Interpret VFD spec for motors

[Cheetos]

Hi, there. I'm trying to use a VFD for my motor. I was reading VFD spec and it says
maximum current/power with 115% overload for 60 seconds; every 10 minutes

Does this mean I can operate VFD at 115% for 1 minute and 100% for 9 minutes or 115% for 1 minute and 0%/no load for 9 minutes?

 

[Marke]

Hello Cheetos

A very good question and is not very clear anywhere.
I always interpret this as continuous run at 100% with an overload up to 115% for 60 seconds every ten minutes. i.e. no cool down period specified.
Interested to see what others say!

Mark Empson
Advanced Motor Control Ltd

 

[edison123]

0% load for 90% of time means VFD (and hence the motor) would be shut off for 90% of time, which does not make any sense.

Muthu

http://www.edison.co.in

 

[Tanweer Ahmed]

Hi Cheetos,
That simply means you can overload the VFD to 115% for a maximum of one minute. But it does not mean you run it at 100% for 9 minutes and every 10th minute you run at 115%. Why it is like that? Because when overloaded you heat up the power electronic components to it limit. Now you need to cool it down. If you continue to run at 100% load, you are not allowing them to cool down.
I am sure if you look into the manual they would explain that once you have run the VFD at 115% for 1 min, in order to allow the power semiconductors you have to run it at less than 100% (most of the time 90% or less load) for another 9 minutes to be able to regain the overload capability.

 

[Marke]

Hello Tanweer

That is certainly one interpretation, but equipment that I have designed has thermally been on the basis of 100% rated load plus 120% for 60 seconds every 10 minutes. i.e. the average thermal loading is higher than at 100% rating continuous, so that is also valid.
If the assumption was that you would run at 90% when not overloading to 115%, why would you rate at 100%? Why not rate at 90% with a higher overload limit?

Mark Empson
Advanced Motor Control Ltd

 

[Tanweer Ahmed]

Hello Marke,
You are right. It depends what is your 100%. My statement is based on standard practice adopted by most of the major brands. Exceptions are always there, as you said.

 

[PaulBr]

I think overload to 115% is to address dynamic of rotational load during acceleration or deceleration, when in addition to static load there is load from inertia which needs to be overcome.

PaulBr.