I think that you will find that the majority of respondents will advise that you go the three-phase +vfd route.  I have converted many machines this way, using “cheap” vfds of foreign origin. Only one has broken while in my possession and at about £60 a go, I consider them to be almost consumables. I would encourage you to buy a vfd with the facility to be controlled from an external potentiometer and switches to control speed and motor rotation direction respectively.

There are, of course, many companies who will supply a plug-and-play system, but at a cost.  I am notoriously “cautious” with money – mine and other peoples.

John

What’s the issue with the motor you have?

Three-phase or brushless is the answer, but I would only do it after the single-phase motor has failed, unless there’s an urgent need for speed control.  Though 3-phase is preferred, single-phase motors do a reasonable job.

Why do you want to upgrade?   If for theoretical reasons,  I’d save the money for a rainy day.   If the reason is practical, go for it full ahead!   You get speed control, more torque and a smoother more responsive lathe.  The number of belt changes needed will be much reduced.   Finish should improve too, though I doubt it makes an obvious difference.

As far as I know, no 3-phase convert has ever regretted it.  At least once it’s working – have to get the wiring right, and some manuals are hard work.

Dave

Jack, I’d assumed your Granville was already fitted with speed control.  Is that not the case?

If the lathe only has a 1/3HP driving the spindle via a single pulley pair, without speed control, then it will only work properly over a somewhat narrow range.   Rule of thumb, cutting mild-steel, a 10mm diameter rod should be turned at about 1000rpm, whilst 100mm diameter should be run at about 100rpm.  Divide by 2 for hard cast-iron, multiply by 2 for Brass.    And threading needs very low speeds, say 15rpm.  When a wide speed range is needed, simply increasing the power of the motor isn’t a good answer.  Some form of speed control is needed.  Just as a car needs a gearbox.

Options, add:

• Belt and multiple pulleys.   This is the traditional method.  A single speed motor drives a countershaft on which is mounted a 3-step pulley, paired with similar on the headstock.   Good description here on lathes.co.uk.   Main disadvantage is that constantly shifting the belt can be a serious time-waster.  Also, only a limited number of fixed speeds are available, but that’s unlikely to be troublesome unless carbide is in play.
• Motor (not single-phase) with electronic speed control.  This is the modern approach, often coupled with a simple 2-step pulley providing a slow and fast range.  Slow range covers from about 15 to 250rpm, fast from 250 to 2500rpm.   Rarely necessary to shift the belt, though it can be a nuisance, and speed can be varied precisely to suit the job just by twiddling a knob.
• On older machines, do both!  Change the motor from single-phase to 3-phase, and keep the pulleys.  However, belt changes are minimised because most work can be done on one pulley setting, whilst the electronics change speed over a wide range.  (Modern electronics are much better at maintaining low speed torque than Noel imagines, but if you need more, the belt can be moved.)

Which model Granville do you have?  They vary from pre-war simplistic designs to post-war sophisticated.  A 1/3HP motor and talk of pulleys suggests an older design.

What’s the actual problem?  Is it that the lathe doesn’t cut properly with the existing motor?  That’s not surprising if the spindle can only be spun at one speed.  If so, the question may be “what’s the best way of providing speed control?”, rather than “single or 3 phase???”.

Apologies if my analysis is wrong – I’m joining the dots!

Dave

^^yes this^^

..1/3rd hp is perfectly adequate – the step-down gearing needed to reduce the motor speed in the slow/medium ranges multiplies the available torque which is what is needed for turning larger diameters, and in the high range the smaller diameter of work requires less power to machine.

1/3hp rated motor was a common ‘spec. for the ML.