DC vs AC (Variable Speed)

[Don Nguyen]

From what I know:

-DC motors can run from any speed from zero to max, but torque is dependent on RPM?
-AC motors can run from 1/3 is rated RPM to max, but has constant torque throughout the speeds.

What other differences are there? Reliability? Noise? "Smoothness"?

If buying new, I can get a set-up for either around 1.5hp or 2hp for about the same price; I've noticed that 3ph motors can be found cheaper, but the VFD's are generally more expensive that DC speed control counterparts. If I go used an AC set-up will almost probably definitely maybe be less expensive.

From your experiences, which do you like more? What issues have you had with either?

Thanks guys (I've done searching and researching for a while now, but haven't quite found the answers I'm looking for here)

-----


Off-topic, I think this grinder is GOOOOOOD LOOOOKIN.

 

[timgunn1962]

DC motors can run from zero(-ish) to max rated speed and will provide constant torque.

When run from a VFD, AC motors can also run from zero(-ish) to max rated speed and will provide constant torque, but can also be run at somewhat higher-than-rated frequency, whereby they give constant power above the rated speed (torque declines as speed increases).

There is a caveat though. A TEFC motor relies on the fan on the end to provide cooling air. This slows with the motor, so usually sets a lower limit on the usable speed at full torque. A TENV motor is designed to dissipate the heat produced at full power without a fan, so can be slowed right down without problems. Most cheap AC motors are TEFC, at least this side of the pond.

It's usually best to use a 4-pole (1800 RPM, or a little less) motor and run it up to a maximum of 100 Hz (3000 RPM or a little less) when you can, to get the widest speed range you sensibly can, assuming you can design/modify the machinery to suit.

There's a limit to how high you can take the frequency with a "normal" 3-phase motor before the electrical losses really start to mess with the relationship between speed and frequency. 100 Hz is usually pretty safe.

At the bottom end, something similar happens with the simpler "V/Hz" drives and about 10Hz is the "pretty safe" minimum. Using a "Sensorless Vector" drive will let you go slower. The drive electronics measure the power factor and make adjustments automatically. I found my lathe felt like it was "cogging" on a V/Hz drive at 8 Hz, but is still smooth on an SV drive at 3 HZ. SV doesn't cost much more now.

Essentially, the big difference between AC and DC for the last decade or so has been that AC uses cheap motors and expensive electronic drives, whilst DC uses cheap drives and expensive motors. Now the AC drives are so cheap, AC is pretty much a no-brainer, at least industrially.

For the hobbyist, second-hand DC stuff might still be a bargain, of course.

I like AC. The motors are simple and reliable. At work, we tend to find the AC drives last about as long as the motors; 5 years or so of constant industrial use, so they are pretty reliable as well.

The days of low-frequency switching are long gone. Unless you are hyper-sensitive to certain frequencies, the newer drives can run at a switching frequency that takes the whining noise, that used to be associated with AC drives, above the audible range.

Regards

Tim

 

[12345678910]

I have found that 3 phase AC motors are SO much cheaper than DC

I can buy a 1.5 HP grinder suitable motor on Ebay everyday for about $70 plus shipping

DC motors, maybe 10 times that.

I can easily hook up one controller to multiple motors with either setup, so AC was the way I went.

Once you already have a VFD, it's much easier to just keep adding motors - maybe some switches, or plugs to buy,
but once you already have the vfd, it's even cheaper than step pulleys.

 

[synthesist]

That's a fine looking grinder. What brand is it?

Syn

 

[Don Nguyen]

That is a Wilmont. They also made the EERF grinder ('Free' backwards).

Thanks guys. Lots of food for thought for me.