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MG TD TF 1500 - Lathe speed control via inverter

I am about to replace my Myford ML7 lathe, which has made many parts for both my TF and M Type, with a top of the range Myford Super 7. The new machine has an inverter which controls the motor speed. I assume I set the belts in the lathe to the fastest speed and let the inverter do the rest. I don't want to exceed the maximum spindle speed of the lathe. The other option is to set the speed of the motor to maximum and then use the stepped pulleys. I am also concerned about reversing the spindle with the inverter. The Myford doesn't have a cam lock chuck and I don't want it to unscrew when I flip its direction.

I know the above isn't directly T series stuff but without my lathe I wouldn't have been able to complete the restoration of my MGs. I think a lathe should be an essential part of a T series tool kit.

Jan T
J Targosz

I have been using one of those for years now. Over here they are called a VFD (Variable Frequency Drive).

Its important to know that that as the motor is synchronized to the converter frequency. As the output frequency of the converter changes the motor speed changes. With electric motors the TORQUE, will be relatively independent of the speed. The HP will directly relate to the speed. The slowest you can make the motor turn is when you no longer have enough power to do the job.
So does a turning tool use force (Torque) or power. If the work gets hot its using power, if the tool is nice and sharp and work does not heat up its torque.

It is also permissible to over-speed the motor by 50%. Most motors will not care that you are running it 150 Hz.

I set my belt on the middle step. I over speed the motor to get it up to high speed and then reduce speed as needed, say for threading. No remember at low speed although the motor still is delivering the same torque If you are not using back gear you will have significantly lower overall torque that wit Back Gear, still if you are only taking 1/10 mm or less on a pass it might be sufficient.

Generally the controller will allow you to set a maximum and minimum frequency, so once you have calibrated it and set the maximum frequency, you cant over-speed the lathe.

I have a app for my iPhone turns it into a strobe so spindle speed measurement is easy.

Jim B.
JA Benjamin

I though I posted this before.

Top frequency should be 75 Hz NOT 150 Hz.

Sorry about that.
Over here I run 90 Hz.

Your pulleys would be set for 50 Hz mains.

Jim B.
JA Benjamin

I have a 3 phase VSD (Variable Speed Drive in UK= VFD in US), on my lathe. I have a switch for forward/reverse which I fitted years ago when VSD's first became popular in the 1980's and then known as VVVF (variable voltage variable frequency) drives. In industry the then common DC drives were gradually phased out at the time,
Modern VFD's can be configured for forward/reverse via inputs without complicated three phase switching. You can also configure the torque compensation for slow speed applications. I have been dealing with these as my daily bread all my working life. The ones that we use on our roller mills go up to 10,0000kW (10MW)power rating!
Unlike here in Germany where every house has three phase for water flow heaters (showers)/cookers etc, it is not common in the UK where you generally only have a single phase per house.
There are plenty of inverters on the market (even on eBay) at relatively good prices for connection to single phase.
I use the ABB ACS series (3ph and 1 ph) and they are excellent and are more or less plug and play-ideal for retrofitting to workshop equipment. I would highly recommend them.
I have the belt on my lathe on the fastest speed and use a potentiometer to speed down. You can also use the +/- button on the inverter display. Just watch the cooling but you can also use a forced ventilation (computer fan) if the motor runs hot.

Declan Burns

Should get myself a new lathe but the trusty old Myford has earned a spot for as long as it keeps behaving itself--The way it's going it will outlast me I reckon----
William Revit

Got to be lucky as we were. Picked up a 1944 Monarch EE lathe. Old Navy lathe that is 3 phase 220 that drives a DC generator which powers a variable speed DC motor. Lathe has pretty much infinite speed control and is a great shop lathe.
Bruce Cunha

While we are on lathes--
In the workshop up at the smelter here a year or two back one of the machinists poked his hand in amongst the chuck while it was going and swore that it wasn't spinning
The workplace safety guys found that it had a fluro light over it and at 3000rpm at which the lathe had been spinning at, the light which was on 50hz supply made the chuck appear stationary
They run double lights on two seperate phases now which removes the strobe effect

Don't get caught out-----
William Revit


My Myford lathe has an original articulated, filament lamp fitted to the rear of the drip tray. I have seen similar ones on the industrial antiques salvage programmes on the Discovery Channel and they are worth hundreds of pounds!

Jan T
J Targosz

This thread was discussed between 28/11/2017 and 08/12/2017

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