Category: electric motors

I have an active but paraplegic friend who is building an electric off-road scooter using DC motors. Those motors will have to reverse directions frequently while under load. Will they tolerate immediate reversals, or must there be a delay? — JO, Valley Springs, California

Modern brushless DC motors are amazing devices that can handle torque reversals instantly. In fact, they can even generate electricity during those reversals!

Instant reversals of direction, however, aren’t physically possible (because of inertia) and aren’t actually what your friend wants anyway. I’ll say more about the distinction between torque reversals and direction reversals in a minute.

In general, a motor has a spinning component called the rotor that is surrounded by a stationary component called the stator. The simplest brushless DC motor has a rotor that contains permanent magnets and a stator that consists of electromagnets. The magnetic poles on the stator and rotor can attract or repel one another, depending on whether they like or opposite poles—like poles repel; opposite poles attract.

Since the electronics powering the stator’s electromagnets can choose which of the stator’s poles are north and which are south, those electronics determine the forces acting on the rotor’s poles and therefore the direction of torque on the rotor. To twist the rotor forward, the electronics make sure that the stator’s poles are always acting to pull or push the rotor’s poles in the forward direction so that the rotor experiences forward torque. To twist the rotor backward, the electronics reverses all those forces.

Just because you reverse the direction of torque on the rotor doesn’t mean that the rotor will instantly reverse its direction of rotation. The rotor (along with the rider of the scooter) has inertia and it takes time for the rotor to slow to a stop and then pick up speed in the opposite direction. More specifically, a torque causes angular acceleration; it doesn’t cause angular velocity. During that reversal process, the rotor is turning in one direction while it is being twisted in the other direction. The rotor is slowing down and it is losing energy, so where is that energy going? It’s actually going into the electronics which can use this electricity to recharge the batteries. The “motor” is acting as a “generator” during the slowing half of the reversal!

That brushless DC motors are actually motor/generators makes them fabulous for electric vehicles of all types. They consume electric power while they are making a vehicle speed up, but they generate electric power while they are slowing a vehicle down. That’s the principle behind regenerative braking—the vehicle’s kinetic energy is used to recharge the batteries during braking.

With suitable electronics, your friend’s electric scooter can
take advantage of the elegant interplay between electric power and mechanical power that brushless DC motors make possible. Those motors can handle torque reversals easily and they can even save energy in the process. There are limits, however, to the suddenness of some of the processes because huge flows of energy necessitate large voltages and powers in the motor/generators and their electronics. The peak power and voltage ratings of all the devices come into play during the most abrupt and strenuous changes in the motion of the scooter. If your friend wants to be able to go from 0 to 60 or from 60 to 0 in the blink of eye, the motor/generators and their electronics will have to handle big voltages and powers.

What is the difference between a magnet and an electromagnet? Why are some metals automatically magnetic?

Some metals are composed of microscopic permanent magnets, all lumped together. Such metals include iron, nickel, and cobalt. This magnetism is often masked by the fact that the tiny magnets in these metals are randomly oriented and cancel one another on a large scale. But the magnetism is revealed whenever you put one of these magnetic metals in an external magnetic field. The tiny magnets inside these metals then line up with the external field and the metal develops large scale magnetism.

However, most metals don’t have any internal magnetic order at all and there is nothing to line up with an external field. Metals such as copper and aluminum have no magnetic order in them—they don’t have any tiny magnets present. The only way to make aluminum or copper magnetic is to run a current through it.

How does electric current create magnetic poles in metal? When the current goes through the metal, what makes it positive and negative?

An electric current is itself magnetic—it creates a structure in the space around it that exerts forces on any magnetic poles in that space. The magnetic field around a single straight wire forms loops around the wire—the current’s magnetic field would push a magnetic pole near it around in a circle about the wire. But if you wrap the wire up into a coil, the magnetic field takes on a more familiar shape. The current-carrying coil effectively develops a north pole at one end of the coil and a south pole at the other. Which end is north depends on the direction of current flow around the loop. If current flows around the loop in the direction of the fingers of your right hand, then your thumb points to the north pole that develops at one end of the coil.

In a three-phase induction motor, there is a rotating magnetic field in the stator, which induces a rotating magnetic field in the rotor. Those two magnetic fields will interact together to make the rotor turn. Is the interaction attractive or repulsive? — G

The magnetic interaction between the stator and the rotor is repulsive—the rotor is pushed around in a circle by the stator’s magnetic field; it is not pulled. To see why this is so, imagine unwrapping the curved motor so that instead of having a magnetic field that circles around a circular metal rotor you have a magnet (or magnetic field) that moves along a flat metal plate. As you move this magnet across the plate, it will induce electric currents in that plate and the plate will develop magnetic poles that are reversed from those of the moving magnet-the two will repel one another. That choice of pole orientation is the only one consistent with energy conservation and is recognized formally in “Lenz’s Law”. For reasons having to do with resistive energy loss and heating, the repulsive forces in front of and behind the moving magnet don’t cancel perfectly, leading to a magnetic drag force between the moving magnet and the stationary plate. This drag force tends to push the plate along with the moving magnet. In the induction motor, that same magnetic drag force tends to push the rotor around with the rotating magnetic field of the stator. In all of these cases, the forces involved are repulsive-pushes not pulls.

Why does a single phase 220 volt motor run off two legs of a three-phase circuit?

In three-phase power, the voltages of the three power wires fluctuate up and down cyclically so that they are “120 degrees” apart. By “120 degrees” apart, I mean that each wire reaches its peak voltage at a separate time—first the X wire, then the Y wire, and then the Z wire—with the Y wire reaching its peak 1/3 of the 360 degree cycle (or 120 degrees) after the X wire and the Z wire reaching its peak 1/3 of the 360 degree cycle (or 120 degrees) after the Y wire.

