# Using SCR Voltage Adjustable Regulator Motor Speed Control to drive treadmill motor

Joined Aug 27, 2017
10
I have this ongoing saga with a Smooth 5.25 treadmill that I've been trying to fix.
The motor control board is bad and replacement costs $300. So in another thread I read about using a 4000W AC 110V 120V SCR Voltage Adjustable Regulator Motor Speed Control (available for less that$10 on ebay)
plus a bridge rectifier to drive the treadmill motor bypassing the original control board.
I got the parts, wired it up, and - Yippeee - it started up just fine. However, as soon as I put resistance on the running belt (it was not running true, so I was trying to force the belt away from an edge, so I could then adjust the tension on the back roll) is stopped after about 2 minutes. Any attempt at starting it again ends with popping circuit breaker.
I checked the voltages: the regulator no longer regulates the voltage, it is 110V AC in and 110 V AC out, regardless of the potentiometer setting.
It is rated at 4000 W resistive load, the treadmill motor is a 2.5 HP DC continuous duty (Could that just be too much against 4000 W?.
I see the TRIAC on the controller is stamped BTA416008, which should be 40A, but the third line includes: 7A - could that be the problem? By the way, the controller itself is NOT stamped/labelled anywhere, so the 4000W is just a posted rating on ebay, nothing "official".
What could be the problem here?

#### Alec_t

Joined Sep 17, 2013
11,304
It is rated at 4000 W resistive load
Unfortunately a big motor is far from being a resistive load. Does the controller have back-emf spike protection? Can it handle repeated motor starts? N.b. start-up current will be many times the full-speed running current.

Joined Jul 18, 2013
20,709
What is the No of the original board?
Do you still have the flywheel on the motor?
T.M. controllers have soft start so that the full impact of the flywheel is not immediate.
Any controller that does not have this, you may be better off to remove the flywheel if fitted.
Do you have the bridge between the Triac and motor?
Max.

Joined Aug 27, 2017
10
The original board is marked as follows:
JD-110E/E 07/01/1
301S044 DF-1H [logo] 94V-0

The flywheel is on and yes, I have a bridge between this board and the motor.
I'll try attaching a picture of this cheepo control board.
TRIAC on the controller is stamped BTA416008

Joined Jul 18, 2013
20,709
Using that simple controller that does not have current limit, soft start etc, I believe will give you problems if at least not removing the flywheel.
Max.

Joined Aug 27, 2017
10
Thank you.
Is there a better off-the-shelf item you could recommend for the same purpose?

Joined Jul 18, 2013
20,709
There is the KB versions, the low end ones use a SCR bridge for control, the higher models use PWM.
There is also the T.M. MC-60 which is fairly close to the KB SCR type, and has current limit features.
Max.

#### IamJatinah

Joined Oct 22, 2014
122
The current requirement for that treadmill motor is rather large. The "DC" required would need to be controlled by a board or drive like a treadmill controller, a pot would need to be huge to regulate, the Triac/SCR idea is a bad idea with no capacitor filtering and or added serial inductance, and causes a DC motor to growl with AC components riding all over the sliced or rectified DC. Find a "motor controller" suited to the ratings on the motor plate, which should be a good 30A at 100vDC or better on that motor ;o) The original controller can also be repaired in most cases unless the proprietary ceramic IC on some is damaged. Good Luck.

#### MrAl

Joined Jun 17, 2014
7,594
Unfortunately a big motor is far from being a resistive load. Does the controller have back-emf spike protection? Can it handle repeated motor starts? N.b. start-up current will be many times the full-speed running current.
Hello there,

Triacs can not turn off with current still flowing through them. They must turn off when the current gets very near to zero. Maybe a little spike perhaps? They usually are shown with a snubber installed too.

#### Alec_t

Joined Sep 17, 2013
11,304
True, but if the main power switch went off part way through a cycle there could be a major spike.

Joined Jul 18, 2013
20,709
The original board is marked as follows:
JD-110E/E 07/01/1
301S044 DF-1H [logo] 94V-0
[/ATTACH]
Post a pic of the original board.
Max.

#### MrAl

Joined Jun 17, 2014
7,594
True, but if the main power switch went off part way through a cycle there could be a major spike.
Hi,

Sounds like a very good idea to investigate.

The question then becomes, does the triac see a differential voltage that is very low because it is 'on' or does it still somehow get a high diff voltage.
Also i was thinking about the effect of the flywheel which i did not know existed until now. That would make the motor act like a generator too.

I can say for sure that i had created triac controllers in the past and never saw a failure. This was for motors up to about 10 amps 120vac.
I also did a static switch which was 100 amps that could work up to 240v three phase, and that used very high current SCR's (not triacs).

Since i never saw a failure when not using a high power clamp or snubber such as what you are suggesting, i might think that perhaps the triac
sees a low differential voltage during power disconnect (as they call it in the power industry) but i think your idea still deserves investigation.
Perhaps you can do a simulation using a triac (or two SCR's set up for AC operation) and maybe an inductor, and try disconnecting the power while the current is still flowing in the triac and while monitoring the triac voltage and maybe some other nodes. Might be interesting to see the results.

#### Alec_t

Joined Sep 17, 2013
11,304
Perhaps you can do a simulation
That would require a good realistic model of a treadmill motor, including back-emf and generator effects. Do you know of one?

Joined Jul 18, 2013
20,709
The 'simple' T.M. controllers and the low end KB DC motor controllers use a bridge set up with two rectifiers and two SCR's . In place of the Triac method.
Max.

#### MrAl

Joined Jun 17, 2014
7,594
That would require a good realistic model of a treadmill motor, including back-emf and generator effects. Do you know of one?

Hello,

There is one on the web somewhere that includes back emf, but i dont think it includes the generator effect. However, the inductnace would act just like an inductor and the generator would act like an AC source, so one thing to do would be to use an inductor and AC voltage source.

Now that i think about it, the circuits i did in the past that worked with motors would have some generator effect too, because there is always some rotational inertia. They all seem to survive. This could be due to a low differential voltage across the triac or perhaps the triac turns back on if it turns off due to the high dv/dt thus keeping the differential voltage low anyway.

#### Alec_t

Joined Sep 17, 2013
11,304
Perhaps you can do a simulation
I've now done one. The sim shows that a mains disconnect near a half-cycle peak does cause a spike, but it's not as big as I'd assumed and is easily damped with a standard RC snubber.