Myford super7 motor U/S

[c wastellParticipant @cwastell25217]

I have an old Super7 with a big old motor (induction). It was working then I went to switch on and it just buzzes. I am useless with electrics but could there be a straight forward fix?

[c wastellParticipant @cwastell25217]

Myford super7 motor U/S

[Kevan ShawParticipant @kevanshaw32462]

First guess would be failed start capacitor, that is if there is not an obvious duff connection. What kind of switch do you have?

[not done it yetParticipant @notdoneityet]

Posted by Kevan Shaw on 08/09/2020 21:18:50:

First guess would be failed start capacitor, that is if there is not an obvious duff connection. What kind of switch do you have?

Also, if it has a start winding with ‘centrifugal’ switch, something could be amiss with the start winding or switch….

[SteviegtrParticipant @steviegtr]

Yes as said above most likely is the capacitor followed by the cent sw failure or sticking.

Steve.

[c wastellParticipant @cwastell25217]

Thanks for the quick answers. Capacitor change is within my limited capabilities🙂. I'm guessing its the big thing screwed on the side of the motor. I'll make that my first check. When you say what kind kind of switch, do you mean on the lathe? That is, I assume an original Myford fitting, with forward and reverse positions.

Edited By c wastell on 09/09/2020 07:29:11

[Brian MorehenParticipant @brianmorehen85290]

I take it this was working o/k and now only busses and will not start almost definitely the centrifugal switch which is built inside the motor you will have to remove the end cover on the motor get acces to the switch which may be brocken or stuck a common fault ( Capacitor less likely) Investigate further

Good Luck Brian

[SillyOldDufferModerator @sillyoldduffer]

Be good if you can to post a photo of the 'big old motor', especially the motor plate. It's almost certainly a single-phase motor, but these come in a few different configurations. One of the experts may recognise it. (Photo posting Instructions here.)

Single-phase motors all have 'start' and 'run' windings. The purpose of the start winding is to create a temporary magnetic field for the run winding to push against, and the field is usually angled electrically with a capacitor or two. Once the motor is turning, the 'start' winding might be disconnected by a centrifugal switch inside the motor (behind a removable cover on the back end) but not all motors have them. Myford motors usually do, so well worth a look.

Capacitors fail open-circuit. Not unusual and they are typically easy to replace.

So:

• If the centrifugal switch is stuck open or the contacts are filthy, the start winding gets no power and the motor buzzes
• If the capacitor fails the start winding gets no power and the motor buzzes.

As the start winding is only needed to get the motor turning when power is first applied, it should be possible to start the motor by spinning it manually. In the good old days when men were men and losing a few fingers was all in a day's work, early-design single-phase motors were started by pulling a rope wound around a drum on the shaft. Basically pull rope hard to spin the shaft and apply power. Yuk. You may be able to start the lathe by applying power and spinning the chuck by hand: if it runs, start problem as above confirmed.

If there's nothing obviously wrong with the centrifugal switch (does anyone have a photo?), I'd replace the capacitor. Nothing lost by replacing it as the first step because its fairly cheap and easy to do.

Also inspect connections & wiring carefully – not unknown for wires to come adrift from terminals or to chafe through. Make sure all is firm. Could be as easy as a loose connection.

Most serious possibility is a winding has failed, not easy or cheap to fix. If the motor has died I'd consider upgrading to a 3-phase motor and VFD. Speed control is lovely, and the motors are smooth…

Dave

Edited By SillyOldDuffer on 09/09/2020 09:15:18

[noel shelleyParticipant @noelshelley55608]

See recent article in ME for a few ideas. Noel.

[Andrew TinsleyParticipant @andrewtinsley63637]

Hello SOD,

Not all single phase motors have start and run windings. A capacitor run motor has two windings which are permanently energised, for example.

The OP's problem is almost certainly a sticky, or more likely dirty contacts on the internal centrifugal switch as detailed by others.

Andrew.

[Dave HalfordParticipant @davehalford22513]

The cap might have black stuff leaking out, that means it's dead. Some lathe suitable motors are cap start some are cap start & cap run (there are 2 values of cap on the motor plate) buy the right replacement.

The centrifugal switch is in the wiring end cap.

Check the winding air passages are clear of rubbish like plastic swarf before you reassemble.

[Brian MorehenParticipant @brianmorehen85290]

Dave ?

