The safety department will say zero percent.
If the ambient temperature is less than the temperature that the motor is rated for, some overload may be possible. It is difficult to put a percentage on it.
A motor with a service factor greater than 1.0 may be overloaded to th limit of the service factor.
Bill
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"Why not the best?"
Jimmy Carter
But there is no free lunch... Running a motor with a service into that service factor* continuously means you are acknowledging that the motor performance and life will be compromised, but there is no way to know how much.
*Since this s an international forum, "Service Factor" is a term from NEMA MG-1 motor design specifications for North America that denotes an overload capacity in a motor, but is intended to be "temporary", or rather non-continuous, with "continuous" being defined in different specs as typically meaning 3hours or more. But the actual time references in the definition of Service Factor were all removed some years ago, likely under lobbying pressure from OEMs who did not want to have to use the next size up for a +15% need. So now all the spec says is something weakly stated to the effect of "you can expect shorter life and torque or current specs may be different..."
The closest analog in IEC motor design specs is the "Duty Cycle" rating system, S1 - S8, but as far as I can tell, it is to allow for different ratings with LESS continuous load, never with more. In other words the highest rating is a continuous load at the nameplate rating.
Remember too that if on a VFD, the SF is normally considered 1.0 no matter what is written on the motor nameplate.
In machine tool world the motors are often rated with the S1-S8 rating system. Most common is the S6-30 for higher than continuous rating for 30 minute duty cycle.
Example attached: 20hp continuous, 25hp for 30 minutes, 30hp for 1 minute....
In the Cowern papers (Baldor Website) There is an explanation and the calculations for RMS HP loading. This technique is used to determine if an application is reasonably safe when a motor duty cycle varies between overload and underload.
Normally, a motor should be supplied with a thermal o/l curve. Exceeding the values on the curve would damage the motor.
Thus, you would be programming a relay to lie below the thermal curve, so that in the event the overload exceeds the relay curve, the relay will trip out.
Now, if you want occasional o/l on the motor, exercising the service factor may be the only option. As long as the thermal content does not exceed the prescribed limits by the manufacturer, you are ok.
Hi, electricaleasy ,
If you are thinking for S1 duty cycle (continuous work), then I agree with Waross: "zero percent."
Also, I agree with the jraef's explanation regarding the "service factor". I think it is rather a commercial then a technical term.
Service factor is not a commercial term. It has a technical definition given in NEMA MG-1. The winding must remain below a certain temperature rise at service factor load (the limit for service factor rise is of course higher than the limit for rated-load rise for a given class insulation).
However I agree service factor is still an ambiguous concept from a user's standpoint... what is he to do with this info. Not clear and open to interpretation.
Specifying a NEMA motor Service Factor is analogous to specifying an IEC motor with an F ( 155 deg C ) insulation and a B ( 100 deg C ) temperature rise at rated current.
Assuming the motor is rated for an ambient temp of 40 deg C, the motor winding temp will rise by 100 deg C above the 40 deg C ambient when it is running at rated current.
The winding temperature of 140 deg C is 15 deg C below the winding insulation rated withstand temperature. RTDs would be typically set to trip the motor at 150 deg C. This allows for short time overloads without exceeding the motor winding temperature rating.
