voltage regulators vs. speed controlers

[Greg Needel]

i am working for a side project for my schools DARPA team and i am trying to figure out the differences between using a voltage regulator (variable resister) or going and getting speed controllers. i know that the voltage regulators will use more power and are probably not as accurate i am just looking to find out what the real differences are.

[Billfred]

This experience comes from R/C cars, and it may or may not be applicable. Just thought I'd warn in advance.

In R/C cars, you've got manual and electronic speed controllers. ESCs are just like Victors in FIRST (except usually designed for 7.2 volts, standard radio gear, and are cheaper), and basically blip the throttle hundreds to thousands of times a second, so fast that it seems like you're driving it at various amounts of power. MSCs are basically a servo sweeping the electrical connection across two, one, or no ceramic resistors, thus giving varying amount of resistance and changing the speed of the drive motor. (This is what I think you're talking about by variable resistors...correct me if I'm wrong) It gets the job done, but there's lots of energy lost to heat. Most people immediately ditch the MSC at their earliest chance for an ESC...if they can afford it.

Hope this helps!

[Max Lobovsky]

If I'm not mistaken, that is how all none transoformer-based speed controllers work, right? I believe the pulses in the PWM signal sent to a Victor directly correspond to it's output.

[phrontist]

Originally Posted by Max Lobovsky

If I'm not mistaken, that is how all none transoformer-based speed controllers work, right? I believe the pulses in the PWM signal sent to a Victor directly correspond to it's output.

I was under the same impression, and would be suprised to find out otherwise. I think the victors are simply "relays" in that they switch the current from the batteries "on and off thousands of times per second" in order to drive the motors.

[Tristan Lall]

Originally Posted by Max Lobovsky

If I'm not mistaken, that is how all none transoformer-based speed controllers work, right? I believe the pulses in the PWM signal sent to a Victor directly correspond to it's output.

I believe you mean transistor. (As in the FETs in the Victors.)

[seanwitte]

An ESC, like the Victor, uses a circuit called an H-bridge to control the speed and direction of the motor. It uses transistors to control the polarity of the power supply seen by the motor terminals. Heres a sample schematic:

     --T2--o--T1--
     |     |     |
+ ---o     M     o--- -
     |     |     |
     --T1--o--T2--

In the diagram, T1 and T2 are transistors that are either open or closed. If T1 are closed and T2 are open then the motor will turn one direction. If T1 are open and T2 are closed then the polarity at the motor leads will be reversed and the motor will turn the opposite direction. If they're all open then the motor will turn freely (coast). If they're all closed then the motor will not turn at all (brake). By controlling the state of the transistors you can control the direction of the motor.

Now, if instead of closing the circuit you switch it on and off very quickly then you can also control the speed. The Victor measures the width of the incoming PWM pulse to determine the speed and direction it needs to drive T1 and T2. The actual frequency of the PWM signal applied to T1 and T2 is much higher than the PWM output from the RC. You could replace T1 and T2 with a push-button switch or a relay and get the same end result. Transistors work better because they can turn on and off much faster.

[Al Skierkiewicz]

OK,
Here's the real reason. The speed of a motor is directly proportional to the current flowing through it and the load placed on the output. For a fixed input voltage and a fixed output load the current will remain constant. For a fixed input voltage, as the load becomes greater the current will increase. Now with that said, the motor winding is essentially a resistor, so using Ohm's Law, if you have a fixed resistor value and a fixed voltage source, then the current will also remain fixed. The only way to force more current through a motor winding is to increase the voltage.
Now comes the hard part. The motor has other losses, frictional, electrical and magnetic, that require a certain amount of current to overcome. Once those initial losses are overcome than the motor will speed up even if you hold the voltage constant.
The one thing that speed controllers do is to switch on and off the full supply voltage from the power supply, and vary the length of time the voltage is applied to the motor. This high voltage overcomes the losses (especially at low RPM) so the motor has greater torque and more reliable speed regulation at low speeds. The length of time that the controller is turned on determines that average current flowing through the motor. This output voltage switching is commonly referred to as PWM, Pulse Width Modulation. When the "ON" pulse is very short, a small current flows through the motor and when the pulse is very long, a large amount of current flows through the motor.
If anyone still has a hard time understanding this, please PM me and I will try to answer this better for you.
The simple answer is, Yes speed controllers are far superior to variable voltage sources.

[Max Lobovsky]

Originally Posted by seanwitte

The actual frequency of the PWM signal applied to T1 and T2 is much higher than the PWM output from the RC.

Ah, good, that gives me an excuse for not building an ESC myself because if the output signal was just the PWM signal amplified, it probably would be relatively simple to build. (At least a low power one where heat and stuff isn't a problem)

[Greg Needel]

thank you all for your help...and i will show you what i am working on soon....