DC Motor

genejohnson · Post by **genejohnson** » Sat Jul 26, 2008 11:33 am

I have read where 3 phase motors are used sometimes on lathes to make speed changing easy. Has anyone tried a DC motor?
I have one out of a 3 wheel handicap vechicle that has a controller and is direction and speed controllable. I don't know what the speed of the motor is as of now. It is a 24 volt motor. If it could power a vechicle that carried a lady that was over 300 Lbs, It surely could turn a small benchtop like mine. Comments anyone? Gene

Frank Ford · Post by **Frank Ford** » Sat Jul 26, 2008 1:25 pm

My pal, Steve, has a very fine little old 7" Hardinge Cataract lathe, and he rigged it up with a 1-HP DC motor and speed controller. He used it that way for turning very small items (half inch or less) until he couldn't stand it any longer. He said the low speed torque just wouldn't hack it, so he switched to a three phase and VFD.

I use a similar 1-HP DC motor on my little 1x42 belt sander and it works just fine, although the torque in the low speed range is just enough to work if I don't use too much pressure on the belt.

mechanicalmagic · Post by **mechanicalmagic** » Sat Jul 26, 2008 3:17 pm

The proper speed controller will provide very high torque at low speeds. But, they are more expensive to design and build.

Treadmills are a good source of motors with controllers. If you keep the display, they have feedback, and will provide high torque and high speed. Without the display, a simple pot can be substituted.

I have two, waiting to come up on the project priority list. both over 1 3/4 hp, 6000+ rpm. Only problem, they are not reversible. (Well, actually they are, but since the pulley is threaded to the motor, it's not a good idea.)

Free treadmills pop up all the time, often with dead displays.

Dave J

kenh · Post by **kenh** » Sat Jul 26, 2008 7:50 pm

A PWM controller comes to mind.

J Tiers · Post by **J Tiers** » Sat Jul 26, 2008 8:27 pm

The whole low speed torque thing comes up with ANY variable speed motor.

The VS is NOT EVER a substitute for a gearbox, even if you can get 100% torque at low speed. Even then, you would get power strictly proportional to speed.

At low speed you need MORE than 100% torque. With the belts that generally are used to slow the spindle you GET more than 100% torque. Actually you get CONSTANT POWER, which is what you need.

There is NOTHING about a DC motor which will lead to low torque at low speed. Generally you can get any torque within the capability of the motor, if the controller is made by an intelligent person.

A PM motor will have 100% torque at any speed. A wound field motor will not, UNLESS you do the smart thing, and keep the field on FULL VOLTAGE, varying ONLY the armature voltage.

That is how the Monarch 10EE works at lower speeds, and it is not a poor substitute for AC power..... Actually the full 10EE system is far better than virtually any VFD system. The very best VFD systems can really only hope to equal it.

But, the 10EE has back gear, and belt drive...... that is how you get the low speed torque and the high speed as well.

If you assume that the speed pot will do for your ONLY speed changing method, you are due for an eye-opening disappointment.

genejohnson · Post by **genejohnson** » Sun Jul 27, 2008 11:52 am

I think all of this might be a mute point for me because I can't seem to get the motor to run at all. I was told the batteries in the cart was kaput but now I'm beginning to think the motor might be bad.
It had what I take to be an electricomechanical brake attached to the end. I didn't know what voltage or how it is activated so I removed it. I then hooked the two batteries in series (24V) and the motor did nothing. The voltage reading I took from the batteries (not fully charged yet) was about 20volts. Shouldn't that have been enough to get some reaction from the motor? Does connecting the batteries directly to the motor instead of thru all the electronics keep it from turning? Gene

J Tiers · Post by **J Tiers** » Sun Jul 27, 2008 3:33 pm

Depends if it really is a DC motor..... A brushless DC that requires an external controller won't run at all without it.

The "brake" may have also had a tach or other essential part in it.

Or a connewction may not be what you think, Or the motor could be bad.

