Overload settings.
Overload settings.
(OP)
What is the current (har!) school of thought on what value to set an overload at on a standard motor starter? I've always used the FLA from the ID plate of the motor (if available) but it seems that some agencies recommend a significantly higher value (for motors with better than 40 degrees ambient rating and better than 1.15 service factor and with the moon in the right phase, etc. etc.)
Mike
Mike





RE: Overload settings.
For 1.15 service factor motors the service factor is already reflected in the motor FLA so O/L selection is still based on nameplate FLA.
For interchangeable heaters in particular use the right chart for the contactor size and enclosure.
The heater sizing chart for a given contactor in an enclosure may not be the same as the one for an open contactor or a reversing contactor in an enclosure.
respectfully
RE: Overload settings.
RE: Overload settings.
NOTE 1: For motors with a nameplate temperature rise rating not over 40ºC, size the overload protection device no more than 125 percent of the motor nameplate current rating.
NOTE 2: Motors that do not have a temperature rise rating of 40ºC and less, must have the overload protection device sized at not more than 115 percent of the motor nameplate ampere rating.
RE: Overload settings.
RE: Overload settings.
Anything below is legal but it is a matter of proper application and objectives and are subject to judgement of the end user.
If protection of motor itself is more important than interrupting the load it drives, it is best to set the OL setting just above (say 10%) the actual running amps. On the other hand, in many cases it may be more critical to keep the load running even if occassionaly the motor may get damaged, then it can be set to upto 125% its rated amps.
Derating due to high ambient etc., should be taken care of prior to applying the O/L and the derated amps should be treated as FLA.
RE: Overload settings.
SF should not be factored into OL protection unless it is absolutely necessary to use it. It used to be more vague but now is a NEMA recognized condition that running a motor into the SF will shorten it's life.
RE: Overload settings.
Mike
RE: Overload settings.
RE: Overload settings.
- Insulation Class eg. F insulation for 155°C
- Temperature rise above ambient temp at rated load eg. B rise for 80°C rise above an ambient of 40°C giving absolute temp of 120°C.
A 10°C 'hot-spot' allowance is made giving a permissible max temperature of :-
- F insulation = 155°C
- B insulation = 130°C
The permissible temp rise above a 40°C ambient temp is :
- F insualtion = 105°C
- B insulation = 80°C
Winding temperature is proportional to the square of current.
The difference in FLC for a motor with F insulation and B temp rise, and the same motor with an F temp rise is :-
( F rise / B rise )^0.5 = ( 105 / 80 )^0.5 = 1.1456
I have always assumed that an american motor with F insulation and an SF of 1.15 has equivalent FLC rating to a motor with F insulation and B temp rise.
Not fully utilizing the winding temperature rating provides a reserve for factors such as :-
- unbalanced voltage ( -ve phase sequence current )
- higher ambient temperatures.
- harmonic current
Can anyone confirm the above ?