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PWM Motor Controller and Torque

rups22

New Member
Hi all
I'm building an gate opener (90 degree boom arm) using a 12v car windscreen wiper motor. These motors use a worm gear and have very high torque.
I found a 20A PWM motor controller on eBay and I've reduced the speed however I'm losing a lot of torque. Is there any way to reduce speed and maintain torque or should I be looking at a different approach?

Many thanks
 

alec_t

Well-Known Member
Most Helpful Member
Welcome to ETO!
PWM alone should maintain instantaneous torque, but the effective average torque over a PWM period will be reduced in dependence on the duty cycle. To boost torque, additional reduction gearing can be used.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
An "open loop" system such as PWM with no feedback means the motor speed will be very dependant on physical load (torque).

Controlling servo motor speed accurately with varying loads needs a two stage process: You compare the actual speed to the wanted speed, then adjust the motor drive current up or down to correct the speed.

It's called a "double loop" system; the velocity outer loop and inner current loop.
[If it's being used to control the position of a load, the position loop is outside both of those].

Example block diagram, with a position loop as well:
 

MaxHeadRoom78

Well-Known Member
Although in modern servo applications, the double, inner and outer loop, is not often seen now as it once was, it can be achieved now with a single current Loop, usually obtained with a Trans-conductance servo amplifier, rather than the older voltage/tach inner loop.
Max.
.
 

rups22

New Member
Thanks
I think reduction gearing will be the more straightforward option but I will look into feedback loops in future projects
 

rjenkinsgb

Well-Known Member
Most Helpful Member
Another option:
If you are using an AC power source, you can use a very simple speed control circuit (as long as it is a permanent magnet motor).

For power, just AC from the transformer to a bridge rec then a suitable thyristor in series with the positive feed to the motor (cathode to motor). No smoothing capacitor.

Add an extra diode from the rectifier positive and a reasonable side smoothing cap to give a separate smoothed DC supply, with negative also the bridge negative.
From that, an adjustable voltage regulator such as an LM317 (with appropriate components for adjustment and stability).

The output of the 317 goes to the thyristor gate via a ten ohm resistor and a 1N4000 series diode.
Add 100 Ohms between gate and source on the thyristor and a back EMF diode across the motor.

That forms a simple "hit and miss" back emf regulated speed control; they can work extremely well.

The thyristor only fires when the back EMF from the motor is low enough so thyristor gate current is drawn from the regulator. The regulator voltage sets the speed that happens at.
 

rups22

New Member
Interesting. I'm using DC 12v for this project but I have another project using an AC motor
Does this circuit have a common name I can lookup to find a schematic ?
 

rjenkinsgb

Well-Known Member
Most Helpful Member
It's a low voltage version of a classic "drill speed controller". I saw the original low voltage circuit in an electronics mag back in the 70s, published as a model train speed control. It can control a train down to an inch or two per minute...

It can only work with a brushed motor, DC or "universal" types.

This is the simple half-wave drill speed controller version; at higher voltages and lower current they do not need the regulated gate supply; I added that for your 20A system...

The principle is the same; as long as the motor is producing more back EMF than the thyristor gate voltage, the thyristor does not fire.
When the back EMF drops due to the speed reducing, the thiristor fires and accelerates the motor.


 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
PWM will generally reduce the speed with similar torque, but it's still dependent on load. So, if the motor is just too fast PWM should work.

Torque is proportional to current and speed is proportional to open circuit voltage in a DC motor.

PWM allows yu to slow down the motor without an appreciable loss in torque.
 

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