Transformers, Motors and Generators

All three devices work by electromagnetic induction, and all three are asked as questions about which direction the conversion runs.

Transformers

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A transformer is two coils on a shared iron core. Alternating current in the primary creates a changing magnetic field, the core carries that field to the secondary, and the changing field induces a voltage there.

There is no electrical connection between the two coils. The energy crosses as a magnetic field, which is also why a transformer can isolate one circuit from another.

The turns ratio

The voltage ratio equals the turns ratio.

• More turns on the secondary: step-up. Voltage rises.
• Fewer turns on the secondary: step-down. Voltage falls.

A primary of 100 turns at 120 volts feeds a secondary of 500 turns. What is the output voltage?

The secondary has five times the turns, so the voltage is five times: 600 volts.

A primary of 400 turns at 240 volts feeds a secondary of 100 turns.

One quarter the turns, so one quarter the voltage: 60 volts.

Set it up as a proportion if the numbers are awkward: primary volts over primary turns equals secondary volts over secondary turns.

Current goes the other way

A transformer does not create power. Ignoring losses, the power out equals the power in.

Since power is voltage times current, stepping the voltage up steps the current down by the same factor, and the other way round.

The 600-volt output above: if the primary drew 10 amperes, what does the secondary deliver?

Voltage went up five times, so current goes down five times: 2 amperes.

Check: 120 times 10 is 1,200 watts in; 600 times 2 is 1,200 watts out. Equal.

That check is the reliable way to answer transformer current questions, and it also rules out the answer that the current rises with the voltage, which is the distractor.

AC only

A transformer needs a changing magnetic field, so it works on AC and does nothing useful with steady DC.

This is the reason the grid is AC, as the AC and DC lesson argues: high voltage for transmission means low current, and low current means small losses, and only AC can have its voltage changed simply.

Where a transformer loses energy

A real transformer is very efficient but not perfect, and the losses have names:

The core is laminated - built from thin insulated sheets rather than solid iron - to reduce eddy current losses. The insulation between the sheets breaks up the circulating currents. Motors use laminated cores for the same reason.

Even with nothing connected, a transformer draws a small magnetizing current to set up the flux in its core; what limits that current is the inductive reactance of the primary winding.

Special transformers

• Isolation transformer: a 1 to 1 turns ratio, so the voltage is unchanged. Its purpose is safety: it separates the equipment from the supply, because the two coils are not electrically connected.
• Autotransformer: a single winding with a tap partway along. It is smaller and cheaper, but its main drawback is that it provides no isolation.

An ideal transformer's volt-amperes in equal its volt-amperes out, which is the same power-is-conserved rule written in the unit transformers are rated in.

Motors and generators

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A motor converts electrical energy into mechanical energy. A current-carrying loop in a magnetic field experiences a force, so it turns.

A generator converts mechanical energy into electrical energy. Turning a loop in a magnetic field induces a current.

They are the same machine run in opposite directions, which is the sentence to carry. Read the question for which quantity is supplied and which is produced.

The parts

The commutator is the part that distinguishes a DC machine. Without it the loop would turn half a revolution and stop, because the force would reverse. The commutator flips the current at the right moment so the turning continues in one direction.

Commutator for DC, slip rings for AC is the distinction asked.

Brushes wear down from rubbing on the commutator and need replacing; worn brushes show as excessive arcing and reduced performance. A brushless motor has nothing to wear out or arc.

Counter electromotive force

A spinning motor is also a generator. As it turns, it generates a voltage that opposes the supply, called counter electromotive force (counter EMF). That one idea explains three questions:

• A motor draws a large current the instant it starts, because it is not yet turning, so there is no counter EMF to oppose the supply. That inrush current is why large motors use starters or reduced-voltage starting, and why a vehicle's starter motor is engaged only briefly.
• The current falls as it comes up to speed, because the counter EMF builds.
• A mechanically overloaded motor draws more current and overheats, because it slows, and the counter EMF drops. A thermal overload device disconnects it before the heat damages the windings.

The same effect runs the other way in a generator: a generator gets harder to turn when a load is connected, because the current it delivers creates a magnetic force opposing the rotation.

AC induction motors

The most common motor in industry has no electrical connection to its rotor at all. In an induction motor, the stator's rotating magnetic field induces the current in the rotor, usually a squirrel-cage rotor of conducting bars shorted together at both ends.

Synchronous speed is the speed of the rotating field, set by the supply frequency and the number of poles:

Synchronous speed in rpm = 120 x frequency / number of poles

On 60 hertz: 2 poles, 3,600 rpm; 4 poles, 1,800; 6 poles, 1,200; 8 poles, 900. More poles, slower speed.

The rotor always turns a little slower than the field, because if it kept pace there would be no relative motion and nothing would be induced. The difference is slip, usually quoted as a percentage of synchronous speed:

Synchronous speed 1,800 rpm, actual speed 1,746 rpm. The slip is 54 rpm, and 54 / 1,800 = 3 percent.

A single-phase induction motor needs a start winding or a capacitor, because a single phase alone cannot create a rotating field; a three-phase motor starts on its own. Swapping any two of a three-phase motor's supply leads reverses it, and running one on only two phases - single phasing - overheats it.

Other conversions

An alternator is a generator producing AC, which is what a vehicle has.

A solenoid is a coil producing straight-line motion rather than rotation, used for starter motors and valves.

A relay is a switch operated by an electromagnet, letting a small current control a large one.

A stepper motor moves in precise, fixed increments rather than spinning freely, which is why it positions printer heads and machine tools.

A magneto is a self-contained generator using permanent magnets - the ignition source in small engines and piston aircraft.

Magnetic materials and circuits

The strength of a coil's magnetizing force is measured in ampere-turns: turns times current. A 200-turn coil carrying 2 amperes gives 400 ampere-turns. The same ampere-turns from many turns at a low current generates less heat than few turns at a high current, because heating rises with current squared.

Soft iron magnetizes and demagnetizes easily, which is exactly what an electromagnet or a transformer core needs: it must switch on and off with the current. Hard steel and alnico hold their magnetism, which is what a permanent magnet needs.

Magnetism has its own vocabulary, and the questions ask it directly:

A magnetic shield is made of a high-permeability material, which gives the flux an easier path around whatever it protects.

What you can skip

Across all 1,358 Electronics Information questions in our bank:

• Winding design and turns calculations beyond the ratio. Every turns question is the proportion shown above.
• Motor torque curves. One question asks how torque changes, as slip rises under load, at the hardest difficulty.
• Bearing maintenance. Grease and bearings come up once, at the hard end.
• Universal motors, servo motors and capacitor-start variants are not named. Knowing that a single-phase motor needs help to start is enough.

Where people lose points

Saying a transformer works on DC.

Saying a transformer changes frequency. It does not.

Saying stepping up voltage steps up current. Power is conserved, so current falls.

Swapping motor and generator.

Giving the commutator to an AC machine. It is the DC part; slip rings are AC.

Saying a transformer can rectify. That needs diodes.

Work one in under a minute

A transformer steps 240 volts down to 12 volts. The primary has 800 turns. How many turns does the secondary have, and if the secondary delivers 5 amperes, what current does the primary draw?

Voltage ratio: 12 over 240 is 1 to 20, so the secondary has 40 turns.

Current goes the other way: the secondary's 5 amperes at one twentieth the voltage means the primary draws one twentieth of 5, which is 0.25 amperes.

Check the power: 240 times 0.25 is 60 watts; 12 times 5 is 60 watts. Equal.

Where this leads

The rectifier that a transformer is usually paired with is the diodes lesson, and the grid argument this completes is the AC and DC lesson.