The Not-So-Odd Couple

Like an Old Married Couple, a Motor and Its Generator Are Bound Together. One Does Not Exist Without the Other

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By Ernst Dummermuth

Some important properties and characteristics of electric motors and electric generators can be demonstrated easily using an actual motor and generator. Grade or middle school students could conduct these experiments. Through my own experience, I’ve observed that these experiments, executed on a laboratory table, can be very useful as a teaching tool.

Basically Speaking

Motors come in all shapes and sizes and most often are used to convert electrical energy into mechanical energy via the rotation of a shaft. Since the primary electric source in a home is single-phase, 110 Vac, the motors that drive a washing machine, vacuum cleaner, all other gadgets and tools are designed for that supply. For simplicity and reduced control circuits, many of these household motors turn only in one direction.
In your automobile, there are many motors of different power levels that run on 12 Vdc. The starter is by far the strongest electric motor in a conventional automobile.

Industrial applications often require much higher mechanical outputs than the typical appliance or car electric motors and thus use motors that are much larger in size and are designed for higher DC voltages or multi-phase AC voltages. In addition, these motors also have a much higher current rating.

Generators usually are not found in a household. Some houses have an emergency generator, in case of loss of electric power.

Every automobile has a generator to provide electric energy for ignition, various fans, accessories and lights, as well as to recharge the battery.

I used to ride a bicycle with a small generator attached to the frame. At night, this human-powered generator could be leaned against a wheel to produce electric energy for the headlight and the taillight.

On the whole, most generators are big and used to supply the power grid with electric energy. It takes a lot of mechanical power to crank the shafts of these generators. Mechanical power is obtained in steam power plants from coal, gas and nuclear sources, with diesel engines or hydro turbines, or more recently with wind turbines erected on hilltops, often in groups as windmill farms.

Basic Motor Designs

Motors are based on the principle that opposite polarity magnets attract each other, while magnets of the same polarity repel each other. One magnet is located on the stator and one magnet is on the rotor. If those magnets are not lined up, the rotor will turn until they do. Once the magnets are lined up, rotation stops.

One of these magnets could be a permanent magnet—a rare-earth magnet—or a quasi-permanent magnet—an electromagnet with coil windings driven by a DC current. The other magnet is an electro-magnet, the orientation of which is modified continuously. As a result the rotor keeps trying to line itself up. If this magnet modification is done in an intelligent way, such as in a rotation to the right, then the rotor will rotate to the right.

DC Motors

To sustain rotation, and since the permanent magnet cannot be reoriented, the electromagnets must be activated or deactivated. In a typical DC motor (Figure 1), the stator has the fixed, permanent magnet and the rotor is composed of a number of windings.

Figure 1

Basically one or two windings are activated at a time via commutation, depending on the number of coils on the rotor. At start-up, referring back to Figure 1, the green winding is activated. With the polarity shown, the rotor will turn clockwise, trying to align the green magnet with the stator magnet. As the rotor turns, the blue coil becomes activated, while the green coil is deactivated, and so on as shown after first commutation. The rotor field is reoriented in discrete steps, according to the number of rotor windings, while the rotor turns. Details about pole shaping and arrangement of air gaps are not shown. The rotor could have many more individual windings, and for that reason, two or three adjacent windings could make contact through commutation at any one time.

In another form called a brushless DC motor or an inside-out DC motor (Figure 2), the permanent magnet is on the rotor. This magnet again can be a rare-earth magnet or a fixed electromagnet connected via slip rings. External solid-state switches turn on the proper stator winding as a function of the rotor orientation. The commutation that took place on the rotor in the previous example takes place in the stator windings.

Figure 2

When, the red stator coil in Figure 2 is turned on, the rotor moves clockwise trying to align the opposite poles. At that time commutation moves to the blue coil and then to the green coil and so on. At nominal speeds, the excitation moves from one phase to the other, turning on and off the different stator coils. At standstill and low speeds, the active stator phase is not a pure DC signal but a pulsed DC signal to avoid saturation. For higher power systems an almost perfect three-phase excitation is created through solid-state switches.

Synchronous Motors

A synchronous motor is basically the AC equivalent to the brushless DC motor just discussed. In these motors, the rotor turns at the same speed as the outside field. In general, the outside field for a synchronous motor is a three-phase, sinusoidal excitation obtained from the three-phase, 60 Hz line. When no mechanical load is connected to the motor, the rotor is lined up almost exactly with the stator field (Figure 3). As the mechanical load increases, the rotor lags the stator field by several degrees but is still in step with the field. If the load is increased beyond the rated value, the rotor will fall out of synch and come to a halt.

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