Thread: Lathe motor

The best option would be to use a single phase motor with the same RPM's.
More torque might sound good but also means in moments of "OOOPS I missed it" damage can happen to parts of the lathe that would otherwise have no problems.
Easiest way to check the CAP is with a tester, either in a multimeter or with a dedicated tester.
You can't see from the outside if these Caps are dead, if in doubt invest the 10 bucks and replace it, but double check that you the right size for the motor.
Wrong seize, e.g. 25µF instead of 10µF can mean the cap does not like it or the motor can overheat.

Thanks Downunder, the motor is geared down with a jack shaft and reduction pulley setup before it inputs to the lathe.
I have a good multi meter but don't know how to go about testing the start cap. it is a 50 uF. Is that microfarads?

Yes, 50µf is your value, but AFAIK this is not a correct value as usually the single phase motors I encountered had caps between 6.8 and 25µF.
Could be that your motor was used for something that needed a lot of torque from the start but would recommend to check if the cap value is really correct for your motor.
By using a simple multimeter you might be able to find out if the cap is dead already but you won't be able to check if the value is still correct.
Most cps tend to burn out, that means you get shorts in the foil that cause a hloe, up to a poit they can heal themself this way but of course the cap shrinks in vaue each time.

Thanks Oceanboy, for all the trouble of those videos, I will use them.

Downunder, I bought the motor new specialy for the lathe about 5 years ago but because I had a smaller motor already on it I could use it in the lower gears so I was a bit uninspired to go to all the trouble of making the jack shaft and the frame to hold it all together, so I've only installed it relatively recently.
It had the 2 caps installed in a housing on top of the motor from the factory, 1 start cap 250uF and 1 run cap 50uF ( I've had another look since watching the videos)
The place I bought it from I don't even recall the name of and they're no longer there (was at Rocklea Brisbane) the manufacturer is International Electric Motors Melbourne who I also can't find so it looks like I have to sort it out myself, with the help of my mates here on Austech, I'll let you know how it goes.

Ok, so the start cap has failed the test.
The next question is of course why.
I was working in high gear when it failed and there is obviously too much stress on the start cap unless the cap was faulty from the start.

Is there anything I can do to help the motor out, maybe change the value or voltage of the start cap (it is a 250 volt) or make it a soft start?
When I say soft start I don't really know what I'm talking about but I have a roller starter that I made for my push start race bikes, I made it using a supercheap 9" angle grinder rated at 2,200 watts, it has a soft start and I think this may be the key as to why it works so well as a roller starter.
If I could incorporate a soft start on the lathe motor that may help it, hey?
I know I can get an inverter to do that but then I could have bought an inverter to run the original 3 phase motor but went for the 250 volt motor as a cheaper option.
Is there a cost effective method of making it a soft start?

Originally Posted by mickc

Is the motor under load when started
If it is taking to long to get up to speed this will kill the cap
How is it changing from start to run, centrifugal switch or relay or controler

In top gear the lathe takes about 3 seconds to get up to speed. Unfortunately the thing has to be stopped to change gears.

The motor has a centrifugal switch, start cap and run cap, the lathe electrics starts the motor with one of two relays, one for forward and the other for reverse.

Originally Posted by crazy

For reverse, is it wired into the motor or just switch incoming lines? as if it was the later i assume it wouldn't work?
the caps are the original ones for the motor?replace the faulty caps and see how it goes
Check if it gets too warm when running.

The motor was working ok, it's a 4 pole so I am using the two relays to select the input pairs for forward and reverse. When I bought it, the guy at the shop said this is the one to use, its got heaps of grunt, if ya gear it down to half the output shaft speed it will have twice the grunt and it will be easier on the motor, sounded like good advice to me.

The caps are the original but the start cap failed after a few weekends of hobby work (so the machine is not constantly working) so I am looking for advice to try and avoid a repeat of the start cap failure. Any ideas?

Ok, my five cents on how to do it properly:
It is a 3 phase motr running on a single phase with a cap...
So get rid of the caps altogether!
Use one these electronic motor controllers instead.
They cost a few $, but are far easier for you than anything else that is not the original motor for the lathe.
Why?
1. The controller is fed by a single phase but gives a three phase, frequency controlled output.
This means you can run the motor from standstill up 25% over currend max speed by just turning a knob.
2. If break resistors are added the lathe can perform an emergency stop in under a second.
3. You will never have to replace a cap again.
4. You don't need to gear to motor down as you adjust the torque and speed over the controller - even at very low speed the motor will be able to produce full torque.

