Thread: Calling all electrical experts - DC motor conversion

Most of these DC treadmill motors are designed to spin in one direction. They will spin the other way if you reverse the polarity but they are not designed for that. If you look at the brushes in the motor you will see that they do not make contact at 90 deg, they make contact with the commutator at some angle so as to point "away" from the rotation. Like I said, they will run in the other direction, but for how long I don't know.

Simon

My motor looks like the brushes are 90 degrees, plus reversing the polarity works fine. Problem is the attached fan which now works great for sucking air out of the motor, not pushing air in.

Originally Posted by neevo

My motor looks like the brushes are 90 degrees, plus reversing the polarity works fine. Problem is the attached fan which now works great for sucking air out of the motor, not pushing air in.

I would have thought that as long as air was moving through, that was the important thing. I could stand to be corrected, I usually am.
Kryn

Dunno. I was going to run it and see what happens. Worst case I make a replacement.

Give it a crack, see how it goes. At least you won't die wondering!

Simon

True.

The problem with fans is that the efficiency drops off faster as it gets slower. Forced air cooling from an independent fan would be better. The major problem with these permag motors, is that apart from the air flow through them, the only other way heat can be got rid of is by conduction through the armature shaft and bearings. Where as an induction motor has good thermal conduction directly to the case.

Originally Posted by simonl

Most of these DC treadmill motors are designed to spin in one direction. They will spin the other way if you reverse the polarity but they are not designed for that. If you look at the brushes in the motor you will see that they do not make contact at 90 deg, they make contact with the commutator at some angle so as to point "away" from the rotation. Like I said, they will run in the other direction, but for how long I don't know.

Simon

This is called "commutator timing" in DC motors. Motors designed to run in both directions sacrifice some efficiency by using a symmetrical commutator timing. Motors designed to run only or mostly in one direction use an asymmetric (aka advanced) timing. Asymmetric means that the brushes are advanced a few degrees against the rotation direction.

- Motors with asymmetric timing do not run well in the other direction, the commutator sparks and power/speed is reduced.
- It is usually easy to change motor timing from L or R to symmetrical, and vice versa. Often just the brush carrying flange needs to be rotated. Sometimes a little locating notch needs to be filed away first to allow rotation.
- Sometime though a motor with asymmetric timing uses a brush setup, where the brushes are set at an angle to the commutator (not at 90 degrees to the commutator, but angled in rotation direction). In this case it is not possible to change rotation direction, because the brushes would quickly wear away and possibly even chip.

As a practical example: You may have noticed that some better handheld drills (metabo, bosh come to mind) have a reversing lever that mechanically rotates the whole brush carrier. This is a superior method to change rotation direction, over the cheaper handheld drills that have a fixed symmetric timing and only switch direction by reversing polarity. It allows for best efficiency and performance in either direction.


see here if you like to know more about this:
http://en.wikipedia.org/wiki/Commutator_%28electric%29

Originally Posted by NCArcher

I'll have a go.
Prox switch is speed feed back as mentioned. I think for the digital display only.
I think the speed control is multi speed not variable.
Pressing the up/down buttons increases or decreases a counter that varies the pulse width to a pre determined value that corresponds to the counter value..
Pressing the stop button resets the counter to zero.
Only a guess but if it is correct I don't think you could adapt a pot easily if at all.

I think that this is probably correct, and any modification to use a pot to achieve variable speed is most certainly unlikely to work without some additional Analogue to Digital interface electronics.

I would try to adapt the push button "speed up down" aspect which is already built in.

mike

Will do, watch this space as I've ordered a larger control box and will look to incorporate the control buttons and leds.

Parts ordered, plan is to redesign the control box, a little larger so I can include the control unit for the motor.

Also ordered a few fans so I can try and get the cooing right once I get the motor running.

Also finalising the motor mount which should be relatively easy as I have a TIG here, just need to find some steel plate.

I'm really annoyed with myself. After receiving all the necessary parts to build my new box, building the box, chasing out the lid to incorporate the control board and speed indicator leds... I accidentally let the smoke out of the board by shorting it on my welding plate!

BALLS!

I found the buttons were the things that took the hit and went to an electronics store today to buy replacements. Soldered them in fine but unfortunately it's not come back to life

No burnt looking parts on the board so probably not worth chasing the damaged parts.

Looks like another box ruined (shame as I had done a good job on this one):



So now I'm on the hunt for either another replacement board, I may pop down to a local treadmill repairer to see if they have anything or I may go down the new PWM route.

Sorry to hear about your magic smoke early release program. Been there & done that!

Is it a standard treadmill speed controller you have used? If so then I may have one or two floating around. I could post them to you if you think it will help you out. They may or may not work but they may be worth a try.

Simon

Cheers Simon, appreciate the help. I've been in contact with a treadmill repairer in Penrith who may have a replacement controller board.

If not I will let you know and see if you have something that can help.

Appreciate the offer.