Motor Overload Duration

It is said motors are can be overloaded for a duration of time but I can not find a document or formula which clearly states how long I can overload a motor. For example, in vfd's manual I can find some information like it can be overloaded 5s in a minute with %120 of rated current but for motors the manual gives peak torque for example %200 of rated one but doesn't say how long I can overload ? Is there any formula or something to derive this relationship.

bodoo23 said:

It is said motors are can be overloaded for a duration of time but I can not find a document or formula which clearly states how long I can overload a motor. For example, in vfd's manual I can find some information like it can be overloaded 5s in a minute with %120 of rated current but for motors the manual gives peak torque for example %200 of rated one but doesn't say how long I can overload ? Is there any formula or something to derive this relationship.

Click to expand...

"What is trip class when working with overload relays?"

Select the above dropdown from the link below:

https://www.google.com/search?q=mot...i22i29i30.14205j0j15&sourceid=chrome&ie=UTF-8

Hope this helps you.

saultgeorge said:

"What is trip class when working with overload relays?"

Select the above dropdown from the link below:

https://www.google.com/search?q=mot...i22i29i30.14205j0j15&sourceid=chrome&ie=UTF-8

Hope this helps you.

Click to expand...

Thanks for answer. I already check these curves but I found out that I couldn't explain my question well. For example, I check this curve and see that %150 overload is ok for 2 min in terms of motor and overloaded it. How long after that can I overload the motor to %150 again safely ? 10 min 1 hour ? How can I calculate this ?

It will depend on what percentage of full load you're drawing following the period when it was overloaded.
The RMS current over the entire time of operation should be within the motor's nameplate rating.

That would be based on the motor's Service Factor (SF) on nameplate.
But it is not necessarily good for the motor to run in its service factor for extended periods, unless designed to do that.

we have a custom wound motor for a cooling fan installation that is rated 1.25 (SF) continuous.

More info...
https://www.thesnellgroup.com/blog/what-you-need-know-about-service-factor

There are two issues involved; one is the Overload Class of the protective device, which is designed to emulate the thermal damage curve of the motor, the other is a "cooling time constant" of the motor. Unfortunately most motor manufacturers do not provide you with the cooling time constant value. They trust that the Overload Relay curve is going to take care of that, and for the most part it does. The problem I see you having here is that you want to play with that motor thermal damage model in terms of getting the most out of the motor under a known overload condition. You are (almost literally) playing with fire here. If you want to get an idea of how complex it is, try to read this paper;
https://library.e.abb.com/public/58...Section_08p11_Motor-Protection_757291_ENa.pdf

The bottom line though is that the cooling time constant is highly variable, because it has to do with the ambient air, the method of cooling of the motor, the mass of the motor stator and Rotor, the duty cycle involved etc. etc. etc. Here in North America, we rely upon formulae made by NEMA that relate to a "Starts Per Hour" rating of a motor, allowing us to over size a motor based on typical values from known motor designs. Those would NOT be appropriate for IEC motors however, and seeing you are in Greece, you should be using IEC motor data.

For IEC motors, you have a "Duty Type" system, S1 through S10, and within that list, Types S3 through S8 are what are referred to as "intermittent duty" cycles, relating to how often they cycle, for howl long and by how much. This is a decent article on that issue, I suggest that you read it and understand what you are getting in to..
https://electrical-engineering-portal.com/10-duty-types-three-phase-asynchronous-motors

NetNathan said:

That would be based on the motor's Service Factor (SF) on nameplate.
But it is not necessarily good for the motor to run in its service factor for extended periods, unless designed to do that.

we have a custom wound motor for a cooling fan installation that is rated 1.25 (SF) continuous.

More info...
https://www.thesnellgroup.com/blog/what-you-need-know-about-service-factor

Click to expand...

The OP is in Greece, most likely using IEC motors, which do not have a "Service Factor" the way NEMA motors do.

I would consider getting a motor with an internal temperature sensor. Monitor the motor temperature and figure out what is the normal allowed max temperature.
Then use that temperature to log heating & cooling times to see how you could run it without exceeding the limit.
Yes, it's reverse engineering a motor & it's control, but that probably would be the only way to know for certain.

Thanks you for your answers. They helped a lot