This notes sheet looks at electric generators and motors, general electric machine properties, motoring graphs and nominal voltages.
Generators & Motors
- A torque is applied to the coil, causing it to rotate with angular velocity
- The right-hand rule shows that a current is induced clockwise around the coil
- The emf can be calculated using Faraday’s law
- There is an anticlockwise current in the coil, as shown
- Fleming’s left hand rule shows that this induces a clockwise torque couple on the coil, due to the magnetic field
- The motor rotates clockwise
A split-ring commutator is required where the current and emf is applied to the coil in order to maintain continuous motion (without this, the motor would change direction every half-turn).
The torque varies hugely with the position of the coil, as the Lorentz force depends on angle (it is
maximum when perpendicular, zero when parallel). This is known as the ‘cogging effect’, and can
be minimised by adding more coils:
Despite the disadvantages, single coils are usually still used as they are cheaper and lighter.
Both generators and motors can be modelled as a Thevenised generic electric machine with
internal (armature) resistance:
- For a generator, the induced emf goes in the same direction as terminal voltage
- For a motor, the induced emf opposes the terminal voltage (it is modelled as a load)
We express Faraday’s Law (see notes sheet on Magnetic Fields & Electromagnetism) in terms of
the angular velocity and a machine constant:
- is the induced emf
- is the machine constant, in Vs/rad
- is the angular velocity, in rad/s
The power of the generator is given by the mechanical input power minus the power dissipated
in the armature resistance:
- is the mechanical power,
- is the power lost in the armature resistance
for a generator
Therefore, the mechanical torque is:
In a generator, the mechanical torque that is applied, T, is not the same as the mechanical torque
applied to the electrical circuit. This is because some torque – knows as shaft torque – is lost as
- is the torque applied to the circuit
- is the total mechanical torque input
- is the torque lost as friction
Meanwhile, the power for a motor is the terminal power minus the power lost in the armature
for a motor
The mechanical torque is:
In a motor, the mechanical torque output is slightly less than the electrical torque output. This is
again due to friction in the system:
- is the mechanical torque output
- is the torque output from the circuit (Ti in the equations above)
- is the frictional torque
Electric Motor Graphs
The relationship between torque and speed for a motor is:
See derivation for this relationship
- The maximum speed is the no load speed, when there is no torque output, only the frictional shaft torque is overcome.
- Stall is when the speed is zero. There output torque here is the stall torque.
- At stall and no load, the power output is zero.
Electric Machines will have a specified nominal voltage. This is the recommended terminal voltage
they are designed to operate at and does not have to be stuck to.
- Operating a motor above its nominal voltage gives greater performance, but reduced lifespan
- Operating a motor below its nominal voltage reduces performance but increases lifespan.
A motor can operate anywhere in the triangle beneath its torque-speed line. Often, you want to
limit where the motor operates. This can be done in three ways:
1. Voltage Regulation
As seen above, reducing or increasing the voltage reduces or increases the area in which the
motor can operate respectively.
2. Speed Regulation
This is when a fixed speed is set for a certain voltage range, as torque changes. However, once
the voltage passes the upper limit, the speed follows the torque-speed relation.
This is the opposite: a constant torque is set as speed varies over a voltage rage. Once the voltage
is out of the range, the torque will decrease as speed increases.
- The two basic electric machines are generators and motors:
- An emf is induced in a generator – this can be calculated using Faraday’s Law, and the direction is found with the right-hand rule
- Electric potential is converted to mechanical energy in a motor, and Fleming’s left-hand rule shows the direction of rotation.
- It is easiest to deal with electric machines when thevanised in series with an armature resistance and a terminal p.d.
- Faraday’s law in terms of angular velocity and machine constant:
- For a generator:
- For a motor:
- All electric machines are specified with a nominal voltage – this is the recommended terminal voltage, and is a balance between lifespan and performance.
- The operating point of the machine does not have to be at the nominal voltage, but can be regulated through voltage, speed or torque.