overload above hot stall time

[COelectrician]

Saw this post and was wondering the same thing. Has this been asked/answered anywhere else? rockman7892's post was never answered.

"rockman7892 (Electrical)
(OP)
16 Jan 10 11:42
I have recently been looking at some motor protection relay curves and noticed some curves that were set below both the the Hot and Cold locked rotor safe stall times, and other curves that were set inbetween the two points.

I have heard that setting the overload curve between the two curves somtimes helped with acheiving more starts, which in most cases is 2Cold, 1Hot start.

I was wondering typically where the relay curve should be set in relation to the two curves and if setting it between the two curves was risking damage to the motor?"

Any insight would be appreciated.

Thanks.

 

[waross]

It depends!
Damage curves are often based on worst case assumptions:
Maximum ambient temperature.
The motor running fully loaded prior to a hot restart.
Consecutive starts with no time for the motor to dissipate the extra heat of one start before the next start.
Load inertia, starting time.
If one or more of the parameters on which the damage curve is based on are less than the worst case, an extra hot start may at times be made without damaging the motor.
Does the protection scheme inhibit a second hot start, or is a second hot start allowed by the protection devices or settings?
Is the weather/ambient temperature very cold?
Is the motor starting time less than the starting time that the curves are based on?
What was the motor load profile prior to an "over the curve" hot start?
A lightly loaded motor in a cool ambient that has been running long enough to dissipate the extra heat of starting may at times be allowed an extra hot start without the motor temperature crossing the damage curve.
A failed motor following excessive hot starts may be an indication that the settings are too high.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

Often it is a compromise.

A good motor protection relay will model the motor heating and reduce the trip time once the motor has been running.

 

[COelectrician]

@waross - Great response, that makes sense. So without knowing all of those operating characteristics and not wanting to risk motor damage, it sounds like setting just below the hot stall curve would be the most conservative approach. I've also read some papers which indicate to set the the protection between the hot & cold curves, but in the same paper indicate if only the hot curve is given, then set below. Still struggling with this logic.

 

[Parchie]

AFAIK, decisions will be considering financial reasons also. Do you sacrifice motor life so your plant keeps humming and earning money or does burning the motor out mean the death of your business?
Big, more expensive motors have temperature switches/sensors embedded in the windings to stop motor when excessive temperatures are reached. But that doesn't mean 100% motor protection. IMO, it's not just thermal overload relay setting that matters-->it all depends on how the organization value a specific motor and the related costs of replacement, including the required downtime.