Overload Relay selection tripping class
Overload Relay selection tripping class
(OP)
We have been having trouble with tripping on start up of a screen using a 10 hp design A high efficiency motor. Larger overload heaters had to be installed in order to get the machine to start up without tripping.
Start up current was about 90A and slowly dropped over an elapsed time of 30s.
We recently installed a 10hp design C motor and have now dropped the start up current to 78A dropping down to 9A within about 10s.
We then were told to install an IEC contactor and overload relay and test the machine. The overload still trips when set at 13A wich is the FLA of the motor. The overload class is 10. Would there be any disadvantage to installing a class 20 overload. I'm thinking of ordering a solid state overload relay with adjustable classes for experimentation.
I checked for voltage drop today and noticed that when the crusher started the screen wasn't up to speed and caused a voltage drop down to 420V. We adjusted the timer to allow for the screen to come up to speed before the crusher starts.
Start up current was about 90A and slowly dropped over an elapsed time of 30s.
We recently installed a 10hp design C motor and have now dropped the start up current to 78A dropping down to 9A within about 10s.
We then were told to install an IEC contactor and overload relay and test the machine. The overload still trips when set at 13A wich is the FLA of the motor. The overload class is 10. Would there be any disadvantage to installing a class 20 overload. I'm thinking of ordering a solid state overload relay with adjustable classes for experimentation.
I checked for voltage drop today and noticed that when the crusher started the screen wasn't up to speed and caused a voltage drop down to 420V. We adjusted the timer to allow for the screen to come up to speed before the crusher starts.





RE: Overload Relay selection tripping class
That said, if it is a NEMA design motor, chances are very good that it will take a Class 20 OL relay. If it is an IEC motor, chances are that it will only be capable of having Class 10 OL protection. That is why IEC starters will come only with Class 10 OLs on them and require special attention to be ordered otherwise. With some manufacturers, you may not even be able to buy Class 20 OLs for an IEC starter.
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RE: Overload Relay selection tripping class
If its a NEMA cast iron T frame motor, its probably Class 20.
If its a cast iron U frame, its probably Class 30.
Otherwise, jraef's rules above are more precise and I agree with them fully.
A NEMA Design A motor on a crusher flywheel is most definitely a poor choice. Your change to NEMA Design C is the right choice, in my view. Your only other choice would have been a NEMA Design D which would do the job maybe a little better than the Design C but with a large efficiency penalty. Good choice on the Design C.
RE: Overload Relay selection tripping class
RE: Overload Relay selection tripping class
Given that, the only correct course of action, in my view, would be to change to a Class 20 overload curve.
With a Design C motor, a cast iron T frame, the heater set for the correct NEC value, and a Class 20 time curve, I would expect the starting to be reliable and the motor to be protected properly.
I'm not at all clear on what the 10, 15, and 20 second time delay is but, the above would be correct for the overload protection.
RE: Overload Relay selection tripping class
RE: Overload Relay selection tripping class
Sounds like a true VFD application.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Overload Relay selection tripping class