Crabtree motor starter cutting out on Myford lathe

[Mike DonnerstagParticipant @mikedonnerstag]

Dear all,

I’m hoping someone is familiar with the Crabtree motor starter I have on my Myford Super 7 lathe to start the (I think original) single phase motor. Very intermittently, as in every few weeks, the starter cuts out and doesn’t allow me to restart the lathe. I’m wondering why this is, though I am assuming something is overheating or overloading as it allows me to restart the lathe after several minutes have passed, often after pressing the black stop button, which may act as some kind of reset.

Any information on these starters and why this might be happening would be very gratefully received.

Thanks,

Mike

[Clive FosterParticipant @clivefoster55965]

Almost certainly an overload issue.

Could be the setting for the current detecting system has shifted due to mechanical vibration, old age or just a build up of dust.

Motor current is generally monitored via a bi-metallic strip which heats up and bends in proportion to the current flowing through it. Too much current causes it to bend far enough to open a set of contacts cutting current to the contactor coil. Contacts don’t close until it has cooled down allowing the bi-metallic strip to straighten out.

There is a simple mechanical adjuster to set the amount of bend needed before the contacts open. Calibrated in amps of current. These can shift. Old bi-metallic strips may take on a small set so the adjustment needs to be tweaked. But the calibration isn’t super accurate. Near enough is good enough because the current draw of failing or seriously overloaded motor is vastly greater than the normal current.

Significant quantities of dust can insulate the bi-metallic strip so it needs less current to heat up enough to bend fare enough to open the contacts. I’ve seen some right furry ones associated with intermittent failures. Blowing off the dust was a quick and easy fix.

Worst case is a failing motor which sometimes draws enough current during fairly extended use to cause things to trip.

Clive

PS pictures don’t show for me.

[EmgeeParticipant @emgee]

Hi Mike

Clive has provided a really good description of what may the cause of the motor cutting out, you indeed sugested overloading so that seems to make sense.

Have you started using the lathe for longer periods ? doing a lot of stop starts ? taking much heavier cuts with some new tooling ?

Any of those may cause an overload that trips the thermal overload unit which sits under the contactor in you starter.

After isolating the power supply you can gain access to  the interior of the starter by slacking the hex screw at the bottom and “hinging” the cover off.
You will see the pointer indicating the overload setting, check that is at the FLC stated on the motor nameplate.
Check all connections for tightness and security of any fixings.

Best put the pics in the post rather than your Album.

Emgee

 

[Robert Atkinson 2Participant @robertatkinson2]

I concur with Clive and emgees’s comments and add the following,

Have you checked the motor temperature after a trip. If it isn’t very warm it’ likely to be a contactor or wiring issue rather than a mechanical overload or motor fault.

Robert.

[Mike DonnerstagParticipant @mikedonnerstag]

Thanks everyone for your replies and advice. I now know far more about motor starters than I did previously. I’ll check motor temperature next time it trips, and will probably take the starter apart and check it for dust and dirt in due course. I hadn’t realised the current trip is based on a bimetallic strip, a bit like a room thermostat.

Thanks for all your information,

Mike

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