Treadmill/Variac controller

No. Just saw video little while ago. Have a kb controller but that’s too easy. Not really. Too damn cold for me to mess with much.
 
I watched the entire video. The presenter is a bit fuzzy on some of the stated facts. The rating for current on a Variac is the current that it can safely handle without long term failure. The current drawn by the motor is a function of the applied voltage, the motor windings, and the applied load. Diode bridge failure as he has configured it is more than likely due to surge current. when the motor is switched on with a non zero voltage, there is a very large current surge because the motor hasn't yet developed the back emf. Once the motor is at operating speed, for that voltage, the current drops to the run level. If one is starting the motor under load, the current is higher for a longer period of time.

Fuses, semiconductors, and the like have a characteristic called i2t. They can handle a very large current if the time is very short. Increase the time and they fail. To prevent this, I used an inrush current limiter. It is basically a resistor with a high resistance when cold but a relatively very low resistance once it heats up. The high resistance limits the initial current to a safe level but drops to a low value during operation so there is a low voltage drop.

I used a filter capacitor when I tried the Variac The filter capacitor was large enough to smooth out the pulses of the full wave rectification to provide a reasonablt constant d.c. voltage. However, they can have inrush currents on the order of hundreds of amps. High enough that they welded the contacts on the power switch. The inrush current limiter solved that problem.

As to torque characteristics of a Variac controlled motor, although you can turn the motor at a very low speed, you can easily stop the motor with your hand. When I first tried the Variac control, it proved unsuitable for even moderately low speed operation of my lathe. It is conceivable that you could set your speed under load but there would be a significant variation in motor speed with load. If used on a lathe,

I have since made a device called a Prony brake which can measure output torque of a motor and it would be instructive to put numbers to the motor torque.
 
Old saying might apply here. When it sounds to good to be true.
 
cross thread said:
RJ what you have done sounds like what I need . A shop built PWM , how did you do it ?
Click to expand...
The circuit is a pulse width modulator supplied with d.c. voltage by a bridge rectifier and capacitor filter. The PWM is switched with a MOSFET power transistor which is triggered via an optoisolator by a custom controller. I used a shop-built encoder to monitor motor speed and apply feedback when it sensed a motor slowdown or speed up. As mentioned above, I use an inrush current limiter to protect the diode bridge but bypass it with a time delay relay after about 1 second. This accomplishes two things. One, the motor can operate on full power after one second as opposed to slowly ramping up over a period of several minutes and two, if the power is switched off and immediately switched back, the inrush current limiter is cooled down and will function as intended. The control circuit consists of common discrete components operating in analog fashion. I chose this route rather than digital control via a microprocessor and software because I wanted components which would be available some twenty years from now.

The key to the design is the feedback loop. If you think about a variable speed hand drill, it is possible to turn it at very low speed and still develop adequate torque. That is because there is a biofeedback loop consisting of the operator and his finger on the trigger. When he senses the speed dropping, he depresses the trigger more to counteract the drop in speed. The encoder essentially take the place of the operator. When it senses a decrease in speed, it increases the the pulse width up to full on, if necessary. Thus, even at extremely low speed, the controller can essentially deliver maximum voltage to the motor. The encoder delivers a pulse every 18º of motor rotation so at extremely low rpm, the pulses cause the motor to cog much like a stepper motor but that occurs at a spindle speed of less than 10 rpm on the medium low belt setting or 5 rpm on the low low belt setting. Maximum rpm on the medium low belt setting is just over 1,000 rpm yielding a 100:1 operating range. At the 10 rpm setting, the belt will slip rather than stalling the motor.

I have a detailed description of the entire process which hasn't been released yet as I was waiting to compile some torque data with the recently completed Prony brake. I also want to tweak some of the parameters. Operational amplifier circuits run best with critical feedback applied. That is feedback that causes a small amount of overshoot of the targeted parameter. More feedback causes larger overshoot and possibly oscillations and less feedback results in longer settling times and a larger operating range in terms of speed vs. load.
 
