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Electric motor shorted out, causes?

Firstgear

H-M Supporter - Gold Member
H-M Supporter Gold Member
Joined
Dec 7, 2018
Messages
611
This is a single phase, 3 hp motor. At first I thought it was the drive belt because some smoke came out before it stopped. Upon inspection the contacts located at the top of the motor had some arcing present so those were changed out as was the large capacitor.

The unit fired back up and within 30 seconds of the start, smoke came pouring out of the vent where the belt can be changed out and before I could get to the switch I could see fire inside the vent coming down from the motor and the circuit breaker kicked open.

So what would cause that to happen? This is a knee mill and I had a 1/8” end mill cutting a small piece of aluminum. So I am trying to understand if it just happens from time to time or manufacturer’s defect. Motor has been in use since January. My use at most has been sporadic, I am guessing less than 24 hours of actually cutting with it over that period. It is a motor from Taiwan. Maybe it’s just one of those things where infant mortality occurred. I know electronics have those.

Thoughts or comments?
 

Karl_T

H-M Supporter - Sustaining Member
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Oct 14, 2014
Messages
1,382
You just got lucky, sh*t happens.

I'd consider replacing it with a 3 phase motor and VFD. get you variable speed for about the same price as a new single phase motor.
 

Firstgear

H-M Supporter - Gold Member
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Joined
Dec 7, 2018
Messages
611
You just got lucky, sh*t happens.

I'd consider replacing it with a 3 phase motor and VFD. get you variable speed for about the same price as a new single phase motor.
Got it covered by warranty....they are replacing. I have a variable drive machine now. I do have to flip a handle for high or low speed, otherwise it is variable. Has worked well up to this.....
 

mksj

Active User
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Joined
Jun 12, 2014
Messages
2,630
Agree with Karl, if you can go 3 phase with an inexpensive VFD (Like the Teco L510) you will not have future problems with the starter switch and capacitors. You use the VFD in a fixed frequency or for small adjustments, use the machine mechanical variable speed otherwise. Single phase motors that are switch on/off frequently can blow the start capacitor or the motor start switch can burn out. If you can get a replacement 3 phase motor, then you can do a very simple VFD setup. On a mill I personally prefer 3 wire control which uses momentary push buttons for the run/stop commands, these can be mounted on the knee. Otherwise use 2 wire control with a 3 way rotary switch on the head. Set the VFD parameter to not resume a run command if powered up and you forget to put the switch to the off position. VFD you get a soft start and controlled deceleration.
 

WalterC

Registered
Registered
Joined
Jul 3, 2015
Messages
121
Heated windings usually causes the short. The coating on the wire burns off and shorts out.
Heating and cooling of the windings also causes them to rub together and creates a short in time.
Oils on the windings can soften and eat the coatings.
Tight bearings, dry bearings, weak run capacitor, weak start capacitor, wrong/weak voltage, worn bearing, bad wiring connections, wrong belts, over tight belts, other drive bearings in gear boxes, all support overheating.
Asking a motor to do more than the ratings allow.
 

Bi11Hudson

Artificer00
H-M Supporter Gold Member
Joined
Feb 13, 2017
Messages
637
Since it's a warranty job, I don't feel too bad about saying shorted?, "It looks long enough to me."
 
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