Up With the Slugs and Banana Spiders
January 12th, 2009The Return of 5:30
What a wonderful morning this has been so far. The holidays derailed my schedule; I found myself getting to bed and getting up late. But as of today I am back to my regular routine. I got up at 5:30 and had the world to myself.
I think things reached a crescendo on Saturday. Sunday was coming up, and I was not going to let myself spend that day in the garage, working on my own projects. And I was trying to get the extension for my woodworking megaplex put together. So I kept working, and I got to bed at something like 1 a.m., which is four and a half hours before my preferred rising time. Unacceptable. But that’s all behind me now.
Help me out with VFD questions.
A VFD is a doodad that allows you to run an electrical motor at a speed you like. Readers have been mentioning them. One of the fun applications is running a big, cheap 3-phase motor on single-phase current. You can buy a big ol’ industrial tool cheap and then use it in your garage with single-phase power. Some of these things accept 120 in. You can also get them in 220.
If you stick one of these on a drill press, it can make your life easier. I think. Drill presses with 3-phase motors are not too hard to find. I have read about people using VFDs on their home drill presses.
My questions:
1. Do you lose a lot of power if you slow a 1750-RPM motor down by half, to get a bit speed of around 250 RPM? I don’t know a whole lot about electric motors. I assume there must be some sort of torque band, even if it’s not as peaky as a gasoline engine’s.
2. Do you lose any power running at normal speed? I don’t see why you would. Current is current.
3. Is there something out there that will convert 1-phase 220 to 3-phase 220 without affecting the speed? I might land a nice variable-speed drill press that needs 3-phase but no speed control.
I checked into the place that has the used Powermatics for sale, and their prices are out of control. They want $850 for fairly beat-up machines. I think that’s about twice what a used Powermatic 1150 should cost.
I found a 14″ Walker Turner band saw for sale right here in Miami. Dang it. Too small.
January 12th, 2009 at 4:00 PM
What you want is called a phase converter.
14″ is too small? Are you planning on building the U.S.S. Constitution?
January 12th, 2009 at 4:36 PM
Physics, Steve, physics. VFD drives serve the same voltage to the motor that it expected in a “normal” (direct to the mains) connection. Electric motors don’t have a “power band” like a gas or Diesel reciprocating engine; they have a power curve based on the fact that every electric motor is also a generator. In an induction motor, the speed is set by the input frequency; the self-generated voltage or “back EMF” opposes the input voltage, and therefore restricts the current.
Back EMF increases with increased speed. When you’re running the motor slowly, the VFD restricts the current (not always down to “normal”, but it does restrict it) because the back EMF generated by the motor is less, which would allow excessive current and burn the motor out. That means the power output is less, because the torque is the same or a little greater but the speed is lower. (If you use a VFD with a higher rating than the motor, you should be careful at low speed and perhaps provide additional cooling. The difference between 3/4 HP and 1 HP is too little to worry about, unless you’re running continuously for a long time.)
At normal (=rated) speed the power output is the same as it would be for connection to the mains because voltage and frequency are just what the motor expected to see. The system as a whole uses a bit more power than that because of losses in the VFD.
When you pass through the rated speed and get up to faster than that, the power hits a plateau the same as or a little more than the rated value, but the torque is less because the back EMF of the motor restricts the current.
And yes, you can get single-to-three-phase inverters. The difference in price between them and VFDs is nearly nothing, because the difference in what’s inside is a few simple, cheap control components. The power-handling (=expensive) stuff is the same in both cases. There’s no law of God or Man preventing you from setting a VFD to 60-Hz output, hooking up the START and STOP buttons, and tucking the box out of sight.
Regards,
Ric
January 12th, 2009 at 6:24 PM
Steve,
A VFD is next thing to magic, I highly recommend them. Here’s the answers to your questions:
1. An electric motor is constant torque. That means that if you run the motor at half speed, it’s half the horsepower of nameplate. This is not normally a big deal, as you can ease up on the feed a bit. The variable speed feature more than makes up for it, as it’s SOOOO nice to be able to crank the speed up and down with the twist of a knob.
2. You do not lose any power when the motor is running at normal speed. Be aware that you can crank the motor up to higher-than-rated speeds, half-again is fine.
3. Yes, you can get what they call a Phase Converter, there’s “static” and “rotary” types. The latter is considered to be better, and you can build a half-assed one for next to nothing if you’re a scrounger.
Most VFDs are 240 volt input, 120 volt is not as common. A drill press would typically have a 1Hp motor, and such VFDs are quite inexpensive. New, maybe a couple hundred bucks, half that in the used (read: eBay) market.
If you go here: http://www.practicalmachinist.com/vb/forumdisplay.php?f=11 and read some threads, you’ll get up to speed on VFDs quickly.
I have one on my BridgePort, running from 240 volt single-phase, even though I have 3-phase factory power in my shop. My reason is for the variable speed function, not the phase conversion.
Wiring is pretty simple, programming them looks very intimidating, but isn’t all that bad.
January 12th, 2009 at 6:52 PM
A VFD also derates. You don’t get the horsepower on the nameplate, as a general rule.
Low speeds.. well, if you feel you must, but unless you intend to do milling you’re most likely wasting money. Her’es a site that shows some of the basic calculations for drilling:
http://its.fvtc.edu/MachShop1/drillpress/cutspeeds.htm
As you can see, there, you’d have to be drilling some big holes in some hard material to need speeds below 500 rpm. But a VFR is a good way to do it.
January 12th, 2009 at 7:01 PM
The 14″ band saw is not good for resawing. The Walker Turner 16″ saw, on the other hand, will take boards 12″ wide.