EC-12.3 Energy Conversion, and DC and AC Machines

What this is and why it exists

Every mechanical thing a graduate meets is turned by one of a small number of machines. The same two physical laws explain all of them.

This topic keeps to that argument rather than to machine design. What makes torque, what makes voltage, and what sets the speed. Those three questions cover the direct current machine, the induction motor and the synchronous machine between them.

The synchronous machine is included because everything generating on the grid is one. A reader who has never met it cannot read a power system diagram.

The vocabulary

• Torque — the turning effect produced by a force at a radius.
• Back voltage — the voltage a rotating machine generates, opposing the applied voltage.
• Commutator — a rotating switch reversing the winding connection twice per turn.
• Field — the magnetic field the rotor moves in or produces.
• Armature — the winding in which the working voltage is induced.
• Rotating field — a magnetic field that turns, produced by three phased windings.
• Synchronous speed — the speed at which the rotating field turns.
• Slip — the fractional difference between rotor speed and synchronous speed.

The mental model

Two laws, and both act in every machine at the same time.

A current in a magnetic field feels a force. A conductor moving in a magnetic field develops a voltage.

Because both act at once, the same machine can motor or generate. Which one it is doing depends only on whether energy is going in or coming out. A motor slowing a load down is generating, and a generator being driven too hard is motoring. Holding that as one idea rather than two saves a great deal of confusion.

The direct current machine came first historically and is the easiest place to see it. A winding rotates in a fixed field. Left alone, the voltage it induces would alternate, because the winding reverses its orientation twice per turn.

The commutator is a rotating switch that reverses the connection at exactly those moments, so the external terminals see direct current. That is all it is, and it explains both the machine's dominance and its weakness. Brushes rub, wear and spark, so these machines have been displaced wherever electronics can do the job.

The control properties are what made them dominant. Torque follows the armature current. Speed follows the applied voltage, once the back voltage is accounted for. The field strength scales both, so weakening the field raises the speed.

Those two controls are nearly independent, which is exactly what a variable speed application wants. No alternating machine could offer it until power electronics arrived.

Now the rotating field, which is the central idea of the rest of the topic. Three windings spaced around a stator, fed by three phases, produce a resultant field that rotates rather than pulsating. Its speed is the supply frequency divided by the number of pole pairs.

The induction motor exploits that with a rotor that has no electrical connection at all. The rotating field sweeps past the rotor conductors, induces currents in them, and those currents feel a force in the same field. The rotor is dragged along behind.

It must lag behind. At exactly the field's speed the rotor would see no change, induce no current, and produce no torque. It always runs slower, and that fractional difference is the slip.

The torque against speed curve follows from this and explains everything practical about the machine. At standstill the slip is one, the current is very high and the torque is only moderate. That is why starting a large motor is a design problem. Torque rises to a peak at a modest slip and falls again as the motor approaches synchronous speed. The machine settles where its torque equals the load's.

The synchronous machine completes the picture. Its rotor carries its own field, supplied separately, and that field locks to the rotating stator field. It therefore turns at exactly synchronous speed under any load it can handle, and stops abruptly if overloaded.

Nearly all generation is synchronous, and one further property matters. The excitation of the rotor field sets the reactive power the machine supplies or absorbs, independently of the real power it produces. That is how power factor is managed at grid scale, and it connects this topic directly back to the first one.

What you should now be able to explain or do

• State the two laws and explain why a machine can motor and generate with no change.
• Say what a commutator does and why it is both the machine's strength and its weakness.
• Describe how speed and torque are controlled in a direct current machine.
• Explain how three phases produce a rotating field and what sets its speed.
• Explain why an induction motor must slip, and read a torque against speed curve.
• Say what a synchronous machine does that an induction machine cannot.

Check yourself