Beginner needs help with Colchester Bantam Lathe

[Paul Matthews 4Participant @paulmatthews4]

Thank you Martin, Bob, trouble is I've no idea what happened to the lathe before I got it, so drive pulley may well have been changed when motor was changed. Regards Paul

[john swift 1Participant @johnswift1]

Hi Paul

since I don't have the original Colchester wiring diagrams

I have modified your diagram to show how I think the relay should be wired

 

    John

Edited By john swift 1 on 19/07/2016 12:45:43

[MuzzerParticipant @muzzer]

Posted by Paul Matthews 4 on 19/07/2016 11:14:11:

334rpm actually running at about 360rpm

510rpm actually running at about 560rpm

Presumably you are measuring the speed with no significant load. The nameplate speed is at full rated power, whatever that is. The unloaded motor will spin close to 3000rpm if it's a 2 pole machine.

Personally I'd bin the single phase motor, refit the original (or even better, a modern 2 pole motor of >2kW rating) and spend my time setting up a VFD. You'll never look back….

[Paul Matthews 4Participant @paulmatthews4]

Guys, my apologies if I've been talking nonsense and managed to confuse anybody, probably down to my lack of knowledge when it comes to using the correct terminology. The "reverse switch" as I call it is also the start / stop – move the lever down to run CCW and up to run CW, centre is motor off: Here's a short vid:

https://youtu.be/pETBymJVpI0

[Bob Brown 1Participant @bobbrown1]

Muzzer I assume you mean fitting a 240v 3 phase motor and VDF which would be my choice also probably fit a 4 pole motor and change the drive pulley to suit.

[MuzzerParticipant @muzzer]

Well, my approach would be to fit a larger 2-pole motor so that it still has enough grunt and cooling down at 1500rpm. My Bantam has a 1.5kW 2-pole machine so that I can get full power at the original top speed out of it (plus a bit more with the VFD set to 70Hz or so). You don't want to lose the top speeds if you want to make use of carbide etc tooling – if anything you want a bit more power than std – it's not going to suddenly pack in.

I've not used mine much at low speed and high power but that's where the low speed range of the original motor would be missed. To provide that level of grunt with a single speed 2-pole motor you'd specify a higher rating, like 2.2kW or more. There's plenty of room on the motor mounting plate on a Bantam for a bigger foot mounted motor and it would help at the top end too.

Murray

[Paul Matthews 4Participant @paulmatthews4]

Done a bit more work by checking RPMs / pulley sizes etc, existing old motor is 2HP 2800rpm, which I am planning to replace with modern 2HP 1400rpm motor – hopefully that will give me the full range of speeds. Awaiting parts to be delivered for new circuity so will advise how things progress.

[Phil WhitleyParticipant @philwhitley94135]

This is an interesting fault!! I know single phase motors, but not Bantams, I have a student. How is the motor reversed? is there a switch on the lathe start lever? It seems to me that the start winding is working in both directions, but the run winding is out of circuit when the fault occurs, so as soon as the motor gets up to speed, and the Cent sw opens, the motor slows and the start winding cuts back in. There does not appear to be any greater resistance to rotation in either direction, so I would be looking for loose or dirty contacts in the reversing switch but on the run winding. Z1 and Z2 are the start winding, U1 and U2 are the run winding (sometimes A1 and A2 for the run winding). To reverse the switch swaps Z1 and Z2 over, but if the run winding also goes through the switch, maybe a loose dirty connection that only fails with a higher speed/vibration level? Turn it off, unplug it, and get the multimeter out, and test through from the plug to the run winding connections , but you will have to push the starter/contactor in manually, or test from there to the motor. I suspect the reversing switch. A single phase motor will run in either direction at equal power. If you disconnect the start winding, turn on the motor and spin the shaft manually, it will run up to full speed and power in the direction you spin it. All the start winding does is push the rotor in the same direction every time, and provide extra power to overcome the starting load of the machine. Reverse the polarity of the start winding, and you reverse the motor.

[Phil WhitleyParticipant @philwhitley94135]

Just re read the last postings. the number of poles in a motor sets the speed, not the power, thus a 2 pole motor turns at about 2880rpm on 50HZ supply, and a 4 pole at about 1440rpm on 50Hz. Putting a 2880rpm motor on a machine designed for a 1440 motor is not a good idea, so you need to know if the bantam was fitted with a dual speed motor like some of the Students (mine included) were. I fear modern motors are not as good as the old ones, and rating in watts rather than HP is a bit of a con, as they are telling you how much energy the motor will dissipate, rather than how much power it will actually produce! the difference is of course the efficiency, or how good the motor is at turning watts into rotational horsepower, and cynical old me thinks that ratings in watts are there to cover for the fact that modern cheap Italian and Chinese motors are nowhere near as efficient as they should be. My small compressor has a dual capacitor start 3hp motor 2880 rpm which only just starts a small clarke compressor that it is fitted to (original motor) Without the inlet decompressor, it refuses to start, and will blow the fuse, even though the turning resistance of the compressor is very low. Mind you starting 3HP on single phase is pushing the limits of a 13A fuse, so I have fitted a 15amp! Naughty but it works!! This is entirely due to poor starting on load characteristics of single phase motors. I still think that fault is caused by the run winding being out of circuit in that direction, but without a bantam wiring diagram, I am a bit stuffed, is there one in your manual if you have one?

