View Full Version : DC Motor - voltage or current

Zap-o

09-01-2011, 03:28 AM

I have been thinking about this for a while and it seems the more I consider it the more it becomes paradoxical. will a DC motor run faster with an increase in voltage or current?

I am thinking that the voltage is what makes the motor run faster only because a motors coil has a resistance and its that resistance in a series circuit that results in a constant current. So this leaves me to lean towards a change in voltage will produce a change in speed (like stepper motors).

Whats your take?

You can look at dc motor curves at the Mabuchi web site: http://www.mabuchi-motor.co.jp/en_US/product/p_0304.html

In general, if you double the voltage to a dc motor, the current will also double, and the power input goes up 4X. Depending on where you are at the efficiency curve, output power may not quadruple. For any given voltage, peak power is typically obtained at half the no-load speed times half the stall torque.

Phil Pilgrim (PhiPi)

09-01-2011, 04:10 AM

If it's a matter of which to regulate, always choose current over voltage for inductive loads like motors. By regulating current, you have the advantage of providing a voltage boost at turn-on to get the desired amount of current flowing more quickly. This, in turn, yields positive performance benefits.

-Phil

JeremyJ

09-01-2011, 09:38 AM

A good book to read to understand the basics of electric motors, particularly brushed DC motors, is "Electric Motors and Drives: Fundamentals, Types and Applications" by Austin Hughes.

Like Erco mentions, if you double the voltage of the motor the current will also double, since the motor is nothing more than a resistor (V = E + I * R, where E is the motor's induced EMF and zero) at rest. Power will go up by a factor of 4 also, since it is equal to V * I. The back EMF of the motor, which is generated as it begins to spin, will increase as the current in the windings decreases until the motor reaches an equilibrium point, which would be its no-load speed.

The big advantage of raising the voltage of your motor is to get similar power at a lower current. Since heat generation in the windings is equal to I^2 * R, you want to keep the current as low as possible. Your motor is ultimately limited by its maximum operating temperature (better said this is what should limit it, if it is well-designed). The voltage may also be a factor, as you cannot exceed the insulation resistance of your windings.

prof_braino

09-01-2011, 08:26 PM

Your motor is ultimately limited by its maximum operating temperature (better said this is what should limit it

The voltage may also be a factor, as you cannot exceed the insulation resistance of your windings.

Motors always seems like the simplest electronic component, but inductors always seem like black magic.

I never realized this to be the core issue, maybe this is why.

Thanks for these explanations.

Zap-o

09-02-2011, 03:15 PM

Wow,

This is fascinating -

JeremyJ you state that [V = E + I * R] I take it E is extremely minimal in the equation? Almost meaningless perhaps?

Could some one elaborate on what was said in the post by Jeremy.

The back EMF of the motor, which is generated as it begins to spin, will increase as the current in the windings decreases until the motor reaches an equilibrium point, which would be its no-load speed.

I thought that no-load was when a motor was running without anything attached to it -no load? Perhaps I have been misinformed?

JeremyJ

09-02-2011, 08:48 PM

Zap-o,

Yes, the no-load speed is the natural speed at which the motor will run with nothing attached to it. The back EMF helps us understand this speed conceptually.

I looked through a few of my motor texts - most, even if they deal with inductor or BLDC motors - start off by explaining the concept of a "B I L" force that a brushed DC motor uses to operate. I was looking for a great explanation, but I didn't really find one. I'll try to explain it to the point where my knowledge reaches a cliff.

- understanding back EMF

Imagine a couple of parallel plates with a magnetic field between them. Now imagine an infinite, current-carrying wire between these plates. This wire will "feel" a force proportional to its length, the magnetic field and current. It may help to remember that amperes are defined in terms of force. This is the force that causes the motor to spin. Well, according to Faraday's law of induction, an EMF is generated within a conductor moving in a magnetic field - this EMF will oppose the movement of the conductor (why is this? in more fundamental terms? that I cannot explain!). So combine these two ideas. When your motor starts, a portion of the current goes to the stator to energize the magnetic field (unless it has permanent magnets) and the rest goes to the rotor windings. These windings feel a force and begin to turn the motor. As the angular velocity of the motor increases, the induced EMF (opposing the voltage source), grows. If you neglect the frictional (and resistance) losses in the motor, the motor will accelerate until the back EMF completely cancels the source voltage and current ceases to flow - spinning perpetually! In a real motor, a small current will flow to overcome the frictional forces on the motor. In this sense the back EMF represents the real work done by the motor - the resistive losses are converted to (undesireable) heat.

- Stepper Motors

Your original question was directed towards stepper motors, I think. These motors do not actually exploit the "B I L" force mentioned above - they work by varying the reluctance of the motor, sequentially energizing different phases (I understand this conceptually as the magnetic attraction between poles). Though different, the speed of a stepper motor is going to be limited just the same. I'm not very familiar with steppers, but I'd say that you are ultimately going to encounter two problems. As you increase the current, you will reach a point where you can no longer generate more torque as the stator windings become saturated. Another potential problem is switching. As the motor accelerates you must energize and de-energize the coils in the stepper very quickly...you will likely reach a speed where much of your energy is wasted fighting the inductance of the coil, precluding you from generating more torque. Again, not really familiar with steppers, but I guess that with most you'd probably burn the windings out before you reach this speed (or maintain it for more than a few seconds!). Great project for the destructive hobbyist! haha

I'm personally working to design a switched reluctance motor. On my best days, I marginally understand this stuff!

Zap-o

09-03-2011, 05:47 AM

Fabulous way to put it. Thank you.