Speed control using triac based motor controller

Thread Starter

white aurora

Joined Sep 26, 2019
17
Hi all,
I have two treadmill motors with controllers. They are old, from the 90's. They have scr control boards and work very nicely with their upper control boards. These boards are small and I would like to use them as controllers for my machinery (drill press and sander). My problem is that I don't understand what is going on in the control circuit. The cable has a pinout label and has three pins of interest, one is photoeye coming from a reflective rpm counter, one labeled zero cross, and a third labeled scr control. When the motor is running I put an oscilloscope lead on each of these points and this is the result.
Photoeye1.JPG
Above is the photoeye at minimum speed.
Photoeye2.JPG
This is the photoeye at about half speed.
zerocross.JPG
this is the zero cross pin (this does not change with speed of course)
scrcontrol.JPGThis is the scr control pin. I would expect this to change with an increase in speed but it does not.

So what my question is, what is the lower board expecting? I was planning to replace the upper board with an arduino nano but cannot write the program if I do not know what the output needs to be. I was expecting that the lower board would need a zero crossing based pulse to the triac that would change with duty cycle but this does not seem to be the case. Is the photoeye contributing to the speed control perhaps?

I currently have a partial schematic of the lower board but it is not complete yet.

Any ideas what might be going on would be helpful.
 

Ian0

Joined Aug 7, 2020
759
An SCR or triac controller is generally controlling a universal motor. A drill press generally has an induction motor, either three-phase or capacitor-run.
 

MaxHeadRoom

Joined Jul 18, 2013
21,636
An SCR or triac controller is generally controlling a universal motor. A drill press generally has an induction motor, either three-phase or capacitor-run.
As the OP said, there are many of the older T.M. DC PM motors running off of SCR bridge type controllers, as well as The DC controller manuf, Minarik and KB eElectronics for two, make gen purpose SCR boards for DC P.M. or wound field motors.
The 2.5+ HP T.M. motor will run OK on a drill press.
The Universal motors that are ran off of AC with a SCR, usually are used inside a bridge. Otherwise it is half wave, some eliminate the bridge by using a Triac instead.

Max.
 

Ian0

Joined Aug 7, 2020
759
What sort of motor is on the drill press? That's what is important. A triac/SCR is not going to control an induction motor successfully. You need to vary the frequency not the power.
 

Thread Starter

white aurora

Joined Sep 26, 2019
17
What sort of motor is on the drill press? That's what is important. A triac/SCR is not going to control an induction motor successfully. You need to vary the frequency not the power.
I am trying to put the treadmill motor and its controller on the drill press. The old motor that came with the drill press smoked quite a bit when I turned it on. It was a 1/2 hp induction motor.
 

MaxHeadRoom

Joined Jul 18, 2013
21,636
I am trying to put the treadmill motor and its controller on the drill press.
That board looks like it may be very easy to reverse-engineer, I would try that first and it should give you a good idea of the circuit and where to apply the Pot, if needed.
The S21MD3V is a triac driver.
If you manage to reverse-eng it, post the DWG here.
Max.
 

MaxHeadRoom

Joined Jul 18, 2013
21,636
I missed the schematic, should be possible to find out the control element, Is this something someone has reverse engineered, some things do not look right?
The motor is SCR bridge controlled.
Max.
 

MaxHeadRoom

Joined Jul 18, 2013
21,636
Did you 'scope the pins 9 & 10 from the upper board?
There does not appear to be any Common going up to the upper board, which I would have thought there would be?
Max.
 

Thread Starter

white aurora

Joined Sep 26, 2019
17
That is a nice quality motor, normally I remove the flywheel when used in other applications.
Max.
Yes, I will probably do that. I have it on its original mounting plate because there is a reflective rpm counter on the flywheel and I am not sure if it needs that but for now the upper board needs it to run so I can test for the different wave forms.
 

Thread Starter

white aurora

Joined Sep 26, 2019
17
I missed the schematic, should be possible to find out the control element, Is this something someone has reverse engineered, some things do not look right?
The motor is SCR bridge controlled.
Max.
circuit diagram 905e.png
This diagram is a work in progress as it has been remade a couple of times to correct mistakes. I am quite sure that it is correct but anything that looks wrong I can certainly compare to the board.

Here is a page from the service manual that may be telling. Pins 8 and 10 have been tested multiple times Pin 8 yields a square wave that rises and falls approx. every 8ms (half wave) I believe. Pin 10 gives a pulse approx every 8ms but this does not seem to change with an increase in speed as I would expect.
905e layout.pdf

I am now suspicious that the output pin on my arduino may not be sending enough current through to trigger the optotriac.
 

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Thread Starter

white aurora

Joined Sep 26, 2019
17
Did you 'scope the pins 9 & 10 from the upper board?
There does not appear to be any Common going up to the upper board, which I would have thought there would be?
Max.
I have attached a pinout diagram in an earlier post. What is it that you mean by a common? The data cable is attached to the bottom of the board in the picture and my not be very visible.
 
Last edited:

MaxHeadRoom

Joined Jul 18, 2013
21,636
The J1 10 pin connector does not appear to have the 5v common (GND) on any pin as I would have expected.
The collector of Q4 does not have any connection?
The photo detector output appears to go up to the console, as the practice on many T.M.'s. Where it is processed and the speed command changed accordingly.
Most types work without the speed sensor when operated on their own.
Max.
 

Thread Starter

white aurora

Joined Sep 26, 2019
17
The J1 10 pin connector does not appear to have the 5v common (GND) on any pin as I would have expected.
The collector of Q4 does not have any connection?
The photo detector output appears to go up to the console, as the practice on many T.M.'s. Where it is processed and the speed command changed accordingly.
Most types work without the speed sensor when operated on their own.
Max.
I looked at the board to follow the collector on Q4 and it goes to a circuit breaker that is attached to the power switch.

I suspected that the speed sensor is not needed once the speed can be controlled in another way.

There are three ground pins on the 10 pin connector, are any of those the ground that you were looking for?
 

Thread Starter

white aurora

Joined Sep 26, 2019
17
I don't seem to see them on the DWG, there is three to the +5v.?
Max.
if you look at the 905e layout page i posted above there is a pinout on the top of the page that identifies the 10 pins. Pins 2,3,and 4 are +5v, and pins 5,7 and 9 are ground pins. I think this is what you were looking for.
 

MaxHeadRoom

Joined Jul 18, 2013
21,636
I am guessing there is a processor in the Upper unit that takes the RPM sensor output and calculates the speed when compared to the entered value.
Then provides the commutation angle for the Triac bridge device .In that case, you would need to come up with a duplicate method.
Max.
 
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