Switching on/off vacuum motors

[Yham]

Hi,

A question bothers me for a while and haven't found an answer for it. Hope community knowledge will step up. People are saying that it is not good idea for high rpm motor to be switched on/off very often. Motor I am talking about is used for vacuum holding wood panels during cutting on a cnc machine. Owners rather use valve to turn on/off vacuum while load / unload panels than switch motors on/off.

Is there anything what may damage motor by repeating on/off cycle during the day. Cycle is like off is 30 sec or so, and on is a few minutes.

Thanks,
Milosh

 

[ScottyUK]

Some large motors are only capable of starting a handful of times a day.

The answer depends on factors we don't know, starting with "How big is the motor?", and "How is it controlled?".

 

[dArsonval]

The answer centers around "HEAT".

Every time an electric motor is started, it generates heat. And components supplying electricity to the motor endure heat as well.
Fuses get hot, circuit breakers get hot, and most of all... the coil windings within the motor get hot.
As the motor runs, it has an opportunity to cool from its initial startup.
If the motor is not given ample time to level off to its designed operating temperature, degradation of the coil winding(s) can begin.

The larger the motor, a longer time between starts is required.

Electric motor duty cycles have been labeled; continuous, intermittent, periodic, short-time, varying, etc.

John

 

[waross]

Turning a motor off for 30seconds is a bad idea. Use the unloading valves or controls. That is why they are there.
Depending on the size, the inertia and the load turning a motor momentarily (part of a second to several seconds, it depends) may generate torque and current transients that may damage or break shafts and blow fuses.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[MikeHalloran]

There is presumably some flow through the motor even during the 'on' portion of the cycle, due to imperfect sealing around the workpiece and such.

Using a valve that completely shuts off the vacuum motor inlet reduces the load on the motor, typically producing a large increase in motor speed, but also reducing the cooling air flow.

In this case, it's probably a good idea to vent the vacuum motor's inlet during the 'off' portion of the cycle, to provide some cooling air flow through the motor.




Mike Halloran
Pembroke Pines, FL, USA

 

[waross]

We should ask; What type of motor is it?
What type of vacuum pump is it?
How is the motor cooled?
Is this a vacuum cleaner type of setup?
Universal motors, small AC-DC motors with brushes, do not suffer as much larger induction motors from repeated starts. Universal motors do tend to speed up noticeably when the load is removed, but they are designed to handle the no-load speeds.
Can you give us a make and model of the vacuum pump?


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[edison123]

What is the duty cycle that is stamped on the motor nameplate?

Muthu

http://www.edison.co.in

 

[Yham]

Thank you guys for all your suggestions and sorry I didn't give you any info about the motor.

It is an AC motor with brushes. It is like one used in a central vacuums. Spec says 8A at 220v and run at 22000 rpm and there is no nameplate. Here is the link where I got it from. Model is Lighthouse LH7123. There is no data regarding the temperature of the core (coils)s well. Is there any rule of the thumb what temperature shouldn't be reached? I understand, cooler the better, but they get hot during normal operation, and it looks like they are designed to be like that. What would be their "normal" working temperature?

Thanks,
Milosh

 

[jraef]

That is called a "Universal Motor" which is essentially a DC motor designed to accept AC power. The duty cycle of that type of motor is typically very low because the efficiency is low so the waste heat is high. The intended use is typically for small household appliances and hand held power tools that do not get turned on and off very often.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington