Electrician Talk banner
1 - 9 of 9 Posts

·
Registered
Joined
·
420 Posts
Discussion Starter · #1 ·
Doing a small job and have a three phase motor question, this motor is a 208 volt 7.5 hrp and says 21.5 amps. Do I have calculate 1.73 just like I would a three phase transformer? I'm getting 12 amps on each leg but name plate on motor says 21.5 amps, trying to figure out what overload setting I should set my starter at.
 

·
Registered
Joined
·
3,450 Posts
You base the overload setting off the nameplate amperage and service factor. No calculations required unless the the particular overload you are using doesn't take into account the 115/125%.
 

·
Registered
Joined
·
420 Posts
Discussion Starter · #3 ·
Cow said:
You base the overload setting off the nameplate amperage and service factor. No calculations required unless the the particular overload you are using doesn't take into account the 115/125%.
Why set the overloads at 21.5 amps or 125% of that and not the actual current I'm getting on my meter which is 12 amps?
 

·
Registered
Joined
·
2,916 Posts
Why set the overloads at 21.5 amps or 125% of that and not the actual current I'm getting on my meter which is 12 amps?
Overloads are set to the FLA of the motor, not the load on the motor.. If the load should change from where you are at now, nuisance tripping could result if the overloads are set improperly...
 

·
Registered
Joined
·
420 Posts
Discussion Starter · #5 ·
glen1971 said:
Overloads are set to the FLA of the motor, not the load on the motor.. If the load should change from where you are at now, nuisance tripping could result if the overloads are set improperly...
FLA of motor without using 1.73 formula? So I will be fine if I set the overloads at 21.5 amps? even if I'm getting 12 amps on each leg .
 

·
Registered
Joined
·
420 Posts
Discussion Starter · #6 ·
glen1971 said:
Overloads are set to the FLA of the motor, not the load on the motor.. If the load should change from where you are at now, nuisance tripping could result if the overloads are set improperly...
I'm so use to three phase transformers, this is why this is confusing me.
 

·
THE "BIG RED MACHINE"
Joined
·
3,416 Posts
is there a table in the uglys book you can use?
 

·
Premium Member
Joined
·
20,983 Posts
Dataplate, dataplate, dataplate.

Don't do math. Don't look anything up. Look at the dataplate.

That will be all.
Carry on.
 
  • Like
Reactions: glen1971

·
Registered
Joined
·
5,522 Posts
FLA of motor without using 1.73 formula? So I will be fine if I set the overloads at 21.5 amps? even if I'm getting 12 amps on each leg .
Forget the 1.73 issue here. The motor is ALREADY 3 phase and your are FEEDING it 3 phase. There is nothing to convert here.

The current you are measuring is just based on the LOAD attached to that motor right now at this voltage, but will fluctuate as the load and/or voltage changes. The motor nameplate tells you the FLA of the motor. The Over Load Relay is there to prevent damage to the motor or conductors, so it only operates IF the motor gets over loaded, meaning it is trying to draw current BEYOND the FLA it is rated for. So you NEVER set the OLR to measured current, you set it to MAXIMUM current for the circuit.
 
1 - 9 of 9 Posts
Top