Warco WM16 Mill - Conversion to AC motor with VFD

Having rammed a flycutter into a lump of steel on my unmodified mill, those crappy plastic gears are not looking too healthy. I'm now at a stage where I'm 50/50 on whether to simply replace the gears or convert to an AC motor with VFD. I already have the expensive bit - a 1.5kW Altivar inverter with 240V supply and 3-phase out, so conversion is going to require a suitable motor and whatever pulleys and belts are required.

I don't want to strip the machine yet as it is still operational, and I may need it to make some parts for the conversion. Have any of you guys done the conversion already? All pictures, diagrams, advice, etc are welcome. In particular, any information regarding the spindle diameter, length, gear sizes, etc will be useful as I can't measure anything without taking the head off. Bit of a catch 22 really.
:apthy:

Ideally do both - keep the gears (or much better replace them with timing belts) and then add the induction motor. The power output from an induction motor will reduce with speed, when driven from a VFD, so it's best to keep some form of gearing so that you can still have a useful amount of power at low speeds. The other option is just use a huge motor, but that's often not practical/sensible/economical.

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Originally Posted by Jonathan

Ideally do both - keep the gears (or much better replace them with timing belts) and then add the induction motor. The power output from an induction motor will reduce with speed, when driven from a VFD, so it's best to keep some form of gearing so that you can still have a useful amount of power at low speeds. The other option is just use a huge motor, but that's often not practical/sensible/economical.

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If I go to an induction motor, I'll most likely get rid of the main input gears between the motor and gearbox and replace them with a belt drive. Vee belts are not out of the equation as they WILL slip in the event of an accident. But then again, do we need to minimise slip and use timing belts?

The existing 600W DC motor is a gutless thing at low speeds, so I'd be looking at maxing out the VFD with a 1.5kW motor. IIRC, the Altivar drives have a few functions for "torque boost" and "constant torque" which may help.

I would go with a J type belt rather than timing pulley belts. They give a lot of grip but also ca give you a second chance if all goes wrong. Do you have access to lathe to make them? a two or three step pulley will give you all the range you would need.

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Originally Posted by birchy

VFD with a 1.5kW motor. IIRC, the Altivar drives have a few functions for "torque boost" and "constant torque" which may help.

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Help, but not solve. Power is torque multiplied by angular velocity (i.e. speed), so if the VFD manages to keep the torque constant, then the power you get is the speed multiplied by some number. Therefore if you reduce the speed, the power is a smaller number multiplied by the same constant, hence the power is smaller. So if the motor torque output is constant, the power output will be proportional to the speed. For example if you use a 2-pole motor, you'll get the rated power at (just under) rated speed, i.e. 3000rpm, but if you want 600rpm then the power output (again assuming the torque is constant) is simply 1.5*600/3000=0.3kW.

One way to at least partially get round this is to operate the motor above its rated speed. For example if we take a 4-pole motor, so just under 1500rpm @ 50Hz, and run it at 100Hz, you get twice the speed, and get rated power from (roughly) 1500rpm to 3000rpm. Recalling the formula above, it seems reasonable to expect higher than rated power above rated speed, but unfortunately this isn't the case. The reason you (probably) can't get higher than rated power, when you exceed rated speed, is that the drive would need to supply a higher voltage to the motor, however it's limited by the mains input voltage and wont be able to increase that. Having said this is still a good setup - for example if you want 600rpm now, the power output will be 1.5*600/1500=0.6kW, so twice as much. If this still isn't enough then that's when you include pulleys to reduce the speed further - 1:something and 1:"something bigger" would probably be adequate. You need to decide what range of speeds you need, select the motor speed (poles) from that, then work out the required pulleys.

It looks like the maximum speed your machine currently does is 2250rpm, so if we take that as the limit, then a 2-pole (1500rpm) or 4-pole (3000rpm) motor is closest. From the above it's clearly better to use the slower motor. Generally (i.e. might not be a good idea if you use the cheapest motor you can find) it's fine to run a 4-pole motor at twice the rated speed, so in this case 100Hz, accounting for slip it'll be about 2950rpm, but we want 2250rpm, so make the ratio 2950/2250=0.76... pretty close to 3:4 (0.75) so lets use that. Now pick the other ratio to optimize for low speed...

