Motor Service Factor 4

[zak]

Hi,

I have a 200kW air compressor motor operating at 415V 50Hz drawing a current of about 103% of the full load current(FLA). According to the manufacturer, the motor has a service factor of 1.1. Obviously the current is still within the allowable service factor. Can I run this motor continunously at that current year after year? Will the life span of the motor be affected by this continuous'overloading'and by how much? How do I improve the situation if this prolong 'overloading' is not desirable?

Appreciate your prompt reply.

rgds,

Zak

 

[r2d2]

Zak,
The overload your motor is experiencing (103% of rated load current)should not be a concern, provided the winding temperature does not attain a level that could be damaging.

Looking at the safety aspect, from the motor's point of view, the winding insulation being the weakest component in the motor, can and will get damaged by excessive heat.

The temperature class of the insulation system as a whole, will govern how long the motor will be able to edure the excessive temperature.

Prevention being better than cure, I would advise investigating the cause of the excessive Amps being drawn. This could help reduce your power bill!!

Check the Voltage at the motor terminals, it could be experiencing a drop from 415 Volts resulting in the excessive Amps. Next check the system Power Factor, if it is low, it could also be the cause. Also check all mechanical components, to ensure that there are no "phantom" loads acting on the motor besides the compressor.

Hope this will help you.
Regards,
r2d2.

 

[teco]

zak:-
can only agree with the points suggested by r2d2. to add further :- a motor of this size should come with thermistors, connecting these will give the windings and insulation system some type of protection against thermal damage.
some compressor manufactures rely on thermistors to protect the motor against under volts ,prolonged overload and high temp rise (restricted air flow across the motor)as well as mechanical faults.
regds.

 

[zak]

Dear Teco & r2d2,

Thanks for your responses.
If the said motor construction is designed based on class F insulation (IEC 72 & 72A) use to Class B temperature rise, will the 103% operating current still affects the life span of the motor even though with the service factor of 1.1 and runs continuously?

We believe the Contractor under size the motor. How do you consider a save/adequate design or motor selection? Do you assume 20% additional rating of the absorbed load?

We had done some checking on the 'phantom' loads particularly on the mechanical portion and also changed the tap changer on the transformer to ensure the voltage at the motor terminal is at 415V.

Looking forward to your prompt reply.

rgds,

Zak

 

[rhatcher]

It is common for original equipment manufacturers of package air compressor units to size the motor such that it must run at the service factor. I do not agree with the practice, but it is what they do (to save money). In your case it is not as bad as some, a lot of times you will find these motors running at the full rated service factor. A 3% overload should not cause a major reduction in your motor life based on the 1.1 (1.15?) service factor. My posts tend to be somewhat long so there is my answer in a nutshell. Read on if you want some more info on the subject.

With respect to the insulation temperature classification and allowable temperature rise ("motor construction is designed based on class F insulation (IEC 72 & 72A) use to Class B temperature rise&quot, this combination is standard for industrial motors with a service factor of 1.15. The Class B temperature rise indicates that at nameplate HP the temperature rise will approximately 80C. Added to the rated ambient temperature of 40C, these ratings indicate a total temperature of 120C and a hotspot temperature of 130C at nameplate HP and Voltage. The Class F Insulation temperature classification allows for a maximum allowable hot spot temperature of 155C. This is what allows this design motor to operate at the service factor without exceeding the temperature rating of the insulation. The additional 25C of allowable temperature rise equates to the ability to exceed namplate power (or current) without overheating the insulation, which in turn equates to the service factor rating of 1.15.

Will you reduce the life of a motor by operating at the service factor? Based on insulation life, the answer is no. Motor insulation service life calculations as a baseline use the case where the temperature rise is equal to the temperature class at nameplate power, ie. a Class B temperature rise with Class B insulation (ie. rated for 1.0 service factor) operated at 1.0 service factor (nameplate HP). The average insulation service life under these conditions is 20-25 years. A motor with Class F insulation and a Class B temperature rise ( ie. rated for 1.15 service factor) will have an insulation service life 6 times greater if operated at 1.0 service factor. When operated at a 1.15 service factor though, it will have a temperature rise equivalent to Class F and the insulation service life will be reduced back to 20-25 years. This is because as a rule insulation life doubles for every 10C reduction in temp and halves for every 10C increase (up to insulation temp rating, after that it drops off pretty quick). So yes, you could say that the motor insulation life will be 1/6 of what it would be if operated at nameplate HP or 20-25 years versus 120-150 years.

