Motors and generators have similar structures but opposite functions. Organize and understand the relationship between them.
Basic Knowledge
Motor (electric motor): electrical energy → kinetic energy
Mechanism: Current flows through a coil in a magnetic field → a force acts according to Fleming’s left-hand rule → the coil rotates → the current direction is switched by the commutator for continuous rotation
Generator: kinetic energy → electrical energy
Mechanism: A coil is rotated in a magnetic field → current is produced by electromagnetic induction
The relationship between them:
・If the shaft of a motor is rotated from outside, it operates as a generator
・If current is supplied to a generator, it rotates as a motor
This is because their structures are essentially the same. Hybrid systems and regenerative braking in electric vehicles make use of both functions.
Motor (a machine that changes electricity into motion. Fleming’s left-hand rule)
Generator (a machine that changes motion into electricity. Electromagnetic induction)
Fleming’s right-hand rule (shows the direction of induced current. Generator side)
Energy conversion (changing one form of energy into another form)
Regenerative braking (a system in electric vehicles and other vehicles that switches the motor to a generator to slow down and charge the battery)
Conversion efficiency (the proportion of input energy converted into the intended form)
Further Exploration (Background and Significance)
Fleming’s right-hand rule applies to the generator side (induced current):
・Thumb: direction in which the coil moves
・Index finger: direction of the magnetic field
・Middle finger: direction of induced current
The left hand represents “electricity → magnetic field → force” (motor), while the right hand represents “motion → magnetic field → current” (generator).
At power plants, huge generators are directly connected to turbines and rotate coils 50 times per second (eastern Japan) or 60 times per second (western Japan) to produce alternating current.
There are many types of motors for different purposes, including DC motors, AC motors, stepping motors, and servo motors. Three-phase AC motors are the most common in electric vehicles.
Conversion efficiency reaches more than 90% for both motors and generators, making them far more efficient than internal-combustion engines (about 30%). This is one reason electric vehicles are considered energy-efficient.
- Motor: electrical energy → kinetic energy
- Generator: kinetic energy → electrical energy
- They are two sides of the same machine
- Fleming’s left hand = motor; right hand = generator
- Regenerative braking = charging by switching from motor to generator
- Motor and generator efficiency is more than 90%
- Power-plant generators produce AC (50/60Hz)
Points to Note (Easily Confused)
① Use the left hand for motors and the right hand for generators. Remember: “The right hand makes electricity.” ② In both laws, the three factors—“current, magnetic field, and force/motion”—are mutually perpendicular. ③ Back electromotive force (the generating action) also occurs inside a motor. As the motor rotates faster, the current flowing through it decreases. ④ The power source that turns a power-plant turbine (thermal, hydroelectric, nuclear, and so on) varies, but the generator at the final stage is the same.
Practice
- What energy conversions do motors and generators perform? Answer for each.
- Which law, the right-hand rule or the left-hand rule, shows the direction of induced current?
- Briefly explain regenerative braking.