Phil.

[not done it yetParticipant @notdoneityet]

 

All the start winding does is push the rotor in the same direction every time, and provide extra power to overcome the starting load

There is bound to be a slight (at least) difference in speed at which the start windings are turned off for forward or reverse.  The rotor is accelerating at the time, so the resulting force on the switch will be different, dependent on which direction of travel. Physics fact!

 

I still think that fault is caused by the run winding being out of circuit in that direction

 

If that were the case, please explain how the machine works OK at lower speed settings

 

 

 

 

Edited By not done it yet on 21/07/2016 07:14:59

[Paul Matthews 4Participant @paulmatthews4]

Hi Phil, if you look up about 8 posts, I've posted a small video clip of how the forward / reverse / stop lever operates on the lathe – I haven't dismantled this switch yet but looks like I'll have to at some time so I'll try and get in there today. I've posted the manual diagrams below – plus my updated "as is" wiring diagram. I assume my lathe was the two speed version as it has the two speed switch blanked off on the control box plus the red / blue speed options on the headstock plate.

Now don't laugh, but I was taking some chuck speed measurements last night and set the lathe to run in forward at the 800rpm – that is the speed continually cutting in and out as described above, and the motor ran no problem! I can only assume, as was identified above, that the motor being unable to run in forward but OK in reverse was very marginal, and with the very hot weather, the gearbox oil has become less viscous and therefore not offering so much resistance to the motor. I'm still planning to change the motor so don't worry all the advice has not been in vain!

[john swift 1Participant @johnswift1]

Hi Paul

now I have looked at the original wiring diagrams I have modified the 3 phase single speed diagram to use a single phase spindle motor

John

[MuzzerParticipant @muzzer]

Posted by Paul Matthews 4 on 20/07/2016 14:49:15:

Done a bit more work by checking RPMs / pulley sizes etc, existing old motor is 2HP 2800rpm, which I am planning to replace with modern 2HP 1400rpm motor – hopefully that will give me the full range of speeds. Awaiting parts to be delivered for new circuity so will advise how things progress.

It's not clear to me how a 1400rpm motor will give you the full range of speeds? Given that we determined on page 1 that it was originally a 2-speed (1400 / 2800 rpm) machine, your solution would lose you the upper half of the speed range. I was trying to help when I suggested that a slightly higher power 2800rpm motor would be a solution.

As for the "cynical" post about motor ratings, motor power is generally "shaft power" ie mechanical output. Try reading up before you post! Here's a nice intro to motors. BTW, the Bantam is designed for a 1400 / 2800rpm motor, so I don't understand your concern fitting a 2800 rpm motor??

A lot of hot air going round in circles…

[Paul Matthews 4Participant @paulmatthews4]

Hi Muzzer, as you can probably guess, I'm totally out of my depth with most of the discussion on motors – I'll be very happy with speeds up to 800rpm – I have an existing smaller lathe and don't think I've ever needed to use any speed above that. Thanks Paul

[Bob Brown 1Participant @bobbrown1]

There is an advantage to the lower speed range as that gives you lower speeds for screw cutting, 72 rpm in the high would seem a little high.

[Phil WhitleyParticipant @philwhitley94135]

Not done it yet?

All the start winding does is push the rotor in the same direction every time, and provide extra power to overcome the starting load

There is bound to be a slight (at least) difference in speed at which the start windings are turned off for forward or reverse. The rotor is accelerating at the time, so the resulting force on the switch will be different, dependent on which direction of travel. Physics fact!

The centrifugal switch is concentric with the motor shaft, works on the same principle as a bob weight governor and will operate at exactly the same speed in either direction. If you think this is not the case, please explain WHY?

I still think that fault is caused by the run winding being out of circuit in that direction

If that were the case, please explain how the machine works OK at lower speed settings

Because at lower speed setting there is less rotational resistance due to the gearbox, and less vibration. To cut a long story short, when the fault occurs, the motor is behaving exactly like a centrifugal switch motor with an open circuit run winding, but we know the run winding is not open circuit, as it runs perfectly on the same winding in the other direction, leaving us to come to the conclusion that the fault lies in the switching or reversing circuitry. I did say it was an "interesting" fault.

[Paul Matthews 4Participant @paulmatthews4]

Hi all, just to bring this thread to a close, I'm pleased to report that the new motor is now fitted, switching rewired and all seems to be working perfectly. Thanks once again for everybody's help.

[Ken Price 1Participant @kenprice1]

The people to help you are the Colchester specialists Nobilla Machine tools , they are Colchester trained and can be contacted on 01438 812409. I can thoroughly recommend them.

Ken Price

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