OT - DC Motors In Series Question For An Electrical Engineer

I am working on a display. I have 2 small (3 watt) 12VDC permanent magnet gearmotors that run about 30RPM on a 5VDC USB hub power pack. I have a faceted ball on each motor and 2 USB snake lights behind to light them, the effect is similar to a star ball. But I would like the speed lower.

I thought wiring 2 motors in series would give each motor 2.5V and reduce the speed, but when powered up one motor spins full speed with the other motor stopped. Applying pressure to the shaft of the spinning motor causes it to stop fully and the other motor to spin full speed. If I apply pressure to both motor shafts the motors go full speed one at a time, switching back & forth quickly.

I am not an electrical engineer and was wondering if an electrical engineer could explain this, or if this is not normal for DC motors and they should share the power.

That's normal. Your voltage is so low that you have very little torque.
Connect both in parallel. Put a 2.7V Zener diode in series with each one to drop voltage. Not sure what wattage you need.
Another way would be to use LM317 regulators. That would give you adjustable speed.
You may find that you need motors with a higher gear ratio.

keithkyll,

Thanks. I will try the Zener diodes. I already decided against adjustable voltage regulators as I want to keep this simple.

And I am stuck with the gear ratio - these motors were scrap from a robotic camera control system. Each has an encoder on it that failed and my customer was going to throw them in the garbage and let me have them. The only thing wrong with them is the encoder and the motors work great on 1.2 to 12 volts. (and the cost from the OEM for these is $590.00 each!!!)

is there any electronics inside to control the speed?
maybe it is a BLDC motor?

if you want more control use an arduino UNO.

Paul,
No, it's a straight 2 wire 12VDC motor. The original application varied the voltage to control it, based on the count from the (fragile, delicate) encoder. (and when the encoder goes, it's only $580 for a new motor, but the motor itself still works great)

And I want a simple display that runs directly off a 5V USB power pack I have plenty of.

A couple other ways to do what keithkyll is suggesting;
1) Add several rectifier diodes in series with the motors. Each one will drop the voltage about 0.7 volts.
2) Add a low resistance high current wire wound trimpot in series with the motors to adjust the voltage.
Once you know what voltage you will need then purchase the appropriate zeners.

so you are sure they have brushes on them, for this price?
a BLDC also has two wires only.
this would clair their behavior.

You can use PWM to control DC motor speed using the famous IC NE 555
http://www.dprg.org/tutorials/2005-11a/

Get 2 adjustable Boost/Buck converters or adjustable switching voltage regulators (eBay or local electronics shop). Use 1 on each motor and supply them in parallel from your USB power source. Optionally add switches, fuses, capacitors and flyback/free-wheeling diodes as required.

This is a switching solution that gives you a usable level of adjustment over your motors for just a few dollars.

i like tariks solution.

An update:

I tried the Zener diode suggestion and it works great. As they are 3 watt motors [at 12 volts] I put 2- 2.7 volt 1.3 watt Zener diodes (in parallel) inline with each motor.

They spin at a nice respectable speed now. Thanks Keithkyll.

Glad to hear you got it working. If diodes get hot, you might need to use a 5 Watt.

Keithkyll,
I doubt they will ever get the chance to run hot, it is for an intermittent use and will not be running continuously - probably on for less than 1 minute, then off for a few minutes. But if this changes [why would anything ever need to be changed??] I will go to the larger 5 watt diodes.