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PWM vs VFD

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Chazz:
Forgive my search skills, I'm sure there must be Ad Nausium posts, but I can't seem to find the difference, (besides cost), an AC motor wih VFD vs a DC motor with PWM.

Long story short, I'm replacing my .75HP lathe motor (that just died), with a 2HP motor, should I go AC or DC?

I have 220V single phase available, and my research shows that an AC variable speed set-up woud be around $700~1,000 drive and motor vs $1,200~1,600 for a DC setup?  Is a DC drive worth the 25~40% investment over AC?

If I understand correctly, both AC and DC motors can be controlled by their respective drives and maintain a constant or rather, a higher tourqe curve? at lower RPMs?

Thank's for links, insites.
Chazz
 

sparky961:
I wish I could offer something a bit more definitive, but I've considered doing something similar with my own machine at some point or another in the past.  For starters, search "VFD" on ebay and you'll find a vast number of fairly reasonably priced VFD's with various specs for sale.

If my memory serves, PWM offers very low torque at low speeds while VFD's are better in this respect - though I'm not sure by how much.  Someone please do correct me if I'm mistaken here.

It also seems to me that a VFD would enable more choices of motors, even 3-phase driven off single phase.  I know when driven off standard 3-phase, a benefit is that you can instantly reverse a motor (great for tapping and single point threading!).  This doesn't work with single phase.  I'm not sure if this works the same way with 3-phase from a VFD.

I'm interested in hearing what others put forward as well, perhaps from someone with a bit more experience instead of my own foggy recollections.

-Sparky

picclock:
AC motors powered by VFD's are brushless by nature, with only the bearings and wiring insulation to contribute to failure. DC motors use carbon brushes which wear out, and also incorporate magnets which age and reduce in field strength as they get hot. The best solution IMHO is the brushless DC motor, which in working is more like a stepper motor, As this produces the best torque characteristics.

Of the two you mention the AC motor VFD combination is the best, IMHO. As a bonus it should also work out the cheapest as there are a lot of three phase second hand motors around.

Best Regards

picclock

andyf:
I don't know enough to comment on the VFD vs PWM question, but from what everyone says, VFD must be better than PWM.

But if cost is an issue, consider using the DC motor from a discarded treadmill and a PWM controller from somewhere like KB Electronics. Here at Poverty Acres, that's what I did with my old milling machine. A non-working treadmill cost £15 and furnished a 2HP (actually, I think it's more like 1.5 HP) motor, and a new KB controller was around £70. With bits and bobs, the whole sustem cost around £120 (US$200). I didn't need a reversing switch on my mill, but on a lathe this would require some design work to make sure AC power to the control board is turned off before the reversing switch in the DC supply from the board to the motor is operated.

Andy

kvom:
My mill's 3PH 1HP AC motor has decent torque down to about 20% of the rated speed; below that I need a different belt or back gear.  OTOH, I can get higher speeds than rated by ramping up the frequency to a max. of 150%.

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