VFD + motor

[John S]

If you need 4000 you want the two pole motor but you will suffer at lower speeds.
VFD's are brilliant pieces of kit but unlike gearing they can't give more torque in fact ignoring all the bullshit they don't actually give the torque they say as they fudge the figures.

Can you get a two speed reduction in the design some way ?

Yes the motors were rewound for 200 hz operation, done 8 so far and all but one have had no problems running 8 hours per day on 2 and 3 head large routers. One had a problem after they bent the spindle because of a crash.

[russell]

Even if your 2,800 rpm motor gives full torque (which it won't) at 1000 rpm, the power will be reduced from 2.2 kW to 0.78 kW, in practice probably nearer to 0.5 kW. So, if you need the power at low revs you need either a much bigger motor or some form of gearing.

Russell

[wiatroda]

Can you get a two speed reduction in the design some way ?.

Looks like I tried to avoid unavoidable. I must rethink the whole idea.

[Jonathan]

It might be easier (than making pulleys etc) to just get a bigger motor and VFD than you were planning to compensate for the reduced power at 1000rpm.

A reduction is clearly the best solution though.

[wiatroda]

Yes the motors were rewound for 200 hz operation, done 8 so far and all but one have had no problems running 8 hours per day on 2 and 3 head large routers. One had a problem after they bent the spindle because of a crash.

John, how much do you charge to rewind the motor??
I have one old english made 3 ph 1450 rpm 2hp from very old power hacksaw, but it's damn heavy. Maybe it's better to buy some modern 2nd hand to rewind? Can any rewinding service do it for 200Hz operation??

[Jonathan]

John, how much do you charge to rewind the motor??
I have one old english made 3 ph 1450 rpm 2hp from very old power hacksaw, but it's damn heavy. Maybe it's better to buy some modern 2nd hand to rewind? Can any rewinding service do it for 200Hz operation??

I think given what John has said ('Rotors on modern metric framed motors are OK'), and what I've read it would be safer to get a new motor.

If you're rewinding it to work for 200Hz then you'll have even less torque at 18Hz.

You've said the reason for wanting 1000rpm is for a fly cutter ...
For mild steel, 1000rpm is 32mm cutter diameter... 1mm depth of cut, 155mm/min, power is 63W (Wrong! See post #12)
For aluminium, 1000rpm is 140mm cutter diameter...1mm depth of cut, 310mm/min, power is 38W

If the VFD does manage to keep the torque constant at lower rpm (which it wont quite) then the power will be roughly, with 2.2kW motor, 1000/2800*2.2=0.79kW. ... so many times more than a fly cutter will use. A face mill will be the power above multiplied by the number of inserts - still probably ok (63*4<<0.79kW).

If you do the same power calculations for drilling (with above about 10mm drill) you'll find that it's more of a problem...however if there's a pilot hole that clearly reduces the required power.

[russell]

You've said the reason for wanting 1000rpm is for a fly cutter ...
For mild steel, 1000rpm is 32mm cutter diameter... 1mm depth of cut, 155mm/min, power is 63W
For aluminium, 1000rpm is 140mm cutter diameter...1mm depth of cut, 310mm/min, power is 38W

Doesn't it also depend on the width of the cut or am I missing something?

Russell.

[Jonathan]

Doesn't it also depend on the width of the cut or am I missing something?

Russell.

It does... I made a mistake and forgot to change the width of the cut when I calculated those. If it is full width of the cutter, then for steel it is about 105W and aluminium 160W assuming cutting full width of cutter.

The conclusion for a fly cutter is still the same, however a face mill will require the power above multiplied by the number of inserts... so too much. The thing is does it really matter ... with the machine being CNC you can generally just increase the depth of cut untill you hit the power limit and leave it to take as many cuts as required.