Idiot's question about induction motors

[M_Chavez]

Hi All,

My pillar drill's motor appears to have packed up this morning (serves me right for not using the drill for anything apart from drilling :roll: ).

The motor is now seriously lacking torque when starting - it would barely spin at about 60 RPM if trying to start the drill in high gear (and emit that loud "starting" buzz) , however, it can still start up when the drill is in medium & low gears (albeit slowly). There might be some loss of speed and torque when the low gears are running.

Now, would anyone have any suggestions on how best to approach the fix? It's a single phase induction motor.
Could this be the starting capacitor? I couldn't find a running capacitor, so suspect there is none...
Are there any tests/fixes I can try out?
Or does it sound like the whole motor is buggered and needs to be replaced?

Thank you.

 

[Trevanion]

Could be anything really, I've had a similar problem with a motor before and discovered it was the starter switch which was acting up and not the motor itself, which was after I had bought new capacitors, bearings and stripped and put the motor back together a couple of times to check if it was the centrifugal switch acting up.

Doesn't hurt to try changing the start capacitor first though since they're not that expensive, just make sure you get one with the right microfarad rating.

 

[CHJ]

Not unusual to have just a start-run capacitor on the cheaper motors that don't have a centrifugal switch.

The capacitor then provides a starting phase shift to get it spinning quickly and a pseudo third phase to improve the torque during running.

If you get a replacement capacitor make sure it is Run Rated not start rated so that it will carry the load current continuously.

 

[M_Chavez]

Thanks.

Could long running under load and overheating have caused the capacitor to pack up?

 

[CHJ]

Thanks.

Could long running under load and overheating have caused the capacitor to pack up?

Design concept of Pillar drills is that they will be used in an intermittent mode, depends on how close the person specifying the motor rating has veered towards the 'just adequate'.
Motor could fail by damaging the field winding insulation but this would normally have to be after severe mistreatment as they usually are rated to run way above hand heat.
A struggling capacitor that is getting warm due to the current it is carrying may well be driven beyond its insulation limits if excessive motor body heat is transferred to it.

As already remarked, changing the capacitor is not a high value expenditure and worth a try if you do not have means of checking the windings or know anyone who can do it for you.

 

[sunnybob]

Some multitesters have capacitor checking built in. but if you dont have one its not worth buying a multitester just for this.
A simple way is to remove the capacitor (making note of which wires its connected to, and finding an electrical wholesalers. Most of them will test it for you, or maybe there will be a sparky in there buying stuff who will.

Should be less than a tenner to buy a new one (edit, just realised I'm 12 years out of circulation now, but definitely less than 20.)

 

[CHJ]

Plenty of online sources at reasonable prices. one example

 

[M_Chavez]

Thanks - I've ordered a capacitor to give it a try.
I might also take the motor apart and see if there's any damage to the windings, if I have some spare time before the cap arrives.
I was just trying to figure out whether windings damage would have the same symptoms as a dying capacitor, i.e. loss of starting torque and some loss of running torque/speed, but still capable of drilling at lower and medium speeds.
In any case, a replacement single phase 3/4HP motor won't be too expensive if the worst comes to worst.

 

[sunnybob]

You can check the motor windings by using your eyes and nose.
The miles of copper wire are coated with lacquer, and if the wire is overheated the lacquer melts and then catches fire, which shorts out the copper wires.
Get up close and personal, if it looks burnt and smells burnt, it is.

 

[Sideways]

For any given power of motor a start capacitor that is switched in and out by a centrifugal switch and a permanently connected capacitor will have have different values. Check the power of your motor and the value printed on the existing capacitor. A bit of googling may help you figure out which of these you have. If it's a start capacitor working with a centrifugal switch, it won't be too hard to get the fan off the end of the motor to reach the switch behind it. It never hurts to clean the switch contacts with a fine file or some abrasive like doing the points on a car back in the day.
Failed capacitors and issues with the centrifugal switch are the most common cause of the small motor problems I've come across. Bearings do go too, but good ones are rated for many thousands of hours use and you can make a judgement just by turning the spindle by hand.

 

[shed9]

Be careful when removing the capacitor if you happen to have run the machine just before disconnecting it. The cap will hold a charge for a set period of time and can cause you a nasty shock.

 

[Racers]

Unlikely on an ac motor it’s connected across the winding which will discharge it.

Pete

 

[shed9]

Unlikely on an ac motor it’s connected across the winding which will discharge it.

Pete

Good point, the centrifugal switch should reconnect the cap to the windings and discharge (assuming there is a switch). Given however that the unit in question is possibly faulty I'd still err with caution myself.

 

[M_Chavez]

Thanks gents.

Bearings are fine. I am hoping that replacing the start capacitor will be a nice £4 fix (will find out this week), but I do expect that I have fried the motor. Just need some spare time to have a look inside.
I'll be careful with the cap.
The drill is still running - although the motor has built up a fair amount of heat today after drilling about 40 holes.

What's the best course of action for a fried motor - new motor (upgrade 3/4HP to 1HP perhaps), or try to get it rewound (there seem to be companies around that can do it professionally - not sure about the cost)?
It's a Warco drill with a cheapo motor in it.

 

[sunnybob]

If the drill is still starting and stopping its very unlikely to be the START capacitor :roll:

Pull the pulley belt off and see how fast the motor spins. Then try to turn the chuck by hand. I suspect youve got dust or other foriegn stuff jammed in the drive system.

 

[shed9]

If the drill is still starting and stopping its very unlikely to be the START capacitor :roll:

As CHJ pointed out, it may be wired as a single cap with no centrifugal switch. Some cheaper equipment runs with permanent split capacitor wiring where the cap serves as a starter and assists with running as well. One of the benefits of this apart from no switch parts to wear out is it helps torque which seems to be an issue in this case.

Replacing the capacitor given the low cost is not a bad option at this stage (make sure its correctly rated as CHJ again points out). A new 550w motor won't break the bank either but there will be diminishing returns in going this route. If it was a cheap drill in the first place, may make more sense to put that motor money towards a new drill. You could sell the old one on Ebay to put a little money towards a new one as well.

 

[M_Chavez]

Thanks all - I'll sort her out o e way or another.

Oh well, should have never used her as a winch. (hammer)

 

[shed9]

Oh well, should have never used her as a winch. (hammer)

Now you tell us........

Probably not, no.

Good luck with it, hope it is something small like the cap.

 

[M_Chavez]

Just kidding (about the winch). :lol:

...but it does get used a lot for sanding, safe-t planing and overhead "routing".

Since it gets abused so much, I am rather reluctant to invest into anything better, as I will probably kill it.
A 1HP motor upgrade does look attractive though (as long as the cheapo gearbox can manage it).

 

[sunnybob]

Have you removed the belt and checked how free the quill moves?
Pointless waste of time and money changing motors if the gearbox is no longer fit for purpose.