However, if you are thinking that many motors last 20 years but none will last 120 years no matter what then you are correct. Bearing failure, contamination, voltage problems, or overloading will inevitably limit the total life of the motor. The oldest induction motor I have seen continously running on an original winding was a 1942 unit. It had a bearing failure and although the winding tested ok, a visual inspection determined that the original glass insulated wire was in very poor shape due to oil contamination.

So, in sum, you could expect an full insulation service life of 20-25 years life out of a motor operated at the rated service factor whether it is 1.0 rated or 1.15 rated. In reality though, you will never see it. The problem with this practice is that you have no room for error since the motor is operating at its' maximum temperature. If you do have a voltage problem, an overload problem with the compressor, or a cooling problem then your motor will certainly burn up. A motor with a 1.15 service factor rating operated at 1.0 service factor (ie nameplate HP) would have a greatly increased chance of riding through such an event without damage due to the additional temperature capacity of the motor.

 

[jbartos]

Suggestion: Visit Reference
1. ANSI/NEMA Standards Publication/No. MG 1-1978 (or subsequent ones) "Motors and Generators"
a) Reference 1 Paragraph MG 1-1.43 "Service Factor-Alternating-Current Motors" defines the Service Factor as follows "The service factor of an alternating-current motor is a multiplier which, when applied to rated horsepower, indicates a permissible horsepower loading which may be carried under the conditions specified for the service factor (see MG 1-14.35). So, it is a good idea to look at it further.
b) Reference 1 Paragraph MG 1-14.35 "Application of Alternating-current Motors with Service Factors" that states:
b.A) General. A general-purpose alternating current motor or any alternating-current motor having a service factor in accordance with MG 1-12.47 is suitable for continuous operation at rated load under the usual service conditions given in MG 1-14.02. When the voltage and frequency are maintained at the values specified on the nameplate, the motor may be overloaded up to the horsepower obtained by multiplying the rated horsepower by the service factor shown on the nameplate. When the motor is operated at any service factor greater than 1, it may have efficiency, power factor and speed different from those at rated load, but the locked-rotor torque and current and breakdown torque will remain unchanged.
b.B) Temperature Rise - Integral Horsepower Alternating-Current Motors. When operated at the service factor load, the motor will have a temperature rise as specified in MG 1-12.42.1.b.
b.C) Temperature Rise - Fractional Horsepower Alternating-Current Motors. When operated at the service factor load, the motor will have a temperature rise as specified in MG 1-12.41, par. A.
That what the "Bible" says. Therefore, one must watch nameplate motor rated voltage and motor rated frequency shown on the motor nameplate when the service factor is used. This is a big risk since everyone knows what those motors are getting at their terminals. Therefore, simply, risks are fairly high, if the motor does not have a rated frequency and rated voltage stabilizers (which can get expensive!) while running within the service factor horsepower range.

 

[sparky1976]

Hi zak,
Just add the basic, How about the size of the conductor and the distance of your motor from you MCC, is the ampacity OK ?, you said change the tap on transformer, I'm afraid you will feed other motor on higher voltage than it's rated.

 

[don01]

just another small observation
Most thermal type overloads will carry a 3% overload almost indefinately.
I agree with rhatcher that to use the SF and run a motor in that region is cost cutting and its something I have 0 time for. Why not ask the compressor supplier to guarantee the motor in that configuration. After all they are telling you its all just jim dandy etc etc.
If you have thermistors in the motor all should be ok from the protection aspect.
I have reviewed our standard contract specs today and added a clause to say that the motor duty will not exceed .9 of the name plate.
After all if the maker puts x amps on the plate x amps it is not x + a fudge factor to save a couple of hundred bucks (remember these guys are oem purchasers)

Regards Don