Motor full load amps
Motor full load amps
(OP)
Is the full load amps rating on a motor's nameplate at the rated horsepower on the nameplate or at the rated horsepower times the service factor, if any?
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RE: Motor full load amps
In general the current rating which is on the name plate is given for continuous operation.
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Azmi Demirel
azzmi@elk.itu.edu.tr
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RE: Motor full load amps
RE: Motor full load amps
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.
RE: Motor full load amps
To answer your question directly, the nameplate information on a motor is at 'full load'. Specifically, when operated at nameplate voltage and horsepower, the motor will draw nameplate amps and run at nameplate speed. If the voltage or load varies, the amps and speed will vary as well. Keep in mind though that the nameplate values for a 'production' (ie. mass produced) motor are based on a general standard for that motor and that individual motors may vary to a small degree.
RE: Motor full load amps
jbartos - Your response sounds like NEMA allows operation at S.F. load without any loss-of-life penalty as long as we maintain voltage and frequency. But don't forget the last paragraph of section 14.37.1 of MG1-1998 which reads:
"A motor operating continuously at any service factor greater than 1 will have a reduced life expectancy compared to operating at its rated nameplate horsepower. Insulation life and bearing life are reduced by the service factor load."
In other words, you can expect reduced life if you operate continuously at S.F. load, even if you do maintain nominal voltage and frequency.
For more info on definition of service factor, I suggest you see responses to the thread "service factor and safety factor of motor" which was posted in this forum on 6/5/01 (sorry for quoting myself).
RE: Motor full load amps
My equation:
'HP("nameplate"="rated") = Sqrt(3)*FLA*Vnominal*eff*pf'
did not include proper unit conversion constant to make the left-hand side read in horsepower.
RE: Motor full load amps
1. 1HP = 746Watts, therefore,
HPshaft = 3**0.5 x Vratedterminal x I x Pf x Eff / 746
2. "A motor operating continuously at any service factor greater than 1 will
have a reduced life expectancy compared to operating at its rated
nameplate horsepower. Insulation life and bearing life are reduced by the service factor load."
///This statement may need some interpretations, namely, it is hard to maintain the exact rated frequency, ambient temperature, rated motor terminal voltage. Therefore, the motor reduced life expectancy is almost certain. However, one could possibly select material for the higher service factor motor that would even exceed the life expectancy of the regular motor at SF=1.\\\
RE: Motor full load amps
However, it is refreshing to see that you can not only recognize your mistakes but admit to them (definitely worth a star). I wish that all here were as forthright as I have seen some of our 'experts' go to great lengths to avoid being wrong. I won't mention any names but I think that you know who I mean.....
RE: Motor full load amps
RE: Motor full load amps
Motors having a service factor of 1.15 or higher will operate satisfactorily at unity service factor at an ambient temperature of 40 C at altitudes above 3300 feet up to 9000 feet.
Machines that are intended for use at altitudes above 3300 feet at an ambient temperature of 40 C should have temperature rises at sea level not exceeding values that would be calculated from the following formula:
Trsl=Tra[1-(alt-3300)/33000]
Where:
Trsl = test temperature rise in degrees C at sea level Tra = temperature rise in degrees C
Alt = altitude above sea level at which machine is to be operated.
For instance a 500 hp motor running at 8700 ft would need to be derated to approximately 420 hp.
In summary I feel that altitude should be considered for those of us in the high country. It shows that a motor rated for a certain horsepower must be derated to run at the proper rise.
Keep up the good work gentlemen,
Kind Regrds
motorhead1
RE: Motor full load amps
RE: Motor full load amps
1. Reference
1. ANSI C50.41-1982 "American National Standard for Polyphase Induction Motors for Power Generating Stations"
Reference 1 indicates on page 7 in footnote 6 that "For applications above 3300 Feet (1000 meters), the motor manufacturer should be consulted.
2. Would it be nice to have some References posted when it comes to equations, data, etc. to present the posting more traceable?
RE: Motor full load amps
Also Reference 3.39 section three, AC Machines in EASA Technical Manual for temperature rise at high altitude operation.
The above Aug 2, post formula is from NEMA 14.4.3
Good point jbartos, I will keep that in mind when posting any information that can be reference.
Kind Regards,
motorhead1
RE: Motor full load amps
RE: Motor full load amps
Wirenut