Hi Dave

ll of your suggestions for starting a motor that buzzes I would try myself and have done many times . I dare not suggest this to someone . I thing Health and Safety > Big No/

Regards Brian

[c wastellParticipant @cwastell25217]

having trouble uploading image but I have con tested the capacitor and it is open circuit.

[SillyOldDufferModerator @sillyoldduffer]

Posted by Brian Morehen on 10/09/2020 18:23:30:

Dave ?

Hi Dave

ll of your suggestions for starting a motor that buzzes I would try myself and have done many times . I dare not suggest this to someone . I thing Health and Safety > Big No/

Regards Brian

Thanks for pointing it out Brian. I hoped my reference to losing fingers was sufficient clue, but yes starting motors by hand is risky. Flicking the chuck whilst taking care not to get caught is one thing, pulling on a belt is asking for a trip to Accident & Emergency.

I'm serious about H&S but come from a background where anything can be done provided it's been risk assessed carefully and mitigations applied to minimise risk of injury. Quite a few jobs are intrinsically dangerous: anything maritime, bomb disposal, firemen, police officers, carpentry, electric line repairmen, dustmen, and street cleaners. The best that can be done is to keep them as safe as they can be by constantly reviewing the risks and what's been done about them. Same is true in the workshop; engage brain first, and ask someone else what they think before trying it!

Dave

[SteviegtrParticipant @steviegtr]

How are you testing the Capacitor. Connect one way from meter set at ohms range. Then reverse connections. You will get a deflection on the meter momentary. If nothing either way then definately the capacitor needs changing.

Steve.

[c wastellParticipant @cwastell25217]

Ah. I just put a meter point on each wire with the meter set to ohms.

[not done it yetParticipant @notdoneityet]

A buzzing motor may well start with some help – but generally better to spin the motor while not connected to the drive. Safer that way, and if the load is too much it may well not have sufficient torque to get going.

[Howard LewisParticipant @howardlewis46836]

It is to be hoped that capacitor would be open circuit! If it has a low resistance, it is a faulty surely?

They are dangerous if charged to a high voltage, and are functioning so as to retain the charge!

Sometimes, a motor which buzzes, (Due to a failed capacitor, or centrifugal switch), can be made to run by starting it rotating, by hand. For years, a friend used to start his ML4 in this way.

Hopefully, a centrifugal switch with dirty contacts can be cleaned, or even replaced, and a failed capacitor can be replaced, for a modest sum.

Howard

[EmgeeParticipant @emgee]

Did any electricians on here ever charge a mains working voltage capacitor with a Megger to test ability to take a charge ? even low values give quite a crack when discharged with an insulated handle screwdriver blade.

Emgee

[Mike PooleParticipant @mikepoole82104]

I thought a megger was for seeing if an apprentice could be charged

Mike

[Harry WilkesParticipant @harrywilkes58467]

Posted by Mike Poole on 12/09/2020 14:11:22:

I thought a megger was for seeing if an apprentice could be charged

Mike

They would run off if you got the 1000v megger out

[c wastellParticipant @cwastell25217]

I removed all the wires and a couple of posts were a little bit corroded. There is a sort of sprung pressed metal affair with a contact which locked quite dirty. I cleaned up everything I could find and lubed the armature (?) spindle at each end. Its all back together now and runs forwards but not in reverse. I have looked at the Dewhurst switch and it looks clean. At least It is useable. Thanks again for the advice.

[EmgeeParticipant @emgee]

If you need to have the reverse function you need to find the ends of the start winding and connect those ends into the Dewhurst switch.
The start winding passes through the centrifugal switch, what you call sprung pressed metal affair with a contact, so using your low ohm meter find the other end of the start winding, it will likely be connected to the neutral post with the run winding and incoming neutral supply.

Follow the diagram that came with the switch to connect the start winding ends to the Dewhurst terminals.

Emgee

[SteviegtrParticipant @steviegtr]

The only time you would need reverse is to back up the leadscrew. Assuming you were going to do screwcutting. Otherwise the myford is no good to do any cutting in reverse because of the screw on chuck.

Steve.

[c wastellParticipant @cwastell25217]

Thanks for that further information, chaps. I wonder if I have got all the connections right, in that case. I don't have wiring diagram.

Steve, Screw cutting is the reason for wanting reverse but its not the end of the world.

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