Relay Settings for LV Motors

[cherry2000]

In an article in EC&M, "NEMA standards for overload protection need closer look"
Jul 1, 1996 12:00 PM, Schindler, Robert, there were some interesting conclusions drawn.

Now my question : Is it justified to use a Class 20 relay curve or is the North American industry seriously thinking of adopting the Class 10 curve which is more stringent and gives closer motor overload\stall protection? Also, my another question is, if we use a Class 20 o/l relay, does it mean that the motor needs to sustain the locked rotor current for 20 sec? Is this OK from motor protection viewpoint?

 

[jraef]

It depends. The best course of action is to consult the motor manufacturer if you can.

In general terms though, if your motor was designed around NEMA standards, Class 20 is the typical protection curve they are designed to, and yes, that means NO MORE THAN 20 seconds at 600% current. To default to always using Class 10 is not bad for the motor, but may end up giving nuisance trips if your machine was designed around expecting a Class 20 operation.

If it is an IEC designed motor or one originally designed as such and adapted for use in North America, you should default to using class 10. IEC motors generally do not have a Service Factor either, something else to consider when applying them.

Many submersible pumps are custom design and require Class 10 or even Class 5 protection. Using anything other than what the manufacturer tells you can void your warranty.

"Mill Duty" motors are ones designed around Class 30 protection, but have become extremely rare. I have not seen one in 20 years. I have seen some custom designed Centrifuge motors use a Class 30 protection scheme, but any time you are thinking of it, always consult the motor manufacturer first.


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[cherry2000]

Thanks for the reply. It is not easy to get stall times for small LV motors. Normally we ask for this value only for motors above 100HP. GE manual (attached) Page 6 shows a Fig 9 for hot & cold stall times. Looking at the 20sec Hot stall value, the per unit current is approx. 5 which effectively means that for having a safe stall time of 20 sec, we have to select a Code E/F motor; otherwise the motor is liable to be damaged !!However, most of the LV motors that we have in our plant are Code letter G and above. Any suggestions on this?

 

[rockman7892]

If i'm not mistaking class 30 overloads are used for hard to start motors, or motors with long acceleration times?

 

[cherry2000]

You are right Rockman..for large inertia loads,involving higher starting times, a Class 10 or 20 may not suffice and one may have to choose Class 30 overload curve, which means that the relay operates at 6 times the FLA at 30 sec, giving the motor sufficiant opportunity to start.

 

[rockman7892]

This was a very interesting article. I have been studying overload protection recently for MV motors and have come to learn how MV motor protection build thermal models of the motor based off of operating parameters (current, rtd's etc...)

For the particulr multilin 369 relay that I have been studying I learned that when the motor is running under rated operating current the thermal capacity being used on the motor is modeled as:

TC_used = Ieq x (1-hot/cold) x 100%.

where Ieq= operating current.

After a motor has been started and is running it settles down to a modeled thermal capacity used based upon operating current and the hot/cold ratio. When this equation produces a Thermal capacity used that equal over 100% is when the relay will trip the motor. Of course this is for all operating ranges that fall under the overlaod pickup.

I was curious to see how this same pricipal worked for LV bi-metal overload relays. Anything over the pickup of these overloads will corrospond the published TCC for any given overload but how is the thermal capacity in the metal strips modeled for current values below pickup level. I'm guessing that there has to be some sort of thermal equation that relates the operating temperature to the amount or percent of thermal heat in the overloads compated to when they will trip. Does the hot/cold ratio play into these as well?


One other thing I have been struggling with lately is where to set the overload protection in relation the the hot and cold rotor stall time curves. I have heard both sides of the fence. I have heard to set the protection below the the hot curve in order to fully protect the rotor, but I have also heard that the overload curve should fall in-between the hot and cold curve in order to allow 2 consecutive cold starts per Nema MG-1.