VFD conversion for a Hobbymat MD65 lathe

I'm planning to add a 3 phase motor and a VFD unit to my Hobbymat MD65 lathe.

The lathe has a slow-speed attachment fitted, providing 78 and 156 rpm speed (or perhaps 63 and 125 rpm, according to other source) in addition to the original speeds of 250, 500, 1000 and 2000 rpm.

The original motor is 0.25 kW (1/3 hp) 2840 rpm motor.

I'm going to use a new 0.37 kW (1/2 hp) 3-phase motor and a VFD unit to power it.

I am trying to figure out the following:

1) Buy a 2-pole (same rpm as original) or 4-pole motor (half the speed)?

I've read many discussions of this topic (with regards to various models of lathes), and most people recommend a 4-pole motor (which can then be sped up by the VFD to >3000 rpm, if high speed operation is necessary). A 4-pole motor will give better torque at slow speeds, arguably where it's needed the most. I can't think of a case where I would want the lathe to go faster than 2000-2500 rpm. It also doesn't seem to be a problem, that the motor's torque will somewhat drop if it is run at high speed - I can't think of a case where it would be necessary to have lots of torque available at high speed. I've seen it mentioned by someone that a 2-pole motor might be better with regards to cooling (since it's spinning faster). No one else in the discussions had this concern, though.

2) Would it make sense to remove the slow-speed attachment, or would it be better to keep it? Perhaps the original 4 pulley combinations would be quite enough with the VFD fitted? Especially if I get the (half-speed) 4-pole motor.

I am not sure if the slow-speed attachment will need some modifications, because the new motor is somewhat larger than the original one (frame size 71, original is 63 - moving the motor shaft 8 mm closer in the change gears direction).

3) With the original pulleys & V-belts, would I see the benefit from the additional torque? I've read some stories where people had to convert to poly-V belts (if I understand correctly, that involves making new pulleys with multiple grooves to suit new belts).

4) Tachometer / RPM display

I'd like to measure and display the actual spindle speed, since the VFD would only display the motor speed, regardless of the belts reduction. Can anyone recommend a ready-made product (not too expensive), or a how-to on how to make it (I've seen some solutions involving e.g. a 3D-printed toothed wheel at the back of the spindle, with an LED+phototransistor pickup mounted next to it, but there were not enough details, also there was info about the actual display of the RPM). Would be good not to spend too much money on this. I can solder a bit

Thanks!

Hi,
Not knowing the MD65 I can only address item 1/
I recommend going for a 4 pole motor and make sure your VFD can go up to at least 100Hz output . This will allow you to achieve the high speed on the occasions when you need it and give better torque at the lower speeds you use most.
The fan speed issue is a bit of a red herring, for the same spindle speed (and belt/gear reduction) the fan will be at the same speed regardless of a 2 or 4 pole motor so the relative cooling will be the same. Depending on the actual spindle speed and gearing one may be closer to it's nominal speed than the other so the airflow will be closer to it's design point. The 4 pole motor will be close to this at half the original maximum speed so again has the advantage if you mostly use medium or low speeds.

Robert G8RPI.

Robert,

Thanks for your suggestion. I also read that a motor has it's best cooling performance when operating at the rated frequency/speed. I am just wondering how is this achieved? Does the 4-pole motor has a different fan blade from a 2-pole one, giving same airflow at it's lower rpm? Or is the fan blade not coupled directly to the motor shafts, but via some sort of reduction (different ratio depending on how many poles)?

I have never managed to get my VFD powered 3 phase motors warm, let alone hot, whatever speed they are run at. Don't worry about cooling unless you are going to run the motor at its rated load for long periods. Somewhat unlikely for hobby users. If you want to you could buy a temperature monitor, they are available for fish tanks, and keep your eyes on the temperature if you are loading the motor enough to slow it down.

Search the threads for "LED rpm modules" for people who have added rpm meters.

Martin C

More blades I expect. Motor impellers aren't anything fancy, they're a simple way of providing a reasonable airflow over a motor at its rated operating speed. They fail when the motor is speed controlled because a basic fan can't move enough air at slow speed. A much better solution is to fit an independent fan powered by it's own motor. The cooling fan either runs continuously or can be turned on & off by a temperature sensor.

I agree with Martin though : because most small lathes aren't worked hard enough to get very hot, cooling a 3-phase motor is unlikely to be a problem. And if it is, don't mess about with impellers.

Tachometers are easy; just buy one! Something like this.

Not sure what the slow-speed attachment is: if belts or gears and space allow leave it on because they provide slow-speed torque. If it has to be removed, the VFD will help cover the gap.

