Friday, May 4, 2012

The Least You Should Know - DC Motors 1

I didn't really explain why a motor spins in the piece on AC motors.  Remember how magnets have north and south poles, and while opposites attract, two norths or two souths will repel each other?  In an AC motor, the rotor has magnets on the shaft, and the magnetic field from the current running in the coils of wire on the stator create an opposing force that pushes on the rotor.  Picture a straight wire running in front of you.  Connect it to a battery on both ends so that current flows.  If you were to take your right hand and grab the wire, with your thumb pointing in the direction of current flow, your fingers would point in the direction of the magnetic field caused by that current - and, yes, the field wraps around the wire in closed lines.  This is one of several "right hand rules" that student engineers and technicians have to remember.
With the AC from the wall outlet connected to it, the direction of that current, and the fields, would change every time the current changes direction, every half cycle (50 Hz in much of the world, 60 Hz in the US).  By clever arrangement of the magnets and coils, the motor can be designed so that there's always a force moving the motor regardless of the polarity of the magnetic field at that instant.  It never seizes up.

Did you ever have a slot car?  Remember how they had a hand controller that let you change the speed of the car?  These are probably the most familiar examples of a DC motor, and illustrates one big advantage, that the speed of rotation can be controlled by changing the voltage.  The controller was just a variable resistor. Because the resistor varies the current through the windings, and the strength of the magnetism depends on the current, that changes the speed of rotation.

In a conventional DC motor, the wires are on the rotor, not the stator, and there's an asymmetrical arrangement of coils and magnets. In a small motor like you find in a slot car, there are two magnets and three coils so that they can't line up and lock up (this figure only shows one loop of wire in one coil).  Since the coils are rotating, the contacts go through conductive brushes (braided wire or formed carbon) and rings on the rotor shaft connected to the coil wires. (source)

DC motors come in a wide range of powers, with many more magnets and coils, and in other sorts of configurations.  A very common variation today is the brushless DC motor.  The brushless design is wired much like the AC motor, with the rotating shaft holding the magnets and the coils along the stator.  The trick in these motors is that they have an internal electronic circuit that converts the applied DC to AC to drive the coils.  The electronics adds some cost, but getting rid of brushes that make the DC electrical contact produces a more reliable motor.  High reliability motors like floppy disk drives are often brushless DC motors. The added electronics are pretty cheap for those guys.

DC motors usually have excellent torque vs. speed curves, with the best torque at low speed.  Internal combustion engines tend to have low torque when barely turning, and reach their optimum at thousands of RPM.  I've seen small 4 cylinders (Toyotas and Hondas are notorious for this) that don't reach their maximum torque until they've been rev'ed up to over 4 or 5000 RPM; better towing vehicles have their best torque at lower RPM.  An electric motor frequently has its most torque at stall speed (that is when it's at 0 RPM).  It's something that makes DC motors attractive for cars.  And although I haven't talked about generators at all, a generator and a motor are virtually the same arrangement of parts looked at from different ways.  What this means in practice to the electric car is that with some switching, when you no longer want to drive power into the motors (that is, when you're stopping), you can get voltage out them and dump it back into the batteries.  This is called regenerative braking

By sales volume, it could be that the most common motor today is the stepper motor, the topic for next time.

1. SG,

Not picking nits, but you say that the slot car controller changes rotation speed by changing the voltage, but then speak of a variable resistor doing so by changing the current. I admit to being pretty ignorant (resistive to learning, is more like it ;-) about electricity and electronics, but is it current or voltage? Does the voltage remain the same while restricting the current flow, or does it change too (at least in this instance)?

1. That's an excellent question, Reg. I tend to get sloppy with that terminology, and this is not the place to do that!

The hand controller I'm referring to was a variable resistor that acted to reduce the voltage at the motor. Because of the reduced voltage at the motor's pins, it's coil's resistance consumed less current. It is the current, not the voltage, that causes the magnetic field strength. So by reducing the voltage to a fixed coil resistance, you reduce the current.

As an aside, the modern slot car controllers have gotten more complicated, so this might be a bad analogy, though.

When I was 6th or 7th grade, slot cars were really popular, and saving coins from mowing lawns or rounding up bottles for the deposit bought a half or full hour of racing on the big track in the local shopping center. Can't say I've had one since then.

2. DC motors can have a lot of torque, as you mentioned. I've seen traction motors with 6" diameter shafts twisted off when they ran into something that kept them from turning.

1. Yow. These are the motors used to drive trains, right? They must be outrageously powerful.