Drop voltage for DC Brushed motor

Question

Well, I have a little problem, I'm out of resources for the next month (read: funds for electronics) and I ordered a 12V accumulator/lead battery. The motors I have are rated for 7.4 Volts and are 165W. Full spec's are not available (cannot find them) but I do have a link to the motor.

Here is a quote from the seller page:

Supply Voltage       7,2 - 7,4 V
Nominal Voltage      7,2 - 7,4 V
Turns                12
RPM (no load)        32000 min^-1
Power                165 W
Manufacturer number  2310072
Use as               Tuning-part

Now, I have a motor schield which is 0.018 Ohms and I assume the DC motor has something around just that. I do not have such small resistances to drop the voltage from 12V to 7.4, so what are my options?

Is there any DIY voltage regulator thingy?

The resources I do have are (from the arduino starter kit):

5 x Resistor 220 ohm
5 x Resistor 1K ohm
5 x Resistor 2K2 ohm
5 x Resistor 10K ohm
5 x Resistor 330K ohm
2 x 100 nF Capacitor
2 x 10 nF Capacitor
2 x 100 uF Capacitor
6 x LED
1 x Piezo speaker
1 x 10 KOhm POT
2 x PCB button
1 x LDR
1 x Thermistor
1 x IR Receiver
2 x Transistor
2 x Diode
1 x MOSFET

and some other scrap which is lying around (broken DVD players etc).

Any thoughts on dropping the voltage to 7.4V without losing too much power?

I did look at 30A voltage regulators but some are even in the price as high as $500, but I can't find any 12 to 7 volts 30A regulators, so I think I'm on my own with this?

Anyway, I hope someone here can guide me in the right direction!

Answer 1

The cheap and flexible way to deal with a combination of massive drop in voltage and current drawn for a DC motor is to make an H-bridge from discrete diodes and transistors and control them from a microcontroller. Such converters chop the voltage and ideally wouldn't dissipate any power (but they will from non-zero transition times). There are plenty of H-bridge designs on the Internet, but the basic things to keep in mind are gate saturation voltage (which you'll need to apply to the gates), saturation current, resistance when ON, max drain/source voltage.

Just chop 12V down to 7.4V with one of your transistor and a diode (see buck converter) before feeding that voltage in the motor controller. You'll need to control the transistor with a square wave of a high frequency (bear in mind the lower the frequency the more the motors will vibrate from the current harmonics), say 20kHz or more, modulated in pulse widths with a duty cycle of 7.4/12=0.61. You'll probably need to filter the voltage if the motor controller does anything other than applying it to the motor, otherwise the current is what is of interest and it is already filtered by the motors (but checking that's enough from solving the differential equation could be useful).

Eventually, you could control the converter from a 555 instead of the microcontroller should you need more computational power.

The figure above is a general example of a step-down switching DCDC converter, but you would only place the transistor (properly controlled) and the diode in between the motor controller and the 12V supply; forgetting about the inductor if the motor's is enough, the capacitor if the motors are the only target, and the resistor as you have a different load.