Hello,

I am looking for assistance is selecting a suitable potentiometer to control a brushed DC motor. Please bear with me as I haven't taken any electrical engineering classes yet. I am in middle school working on an extra credit science project to build a hovercraft and I am not allowed to use any form of integrated circuits to control speed. The motor is running off of a 8.4 volt 1600 mAh NiMH battery. I have been researching ohms law but I am totally confused on how to determine the resistance and wattage requirements for the pot. The motor is a Traxxas 3785 Titan 12 Turn 550 Brushed Motor. I cannot find any specs on the motors current draw. I can use a multi-meter to test current but should this be tested while stalling the motor (i.e. holding it so it doesn't spin)? Once I have this reading, what additional information do I need to determine the required ohms and watts of the potentiometer?

Thanks in advance for your assistance.

 

For starters you can power the motor placing an ammeter in series with the motor to measure the normal run current and then briefly grab the shaft and stop the motor to see the stall current. I also looked briefly and could not find a decent specification sheet for that motor. Traxxas does have a forum you may want to visit and hopefully find more data for the motor. You would think for what they cost a data sheet would be commonplace.

As to controlling the speed it is not as simple as using a pot but you can try. With motors like this the popular speed control is PWM. That said " I am not allowed to use any form of integrated circuits to control speed" will complicate things. I suggest you give this little tutorial a read. Eventually we get into a simple square wave oscillator where we can change the duty cycle and use that signal to drive a MOSFET which in turn will drive your motor. They also mention why using a potentiometer is not a very good or practical solution. One problem is as motor load increases the motor current draw will increase.

Ron

 

You could use a simple circuit like this which will control the voltage across the motor using a pot. This will keep the voltage across the motor constant whatever the load on the motor. You will need to make sure that the transistor you use for TR2 can handle the motor current and it will probably need a heatsink.

 

From an answered question at Amazon a person listed the current draw at 10.5 amps on 14.4 volts and 3.2 amps on 7.2 volts at full power.
May or may not be accurate.I would use the oscillator circuit in "question 18" from Ron's link to drive the regulator circuit above from Albert Hall. That would give a PWM and should run cooler at the output.
Steve G

 

I don't think I will be able to use those circuits as the allowed components are very limited. Here is a quote from the rules:

"Electrical components shall be limited to batteries, wires, motors, switches, resistors, potentiometers, capacitors, mechanical relays, fans, and blowers. Brushless motors and integrated circuits are not permitted unless they are an integral part of or embedded into commercially available fans used for cooling electronics or computers."

The hovercraft only needs to run short intervals (approx. 15-30 seconds) at a time and the track is constant (smooth and flat). I just need a way to reduce the speed of the motor enough so the hovercraft completes the track as close to 15 seconds as possible. Given the short run time, I'm not sure if heat would be a major factor.

Assuming the motor draws 4 amps max (just hypothetical until I can test it) and the battery is 8.4 volts, that would give 2.1 ohms resistance (8.4/4=2.1) and 33.6 watts (8.4*4=33.6).

If I want to reduce the current to .5 amps, that would require 16.8 ohms of resistance (8.4/.5=16.8) and provide 4.2 watts (8.4*.5=4.2).

Based on the above, should I be looking for a potentiometer greater than 33 watts and close to 16.8 ohms?

 

The hovercraft only needs to run short intervals (approx. 15-30 seconds) at a time and the track is constant (smooth and flat). I just need a way to reduce the speed of the motor enough so the hovercraft completes the track as close to 15 seconds as possible.

I don't understand why you would want to slow down the motor if you want it to go faster. Maybe I'm missing something in this? Or does it have to fit the 15 - 30 second time frame?

In a single fan, full size hovercraft, they usually just bleed off a certain amount of inflation air(lift air) to control the forward speed. Done with an adjustable air dam.

 

I don't understand why you would want to slow down the motor if you want it to go faster. Maybe I'm missing something in this? Or does it have to fit the 15 - 30 second time frame?

In a single fan, full size hovercraft, they usually just bleed off a certain amount of inflation air(lift air) to control the forward speed. Done with an adjustable air dam.

I want to be able to control the motor speed so I can effect the overall time it takes the craft to travel the track. The target time is 15 seconds but I won't be able to test on the exact track until race day. The craft will have two motors, one for lift and one for propulsion. For the purposes of this question, I have limited it to one motor.

 

The craft will have two motors, one for lift and one for propulsion.

Then why not trim the blades on the propulsion motor? Smaller fan = lower speed.

 

I want to be able to control the motor speed so I can effect the overall time it takes the craft to travel the track. The target time is 15 seconds but I won't be able to test on the exact track until race day. The craft will have two motors, one for lift and one for propulsion. For the purposes of this question, I have limited it to one motor.

You could run the motor at full power and then have a mechanical system to block off part of the propulsion fan which you could adjust on the day.

 

The cost of a suitable potentiometer is likely to be more than the total cost of the rest of the hovercraft! As others have suggested, a mechanical adjustment of the propulsion thrust seems a better option.