Bridgeport Issue

Good Morning

Hoping someone can help me out .

A few months ago I bought a Bridgeport miller (which I believe was single phase from factory)

I’ve got a slight issue which I’m hoping someone can offer me some advice .

- When the spindle stops in a certain position , upon restart the spindle won’t spin and the motor grumbles . When it does I stop the machine and rotate the spindle slightly by hand to another position , then it starts up fine (spindle seem nice and free). It does stop in to positions where it starts without issue , seems to be a 1 in 3 chance of it stopping in the bad spot.
I’ve done a bit of reading online and keep coming across a start up capacitor ? But I’m not too clued up on the electrical side , more mechanical and CAD . Does anyone have any advice ?

- On the back of this does anyone have a single phase wiring diagram , it’s something I’m keen to learn more on.

thanks in advance for any advice , it’s much appreciated .

Mike

Hello Michael,

That sounds more like dirty points on the centrifugal switch within the motor itself than a capacitor fault. It is a common enough problem with single phase motors that have had a lot of starts. On starting, the motor start winding is energised to get the rotor up to speed which in turn opens the centrifugal switch to allow the main run winding to handle the running load. There is always a spark at the contacts when this happens and over time the points become pitted and oxidised.

It is a mechanical issue and it means taking the motor off, opening it to get to the switch inside and cleaning the points with a folded strip of emery cloth so that abrasive is to the outside. Be sure to clean up afterwards as you will have exposed one of the rotor bearings, easily protected with some strategically placed rag. A drop of oil on the bearings would be a useful bit of maintenance while you have the chance

Hopefully that will put things right for you

Regards Brian

Exactly what the switch looks like varies, but after taking the wired end the motor off, it will be found mounted on the axle something like this:

As the motor gets up to speed, a weight, or pair of weights, pulls against a spring and eventually move far enough to open a contact. Every time the motor starts, the contact sparks and they eventually wear out. Or the lever gets out of adjustment so the contact doesn't move far enough to open and close correctly. If the contact can't close when the motor stops, the run winding gets no power, and the motor won't start.

The contact is usually easy to see, and it may only need a clean or adjustment. If it's burned out might be possible to replace the contact points, or to source a replacement centrifugal switch. It depends on the age, make, and type of motor. I'd expect Bridgeport spares to be available, but not necessarily cheap!

The centrifugal switch and capacitors are a weak point in single-phase motors and they vibrate. The advantage of single-phase motors is they can be plugged into ab ordinary mains socket: everything else about them is shades of grey! If the motor has to be replaced, consider 3-phase and a VFD.

Dave

Thank you for the detailed reply’s , very much appreciated. I won’t be able to get to the machine until the back end of next week however I will take the motor off and take a look , fingers crossed it’s just dirty points like suggested.

best regards

mike

If the switch is mounted on the axle, how does it transfer electricity? Last time I looked, there were no brushes or slip rings in my lathe motor.

The switch is fixed to the bell end of the motor. It is the weights that rotate.

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Rather than pulling the motor off and to pieces, slip off the drive belt and see if you can reproduce the problem. If the motor fails to start, mark the pulley position relative to a fixed point and see if the position theory (bad spot) holds up. Given that the drive:driven pulley ratio is unlikely to be 1:1 you will then have separated any spindle issue from any motor issue.

The rotating part actuates the stationary switch by a simple lever bearing on a axially moving (and rotaing) surface. Think of a bobweight engine regulator.

Robert.

Whilst it could be a faulty centrifugal switch the bad spot theory - not sure ! The thrust ring that operates the stationary switch ? The contact points ? Has the start cap gone out of spec ? Or is there a bearing or driveline problem causing an overload on start up. Depending on the drive is it a variable speed drive with a problem or trying to start up at too high a speed setting. If single phase has it got the right motor power ( 2Hp ) ? Good Luck. Noel.