Need Help!- single phase motor gone faulty...

[Numb Nut]

Anyone got any thoughts on what might have happened to my lathe motor? It was worked just fine one moment and then suddenly started to judder.

To troubleshoot the source of the judder I removed it from the lathe and disconnected the V-belt. It judders immediately power is applied but seems to run at approximately the right speed (or perhaps slightly slower) and the juddering stops immediately when power is disconnected even though the spindle continues to rotate under momentum which suggests it's not the bearings. Indeed the bearings seem to be fine, there's no slack in the spindle at all. The spindle can be rotated by hand and rotates smoothly. Any thoughts?

More info about the motor - it's a 750W (1 HP) single phase mains powered motor, 1400 RPM, wired into a reverse switch and to the supply via a no-volts switch. There is a junction box on top of the motor containing a capacitor. The motor is a couple of years old but hasn't had a great amount of use, looks like new infact.

Do these things have brushes?

Any help or advice to get this working again would be appreciated.

[Geof]

Those motors have a centrifugal switch that disconnects the capacitor when the motor is up to speed. It is inside the motor houding but you can sometimes see it through holes, if any, in the ends or through the terminal box.

Often these switches malfunction and either do not switch the capacitor correctly into the circuit for starting or do not open to take the capacitor out of the circuit.

Sometimes you can hear the switch operate, especially when the motor is slowing down you can hear a click when it has slowed down to a few hundred rpm.

Try taking the motor apart to find the switch. The contacts may be burnt badly, sometimes the little springs, either coil or lead, that hold the contacts closed crack.

[Al_The_Man]

As Geof says, it could be the switch, usually they go open, and this usually means it will not start unless you give the shaft a spin, if it is shorted, the start winding stays in, as geof mentioned.
If this is the case, you could put a jumper in series with the cap lead, and when it is up to run, whip the jumper off, this will effectively remove the start cap & switch from circuit .
If it still continues to judder, it could be a winding problem.
Some lower torque motors, have a cap in circuit all the time for start & run.
Al.

[Numb Nut]

Huge thanks for the replies!

So from your explanations, since the motor seems to start fine I guess the capacitor is not being disconnected from the circuit. I'll have a go at trying to find the centrifugal switch as soon as I get a chance and report back.

Thanks again!

[Kipper]

Have a look at the wiring....there are no electro-mechanical switches in the motor....just a single capacitor....Also usually they are dual voltage. The capacitor rating is displayed on the plate. These motors are junk as my 2HP 2800RPM unit failed yesterday...and it's only 26 years old!

[Kipper]

Here's how mine ended it's days...all alone under a bench in a shed....pipe burst and it ran itself to death.....I don't think much of the thermal cutout purported to be fitted (I can't find it either) Motor plate states 240V and yet the wiring connections show it as being dual voltage.....VFD material

[Numb Nut]

Thanks for the pics of your dearly deceased motor! So there's definately no sign of a mechanical switch?

I don't know much about motors but I think the links that you can see in the junction box are for reversing the motor rather than selecting the voltage. Infact, I know this because I wired up the reverse switch for mine so I could try cutting threads on the lathe. The leaflet that came with the motor describes what needs to be done. Pic attached.

Hmmm...I think I should start by testing the capacitor before I get too medieval on it's carcass.

Might try their parts & service phone number on Monday.

[Geof]

I did not notice your location but even if I had I would probably have posted the same advice; Al the Man did comment however, that some low torque motors keep the capacitor in the circuit all the time. I have simply never encountered this on anything larger than 1/3hp. Live and learn they say; next time I will include all possibilities.

[Kipper]

Ahh just noticed there's nothing connected to U1.......Whose the NumbNut now

On the bottom left of your motor plate do you see the µƒ ?

[Numb Nut]

Geof - your earlier reply was gratefully received and understood to be what you know from experience which is exactly the sort of response I was looking for.

Meanwhlie I've been reading up on AC induction motors here:
http://en.wikipedia.org/wiki/Induction_motor

Oddly enough there is a picture of a Clarke motor at the top of that page and a description of why single phase motors need a starting capacitor. I did wonder why there didn't appear to be brushes in the motor and now I know.

[Numb Nut]

Originally Posted by Kipper Ahh just noticed there's nothing connected to U1.......Whose the NumbNut now On the bottom left of your motor plate do you see the µƒ ?

That diagram doesn't show the motor windings which are connected to the various points on the 6 terminals but you knew that, right?

The motor plate states that the capacitor is 25uF / 450V.

Anyway, it's time to get the capacitor out of the motor and hooked up to a multimeter in capacitance measuring mode. Oh yes. They always talk in the end...

[Geof]

Originally Posted by Numb Nut ...Meanwhlie I've been reading up on AC induction motors here: http://en.wikipedia.org/wiki/Induction_motor

That Wiki is quite good but it does not explain how the single phase motor starts and it leaves out the Split Phase motor which is the style I was referring to.

The 3 phase explanation describes how the magnet field 'rotates' because the field coils are energized in sequence; single phase,of course, just reverses the magnetic field but doesn't rotate it. In the capacitor start, or run, single phase motor there is a second coil that has the capacitor in its circuit and this means the magnetic field in this coil either precedes or follows the field in the main coils; this induces a rotation in the magnetic field so the motor starts. In the Split Phase motor the second coil does not use a capacitor but the metal pole of this field has a copper strip across it that influences the way the magnetic field behaves and causes it precede or follow the main field again creating rotation and starting the motor. Once the motor is up to speed it is possible to take the starting coil out of the circuit and on large motors this is done by the centrifugal switch. On a motor that leaves the starting coil in operation normally the starting torque is low, the starting coil does not draw much current, so that this coil does not waste too much energy when it is not needed. On larger motors which need good starting torque the starting coil is designed to draw a large current and if it is left in the circuit once the motor is up to speed it draws too much current and the breaker will kick out or the motor overheat sensor will trip.

If you haven't already done it go and take a few courses in Physics and basic electrical theory they can be quite fascinating and very useful in the long run.