Max induction motor starts per hour with VFD

[Ttrees]

Hello everybody
I was cutting a lot of bits regularly last night, and became a bit concerned after reading on another
forum that induction motors are rated for 15 starts an hour MAX.
I have some query's about this, since a recent thread here suggests that the windings probably
won't get as hot in conjunction with an inverter since there's overload protection in them......
well, at least the good VFD's anyway
This makes it possible, not to require a high temperature tolerant brazed joint, if it were a fixed star wound motor getting made/converted into a dual voltage motor for VFD use forever, thus making me think
its probably a safe enough bet to say that the motor will be allright, if I were to get close to that
15 starts an hour...
Well knowing this rating is for industry, I would think half that for home use is pushing it, for a longer easier life on the motor.
I do have a nice ramp up time on the machine to make things easier.
Would it be common practice to say... go and feel the motor if it has been started....
say, seven times within the hour?

Happy to hear anything about anything, no worries about de -railing the thread
Its all interesting
Thanks
Tom

 

[Sideways]

I can only suggest checking the temp of the motor from time to time (cheap infra red thermometer or hand on if you can get at it)...

Here's my thinking: I'll bet industrial uses don't start and stop between every cut. Theyll know their machine, prep a batch of timber and try to work efficiently. The motor will not spend so much time starting up (pulling peak current) but more time under load (drawing sustained current).
So next time you're doing some heavy cutting, check the temp at the end to give yourself a benchmark for what's normal for your motor. Then as long as you're not exceeding that, you're probably OK.

Induction motors are fairly massive so there's quite a lot of metal to heat up at the start of a cutting session but then they are usually designed for continual use.
Given that they are totally sealed and many don't even have fins, I think the innards will settle at a temperature a fair bit hotter than the outside.
Heat removal is solely by air blowing over the outside of the casing. This isn't very efficient but the motors are designed accordingly and are very reliable.
Induction motors can be designed to run hot - so much so that you wouldn't want to keep hold of them - so the innards will be plenty warm ...
If you're worried about this, I think some VFD's can take an input from a temperature sensor attached to the motor.

 

[Inspector]

The heat is built up primarily when the motor starts. Lots of starts means more heat buildup. Additional heat is added with electrical braking. A VFD can soft start, start over a longer period with lower energy creating less heat. Motors are designed with duty cycles in mind and there are ratings for the different kinds of work they will do. You need to find out what the duty cycle will be and get a motor rated for that kind of work. There is nothing stopping you from adding a cooling fan to blow on a motor.

I searched duty cycles for motors and here is some info of the ratings a motor can have. Keep in mind 'merican and European standards are likely different but have the same goals.
https://electrical-engineering-portal.c ... ous-motors

Pete