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  1. #1
    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.

  2. #2
    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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  3. #3
    Quote Originally Posted by Jonathan View Post
    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.
    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.

  4. #4
    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.
    If the nagging gets really bad......Get a bigger shed:naughty:

  5. #5
    Quote Originally Posted by birchy View Post
    VFD with a 1.5kW motor. IIRC, the Altivar drives have a few functions for "torque boost" and "constant torque" which may help.
    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...
    Last edited by Jonathan; 20-05-2013 at 01:23 AM.
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  6. #6
    John S's Avatar
    Lives in Nottingham, England, United Kingdom. Last Activity: 16 Hours Ago Has been a member for 9-10 years. Has a total post count of 2,072. Received thanks 140 times, giving thanks to others 45 times. Referred 1 members to the community.
    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
    John S -

  7. #7
    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?
    Last edited by birchy; 20-05-2013 at 05:07 PM. Reason: bad pole count!

  8. #8
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Last Activity: 3 Hours Ago Has been a member for 8-9 years. Has a total post count of 1,622. Received thanks 168 times, giving thanks to others 5 times.
    Jonathan, more poles = slower ;)

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

  9. #9
    Quote Originally Posted by m_c View Post
    Jonathan, more poles = slower ;)
    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.

    Quote Originally Posted by m_c View Post
    2pole @ 50Hz theoretically gives 3000RPM, while 4pole gives 1500rpm.
    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.
    Last edited by Jonathan; 20-05-2013 at 01:33 AM.
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  10. #10
    John S's Avatar
    Lives in Nottingham, England, United Kingdom. Last Activity: 16 Hours Ago Has been a member for 9-10 years. Has a total post count of 2,072. Received thanks 140 times, giving thanks to others 45 times. Referred 1 members to the community.
    Normal stated speeds are 1425 for 4 pole and 2850 for 2 pole
    John S -

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