ac motor stalling

[killabyte]

hi, id like to know how to stall an AC motor, if possible to do in one phase AC motor and how to do in three phase ones.

im developing an electronic control for a motor and i like to know if possible to do that.

regards

killa

 

[itsmoked]

You must load the motor until it stalls. This is reeeeal risky to the motor.. and you, if you're not careful.

The motor will let out all its smoke VERY quickly. Like in 2 or 3 seconds. This doesn't include the heating from slowly slowing it down to stall..

Further more single phase motors at about 2/3's rated speed, the start winding will be re-engaged which will cause a large torque discontinuity and also speed the smoke release process.

 

[abcd3286]

Why ????

Most of us beat our brains out making sure this does not happen

Dan Bentler

 

[aolalde]

For safe stalling a motor, keep the current not to exceed the nameplate full load value.
Reduce the line voltage around 1/6 of nominal, the motor shaft must have some load. This condition will drop the magnetic flux to 1/6 of nominal and the motor torque to 1/36. The performance is similar for single or three phases.

 

[fsmyth]

Just curious - do motors generally TURN at 1/6 line?
Or does one lock the shaft and measure current?
Is the 1/36 a constant regardless of construction?
TIA
<als>

 

[LionelHutz]

You connect a load to the motor shaft that you can control and then you apply so much load that it quits rotating. Possibly look at using an eddy current clutch with the output tied to the mounting frame. You can also use a DC motor and drive, an AC motor with an oversized VFD and braking resistor or regen VFD, or a larger AC motor by applying DC to the stator (I'd suggest about 4X the size of the driven motor).

I give 3-phase motors more credit than 1 or 2 seconds before letting the smoke out. I've seen some smaller motors (10-50hp) with a rated locked-rotor time of 40 seconds. This means you can apply full line voltage and frequency for 40 seconds with the rotor locked (not rotating) without damaging the motor. Is this good for the motor? No, not really but if you don't go over the rated locked-rotor stall time you shouldn't cause any damage. Allow the proper cooling off time between tests of course.

1-phase motors are often not as rugged. The start winding that is switched out as the motor accelerates often isn't capable of much more than a few seconds of operation and would quickly be damaged in a locked-rotor condition.

 

[SteveKW]

I agree with Lionel. Also, the start capacitor will not like being energized, even for a few seconds!

 

[killabyte]

what kind of protection is the most used?

i was talking about stalling because i wanted to know if possible to behave an AC motor as a DC one, i mean sometimes it is necessary to stop and keep stopped holding a load and then turn back.

so when is necessary to (suddenly, not emergency) stop an AC motor, how to proceed?

best.

killa

 

[LionelHutz]

Forcing an AC motor to stop via electrical means isn't called stalling it.

If you want to electrically stop an AC induction motor then you would basically apply 0Hz to it (ie DC). It's more complex than this because of the slip so you need to compensate for the slip but DC will get you close. There are VFD's out there that use an encoder to measure exact motor rotation and then they compensate for slip to hold the motor stopped.

 

[killabyte]

thanks for yur valuable help

best

killa