Thread: VFD + motor
Please, can somebody help me. For a motor to have variable speed range from 1000- 4000 rpm is it better to couple VFD with standard 2.2kW 3-phase motor in a way:
-1400 rpm motor: run @140Hz which gives max about 4000 rpm (36Hz gives 1000rpm)? or
-2800 rpm motor: runa @ 70 Hz to have 4000 rpm, and 18Hz to have around 1000rpm
I heard that standard 3phase motor does not like too much Hz from VFD(no more than 60Hz???). I will need quite a lot of torque at low rpm
Which motor to go for?? Did somebody join Chinese VFD with a standard motor??
I would expect the 2-pole motor to be your best bet - synchronous speed is 3000rpm @ 50Hz, but clearly less in reality due to slip. I think running a 4-pole motor, which is only designed for about 1500rpm, at 4000rpm is asking for trouble. At the very least the bearings are going to want replacing, and I wouldn't be surprised if the rotor is not strong enough (centripetal force and all that). With a 2-pole you're not exceeding it much, no doubt a good motor will be safe at about 30% more as they are tested at a lot higher.
When the motor is operated above the rated speed the voltage applied to the motor should be reduced proportionately so that the power rating is not exceeded. This means that at higher speed the torque will be reduced.
The problem with a 2-pole motor at only 1000rpm is it might have lower torque (I'm not sure)... if it needs to be constant (or high) then ideally you would need vector current control, which the Chinese VFD does not do. If the VFD is set for constant V/f ratio the motor should output the same torque from 1000-3000rpm.
A lot depends on what you're using the motor for? If it's not going to be operated anywhere near it's rated power then I should think you'll be fine to just use the Chinese VFD.
I have used Chinese VFD with my Colchester lathe, which is a standard army issue 4 pole induction motor. It works perfectly well - so far I've only dared run it up to 60Hz.
Hope that helps ... I'm not supposed to know about this untill 3rd year
Bearings will not be an issue, they fit the same bearing to a two pole as a 4 pole.
Rotors on modern metric framed motors are OK as well.
I have run two pole motors at 12,000 rpm, no typo, but they have been modified.John S -
Jonathan: I will need this motor to run a mill so low rpm @ big dia cutter or fly cutter.
I understand motors coils must be in delta connection if run at 240V.
John: What sort of mods ? rewind? do you recommend to go for 4 pole motor??
I had idea for a while to stick to a mill chinese spindle via 1:5 or 1:6 reduction. Wonder how it would work. What do you think guys?
btw Jonathan, it is because you're genius :)
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.John S -
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.
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.
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.
Last edited by Jonathan; 30-07-2011 at 04:28 PM.
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