Not much point in multi-groove belts either. Maybe if the existing belts slip, but why are they are slipping? Thrashing the lathe is a bad reason. Putting a big motor on a small lathe and working it hard at high-speed is likely to damage it. MD65 lathes aren't designed to remove lots of metal in the shortest possible time. An MD65 used normally with a VFD & 0.37kW motor should be great, but please don't use the extra power to beat the poor thing up!

Dave

Is 0.37kW already double the power of the original? 1/4HP comes to mind.

This chap has carried out a similar conversion:

**LINK**

He has used a 63 frame size 0.25kW 2800rpm motor.

If you have a VFD you shouldn't need the slow speed attachment. I have only used mine when screwcutting or turning a 125mm diameter CI flywheel.

If you can manage German there is some more stuff on here:

**LINK**

I guess if I follow Dave's (wise) advice of using the lathe reasonably gently (which I intend to do), the motor should have no cooling issues, as mentioned by Martin. When ordering a motor, there is a ~25 EUR option to fit it with 3 PTC thermistors, connected in series - one installed on each winding). The PTC is then connected to the VFD, which can stop the motor in case of overheating.

0.37 kW (1/2 hp) is +50% power, compared to the original 0.25 kW (1/3 hp).

I think that as with cars and motorbikes, a bigger motor that is not used hard will run quieter and cooler - if you use them the same way, and not push them to their limits. So a bit of extra power shouldn't hurt (as long as it's used with responsibility, remembering that this is a rather small and light-duty lathe).

Roger, thanks for the links. The motor used in the first link actually comes from the same manufacturer as the lathe's original motor (there might have been more than one supplier, but this was one of them, had a different name though). They even have a 0.37 kW motor in frame size 63, it's just 20 mm longer (DAS 63G4 for 4-pole version). A motor dimensions table at the bottom of the article has two different shaft sizes (11x23 and 14x30), perhaps this manufacturer offers these motors with two shaft options? I will certainly contact them to find out. They do have a somewhat lower efficiency compared to the ones I was looking at. The wiring in the article is not quite clear. A relay and contactor are fitted into the lathe itself, doesn't make much sense to me - there should be no disconnects between the VFD and the motor.

If you have the slow speed attachment you hardly need a VFD etc . It would be far better to make a mandrel handle.

It's not just the slower speed you get with a VFD and 4 pole motor. Smoother torque delivery, lower noise and more torque are big advantages.

Robert G8RPI.

His wiring is somewhat strange but it appears that he wanted to retain the original controls. The reversing switch between the VFD and motor is also dubious. As the original reversing switch only affected the starting winding operating it while the lathe was running did nothing. Operating his reversing switch while the lathe is running will probably break something.

The VFD can also bring an advantage for screw cutting. The quick stop function can be connected to a limit switch to allow threading up to a shoulder. A similar conversion on a minilathe would stop in 1/3 of a revolution. As it is not a screw on chuck the stop speed is only limited by the motor torque of the drive belts slipping.

If you use the slow speed attachment for threading it takes a long time to stop due to the motor inertia.

Ok, I'm already convinced on the 4 pole motor

If I keep just the original 4 speeds (remove and sell the slow speed attachment), with a 4 pole motor the lowest speed (at 50 Hz) becomes 125 rpm. Running at 10 Hz would give 25 rpm, I doubt any slower would be necessary. When screwcutting, it would be convenient to make a cut at a low speed, then increase speed to return to the starting position. I think that being able to adjust the speed quickly (without changing belts) makes a person more likely to set the correct speed for the job at hand.

Limit switch is a good idea, I was thinking about it. The VFD I'm looking at (Yaskawa V1000) has a special "Fast stop" input function which would be a perfect match for this.

Yeah, that reversing switching between VFD and the motor can be quite dangerous. It would be perfectly possible to retain the original switches, they should be connected to appropriate control inputs of the VFD, and the VFD correctly configured for this arrangement (2-wire control). In my case I'd like to have a few extra features, so I'm thinking about either making a new front panel for the switches, or a separate remote control box. I might reuse the original rev/fwd switch, but for the power I want to have separate start/stop push button switches.

I've contacted EMGR (the manufacturer of the motor used in a conversion, a link to which was posted before) regarding availability of a frame size 63 motor with a 14x30 mm shaft that would fit my Hobbymat without modifications, waiting for their reply. Will post here when I get it. Or perhaps I will just get a frame size 71 motor, which has a higher efficiency (70% vs 59%).

If you did suffer from overheating at lower speeds due to inadequate cooling from the original fan, you could always remove it & fit an external powered fan to provide continuous cooling regardless of motor speed.

That is what Denford did with the 3 phase spindle drive motor on my Triac milling machine & is the standard way that industrial spindle motors are cooled.

Nigel B.