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VFD on a Boxford lathe

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Joe McKean24/05/2019 20:02:26
19 forum posts

My AUD is fitted with an inverter and VFD and up until now I have only used the tallest/fastest pulleys and altered the speed with the pot.

I am now curious if there is any benefit to the motor etc in using different pulleys and running the motor at say 50 hertz in a lower pulley configuration compared to say 25 hertz in the taller configuration.

I hope this makes some sort of sense, as I said just idle curiosity.

Andrew Johnston24/05/2019 22:11:38
4786 forum posts
538 photos

Yes there is, but whether it's useful depends upon how hard you run the lathe.

In simple terms the torque of an induction motor is proportional to the current. So running at 50Hz with rated current the motor will produce a certain amount of power. Now if we run at 25Hz, without changing the belts, the VFD will ensure that the rated current stays the same. But since the frequency has halved the motor speed will be halved and hence the power at the spindle will also be halved.

Now assume we change the belt position (for convenience assume a 2:1 ratio) and run the motor at 50Hz again. Since the motor is running at 50Hz and rated current it will produce the original amount of power. But since we have a 2:1 reduction in speed via the new belt position the spindle will run at a speed as if driven directly by the motor at 25Hz. But the motor is generating the original amount of power, so if the spindle speed has halved the torque at the spindle has doubled. So belt drives via different pulleys allow speed changes while keeping the power transmitted a constant (minus the small losses in the belt).

So varying the motor speed with the VFD and not changing belts results in varying power at the spindle. Changing belts and keeping the VFD at a single frequency results in constant power at the spindle. That may matter if you want to turn large parts at slow spindle speeds, like a flywheel.

In practice one probably would do both, for smallish parts one would just change the frequency on the VFD. But for larger parts it would be advantageous to change the belts.

I'm well aware that VFDs can allow over-current operation, and hence more torque, but that's not a good idea in the long term due to heating issues in the motor windings.


Robert Atkinson 225/05/2019 05:05:18
344 forum posts
21 photos

To add to Andrews excellent response, this means that best use of a VFD is to run the motor faster than the "50Hz" spee but at reduced power and current for small parts and keep the belt ratio high so you maintain power at low speed for larger workpieces. Obviously you need to make sure you do not overspeed the bearings or rotor particuarly if its an older motor.

Robert G8RPI.

Boiler Bri25/05/2019 09:14:24
802 forum posts
285 photos

My AUD is fitted with an inverter and i always leave it set at 50hz. The machine has enough speed selection to run at the recommended settings and defiantly enough torque.

Have a look at where a full description on the machines is placed.


Pete Rimmer26/05/2019 10:36:45
401 forum posts
18 photos

How a person uses a VFD is a matter of preference of course but I find that one of the real benefits of being able to adjust the frequency is chip control. A small adjustment can mean the difference between stringy swarf birds-nesting around the chuck and little tiny curly chips making an easy to clean up pile in the chip tray.

I usually run them up to 100hz. As I see it, if you're running a 4-pole motor at double speed that's no different to running a 2-pole motor at normal speed, as far as the mechanicals are concerned.

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