No way to tell from here.

genejohnson · Post by **genejohnson** » Sat Aug 02, 2008 3:08 pm

I was finally able to get the motor to run on 24 volts (two 12v in series) by removing the brake. The brake appears to be on until voltage is applied, Thus braking when the motor is turned off. But... a different source from the controller. I'm going to experiment a bit and try to find out what voltage unlocks the brake! It might not be getting any voltage from the controller. I might hook the motor to the controller without the brake and see if thats the case.
It is a permanent magnet motor and someone said the torque would remain the same at any RPM? As I mentioned earlier, it certainly has a lot of torque. It carried a 350+ lbs woman around department store aisles with ease.
If I'm going to use it, I've got to find out what is wrong with the controller or find another and 24v power supply. If anyone has any thoughts, please throw them at me because I'm open to any and all suggestions. Gene [/b]

mechanicalmagic · Post by **mechanicalmagic** » Sat Aug 02, 2008 9:22 pm

genejohnson wrote:It is a permanent magnet motor and someone said the torque would remain the same at any RPM? As I mentioned earlier, it certainly has a lot of torque.

Gene,
Given a constant voltage: the max torque is at low rpm. At max rpm, it has no torque (if it did, it would go to a higher speed). It is useless to argue what might happen in the middle, since we have no way to know YOUR motor.

Now, for a controller and power supply. Do you have any numbers on the current draw, maybe a fuse rating? The fighting robot guys had several controllers, I have some links (maybe dead). DC power supplies should not be a problem, if you know the approximate max current.

Well, maybe not. A one hp motor needs about 800 watts of power. At 24 volts, that's 33 amps. That should be an easy power supply to find. The control is another issue, in my mind.

Just some ramblings.

Dave J

J Tiers · Post by **J Tiers** » Sat Aug 02, 2008 11:09 pm

I suspect that statement about constant torque was made with respect to slowing a motor way down to slow machine speed.

The max torque is at some RPM where the back EMF is low enough to draw enough current that the max torque is reached.

There is a limit, since the armature iron will only carry so much, and the PM itself only has a certain ampere-turn equivalent. The wire can only take a certain current without overheating, etc.

A series motor has more possible torque, since the field increases along with armature current. And it truly has max torque at zero speed. But rotten speed control......... lousy machine motor, extreme speed variation.

A PM or shunt motor has good speed control, because a slight slowing produces a rapid increase of current, and resulting torque.

A PM machine is like a machine with separately excited field, where armature voltage does not affect field.

So at some speed, not necessarily a really low one, for a given voltage (like rated) there will be current flow producing max torque, if that current is available. Depends on the DC resistance of the machine, and some other things.

Lower speeds will not generally produce a significant increase in available torque, except in the short term, due to heating, even if the iron etc allow it.

What is true is that the rated torque is theoretically available at any speed up to rated speed, if you vary the voltage.

The practical effect of that is that the POWER available will be generally proportional to speed, and slowing the motor way down will cause a drastic decrease of POWER available relative to power at rated speed and voltage.

Note that rated speed is the speed at which rated power (and therefore actual torque) is available.

The only way to counter that effect is to use gearing or belts to multiply torque and still achieve the low speed. That keeps the motor in the good power range, and the spindle in the desired speed area with good power..

Airhead · Post by **Airhead** » Sun Aug 03, 2008 6:45 am

Here's what made it finally sink into my hat rack.

Horsepower = Torque x RPM

Even in a perfect world where an electric motor always puts out the same torque at any speed, half speed equals half the horsepower. One quarter the speed means one quarter the horsepower, etc..

As JT points out, gearing doesn't suffer this affect. That's because if it halves the speed, it doubles the torque (again, in a perfect world). Because of this you still have full horsepower at the reduced speed.

Rick

steamin10 · Post by **steamin10** » Sun Aug 03, 2008 12:09 pm

Not So!

A properly designed Drive has a ramping of DC pulses. Those pulses act on the rotor for starting and torque requirements, as an average value that is higher than the run current. Your equation is wrong for modern application. The drives help produce the torque at low rpm, and when you feed that into your simple equation, you can see how it will make it vary.

No Drive will make a motor produce more HP than the original full power design, at least not without risk of heat damage.

I am not a specialist on DC drives, but I have gotten into some, and even corrected electronic faults on a 100 HP reliance Drive for a coil steel recoiler. Depending on the job demands , drives can be customized in many ways to suit the need, where a naked Flintstone system will just simply fall on its face.