The Shop > Electronics & IC Programing

PWM vs VFD

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Chazz:
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! :(

dickda1:
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

John Rudd:

--- Quote from: dickda1 on November 01, 2011, 12:26:37 PM ---I would probably go with DC and pwm if I was thinking of cnc because of ease of computer interface.
Dick

--- End quote ---

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:

DMIOM:

--- Quote from: John Rudd on November 01, 2011, 01:19:04 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...
--- End quote ---

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

dickda1:
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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