Every answer

Sorry Jason, the machine most certainly does know and care if a single or 3 phase motor is fitted.

Despite the appearence of constant speed a loaded induction motors speed is not constant. It accelerates and decellerates between poles due to variation in torqe "torque ripple" for the same numbrf of poles it is worse with a single pahse motor. Some single phase motors are worse than others. This cyclic variation in torque and speed creates vibration and noise from the machine. More importantly it can affect the surface finish. This is particuarly true of surface grinders. Thwy don't do well when fitted with single phase motors.

Another advantage that has not been mentioned on this thread so far is stopping and starting. Single phase motors do not like a lot of stop-start cycles. This is one reason why Myford fitted a clutch to the S7. Fitting a 3ph motor and VFD to a ML7 overcomes the lack of a clutch.

Robert.

I think that is rather too generalised because I am aware of my lathes history from new:

My ML7 is documented as being shipped from the factory to its original owner (not me) fitted with TRI-LEVA attachment, two speed motor, & QCGB. I do know that it has white metal bearings not bronze and the top speed has always tested around the stated (rarely used) 1280 rpm

Edited By V8Eng on 13/05/2023 14:23:35

Edited By V8Eng on 13/05/2023 14:29:18

Agreed with what Robert had to say.

Well documented instances of single phase motors, affecting operation, include Raglan lathes which didn’t operate too smoothly while running at reduced speed and Centec mills (on its factory stand - maybe not all types of stand) which suffered from substantial noisy vibrations. Three phase motors improved the operation of both those machines. 50Hz vibration cannot be seen to improve the finish obtained finish of the product but the finish may well be better without that added vibration.

Not sure why you think that - my post explains how you can tell quantitively if over-speeding a bearing does any damage, and if so by how much. It's a process, not an opinion.

Stephen found his single-phase versus three-phase motor internet research got contradictory replies, anything between "no problem" and "don't risk it". Isn't that variability more unhelpful than my answer explaining how to get a grip on the truth?

As working machines always suffer wear, the question is not 'Does over-speeding cause harm?', it's "How long does it take before the amount of wear becomes unacceptable?" The answer depends on the speed and load, and it's determined by testing many bearings under controlled conditions. If Myford owners really wanted to know, they could find out by running the experiment. Doing so risks writing-off the bearings, and discovering that Myford lathes wear out just like any other machine.

I hope what I said in an earlier post wasn't unhelpful: 'In short, nothing awful happens when a single-phase motor is replaced by a 3-phase motor of the same oomph unless the operator chooses to consistently run a machine not designed for it at high-speed. In the age of carbide inserts, there's a temptation to do this.'

Dave