# Basic DC motor questions.

Discussion in 'The Projects Forum' started by jlatshaw, Jul 20, 2013.

1. ### jlatshaw Thread Starter New Member

Jul 20, 2013
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0
Hello,

I have a few questions regarding DC motors.

I believe that at DC steady state that inductors ideally will act as a short circuit and won't have any resistance (right?). So would it then be a really bad idea to just hook up a battery to a DC motor for fear of breaking the motor?

Or, is there always an internal resistance of the motor to prevent an ideally infinite amount of current flowing through the motor?

If there is no intenral resitance of the motor, can we just add a resistor in series with the motor to regulate the current?

Finally, I know that in AC you can hook up a capacitor in parallel with an inductive load to try to lower the reactive power (and raise the power factor of the circuit to 1). However I've seen people hook capacitors in parallel with DC motors, does the idea of a shunt capacitor work in AC and DC or just AC?

The motor that I am working with is:

Thanks for any help,
-James

2. ### Bernard AAC Fanatic!

Aug 7, 2008
4,248
424
There is some DC resistance, in this motor maybe 2/10 Ω, & some inductave reactince, so initial inrush might be up to 100A. In the old days, startup and speed was controller with stepped recistance, but now mostly controlled with electronics, PWM, pulse width modulation.
C asross DC motor is to reduce sparking at the brushes.

3. ### Kermit2 AAC Fanatic!

Feb 5, 2010
3,854
968
The inductors, also know as the motors rotor and stator windings, will indeed act as a short circuits to DC voltage WHEN THE MOTOR IS NOT TURNING.

Once the motor begins to 'motor', the movement of the windings through the magnetic fields inside the motor will create a voltage. The motor windings are acting as a generator when the motor is actively rotating. The voltage produced is known as the 'back EMF'. This is what prevents the battery from seeing the motor windings as a dead short. This is also why a motor draws a large amount of current at start up, since the generator action has not had time to build up and counteract the battery voltage.

The supply should ALWAYS have a protective fuse or circuit breaking device that will protect the battery and motor in the situations when the motor is overloaded or stalled.

Adding a resistance is a good method of regulating the start up surge and most schemes using resistance include a method to switch the resistance out of the circuit when the motor achieves something close to its operating speed range.

4. ### jlatshaw Thread Starter New Member

Jul 20, 2013
28
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OK great. I am working on an ebike project and the reason why I am asking this is because I am trying to design a small circuit to help regulate the speed of the motor by regulating how much current is seen across the motor. So now that I know that there isn't a whole lot of internal resistance, I can go ahead and design something.
I was thinking that I could include resistor(s) in series with the motor, and this would control how much current is seen across the motor and thus the power/rpm of the motor. Does that sound like it would work?

Also, do shunt capacitors work in DC like they do in AC?

Thanks,
-James

Jul 18, 2013
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2,528
Motor regulation usually requires some kind of feed back, this can be from a simple current sense in the motor/semiconductor lead to a DC or digital feedback, but something is usually required, the current sense resistor is usually the simplest way.
You might also want to look at a simple PWM controller.
Max.

6. ### jlatshaw Thread Starter New Member

Jul 20, 2013
28
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OK I'll look into those.

However another quick question. Imagine a motor in series with a resistor and everything then in parallel with a 24V battery. How would I go about choosing my value for resistance so that it is at 10A. Do I just treat the motor as a short circuit and then apply ohm's law? V=IR, R = V/I=24/10 = 2.4 Ohms.

So if I choose a 2.4 ohm resistor and place that as described above, my motor will have 10A going through it, but technically won't the 24V drop from the batteries be directly across the resistor?

My motor says that it needs 24V and varying currents for varying output powers, in this case let's just say I needed a 10A current to get my desired effect. How can I use a resistors to give me my desired current while maintaining the correct voltage drop across my motor?

Thanks for any help,
-James

7. ### crutschow Expert

Mar 14, 2008
13,523
3,391
You missed the post by Kermet2 about motor back EMF when the motor is running. This back EMF means it only draws the current it needs as determined by the motor mechanical load. You don't need a resistor in series to limit the current under normal operating conditions (and don't want one since it will limit the motor power). You only need a resistor to limit the current during startup.

Jul 18, 2013
10,867
2,528
The current detect resistance is often in µΩ's much less than 1 ohm, dependent on the actual motor power.
It is only there to detect a minute volt drop, commiserate with current.
So it has virtually no effect on the actual performance of the motor.
Max.

9. ### jlatshaw Thread Starter New Member

Jul 20, 2013
28
0
Thank you all so very much for helping me out. But I still have one question:

I plan on using this motor for an ebike project, so I would like to have some control as to how much power I want the motor to put out at given times. I know that for different voltage/current levels will result in different power outputs. How can I go about controlling how much the motor outputs at a given time, or will the motor always try to maintain a constant rpm and thus increase its output power as the load increases?

Thank you all,
-James

10. ### tubeguy Well-Known Member

Nov 3, 2012
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This is an efficient method for controlling DC motor speed.

11. ### jlatshaw Thread Starter New Member

Jul 20, 2013
28
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Alright, I think a PMW looks like my best bet.

Thanks again everyone,
-James

12. ### jlatshaw Thread Starter New Member

Jul 20, 2013
28
0
I think a PWM is probably my best bet.

Thanks again everyone,
-James