The specific voltages and their relationships with ground or a possible fourth “neutral” wire depend on the exact type of transformer arrangement that supplies your home or business. In the standard “Delta” arrangement (which you can find discussed at sites dealing with power distribution), the voltage differences between any pair of the three phases is typically 240 VAC. In the standard “Wye” arrangement, the typical voltage difference between any pair of phases is 208 VAC and the voltage difference between any single phase and ground is 120 VAC. And in the “Center-Tapped Grounded Delta” arrangement, the voltage difference between any pair of phases is 240 VAC and the voltage difference between a single phase and neutral is 120, 120, and 208 VAC respectively (yes, the three phases behave differently in this third arrangement).

If you run a single-phase 220 VAC motor from two wires of a Delta arrangement power outlet, that motor will receive a little more voltage (240 VAC) than it was designed for and if you run it from two wires of a Wye arrangement outlet, it will receive a little less voltage (208 VAC) than appropriate. Still, the motor will probably run adequately and it’s unlikely that you’ll ever notice the difference.

How does a fan motor work? — JM, Toronto, Ontario

A fan motor is an induction motor, with an aluminum rotor that spins inside a framework of stationary electromagnets. Aluminum is not a magnetic metal and it only becomes magnetic when an electric current flows through it. In the fan, currents are induced in the aluminum rotor by the action of the electromagnets. Each of these electromagnets carries an alternating current that it receives from the power line and its magnetic poles fluctuate back and forth as the direction of current through it fluctuates back and forth. These electromagnets are arranged and operated so that their magnetic poles seem to rotate around the aluminum rotor. These moving/changing magnetic poles induce currents in the aluminum rotor, making that rotor magnetic, and the rotor is dragged along with the rotating magnetic poles around it. After a few moments of starting, the spinning rotor almost keeps up with the rotating magnetic poles. The different speed settings of the fan correspond to different arrangements of the electromagnets, making the poles rotate around the aluminum rotor at different rates.

How does an electromagnetic doorbell work? — SH, Sault Ste. Marie, Ontario

When you press the button of an electromagnetic doorbell, you complete a circuit that includes a source of electric power (typically a low voltage transformer) and a hollow coil of wire. Once the circuit is complete, current begins to flow through it and the coil of wire becomes magnetic. Extending outward from one end of the coil of wire is an iron rod. When this the coil of wire—also called a solenoid—becomes magnetic, so does the iron rod. The iron rod becomes magnetic in such a way that it’s attracted toward and into the solenoid, and it accelerates toward the solenoid. The attractive force diminishes once the rod is all the way inside the solenoid, but the rod then has momentum and it keeps on going out the other side of the solenoid. It travels so far out of the solenoid that it strikes a bell on the far side—the doorbell! The rod rebounds from the bell and reverses is motion. It has traveled so far out the other side of the solenoid that it’s attracted back in the opposite direction. The rod overshoots the solenoid again and, in some doorbells, strikes a second bell having a somewhat different pitch from the first bell. After this back and forth motion, the rod usually settles down in the middle of the solenoid and doesn’t move again until you stop pushing the button. Once you release the button, the current in the circuit vanishes and the solenoid and the rod stop being magnetic. A weak spring then pulls the rod back to its original position at one end of the solenoid.

How do electric/magnetic linear drives work?

Linear electric motors are very much like rotary electric motors—they use the forces between magnetic poles to push one object relative to another. But while a rotary motor uses these forces to twist a rotor around in a circle, a linear motor uses these forces to push a carriage along a track. Both the carriage and the track must contain magnets and at least some of these magnets must be electromagnets that can be turned on and off, or reversed. By timing the operations of the electromagnets properly, the linear motor pushes or pulls the carriage along the track smoothly and continuously.

Is it possible to mechanically connect two motors of equal speeds and powers to provide twice as much power as a single motor? — EG, Torrance, CA

As long as they’re both AC induction motors, I don’t see any reason why not. While induction motors would turn synchronously with the power line if they had absolutely no load, they naturally lag slightly behind in normal situations. While a line synchronous AC motor would turn at 1800 or 3600 rpm, depending on how it’s wired, a typical induction motor turns at 1725 or 3450 rpm. The more you load an induction motor, the slower it turns and the more torque it exerts on that load. By coupling two induction motors together mechanically, you’ll make them turn at the same rate. Since the torque each motor exerts on the load depends on rotation speed, they’ll both contribute equally to the task and will together provide twice the power of a single motor.

I wouldn’t try this with any kind of motor that doesn’t have such a clear relationship between rotational speed and power output. If you join two mismatched motors with one another, one may end up doing all the work and the other motor might effectively become a generator rather than a motor!

If increasing the power demand on a generator that is turning at a steady rate simply increases the torque needed to keep that generator turning, why do brownouts occur?

As long as the generator continues to turn steadily, it will produce its normal voltage rise and the frequency of its alternating current won’t change. When the homes powered by the generator draw more current, then the generator simply becomes more difficult to turn and the steam turbine that spins it has to exert more torque on it. But suppose that the turbine can’t exert any more torque on the generator. In that case, the power company can either shut down the generator or it can reduce the strength of the generator’s rotating magnet. This rotating magnet is actually an electromagnet and its strength determines the voltage rise across the generator. During a period of excessive current demand, the power company may choose to weaken the rotating electromagnet to prevent the steam turbine from becoming overloaded. When they weaken the electromagnet, the generator becomes easier to spin but it produces less voltage. The electricity leaving the generator still has the right frequency alternating current, but it voltage is somewhat lower than normal and the light bulbs it powers glow relatively dimly—a brown-out.