motor load

[bwcarter]

Is a motor that has a service factor of 115 manufactured to run at 100% load 24hrs. a day 7 days a week? Would it not be better to over size the motor a little? This question come up in a meeting today and nobody knew the answer but everybody had an opinion.

 

[electricman2]

Service factor is a multiplier that is applied to the motor's normal horsepower rating to indicate an increase in power output (or overload capacity) that the motor is capable of providing under certain conditions.
A service factor of 1.15 means that a 10 hp motor can be loaded to 11.5 hp periodically with out damage.

A motor operating continuously at a service factor greater than 1 will have a reduced life expectancy compared to operating at at its rated nameplate horsepower.

 

[EBFD6]

IMO no motor should ever run at 100% continuously, that is a bad design. You always want a little wiggle room built in to the system. Over size the motor and save from having to change out motors prematurely.

 

[Loffgren]

Re motor

Re motor

A motor is designed to run best at 100%, that is where it becomes most efficient, and where the most cooling takes place. But of course the SF is for temp overloads and if run in SF will reduces life due to heat.(From what i have read)

 

[kingpb]

Some useful info from NEMA MG-1

1.42 SERVICE FACTOR?AC MOTORS
The service factor of an AC motor is a multiplier which, when applied to the rated horsepower, indicates a permissible horsepower loading which may be carried under the conditions specified for the service factor.

12.42.1 Alternating-Current Small Motors?Motor Nameplates Marked with Insulation System Designation and Ambient Temperature
The temperature rise, above the temperature of the cooling medium, for each of the various parts of the motor shall not exceed 60 deg (A) 80 deg (B) 105 deg (F) when tested in accordance with the rating, except that for motors having a service factor greater than 1.0, the temperature rise shall not exceed the values 70 deg (A) 90 deg (B) 115 deg (F) when tested at the service factor load

* Temperature Rise (based on a maximum ambient temperature of 40?C Degrees C
** (A), (B), (F) designates insuslation class

14.37.1 General
A general-purpose alternating-current motor or any alternating-current motor having a service factor in accordance with 12.52 is suitable for continuous operation at rated load under the usual service conditions given in 14.2. When the voltage and frequency are maintained at the value 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.
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.

 

[weressl]

A motor is designed to run best at 100%, that is where it becomes most efficient, and where the most cooling takes place. But of course the SF is for temp overloads and if run in SF will reduces life due to heat.(From what i have read)

If you look at the efficiency curves motors run most efficently around 75-85% of their nameplate rating.

Tha overloading reduces the life of the motor is stating the obvious, but it would be even more accurate to state that it reduces the life of the INSULATION.

Screw compressors are notorious for variable cyclic loading and are oftendesigned that the peak horsepower reaches into the service factor area. Since the low point of the load cycle could be below 50% of nameplate the I2T heating effect on the compressor still produces less than full load continous thermal loading.