Author Topic: PWM vs VFD  (Read 14769 times)

Offline Chazz

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PWM vs VFD
« on: October 30, 2011, 06:49:13 PM »
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
 
Craftex CT129N Mill & Craftex 9 x 20 CT039 Lathe

Offline sparky961

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Re: PWM vs VFD
« Reply #1 on: October 30, 2011, 10:53:01 PM »
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

Offline picclock

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Re: PWM vs VFD
« Reply #2 on: November 01, 2011, 04:41:51 AM »
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
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Offline andyf

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Re: PWM vs VFD
« Reply #3 on: November 01, 2011, 06:28:21 AM »
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
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Offline kvom

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Re: PWM vs VFD
« Reply #4 on: November 01, 2011, 09:07:16 AM »
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%.

Offline Chazz

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Re: PWM vs VFD
« Reply #5 on: November 01, 2011, 10:15:09 AM »
Thanks folks, this just keeps getting more and more confusing  :bang:, doing further research, yes one can 'VFD' AC or DC motors, however now I've discovered not only are there VFD and PWM drives, there are also SCR drives?  :bang:

Anyway, back to Google and Wikapedia :scratch:

Cheers,
Chazz

P.S. Ultimately, in replacing my lathe motor, I''m shooting for 2~3HP and be CNC ready, i.e. manual speed control on the driver for now and the ability for a CNC system to 'take over' at such point in time I can afford to complete the CNC retrofit.  For now, I just want my lathe back! :(
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Offline dickda1

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Re: PWM vs VFD
« Reply #6 on: November 01, 2011, 12:26:37 PM »
Could I ask, what size lathe are we talking about?  I have treadmill motor and DC controller on a mini mill - compact, cheap and works great.  Although treadmill motors have become expensive lately.  I have a couple of 2 hp 3 phase motors that run off a Hitachi vfd.  I will probably put one of these on my big lathe.
Just me, but I would probably go with DC and pwm if I was thinking of cnc because of ease of computer interface.
Dick
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Offline John Rudd

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Re: PWM vs VFD
« Reply #7 on: November 01, 2011, 01:19:04 PM »
I would probably go with DC and pwm if I was thinking of cnc because of ease of computer interface.
Dick

Dick,

There's no reason not to interface a VFD to a pc if it has the connectivity..The vfd that I have on my lathe has a 0-10v dc control input to vary the frequency...If I had to use a pc to drive it, all it would need is the relevant control output on the parallel port ( or whatever) and maybe a level shifter to raise it......

Just my 2 cents worth... :coffee:
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Offline DMIOM

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Re: PWM vs VFD
« Reply #8 on: November 01, 2011, 02:42:10 PM »
....There's no reason not to interface a VFD to a pc if it has the connectivity..The vfd that I have on my lathe has a 0-10v dc control input to vary the frequency...

Absolutely agree John - PWM is fine if you're writing something on a PIC or similar, but many VFD have a standard +/- 10v (or 0-10v + direction) input - my main CNC mill has just such an inverter, configured for +/- 10v) which runs from the Heidenhain controller, but which I've also run from a PC.

A couple of other points (whether its CNC-operated or with a manual pot):
1/ Setting the speed to 0 doesn't guarantee the spindle won't turn - get some noise on the input, or a slightly imperfect ground reference, or some drift, and it may (slowly) turn the spindle. Good inverters or drives have an enable input - that's the only way to guarantee the motor won't start or creep when you're in the middle of a tool change.
2/ Whichever way you decide to control your motor, just keep an eye on the motor's temperature if it has a shaft-mounted fan for cooling - the fan is sized to give adequate cooling when the motor is running at its nominal speed - if you run it significantly slower for a period, you may not get sufficient air flow throughout the motor.

Dave

Offline dickda1

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Re: PWM vs VFD
« Reply #9 on: November 01, 2011, 09:58:29 PM »
Good points well taken.

If you should decide to use a VFD, they usually have a programmable startup and shutdown profile.  Sometimes, as a machine comes up to speed, you hit a resonant point (a washing machine for example).  You can choose to speed through this point.  Rapid slow to a stop (not always a good idea with a geared lathe) can also be accomplished.
-Dick
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Offline kvom

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Re: PWM vs VFD
« Reply #10 on: November 02, 2011, 09:26:42 AM »
A lathe chuck and work will have a lot more angular momentum than a mill.  So a VFD may well need braking resistors to get it to stop in a reasonable time.  That's because the energy gets retransmitted back to the VFD and must be absorbed.

If you program the ramp-down time shorter than what the VFD can absorb, the VFD will get an overcurrent fault and the motor will freewheel until it stops.  On my mill, I don't have a resistor and the shutdown time is about 5 seconds from max speed.  On a lathe with a chuck, it would probably be twice that.

Offline BillTodd

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Re: PWM vs VFD
« Reply #11 on: November 02, 2011, 10:13:24 AM »
Just to second what KVOM has said. Get a VFD with braking resister option if you can, it'll give you greater control over deceleration times.

My HLV-H uses a Siemens VFD without an external braking resister option,  I have to use the machine's own (automatic) mechanical brake to get a reasonable stop time from full speed (i.e. 3000rpm to stop with a 5" chuck takes about 5 seconds) . Without the mechanical brake it would take an uncomfortable amount of time to stop.

Luckily, the VFD has an option to use the separate JOG acc/dec times instead of the normal ones so  I have arranged .the control system to select these quicker stop/start times in the lower speed (i.e. 1000rpm to stop in <2 seconds).

KVOM,

Have you seen any designs for an external voltage limit/brake unit ? I have a couple of 600v IGBTs from a dud VFD and fancy trying to make a add on over-voltage limiter

Bill
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