Motor overload protection for SINGLE phase

[Jieve]

I know this is an older thread, but came across it as am using a 3-phase overload on a single-phase motor, and the quote caught my attention:

The trip curve is slightly lower so that it will trip at a lower current value than if all 3 phases are present.

Why is this exactly? I'm looking at using an ABB MS132 motor starter for a 7.5hp single-phase motor (FLA ~21A). As described above, ABB gives de-ratings for their starters when used for single-phase. To see if the de-rating has to do with the shifted phases, I calculated and plotted instantaneous power over time in all contacts using the contact resistance given in their catalog. This is assuming a single-phase is looped 3 times through the overload, and in the normal 3-phase case. The instantaneous power over time fluctuates significantly in the single phase case compared to the 3-phase (assuming all power is dissipated into a single element), but integrating to get the energy put into the trip element gives the same result in both cases (assuming same current). So why would single-phase trip earlier?

Also: They say to set the 3-phase trip at the motor FLA, but then that the devices are de-rated for single-phase. If the trip curve is lower, then how should they be set? Still at the same current? Again, it seems from a heating standpoint that there should be no difference.

 

[I_Automation]

I've never derated them myself, and the ones I've encountered weren't either.

A 1.7A motor uses a 1.7A OLR, just looping through

 

[Snyd1]

Not sure about the overloads, but we use a lot of allen bradley MPCB and for 3phase it says set @ motor FLA, for single phase they state to set @ 105% motor FLA.

 

[the_msp]

AFAIK, the site my panels went to set the OL at FLC and I've not been contacted about any issues.

https://matrix-engineering.co.uk/wp-content/uploads/2023/05/IMG_1647-scaled-800x800.jpg

 

[Jieve]

Thanks everyone for the feedback. I'll get the overload that has the current setting to match the load.

 

[BachPhi]

Not that simple I am afraid. You need to read 430.32.
What about fuse and conductor size?

 

[Jieve]

The motor starter has built-in short-circuit protection. Assume conductor ampacity sized to NEC tables with appropriate deratings.



The NEC allows higher overload setting than the actual motor FLA (based on their motor tables) so setting the overload to the FLA would be conservative in this case.



The application is actually for a VFD on single-phase powering a 3-phase motor. VFD protection is 1.25x the rated input current per NEC if I remember correctly, which again would be conservative if the overload were set at VFD rated input.



Really my post was more geared toward trying to understand why a 3-phase overload would trip earlier on single-phase looped through than on 3-phase.

 

[I_Automation]

The application is actually for a VFD on single-phase powering a 3-phase motor. VFD protection is 1.25x the rated input current per NEC if I remember correctly, which again would be conservative if the overload were set at VFD rated input.

Do NOT use a motor overload to power a VFD!


A VFD output can have a OLR on it going to the motor, but the VFD can draw considerably MORE than the motor FLA rating. If the VFD is outputting FLA and the VFD decides to draw additional power to charge the DC bus at the same time the draw could be close to double the rated FLA.


Use a properly sized circuit breaker or fuse for the input power.

 

[Jieve]

This is a manual motor starter.



Both ABB and Allen-Bradley recommend using these starters (their respective brands) for VFD protection in the tables in their VFD manuals, although you may be right about the actual current setting on the overload part.


Example from the ABB ACS355 operating manual:
"Manual motor protectors may require adjusting the trip limit from the factory setting at or above the drive
input Amps to avoid nuisance tripping. If the manual motor protector is set to the maximum current trip
level and nuisance tripping is occurring, select the next size MMP."

 

[I_Automation]

"Manual motor protectors may require adjusting the trip limit from the factory setting at or above the drive
input Amps to avoid nuisance tripping. If the manual motor protector is set to the maximum current trip
level and nuisance tripping is occurring, select the next size MMP."

So start at the VFD input Max amps, not motor FLA, and expect nuisance trips.

EDIT: If this was electrically sound they would be able to state "Manual motor protectors must be x,x times the drive input Amps" and specify the multiplier - I'm guessing at least 1.7 times.



Trial and error to get something that runs for a customer that their engineers can't specify correctly.


Still say it is wrong and to use normal current protection, curve C or D or time-delay fuse, for the drive input amps and add a OLR after the VFD if it doesn't have an internal one, or as redundant protection so the PLC knows it tripped.