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DC vs VFD/3 phase motor
- Thread starter HarryJM
- Start date
- Joined
- Jun 22, 2012
- Messages
- 1,552
Either one would work. I still use the original motor on my number 4. I also use the original 3 phase 3/4hp motor on my southbend lathe with a vfd, it works great. It converts 110v single phase to 220v 3 phase, it gives me programable soft start and braking as well as forward and reverse. The little burk no. 4 really doesn't need much power, a 1hp DC treadmill motor would be more than adequate, but you will need a controller to operate it. I have salvaged 2 DC treadmill motors for free. One is 1 1/2hp and one is 2 hp. Not sure what I'm going to put them on yet. Maybe a shop made belt grinder! DC motors are very durable and last forever.
- Joined
- Feb 13, 2017
- Messages
- 2,138
As an "outsider" to both approaches, there's not much I can say or add beyond comments from an (retired) electrical engineer. With a VFD, there are a number of "side" benefits, such as a soft start. I'm old school and not too much up to date on technology of the last 15 years or so. But I have worked with VFDs in the past. Big ones, high horsepower ones, 50 HP ones.
VFDs work as desired, as well as designed, for frequency shifts of plus or minus 30 CPS.(+/- 50%) They cannot maintain good regulation as you approach zero speed. Speed is controlled with frequency. Higher, over "base" speed, could have deliterious effects, I don't know. A lot depends on the actual characteristics of the motor. And that can vary from one to another.
A DC motor with a "permanant magnet field" has better speed regulation at lower speeds. But also has a severe loss of torque as speed approaches zero. Speed is regulated with voltage, the most common method being "Pulse Width Modulation". More modern DC motors have little capacity for running above "base" speed, beyond what can be accomplished with increased voltage. I guess "field weakening" on a "compound" motor is a lost art. . .
Each method has pluses and minuses. Much depends on how much time and effort the end user will invest in setting up and maintaining the machine.
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VFDs work as desired, as well as designed, for frequency shifts of plus or minus 30 CPS.(+/- 50%) They cannot maintain good regulation as you approach zero speed. Speed is controlled with frequency. Higher, over "base" speed, could have deliterious effects, I don't know. A lot depends on the actual characteristics of the motor. And that can vary from one to another.
A DC motor with a "permanant magnet field" has better speed regulation at lower speeds. But also has a severe loss of torque as speed approaches zero. Speed is regulated with voltage, the most common method being "Pulse Width Modulation". More modern DC motors have little capacity for running above "base" speed, beyond what can be accomplished with increased voltage. I guess "field weakening" on a "compound" motor is a lost art. . .
Each method has pluses and minuses. Much depends on how much time and effort the end user will invest in setting up and maintaining the machine.
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- Joined
- Feb 8, 2014
- Messages
- 11,176
Ya gotta keep up Bill
Modern sensorless vector VFDs have great speed control down to about 1 RPM and will produce 100% (or better) torque at that speed. Normally constant torque from 0 up to the base speed and constant HP above the base speed. You are correct, the motor will affect the overall performance. But I have had very good luck using name brand inverter rated motors.
- Joined
- Feb 13, 2017
- Messages
- 2,138
The motor's the key. Older/used/inexpensive machines probably don't have those motors. As far as decent torque below 20% base speed, I'll believe it when I see it. The relationship between a sine wave and the windings of a motor tells me it don't work that well. Railroad locomotive traction not withstanding. . .
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- Joined
- Feb 8, 2014
- Messages
- 11,176
I wish there were a way I could show the performance of a modern sensorless vector VFD. A lot different than the old V/F drives. I don't understand how they work, but they do. The problem with the older motors is the insulation class, modern motors have class H insulation, and there may be some other differences also. The biggest problem is cooling when operating at low speeds for extended periods. An external fan will mitigate the heating problem.
- Joined
- Jun 26, 2018
- Messages
- 1,733
On my VFD/motor, I have a large computer fan that kicks on at whatever slower speed I want, I have it at 35hz as the motors fan will do its job at higher speeds. If I had to do it again, I would go with a setup that could take a larger resistor/brake, something I'm sure o.p. already has.