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'[EE:] Silly motor question...connecting in series'
2004\06\03@012708 by

Let's say I have two 12 VDC motors and a 24VDC Power Supply.

Can I put the two motors in series and use the 24 VDC supply?  Will each
motor use up 12 Volts?

Are there reasons why I wouldn't want to do this?  I guess I'm putting two
inductors in series.

James Tu
james2-bit-toys.com

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If you could gurantee that both motors would present the same load, then
it would work.  If one motor is working harder than the other then the
unbalance would lead to a voltage imbalance.

In other words, if you hook up both motors with an equal load (or no
load) then they'll consume the same current, and should each drop about 12V.

If one motor is unloaded, and the other is heavily loaded then the
consume a large amount of current.  This creates a larger voltage drop
across the heavily loaded motor, meaning that it'll be running at a
higher voltage while the lightly loaded motor will run at a lower voltage.

So, no, you can't put two motors in series and expect them both to drop
12V each regardless of what they are doing.

but it's an interesting thought:
I suspect that you could balance the motors with one lm317 by setting it
for twelve volts and connecting it's output between the two motors.  I
suspect it wouldn't work for various reasons, but it would be
interesting to pursue exactly why it would fail.

Anyway, good luck in your project.

James Tu wrote:

{Quote hidden}

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{Quote hidden}

In fact the voltage drop across the loaded motor will fall, the voltage
across the unloaded motor will rise proportionaly and will therefore speed
up.

Remember that the current flowing through a motor = ( Applied voltage - back
EMF ) / motor resistance.  Back EMF is proportional to the speed of the
motor, as the motor slows down, the back EMF reduces, and current increases.
If you have a second unloaded motor in series, the larger current draw of
the first motor will cause a greater voltage drop across the second one.

Regards

Mike

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At 01:26 AM 6/3/2004 -0700, you wrote:
>Let's say I have two 12 VDC motors and a 24VDC Power Supply.
>
>Can I put the two motors in series and use the 24 VDC supply?  Will each
>motor use up 12 Volts?

After it settles out, the voltages across the motors will divide

>Are there reasons why I wouldn't want to do this?

Yes.

>  I guess I'm putting two
>inductors in series.

That's not a problem, but think in terms of the back-EMF and torque.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speffinterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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> Can I put the two motors in series and use the 24 VDC supply?  Will each
> motor use up 12 Volts?
>
> Are there reasons why I wouldn't want to do this?

Hi James,

I'm going to make the assumption that both motors are identical.

In a series circuit, all components get the same current.

In a DC motor, current is torque. So here are your two motors, each
producing the same amount of torque. If one motor should become unloaded,
the current goes down and the other motor's torque goes down.

If that is OK with you, go right ahead and do it. It could be useful if
each motor was powering one drive wheel in a rear wheel drive vehicle,
performing the same function as a differential. Or if both motors power

Cheerful regards,

Bob

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Two PM motors in series will behave somewhat strangely. if you load one
then the loaded one will slow down and give very little torque and the
other will try to *accelerate*. If there is no damping then they will try
this at startup, i.e. one will accelerate quickly and the other will
follow slower. This is a useless configuration unless the two motors drive
two independent pumps, in which case they can be used to maintain a
certain ratio of the pumped substances when the density of one of them
changes (and slows down its pump). The concept can be extended to more
than two pumps. The system is prone to slow oscillations. There are other
examples of use. F.ex. 2 fans venting a case. If one stalls the other will
try to double rpm and take up the slack in the necessary air movement.

Peter

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