Motor OL Class Curve Calculation

[DougElectric]

How are motor OL class curves created? There are two limitations for overcurrent protection, to handle 6X the FLA for 10, 20, 30 s etc. (respective of class) and trip at 1.15 or 1.25% of FLA after some time (not including short circuit requirements). Does anyone know how that curve is created? I am trying to artificially create that curve with a current sensing (electronic) device instead of a mechanical one. Thank you in advance.

 

[Jraef]

NEMA motors or IEC motors?

NEMA motors can have a "Service Factor" of 1.0, 1.15, 1.25 or 1.30 (most are 1.15). The OL protection curve starts at a "pick-up point" of the curve, meaning the point at which the device BEGINS to respond and should trip in 2 hours at that level. For a motor with a 1.0 Service Factor that pick-up point can be 115% of FLA, for motors with a 1.15SF it can be 125% and for "Mill Duty" motors with a 1.25 or 1.30SF it can be 140%. The other end of that curve is the 600% level, so either 5, 10, 20 or 30 seconds depending on the Class 5, 10, 20 or 30. The shape of the curve in between is determined by I^2t (current squared x time). The vast majority of Off-the-Shelf NEMA motors are Class 20 with a 1.15SF, but submersible pumps are usually Class 5 or Class 10 and 1.0SF, some OEM machines intended for intermittent duty, like compressors, are sometimes Class 10 and 1.0SF as well. When any motor is run from a VFD, it is 1.0SF and Class 10.

IEC motors follow the same curve, but there is no Service Factor, they are all essentially 1.0SF and Class 10, with the pick-up point able to be between 115% and 120%, most brands of OL settling on 117%, the other end being 600% for 10 seconds.

 

[paulengr]

How are motor OL class curves created? There are two limitations for overcurrent protection, to handle 6X the FLA for 10, 20, 30 s etc. (respective of class) and trip at 1.15 or 1.25% of FLA after some time (not including short circuit requirements). Does anyone know how that curve is created? I am trying to artificially create that curve with a current sensing (electronic) device instead of a mechanical one. Thank you in advance.

The curves you are describing are from the NEMA standard. But read up on the motor relaying white papers on selinc.com. They are very readable. GE and Eaton use something similar but don’t give the specific equations. The curved in the NEMS standard are developed from the math that SEL lays out.

 

[topgone]

How are motor OL class curves created? There are two limitations for overcurrent protection, to handle 6X the FLA for 10, 20, 30 s etc. (respective of class) and trip at 1.15 or 1.25% of FLA after some time (not including short circuit requirements). Does anyone know how that curve is created? I am trying to artificially create that curve with a current sensing (electronic) device instead of a mechanical one. Thank you in advance.

FYI, OL relays do not react to 6X the rated current of motors! The circuit breaker takes care of the short time and short circuit protection of the motors! The NEMA trip classes defines what type of OL will be fit for the types of duty your motor is going to handle.

 

[paulengr]

FYI, OL relays do not react to 6X the rated current of motors! The circuit breaker takes care of the short time and short circuit protection of the motors! The NEMA trip classes defines what type of OL will be fit for the types of duty your motor is going to handle.

Ok so when the NEMA standard for overloads states that it SHALL trip in seconds at 600% of FLA or sooner and with an MCP which has ONLY a magnetic trip unit (instantaneous only) and it is set to trip at 11-17x FLA as per NEC Article 430, then how pray tell does anything trip under locked rotor conditions (roughly 600% of FLA)?

Review NEC Article 430. Review NEMA ICS-2 Part 4 (Overload relays). See table 4-8-1 and figure 4-4-1, the TCC for overload relays. Pay particular attention to where it states that it SHALL trip at 600% of FLA and the specific time (==trip class) or faster that it MUST trip. And pay attention to the fact that the overload curve is defined well above 600%. The test points are just 1x, 2x, and 6x.