I think you are going to struggle getting a 1.5Kw 3 phase motor to sit on a WM16.
1.1Kw is available in a reduced frame size but 1.5 kw isn't.
Smallest i can source is in a 90 frame which meants it 90mm from base to centreline of the motor so roughly 180mm OD

Thanks for the great information, chaps! I'd like to increase the maximum speed slightly to around 2500RPM - 3000RPM for the occasional times that I'm using smaller cutters in the region of 3mm diameter. I was thinking of using a 2-pole motor and reducing the frequency rather than over speeding a 4-pole. Will overheating be a problem if I run the 4-pole faster?

John, I think you're right regarding the 1.5kW frame size. There's about 60mm clearance to the back of the swivel head, and about 95mm to the Z dovetails. It sounds like you know these machines well - is the internal gearbox all metal gears or are there more crappy plastic things inside there?

Do you guys have any recommendations for motor suppliers?

Jonathan, more poles = slower ;)

2pole @ 50Hz theoretically gives 3000RPM, while 4pole gives 1500rpm.

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Originally Posted by m_c

Jonathan, more poles = slower ;)

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I know. I've corrected the post and highlighted that in bold. I'd have hoped it would have been obvious from the 4 other correct statements in my post giving the synchronous speed for 4 and 2 pole motors.

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Originally Posted by m_c

2pole @ 50Hz theoretically gives 3000RPM, while 4pole gives 1500rpm.

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Only if your theory is no good. Those speeds are the synchronous speeds and if the rotor was spinning at that speed, there would be no current induced upon it and therefore zero torque. For that reason the theory shows the rotor speed will always be below those values, by an amount known as the slip, else the machine is not acting as a motor.

Normal stated speeds are 1425 for 4 pole and 2850 for 2 pole

wobblyjelly

OK, here's my dilemma: absolute maximum distance from motor shaft centre to Z dovetail is 90mm. It looks a bit touch and go whether or not a 90 frame motor will fit. According to THIS thread, which is an almost identical machine, they used a 1.1kW 2-pole motor with an 80 frame size and report that it only *just* fits. As said by Jonathan earlier in this thread, a 4-pole motor will give better torque but will have to be run up to 100hz to get the desired speed of around 2800RPM. But I can't find a 1.1kW 4-pole motor with a frame size less than 90...

So, assuming I use an 80 frame size, would I be better off using a 2-pole 1.1kW OR downsizing to a 0.75kW 4-pole? Or can anyone provide a link to a 1.1kW 4-pole in an 80 frame size?

Just spoke to my motor supplier.
You can get 4 pole in 80 frame at 1.1Kw, if you move to 2 Pole you can go up to 1.5kw
They don't have them but about 3 days lead time.

You need to talk to Martin at M&R Controls 01773 875795

Thank you John. The 2 pole 1.5kW might be a nice solution as it means 2800RPM without running the motor at twice its rated frequency.

As a man with experience of these machines, what would you normally choose?

Hi there birchy, I am planing to do the same thing to my WM16 (fed up with changing brushes) but am thinking about keeping the 2 speed gearbox but replacing them with steel gears to give me a degree of backgear. The primary drive will be by toothed belt I think. Have you come across ant major problems with your conversion?

Regards Mark P.

If you have money to burn you can go the AC servo route and get 1.65Kw in a 85mm square flange motor and you will have more power than you will need on that small machine.

http://www.kollmorgen.com/en-gb/prod...-servo-motors/

Graham

Hi All.
I have a WM16 that was converted some 12 years ago by John Stevenson (did a great job!) - it was originally a Warco CNC, which was crap (it still had Acme screws and not the ball screws as I had been told - so accuracy was about as far out as the moon!). In the upgrade, John also fitted a 1:1 tooth belt drive from the motor to the gear spindle, so increased the main spindle speed to around 4300rpm max - I run mainly at 4000rpm and has run happily at this. I did have error problems with glitches in communication between the computer and the mill from time to time, but this was solved with the addition of SmoothStepper. Excellent.

I only use it for small model engineering hobby projects so is not in constant use, but it has done a few hundred hours of reliable running.

My problem now is that after experiencing a sudden build-up of chatter over a short period, I replaced all the bearings in the head and tightened up all the gib strips. Now I still have a vibration at high speed - removing the belt from the pulley and running the motor by itself, there is a vibration there, so need to replace the motor.

Does anyone know of a higher-speed motor (say 5000rpm) that would be a direct replacement for this one? Or will I have to go back to Warco for the motor?

Regards,
GeoffB