Motors, Generators, and Transformers

Lesson 3 of 3: Magnetism

In this lesson:

• Motor: current loop in B → torque → rotation
• Generator: rotation → changing flux → AC voltage
• Transformer: turns ratio

Learning Objectives for This Lesson

1. Explain how a motor converts electrical to mechanical energy
2. Explain how a generator converts mechanical to electrical energy
3. Use the turns ratio to calculate and
4. Apply energy conservation to ideal transformers:
5. Connect transformers to the sec-19-4 transmission problem

Three Devices, One Unit of Infrastructure

1. Mechanical energy (coal / nuclear / water / wind) → Generator → electricity
2. Step-up transformer → high-voltage transmission
3. Step-down transformers → 120 V at outlet
4. Motor (appliances, vehicles) → mechanical work

All three run simultaneously, billions of times per day.

The Electric Motor: Torque from Force

Force pair on opposite sides of loop → net torque → rotation

The Commutator: Maintaining Continuous Rotation

• Without a commutator: torque reverses when loop passes vertical → loop stops
• Commutator (in DC motors): mechanical switch that reverses current every half-turn
• Result: torque always acts in the same rotational direction → continuous rotation

Back-EMF: The Motor Acts as Generator

• A spinning motor coil has changing flux → induces EMF (Faraday's Law)
• This induced "back-EMF" opposes the applied voltage (Lenz's Law)
• More load → slower rotation → less back-EMF → more current

This is why motors draw large starting current (no back-EMF when ).

Worked Example: Force on Motor Loop Sides

Loop sides: A, m, T,

Both sides experience 0.15 N — in opposite directions → torque on the loop.

Quick Check: Explain Back-EMF Behavior

What does the commutator do in a DC motor?

In one sentence — name the problem it solves and how.

The Generator: Motor in Reverse

• A motor uses current → force → rotation
• A generator uses rotation → changing flux → induced EMF (Faraday's Law)
• Same physical device — different energy direction

Generator Output Is Sinusoidal AC Voltage

Flux: → EMF:

All Power Sources Use This Principle

Only solar PV is different — direct conversion (no generator).

Quick Check: Calculate Generator EMF Output

How would you increase a generator's peak EMF?

List two methods using .

The Transformer: Faraday's Law Applied

• Primary coil: AC current → continuously changing in iron core
• Iron core: channels changing B into the secondary coil
• Secondary coil: changing flux through it → Faraday's Law → induced EMF

No direct electrical connection between primary and secondary — energy transfers through the field.

The Turns Ratio and Voltage Transformation

• : step-up (voltage increases)
• : step-down (voltage decreases)

Power Conservation in an Ideal Transformer

• Step-up voltage → step-down current by the same ratio
• Transformers conserve power — they do not amplify energy

Transformers Only Work With AC

• AC primary: current changes → B changes → flux changes → EMF induced ✓
• DC primary: constant current → constant B → constant flux → zero EMF ✗

This is why AC won the "War of Currents" in the 1880s.

Worked Example: Step-Up Transformer Calculation

Given: kV, ,

This is a step-up transformer — 50× voltage increase.

Worked Example: Step-Down and Power Conservation

From the previous example: kV, kV, A

Power conservation:

Voltage ×50 → Current ÷50 → Power unchanged.

Quick Check: Find Secondary Voltage Here

A transformer has .

• Does voltage increase or decrease?
• By what factor?
• What happens to the current?

The Complete Electrical Power Grid

Generator → 15 kV → Step-up → 500 kV → Transmission → Step-down → 120 V home

Voltage Levels and Why They Matter

Worked Example: I²R Losses with Transmission

MW,

High voltage → 10,000× less loss.

Quick Check: Explain High-Voltage Transmission

Which stage of the electrical grid uses the highest voltage?

And why — what physics principle governs that choice?

Key Takeaways: Motors, Generators, Transformers

✓ Motor: current in B → force pair → torque → rotation; commutator sustains direction

✓ Generator: rotation → changing flux → (AC output)

✓ Transformer: ; (ideal); AC only

Watch Out: Avoid These Three Errors

Motors and generators are not different devices — same machine, different energy direction. EVs use the motor for regenerative braking.

Transformers conserve power — step-up voltage means step-down current. No energy amplified.

Transformers require AC — DC creates constant flux, zero induced EMF in secondary.

Unit 4 Complete — What's Next

sec-21-1: Quantum Nature of Light

• Maxwell's equations predict electromagnetic waves
• Photoelectric effect: light behaves as photons
• Einstein's explanation:
• The start of quantum physics