DC Motor for a planer?

[Allan Speers]

I need a motor for my 12" Powermatic planer.

I had decided to look for a used 3-ph motor, as they can be found for a pretty good price, then add a VFD.

The VFD also saves me the cost of a mag starter, and good mag starters are fairly expensive.

A minor upside of this plan is that I'd also have vari-speed. I figure that if I do my pulleys so I'm getting the maximum recommended feed & cutterhead speed, I then have the option of slowing it down a little, say 25%, for really tough, figured woods.

But I just thought maybe a DC motor & controller is also worth considering. So a few questions:
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1: If a DC motor is rated for 3 HP, is it really as powerful (in a machine like a planer) as a 3 HP Leeson induction motor?

2: If the above answer is yes, do both types of motor maintain the same power when slowed by 25%? I'm guessing that the DC motor is superior here, since it has brushes. Yes?

3: When using a VFD, the VFD protects the motor against thermal overload. It also protects against accidental "power on" starts, like a mag starter does. Does the controller for a DC motor do these two things as well?


4: Any other considerations, such as noise, or life expectancy, or .... ?

[Andy Booth]

I do not recommend a DC motor for this for a couple reasons.
- Brushes are a source of ignition and wood chips/dust are fuel. All you need is oxygen.
- DC motor brushes need replacement and motors need to be cleaned.
- I suspect a 3HP induction motor and VFD is less expensive than a 3HP DC motor w controller.
- DC motors excel at high torques when starting and at low speeds. AC motors are more economical at constant speeds

A motor will develop as much torque as required. HP = Torque x speed
3HP AC @1750 RPM is the same for AC or DC.

VFD's have replaced DC motors in most applications due to the the simplicity and low cost of an inductions motors
You will need to buy an induction motor rated for invertor duty. Wire insulation voltage rating is higher for invertor rated motors. Be wary of used motors as they may not have the right insulation.

At that power you will have to use 230VAC
3hp, 230V single phase is ~18A. A 25A manual motor starter is ~$50.
Grizzly sells a 220V 3HP single phase magnetic starter for less than $100.
A 3Hp VFD is ~$300.

Pulleys are a lot cheaper.

[Allan Speers]

Thanks, Andy, that's helpful. A couple of follow-ups:



- DC motors excel at high torques when starting and at low speeds. AC motors are more economical at constant speeds


But my primary question was about torque at a 25% speed reduction. You've pretty much talked me out of a DC motor, but I'd still like to know how they compare. I know that 3-ph induction motors DO lose some power when you slow them down, I just don't know how much. I don't know if I'm asking about torque or HP, but you know what I mean. My planer needs 3 HP to hog off 1/4" at 18 FPM. Granted, I'll never push it that hard, so this question is academic, but if I slow the motor 25%, giving me around 14 FPM (but also a slower cutterhead) will it have the same apparent power as at full speed?

- Or to put it another (more appropriate) way: Would it have the same power as if it were at full speed but I changed the motor pully to something smaller?

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You will need to buy an induction motor rated for invertor duty. Wire insulation voltage rating is higher for invertor rated motors. Be wary of used motors as they may not have the right insulation.

Good thinking ! I assume then that "class B" should be avoided, but the common "class F" will be OK? -here's a difference between them af maybe 50 degrees C, IIRC.

[Rick Lizek]

Slowing down for tough figured wood doesn't work as your machine has one motor to run the cutterhead and feed rollers. Just thinner passes is a better option. You don't want to slow the cutterhead speed. On my 18" Rockwell wedge bed it has a seperate feed motor and the 3600 direct drive handles figured wood very nicely at the lower feed speed.

[Allan Speers]

Slowing down for tough figured wood doesn't work as your machine has one motor to run the cutterhead and feed rollers. Just thinner passes is a better option.

I don't disagree about the thinner passes, but slowing it down DOES help, a little, because with the PM 100's particular drive ratios, at 18 FPM it gives about 65 CPI, while at 15 FPM (using a smaller motor pulley, or a slowed-down motor) it gives about 72 CPI. Cutterhead speed and feed rate do NOT change in a 1-to-1 ratio.

So, while slowing down isn't a free lunch, it DOES make a difference.

[Mike Henderson]

But my primary question was about torque at a 25% speed reduction. You've pretty much talked me out of a DC motor, but I'd still like to know how they compare. I know that 3-ph induction motors DO lose some power when you slow them down, I just don't know how much. I don't know if I'm asking about torque or HP, but you know what I mean. My planer needs 3 HP to hog off 1/4" at 18 FPM. Granted, I'll never push it that hard, so this question is academic, but if I slow the motor 25%, giving me around 14 FPM (but also a slower cutterhead) will it have the same apparent power as at full speed?

- Or to put it another (more appropriate) way: Would it have the same power as if it were at full speed but I changed the motor pully to something smaller?

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Good thinking ! I assume then that "class B" should be avoided, but the common "class F" will be OK? -here's a difference between them af maybe 50 degrees C, IIRC.
I don't remember much of DC motor theory so I'd have to look up your question about DC motors.

But for induction motors, as you slow them down, the torque remains constant. Horse power, however, is torque times RPM times a constant, so as the RPMs decrease, the HP decreases but the torque stays the same.

As someone else commented, at one time almost all variable speed motor applications used DC motors because it was fairly easy to vary the voltage on the motor. But DC motors have brushes which require maintenance. As electronics have improved, almost all new variable speed motor applications have moved to induction motors with variable frequency control because an induction motor is a lot more reliable (very simple design).

If you really want to go variable speed, I'd choose an induction motor.

Mike

[Art Mann]

Slowing down for tough figured wood doesn't work as your machine has one motor to run the cutterhead and feed rollers. Just thinner passes is a better option. You don't want to slow the cutterhead speed. On my 18" Rockwell wedge bed it has a seperate feed motor and the 3600 direct drive handles figured wood very nicely at the lower feed speed.

I agree 100% with this comment. It seems counter intuitive but if you slow the motor down, it will increase the amount of chip out, not decrease it. First, Rick is absolutely right that the number of cuts per inch will not change but each individual cut will be slower. If you cut a profile with a router, you will get much better results at 18,000 than at 10,000 RPM if the router travel speed is the same in both cases (assuming no burning). Same principle with a planer. You want to slow the material feed while maintaining knife speed in order to improve cut quality. That might be accomplished with a simple pulley change or a more difficult sprocket change, depending on the planer design. That is also simpler than fooling with DC motors and controllers.

If you observe a Dewalt 735 two speed thickness planer, the super fine finish setting slows the conveyor speed but doesn't slow the cutter head.