shop.

Things are somewhat opposite in a power plant. It's easy fairly easy to get the space heaters back on after maintenance. It may not be quite so easy to do a megger in a power plant as it is in a motor shop. (For example, if my space heater power was down but motor was not being maintained, I now may have to hang a tagout to megger the motor).

I would like to know how many people have ever seen the practice that you guys recommend (always megger if space heaters off) in a power plant or other non-motor-repair industrial facility.

What is the off-time of space heaters before this mandatory megger requirement takes effect? 5 minutes? 1 hour? 24 hours?

Sorry I am not really trying to be argumentative but trying to play devil's advocate. What you guys are suggestign is foreign to me and I think different than what most plants do.

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[electricpete]

Also I will have to convince plant customers to do this extra work if I make it mandatory, so I have to be prepared with some solid technical arguments.

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[Mendit]

Typical in Western Canada, the motors are Tagged more than 1 voltage supply, when the motor is off the heaters are on. There are no thermostats on the heaters. Most effective here, we use the flexible type (Heatron Kansas) which are tied direct to the end turn on the stator coils. Thet surface temperatures of them are often between 130 and 150 F. They must never be overlapped. If they are too long the ends must pass each other with a minimum of 1/2" space between the turns. Some OEM's use a flat pad heater and glue it with silicone to the bottom of the stator case, (not much good here). The other type hard coil elements typically 440 V but only supplied with 110V to keep the surface temperatures down. These are not effective here unless positoned properly. In Houston with 100% humidity it may be different.