Myford motor playing up

[vintagengineerParticipant @vintagengineer]

<p style=”text-align: left;”>I have a Brooke Compton motor on a Myford lathe. It only runs in reverse if you spin the pulley with the belt off, but cuts in and out whilst running?
Any ideas on the fault?</p>
Thanks

[BazyleParticipant @bazyle]

Are you saying it doesn’t run forward or that it does run forward but is problematic in reverse? Is it a capacitor start motor?

Have you checked the wiring for loose connections and then continuity of start and run windings?

[vintagengineerParticipant @vintagengineer]

It only runs in reverse if you spin the motor and it cuts in and out whilst running. It does have a capacitor and all wiring looks ok.

[noel shelleyParticipant @noelshelley55608]

A single phase motor has 2 windings a start and a run. The most likely configuration will be a capacitor in line with the start and a centrifugal switch. As speed picks up the centrifugal switch will disconnect this winding leaving the run winding to take the load. Tubal Cain s model engineers handbook shows several circuits, including the colour codes used in those days. yellow / blue for start and red/ black for run. If the cap has gone up the swanney this will cause the start to be to slow /long. If the centrifugal switch has not reset then there will be no start, or if it has failed due to burnt contacts. The direction of start will dictate the direction of run. Has it got a reversing switch (dewhurst ) this switch is not for start stop operation and should only be used to change direction of rotation when the motor is at rest. Check the cap and centrifugal switch. If you have the means check the insulation and the resistance of the windings for short circuit. One reason for a clutch is that regular starting can overheat the windings leading to failure in single phase motors. Good luck.  Noel.

Ps if your near me I can test it for you.

[vintagengineerParticipant @vintagengineer]

It has a dewhurst switch but no start stop switch. I intend to fit a starter switch. I assume the centrifugal switch is at the opposite end to the shaft.

[noel shelleyParticipant @noelshelley55608]

When a Dewhurst switch is used as a start/stop the contacts can get burnt this may be your problem as it controls the start windings. Yes the centrifugal switch is at the non drive end. Very often you can hear the switch reset as the motor idles down to rest, an interesting click ! Do you know the wiring diagram for the switch ? The motor can be wired for one direction just to check before stripping down. Noel.

[Keith Rogers 2Participant @keithrogers2]

I have come across similar problems on a friends machine.

In that case it was as Noel said the Dewhurst switch contacts were badly burnt and making intermittent contact in one direction and not working at all in the other.

Keith.

[Howard LewisParticipant @howardlewis46836]

The normal advice is for a No Volt Release switch to be in the line between the power socket and the Dewhurst switch (So that the Dewhurst is only used to reverse the windings when no current is flowing.

I would suggest

1) Wire in a NVR switch as above.

2) With the power lead unplugged, and the socket switch OFF, strip the Dewhurst switch and clean the contacts.

If you take it out, make a note of which wire goes where, BEFORE disconnecting it!

Hopefully the motor will behave itself when the switch has been cleaned and refitted.

You will know if the centrifugal switch is operative; as when you switch off, as the motor runs down, you will hear a slight “click” as it remakes the contacts for the capactor to the start windings

Let us know how you get on!

Howard

[old martParticipant @oldmart]

Most older Myfords only had a Dewhurst switch, our lathes at work when I was an instrument mechanic certainly had only them. We were not likely to ever use reverse, but constant on and off without a primary switch would cause wear on the contacts. Now you can buy a no volt switch with an emergency button for under £20. The capacitor start is also a cause of problems and there might be a centrifugal switch in the motor.

[noel shelleyParticipant @noelshelley55608]

A No Volt Release type switch may protect you from unplanned starting IF there is a supply failure BUT it will do nothing to save the motor in the event of a fault, and a fuse is no protection either, by the time the fuse goes it will be to late. An overload device is needed, rated for the full load amps of the motor and set accordingly. Normally the range over which it works will be shown. In an ideal world there would be a fuse to protect the wiring, an isolator, and a Direct on line starter. This last followed by a reversing switch if needed.  Noel.

[vintagengineerParticipant @vintagengineer]

Taken the motor off the lathe. I have continuity between AZ and A, but nothing between Z and S or S and T.

[Howard LewisParticipant @howardlewis46836]

Some one who is electrically skilled will probably tell you that the start windings have gone open circuit.

In which case the choice will be bwteen repairing this one, (Might be just a broken connection?) or a replacement motor.

Which ever, do update the wiring to make operation of the lathe safer.

Howard

[noel shelleyParticipant @noelshelley55608]

AZ and A are the run windings ! That you have no connection on the others indicates a fault on the start side – check the centrifugal switch. How many capacitors are there ? 1 or 2 ?  NOTE all wires and where they go, see my earlier comment on the colours.  Noel.

[vintagengineerParticipant @vintagengineer]

Checked the switch and checked the terminals with switch open and closed. On capacitor.

[noel shelleyParticipant @noelshelley55608]

There seem to be to many terminals – are they all used ? No capacitor indicates split phase motor which only uses AZ and A. check the colours of your 4 wires, black and red are run, yellow to the switch and then the start winding and blue back to the terminal – check this start circuit.  Noel.

[BazyleParticipant @bazyle]

There are motors with a start winding that do not use a capacitor. Did you check the start winding from the switch to the other end of said winding. Are the connections to the switch ‘good’. I recently recovered a grinder with bad joints on the switch. An added complication of this one (American Delco) was the centrifugal switch is sensitive to end float of the spindle.

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