So you only need a switch for the reverse if that is not managed by the gears on the lathe.
IMHO the advise given to you when you got the motor was total bull:
The two caps are meant to get the motor running and to keep it running - at the full speed.
Every lathe I know needs adjustable speeds and you motor configuration is crap for that

Originally Posted by mickc

3 seconds is to long to be running on the start cap, you will be changing it often.
You could try a timer if you have access to the wires going to the switch.

Yeah, I figured 3 seconds was too much for the start cap to handle.
Could you go into detail with your timer suggestion, I have access to all the wiring but I don't understand what type of timer you mean, what it's function would be or how to wire it in, there is plenty of space to fit extra electronic gear.

Seems I've confused you with my lack of detail Downunder, the lathe originally had a 3 HP 3 phase motor, I swapped it over for a new 2.2 KW single phase motor with twice the rpm of the old 3 phase as that seemed like good advice from the business selling motors, the thing is that in reality it didn't work out too well.
I knew about the controller option but the cost was much more than the option I took.
I know a bit about electronics but my background is mechanical engineering, that is why I turned to you guys for advice.
I could solve the problem mechanicly by making a centrifugal clutch drive between the motor and output pulley so that the motor is free to spin up to a predetermined speed before it softly engages the input shaft to the lathe, but there would be a couple of days work in doing that.
I've had a look at soft starters but the ones capable of handling the 2.2 KW are rather costly and on par with an inverter controller so I may as well just buy an inverter and refit the old 3 phase motor, an easy fix but costly.
I can't work out why you can buy a 2200 watt angle grinder with a soft starter from Supercheap for $100 but the going price for just a 2.2 KW (3 HP) soft starter is around $300.

I've supplied and installed a few of those Phase Converters Fredly.
You are correct, they are $$$, but the end users have been happy.
Ironically, the converters are just a box full of huge Capacitors, that need regular testing and replacing (kind of where you are now)
I've also done the 1phase for 3phase motor swap a couple of times, it always ends up a pain in arse, generally with the size issues of a 1 phase motor of the same output. And, a 1phase motor can never produce the same torque from start up.

The irony is, the people i installed the phase converters for, were people that went down the 1phase motor swap first.
Depends on the size of the equipment really.

The last one of these jobs i did was for a Cabinet Maker to run his 3 Phase table saw.
He first bought the biggest 1 phase motor he could make fit in his saw, saw used to run slow, and bog down under load, blades would get blunt because they weren't being run at the correct speed all the time. Caps used to die on the 1 phase motor.
Although being more than equivalent in Kw rating, it just didn't seem to do the job.
He went the Phase Converter route after that and has never looked back.

I'd be surprised if you couldn't make one, there doesn't seem to be much to them.
A transformer and heap of capacitors, his has no logic boards, its just simple everyday components wired together in a box.

Hope you get it sorted.

There's one relay for each direction Mick, they both have four separate inputs and outputs, I have no idea what you'd call them but they are 50 mm square and 70 mm tall and go clunk rather loudly when engaged.

The stuff about timers and contactors and pneumatic blocks is completely over my head, as you most likely know, having a good laugh are ya mate.

Ok, might be just my confusion as I tend to think too complicated but:
On a lather you need to be able to adjust the speed - how exactly is that being done on your system?
You replaced the stock motor with something that is much stronger - if the speed regulation system is not up for that much power there will be a problem.
I agree, those converters are a bit expensive, although for 1KW motors you can get them for under 200 bucks on Ebay.
But they have much more to offer than any old style manual regulating system - and no, they are not just a bunch of caps
The single phase is chopped and tranformed to DC,
DC is chopped again into three phases,
these three phases are not only frequency regulated (often from about 20Hz up to 120Hz) but also load compensated.
That means you don't have a pahse shift and the motor always runs at full voltage - the frequency is what makes it run slow or fast.
A good system delivers about 120% torque from slowest speed up to normal torque at almost twice the rated rpm's of the motor.
True, there is a bunch of caps in these converters, but they are for chopping of the DC part
If set up correctly (that means right size for the motor and with break resistors if needed) a converter will last many years, in the long run saving money as there are more options for the user and far less dowtimes due to repairs.

It is down to the lazyness of our power suppliers that a normal private home only gets a single phase connection.
Most countries deliver all three phasesand the loads are spread over all of them, so for example lights are on one phase, power outlets on the other and your aircon on the last phase.
But instead the power company decides which single pahse makes it into to your home (if there three in your road anyway).
Guess we have to pay them their rediculous prices for many years to come and will always have a grid that lacks all safety and redundancy systems...