Overload settings. 2

[MetalworkerMike]

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

 

[waross]

I follow the relay instructions. All are based on nameplate current. Some set at relay current exactly, some, such as the old Square D melting alloy heaters recommend a part number that is 125% of nameplate current.
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

 

[LionelHutz]

It is typically the FLA times the service factor. But, there is a modification to this depending on the ambient and the voltage imbalance. Download a Multilin 469 manual to see more details of this, it's explained fairly well in that manual.

 

[whycliffrussell]

O/L Size = FLA (Nameplate) * S.F.
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.

 

[controlsdude]

I remember some time ago looking at what SF means and using it to size the overload is not recommended. I always sized the overload at the FLA of the motor. I believe their are past posts on SF or was it an internet search.

 

[rbulsara]

Only Code related issue is that maximum O/L setting permitted is 125% of rated amps (FLA).

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.

 

[jraef]

The service factor debate has taken place here several times over the years, although the threads may have dropped off by now.

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.

 

[MetalworkerMike]

It's kind of amazing that there are so many schools of thought on this issue. That's why I asked about it. It seems that you can ask five guys and get five different answers. The 125% number is one that I have seen bandied about, and the process outlined by 'whycliffrussel' is the one that I had alluded to in my first message. To make the muck even deeper, it seems like every time I go to program a VFD they have an overload value that defaults to 1.5 or 1.6 x FLA. It makes you wonder.

Mike

 

[whycliffrussell]

SF (IT DEPENDS ON WHAT YOU ARE RUNNING YOUR MOTOR AT)...ie: the load.

 

[EddyWirbelstrom]

In Australia, motors are specified in terms of :-
- 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 ?