What (power) motor have you got on there? Almost sounds as if the starter is kicking out before it's up to speed and then it doesn't have enough oomph to get it from there all the way up to full speed.

Single phase motors won't start up without help from a starter circuit. Personally I would have left the original 3-phase motor where it was and added a small VFD.

Have you tried loading it up ie taking a cut / making some swarf? It may be so under powered it would cut out.

Nice machine. I have a Bantam too. Now you need to start collecting all the accessories!

Have you got the manual / parts list etc? I have them if you want to PM me.

Murray

Paul, I see that the motor power for these lathes is 0.9 kW for the low speed, and 1.3 kW for the high speed version, with a single phase I go for 1.5 kW/2 hp. I think you'r getting too much drag through the gears on start up, and not getting full speed before the start winding cuts out. I'd agree with Murray about keeping the three phase motor, although except with a lot of cash you can't have two speed. The three phase has better starting torque, and runs smoother, sort of comparing a V 8 with a lawn mower.

My slightly larger Taiwanese lathe has a 1.5 hp sp motor, and at times I'd like a bit more, but it's a belt head, and it slips the belt as it is.

Ian S C

Edited By Ian S C on 20/09/2015 13:17:55

Paul – your lathe seems to get up to speed when you run in reverse, which suggests to me that the motor is basically powerful enough but something is definitely amiss in the forward direction. Can you measure the peak speed that it reached running forwards on the 800 rpm setting? What supply side wiring have you got to it – I think I'd be looking to see if everything is OK there before delving much further into the motor.

If the machine was originally 3 phase you might have 2 contactors to start it, one for forward and one for reverse, with the forwards/ reverse switch selecting which one to use, rather than effectively swapping the motor leads. If that is the case then check the current overload setting on the forward contactor it could be set too low. The fact that the motor runs OK in the forward direction at the lower speeds (less current drawn) suggests that the contactor (if fitted) is holding in OK. Looks as though you need to consider everything from the mains input right through to the motor.

The geometry is subtly different depending on rotational direction – one way any sudden decrease in speed may tend to close the switch more than in the other.

Neil

Motor is underpowered. With the start winding energised it has enough torque to accelerate the machine from standstill, but then the centrifugal switch opens (at about 70% full speed or thereabouts) and the motor now hasn't got enough torque to continue accelerating the gear train etc up to full speed. So the motor begins to stall, and slows until the start winding is energised again by the centrifugal switch where it begins to accelerate.

Solution is a bigger motor, or don't use the 800 rpm range. Maybe this isn't the original motor – someone maybe converted it from three phase but fitted an undersized motor?

As Neil says, it's just a quirk of the internal geometry of the motor that it will develop enough torque to run to full speed in reverse but not forward. Single phase motors are like that. The gearbox sounds fine, I wouldn't go investigating the bearings, that's just a can of worms which is probably irrelevant for the time being.

We'd still like to know what the motor rating plate says is the rated power of the motor. I've got a Bantam 2 with a 1.0 HP motor, that's not got enough oomph to run at 2000 rpm but otherwise it's fine.

Measuring the supply current on a single phase motor isn't going to resolve anything, you would need to measure power factor as well, as the current stays very nearly constant as the motor is loaded, it's the power factor that changes in response to the shaft load. Also the supply current changes drastically as the start winding cuts on and off so measuring the supply current under running conditions is far from straightforward.

Lovely machine – lots of good work here even if the top speed is outside its capabilities for the time being,

HTH Simon

Hi Paul,

Well, I was totally confident that the nameplate would show the motor was half horse power or thereabouts, and we could put this to bed. Shows how much I know! 2 HP should be plenty.

However, there are a couple of questions that arise out of the information you have posted, as I fancy the original motor would have run at 1450 rpm, and this motor runs at twice that speed. Do you have a means of measuring the ACTUAL speed at the chuck, not just what the gear lever nameplate says? I just wonder if the whole thing is running at twice the speed indicated, unless whoever did the conversion did the right thing and altered the pulley sizes appropriately. If we're actually running at 1600 rpm then 2HP on a single phase motor might well be marginal. I can't gauge the speed from the video.

My second question is just to confirm the full load current as indicated on the motor name plate, just to check that it correlates with that of a 2HP 230 volt single phase motor. If you can get a piccy of the nameplate so much the better.

Failing that my next best theory is that there is something odd with the motor run winding, though I can't fit that with the proper operation at lower speed at the moment.

Mmm. Interesting. The thick plottens.

Rgds Simon

There is a clear click when a bench grinder runs down and the start winding contacts remake. Quite possibly the OP might hear this continual clicking in and out for the start windings if one's ear can be positioned close enough to the motor?

It may be that the start winding contact springs are aged or worn? Changing them might effect a cure, but there is always the risk that the start windings may not be disconnected at full speed in the reverse direction and result in a motor burn out.

The cross-over points for 2 and 4 pole motors and required starting torques are discussed in this link, and may be of help (but I doubt it !

http://www.engineeringtoolbox.com/electrical-motors-torques-d_651.html

I don't know enough about motors, but I do know I prefer 4 pole to 2 pole varieties!

Has the drive pulley been changed? as a motor running at 2850 should give the higher speed range rather than the lower range.