RJ has brought up some excellent points regarding using a variac with full wave rectifier in controlling a dc motor. The biggest issue is torque. The lower the voltage, the less torque. That is the opposite of what you want for a mill. The bigger diameter the cutting bit, slower the speed but the need for higher torque. This is why a lot of controllers use PWM of pulse width modulation. Each pulse is a full voltage applied to the motor. The width of the pulse is called a duty cycle. If the duty cycle is 100% or voltage full on, the motor is maximum speed. If the duty cycle is 50%, the motor is running at half speed because the voltage is on half the time. 25%, the motor is running at quarter speed. This way the motor is running at full torque. However, there are limits, If the speed is let's say under 250 RPM, then the motor may have very little torque. If you have the motor control unit of your treadmill, I would use that. I'd also would keep the pulleys with the changeability of the belts because you really want to run your motor at around medium speed or may be higher and still have lots of torque. Besides many motors have built in fans that help keep them cool. And yet you still have variation in speed.
 
RJ & silence dogood , thank you for your detailed descriptions . I'm over the Variac , the PWM sounds like the way to go . I had a feeling a Variac would produce little torque at low RPMs . My thought to fix that is to make a counter shaft set-up and gear it so the motor would be running higher RPMs but my spindle would still be rotating slower with sufficient torque . It would be great to have a back gear , however that would be over my head to make . Multiple pulleys sound like a better way to go . RJ I'm looking forward to to your end results with the Porny brake . As of right now I have no components , I would like to purchase everything new with the exception of the treadmill motor .
Again thanks for your posts and patients . This is all new to me .
Mark .
 
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OK, Just for yuks, I hooked my lathe motor up ti the Variac control I described on post #12 and measured torque at various speeds. Dome background info: the motor is a brush type d.c. 2.5hp 130 volt with a nominal speed at 130 volts of 4090 rpm. I am driving my lathe spindle with a nominal overall pulley reduction of 5.75 which would would be equivalent to a spindle speed of 711 rpm. I am applying torque with the :prony brake and measuring the torque required to reduce motor speed by 50%.
No Load Spindle RPMTorque, lb-ft.
10.2
401.0
601.0
801.5
1002.2
1403.0
1804.7
2405.2

By comparison, My PWM control puts out between 10 and 12 lb-ft of torque at a 20% drop in rpm at which point the drive belt begins to slip. This happens at spindle speeds as low as 10 rpm.
 
An added note; I wouldn't expect that the PWM controller without feedback would perform significantly better than the Variac. A PWM motor applies full voltage with varying pulse duty cycles to provide an varying average voltage but the motor responds to the average voltage. (actually, it is RMS voltage). An increase in load would require an increase in average voltage to maintain the motor speed but without feedback, the motor wouldn't know the load had increased.

In contrast, adding an encoder to the controller allows it, with proper modification of the circuitry, to increase the pulse width and thereby the average voltage to the motor. Theoretically, it is possible to apply full voltage at motor speeds as low as 1% of maximum speed. This is analogous to applying a turbo boost to an internal combustion engine.
 
Thanks for the explanation RJ. I have a very limited knowledge of electronics, but I can follow what you are saying, and I can read a schematic, and use a soldering device. I have several applications where this component would be very useful. Will be looking for your write up on your design in the future, and will definitely give it a try. Cheers, Mike
 
I used an MC2100LT on a round column mill. It is the descendent of the MC60/MC70 series, and you have to have a 20kHz signal generator to make it work. You can get them for around $100 on eBay, and the signal generator can be had for about $10 from Amazon, Walmart, etc.

There is no encoder, but these units somehow sense the power going out the line to provide feedback. Driving a 4" facemill in a WrongFu-45, I could get the motor to slow down and then you could feel it slowly pick up speed, then momentarily jump in speed if raised off the work quickly. Drive it too hard, and it would slow WAY down, and not speed up until you restarted it completely.

Another benefit is that the connector used for the signal generator input has pins with 12V that can be used to power and LED gooseneck desk lamp, or other low power accessories.
 
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