Induction motor problem (help wanted)

[jonny boy]

Hello, I have a Delta tablesaw with a three horse motor and recently, the overload protection is cutting off the motor. The manual say's it could be down to too much strain on the blade or a low voltage supply. I don't see any of these as being the reason. Does anyone have any ideas?
cheers,
jonathan.

 

[9fingers]

How quickly from a cold start is this happening?

I assume the overload trip you are referring to is in the NVR switch?

I also assume you have a single phase machine?

Is the motor making more of a buzzing noise than usual?

Can you hear a click from the motor after you have switched off and it is slowing down?

How would you rate your electrical knowledge/experience.

Comfortable/confident to experiment

OK to put on a plug but thats it

or somehwere in between?

Sorry about the interrogation but some more details are needed and it might help us make suggestions.


Bob

 

[jonny boy]

Hello bob, I know a little about motors and can quite confidently strip one down to replace the bearings for example.
The motor is switching off after it's been on about 5 mins.
The motor is getting quite warm. Not too hot to touch but seems hotter than any other machine motor.
It is single phase.
It's the overload trip in the motor itself i think.

 

[jonny boy]

Bob, I should also make it clear that the saw is an American import, but it should quite happily run with 220-240v as it states in the manual. I will say though, the motor cover door does close very close to the motor fan cover, and do you think this could impair the cooling of the motor? It is the proper cover though and shouldn't interfere with this.

jon.

 

[jonny boy]

I've just been thinking Bob, the overload trip might be in the on off swicth because I have to turn the isolator off and back on to enable me to start the saw again.
cheers,
jon.

 

[9fingers]

I suspect that the starting winding is not switching out when the motor gets up to running speed. This will draw more current which will trigger the overcurrent trip.
The starting winding is switched in by a centrifugal switch inside the motor and this may be stuck closed by the contacts welding or the centrifugal switch mechanism is stuck.

A strip down of the motor - as if you were changing the bearings- should reveal the problem. The switch is normally at the opposite end to the pulley and you should only have to remove one end plate.

Delta manuals and parts list are often to be found online and you might be able to get an exploded diagram of the motor to help.

When re-assembling these single phase motors it is vital that no lubricant is used on the switch mechanism as dust can collect and cause it to jam later.

Good Luck

Bob

 

[jonny boy]

Thanks for the advice Bob, is there any way of knowing if this is the cause before splitting the motor?
jon.

 

[9fingers]

Just read your further info:

There could be a thermal sensor in the motor which is wired into the NVR switch. Correct operation of this would mean that you would need to press the start button again to re-start the motor.

The clue will be in the number of wires from the motor to the switch.

2 plus earth would be no thermal trip in the motor

3 plus earth non conclusive

4 plus earth highly likely to have internal thermal sensor.

The wiring diagram in the manual should help here.

Note that motors intended for saw etc only ever run in one direction.
other applications such as lathes quite often bring more wires out to allow running in either direction.

you could send me a scan of the wiring if you like

to bobdotminchinatntlworlddotcom.

Bob

 

[9fingers]

Thanks for the advice Bob, is there any way of knowing if this is the cause before splitting the motor?
jon.

If you can get to the capacitor terminals, either measure the voltage across the capacitor or connect a standard filament light bulb in parallel with the capacitor.

There should be no significant voltage across the capacitor once the motor is up to speed which is the same as the light bulb should not light when it is running.

Obviously make the extra connections safely and I'd take the drive belt off or at least take the blade out of the saw.

hth

Bob

 

[jonny boy]

Bob, I'm afraid my computer skills aren't up to much so scanning things and sending them is a bit beyond me, but I do have a wiring dia. from the manual and by the look of it, it on ly shows two wires and earth that go to the motor.
jon.

 

[jonny boy]

Bob, I think the problem lies somewhere in the motor getting too hot and the overload is comming into play as it should. Would the problem you are suggesting cause the motor to overheat?
jon.

 

[9fingers]

OK no problem about the scan.

2 wires plus earth means no thremal switch interlinked with the NVR switch.
This is quite normal and means that the current sensor in the NVR is reacting to the extra current being drawn by the motor and tripping.

It is just possible that the current trip has been set too sensitive but if you have had the saw a while and it has only just started misbehaving then I don't suggest you adjust the trip to fix the problem.

hth

Bob

 

[jonny boy]

I think i'm getting what you are saying now, the series of events are as follows,
motor turned on,
starter winding not being cut out,
motor overheating due to extra current for starting,
overload trip cutting in when gets hot.

yes?

jon.

 

[9fingers]

Bob, I think the problem lies somewhere in the motor getting too hot and the overload is comming into play as it should. Would the problem you are suggesting cause the motor to overheat?
jon.

The problem would cause overheating but it NOT the heating causing the trip but the excess current being drawn is both generating the heat AND triggering the trip.

hth

Bob

 

[9fingers]

I think i'm getting what you are saying now, the series of events are as follows,
motor turned on,
starter winding not being cut out,
motor overheating due to extra current for starting,
overload trip cutting in when gets hot.

yes?

jon.

9/10 :lol:

overload trips when the current has been in excess for a few minutes

 

[jonny boy]

So Bob, if I start the saw and check the voltage across the capacitor, there should be nothing. If there's a reading, the starter winding is stuck as you say.
jon.

 

[9fingers]

Yes.

Now it is just possible there could be two capacitors. If there are two and they are NOT wired in parallel, then one will be in circuit all the time and the other will be switched in and out for starting.

two capacitor motors are not that common and pretty obvious as they have two bulges hanging off the side or a bigger one holding two capacitors.

hth

Bob

 

[DaveL]

Jon,

I little bit of motor theory, a single phase motor will not start spinning without some type of starter. The starter will be part of the motor and should be automatic in operation. One way of starting the motor is to use an extra winding in the motor with a capacitor, this produces a supply that is phase shifted to the mains and causes the motor to start turning. On small motors this may be left in circuit, but on larger motors it will only be used during starting and a centrifugal switch will disconnect it once the motor is up to speed.

On your motor it sounds like the switch has stuck in the closed position, stopping the starting circuit being disconnected. This will draw more power and this could will be what is tripping the overload in the NVR switch. The overload is setup to allow the extra current needed for starting but if that is continuously drawn then it will trip out.

note to self, must learn to type faster. :roll:

 

[jonny boy]

You must be a mind reader Bob, I was just going to say there are two capacitors!!!

jon.

 

[9fingers]

note to self, must learn to type faster. :roll:

Don't worry about your typing speed, Dave. I've been having difficulty in keeping up with Jon's rate of postings too.

Between us I think we are on a common theme! :lol:

Bob