Service Factor

[rizwan]

Hello,

 

What is the role of the "Service Factor" of a motor in determining the set point of the Over-load relay ? The service factor is normally given on the name plates of the motor.

 

Thanks for any reply.

 

Rizwan.

[jOmega]

Service Factor has no direct relationship to the setting of the motor overload relay.

 

Normally, the overload relay setting is based upon the motor RATED Full Load Current.

 

Service Factor is a Multiplier of the motor's rated power;........not current !

 

As an example, consider a 100kW motor with a nameplated Service Factor of 1.15

 

That means that you can operate that motor continuously at 115 kW (power)....... NOT 115% current.

 

Because of an improvement in motor power factor when increasing the load above 100%, the 115% POWER will occur at a value LESS THAN 115% current.

 

Also, be advised of these considerations.

 

1. Operating the motor at the service factor, will produce more heat that needs to be dissipated by the motor.

Typically, a motor with a Class B temperature rise at 100% power continuous, will be rated at a Class F temperature rise when operated continuously at the Service Factor rating..

 

2. Operating a motor continuously at the service factor, because of the increased heat, it will have a shorter service life than the same motor operated continuously at it's 100% rating. Motors that run cooler typically have a longer service life.

 

So, if you need to obtain the Service Factor power from the motor, you are well advised to ask the motor manufacturer what the corresponding amperage is at the Service Factor. Once you have that value, you can make the correct setting for the overload relay.

 

 

 

[kezetrum]

Service Factor has no direct relationship to the setting of the motor overload relay.

 

Normally, the overload relay setting is based upon the motor RATED Full Load Current.

 

Service Factor is a Multiplier of the motor's rated power;........not current !

 

As an example, consider a 100kW motor with a nameplated Service Factor of 1.15

 

That means that you can operate that motor continuously at 115 kW (power)....... NOT 115% current.

 

Because of an improvement in motor power factor when increasing the load above 100%, the 115% POWER will occur at a value LESS THAN 115% current.

 

Also, be advised of these considerations.

 

1. Operating the motor at the service factor, will produce more heat that needs to be dissipated by the motor.

Typically, a motor with a Class B temperature rise at 100% power continuous, will be rated at a Class F temperature rise when operated continuously at the Service Factor rating..

 

2. Operating a motor continuously at the service factor, because of the increased heat, it will have a shorter service life than the same motor operated continuously at it's 100% rating. Motors that run cooler typically have a longer service life.

 

So, if you need to obtain the Service Factor power from the motor, you are well advised to ask the motor manufacturer what the corresponding amperage is at the Service Factor. Once you have that value, you can make the correct setting for the overload relay.

 

 

service factor (SF) is an indication of how much overload a motor can witchstand when operating normally within the correct voltage tolerances. It is a multiplier that is applied to the motor's horsepower rating to indicate an increase in power output (overload capacity) that motor is capable of providing "under certain conditions". In general it is not a good practice to size motor to operate continuosly above rated load in service factor terms, for a certain conditions, as per jOmega said also.

 

If the motor will operate at or near the service factor, the appropriate FLA of the motor at its

service factor should be used to select the overload size from the manufacturer’s chart. It is better I think.

 

Regards

 

 

[jOmega]

kezetrum,

 

One needs to be careful in the use of terminology. For instance, the term "overload";...... many would define this term as an increase in current; and in some instances they would be correct. However, when discussing Service Factor, it must be made absolutlely crystal clear that overload is motor power .... and not motor Full Load Amps.

Kind regards,

[kezetrum]

ok jOmega..

 

NEMA Standard MGI-143 defines service factor of an ac motor as "...a multiplier which, when applied to the rated horsepower, that indicates a permissible horsepower loading which may be carried under the conditions specified for the service factor..."

 

and I think that Riswans point is connected to NEC 430.32 (A) (1), 430.32 ( (1) and 430.32 ©.

 

Regards

[GGOSS]

jOmega,

 

I fully accept the point you make in your post of November 14 however I am still not sure about how SF is implemented in many of the newer 'high end' electronic motor protection relays.

 

These devices may for example allow the user to enter values for Motor FLC, Motor LRC, Maximum Permissable LR Time and Service Factor. Are you able to comment on how the overload protection curve is altered through entering a value of SF greater than 1? Also, your thoughts on how SF may impact on electronic motor protection systems designed to comply with IEC 60947-4-1 that also offer SF as a user adjustable feature, would be appreciated.

 

Thanks in advance.

 

Regards,

GGOSS