Overload Relay selection tripping class 2

[Thedroid]

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.

 

[jraef]

The OL class you select is based on the thermal capacity of the motor, or really the thermal damage curve of the motor. So only the motor manufacturer can tell you for sure what the proper protection class is.

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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[DickDV]

It's a really rough estimation but, in general, if you've got an extruded aluminum or very light weight rolled steel motor, its probably Class 10.

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.

 

[Thedroid]

It's a reliance electric cast iron T frame. We didn't want to go to a bigger motor and have to rework the base and use a different shieve. The design C definately start the machine much better than the design A. Changing the timing on the crusher start up also helped alot. I added another 5s to it today to allow the screen to get below FLA before we saw the voltage drop during the crusher start up. I'm looking at about a 15-20s start up now. Does this sound excessive, or should I try the Class 15 or 20 overload setting. Right now the class 10 is turned up all the way past 16. It's a GE adjustable thermal overload rated 4-16A. I plan on replacing it with an AB solid state.

 

[DickDV]

You must use the correct heater to get any meaningful motor protection while running.

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.

 

[Thedroid]

I was refering to the time it takes to get the motor up to speed. It's taking about 15 seconds now. I'm still waiting for the new overload relay so that I can adjust the trip setting to the proper level and raise the overload trip class.

 

[itsmoked]

I cannot imagine that motor lasting very long if it takes 15 seconds to stop drawing starting current. Yikes!

Sounds like a true VFD application.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Thedroid]

It doesn't draw full starting current for 15s. It starts at about 78A and drops progressively down to 8-9A over the 15s. The Design A motor drew 90A for at least 10s then made its way down to 10A. This was the reason for changing the motor out to a design C motor. I hooked up an ABB ACS 600 to the original motor, but could not get it start the machine any easier. This screen has a large flywheel with an offset pin to shake the screen. Getting over the hump was impossible to do with the VFD without drawing high current.