Electric Motors
Electric motors convert electric energy into mechanical (rotational) energy.
The most common type of motor is a rotating machine that consists of a rotor (the part
that rotates) and a stator (fixed). The motor turns because of the interaction between
the magnetic fields created by the stator and the rotor.
Operational Principle of Electric Motor
1. The stator generates a stationary magnetic field
surrounding the rotor.
2. The rotor/armature is composed of a coil which generates
a magnetic field when electricity flows through it.
3. The brushes provide mechanical contact between the
rotor and the commutators and help switch polarity of rotor
windings.
4. Commutators reverse the current every half a cycle to
keep the motor from turning.
Electric Motor Family Tree
There are hundreds of types of electric motors.
Electric Motor Selection
Among the specifications that need to be considered when selecting a motor are:
▪ speed range (rpm),
▪ Torque (N.m),
▪ torque versus speed characteristics,
▪ (mechanical) output power (kW) (P = T . ω),
▪ type of power supply (AC or DC, voltage, current, phases),
▪ efficiency (%),
▪ armature inertia, inductance and resistance,
▪ physical characteristics (motor and shaft dimensions, weight, ventilation requirements, etc.),
▪ cost, etc.
Warm-up Exercise:
A DC motor is powered by a battery supplying 0.8 A of current and 12 V of voltage. The motor
operates at 3600 rpm and has an efficiency of 80%.
How much torque does the motor generate?
Second Exercise:
A permanent magnet DC motor is powered by a battery supplying 1 A of current and 12 V of
voltage. The motor generates toque of 0.1 N.m, experiences a loss of electric power through
the rotor coil of 1 W, and losses due to friction of 0.8 W.
What is the speed, rotor coil resistance, and efficiency of the motor?
Brushed DC Motor – the ‘classical’ electric motor
A brushed DC electric motor is an internally commutated
electric motor designed to be run from a direct current
power source and utilizing an electric brush for contact.
Brushed DC motors are constructed with wound rotors and either wound or
permanent-magnet stators.
Generally, the rotational speed of a DC motor is proportional to the electromotive
force in its coil (= the voltage applied to it minus voltage lost on its resistance), and
the torque is proportional to the current.
Speed control can be achieved by variable supply voltage, resistors, or
electronic controls.
Brushes can cause malfunction (e.g. sparks), cause inefficiencies, and have to be
replaced regularly. Brushed DC motors generally have efficiencies of 75-80%.
Brushless DC Motors
Here, an electronic controller replaces the brush commutator contacts. They are sometimes
called Electronically Commutated Motors.
An electronic sensor detects the angle of the rotor and controls semiconductor switches (e.g.
transistors) that switch current through the windings, reversing the direction of the current at the
correct angle, so the electromagnets create torque in one direction.
The elimination of the sliding contact allows brushless motors to have less friction, leading to
longer lifetimes and higher efficiencies. Their working life is limited only by the lifetime of their
bearings.
Additional advantages of brushless motors include:
• high torque-to-weight ratio,
• increased reliability,
• reduced noise,
• elimination of sparks from the commutator, and
• an overall reduction of electromagnetic interference.
A typical brushless motor has permanent magnets that rotate around a fixed armature,
eliminating problems associated with connecting current to the moving armature.
With no windings on the rotor, the windings of the stator can be supported by the housing,
and therefore, cooled by conduction. Thus, they don’t require airflow inside the motor for
cooling. The motor can be entirely enclosed and protected from dirt or other foreign matter.
These benefits come at the cost of less rugged, more complex, and more expensive
control electronics.
Exercise:
An electric motor is used to spin a disk with a mass of 200 g and a diameter of 20 cm.
Calculate the required torque for the motor to accelerate the disk from linearly 0 to 1800
rpm in 4 s.
Induction Motor
An induction motor or asynchronous motor is an AC electric motor in
which the electric current in the rotor that produces torque is obtained by
electromagnetic induction from the magnetic field of the stator winding.
An induction motor therefore needs no electrical connections to the rotor.
An induction motor's rotor can be either wound type or squirrel-cage type.
Electromagnetic or magnetic induction is the production of an electromotive force across an
electrical conductor in a changing magnetic field.
Alternating electric current flows
through the solenoid on the left,
producing a changing magnetic
field. This field causes, by
electromagnetic induction, an
electric current to flow in the wire
loop on the right.
A rotating magnetic field (RMF) is the resultant magnetic field produced by a system of
coils symmetrically placed and supplied with poly-phase currents.
A rotating magnetic field can be produced by a single-phase current provided that two
field windings are supplied and are so designed that the two resulting magnetic fields
generated thereby are out of phase.
Poly-phase current produces a magnetic field which rotates at an angular velocity
determined by the frequency of the alternating current.
In induction motors, the AC power supplied to the motor's stator creates a magnetic field that
rotates in synchronism with the AC oscillations.
The rotor rotates at a somewhat slower speed than the stator field. The difference in speed is
called slip and increases with load. This explains as well why these motors are called
asynchronous.
A squirrel-cage rotor is the rotating part of the common squirrel-cage induction motor.
It consists of a cylinder of steel laminations, with aluminum or copper conductors embedded in its
surface.
By adjusting the shape of the bars in the rotor, the
speed-torque characteristics of the motor can be
changed, for example, to minimize starting current or to
maximize low-speed torque.
Squirrel-cage induction motors are very prevalent in
industry, in sizes from below 1 kW up to tens of MW.
They are simple, rugged, and self-starting, and
maintain a reasonably constant speed from light load to
full load, set by the frequency of the power supply and
the number of poles of the stator winding.
A wound-rotor motor, also known as slip ring-rotor motor, is a type of induction motor where
the rotor windings are connected through slip rings to external resistance. Adjusting the
resistance allows control of the speed-torque characteristic of the motor.
Compared to a squirrel-cage rotor, the rotor of the slip ring motor has more winding turns; the
induced voltage is then higher, and the current lower, than for a squirrel-cage rotor.
The speed-torque characteristics of a wound-rotor motor can be adjusted by changing the
external resistance, unlike a squirrel cage motor, which has a fixed characteristic. This is useful
for speed control of the motor.
Induction Motors
Advantages
▪
▪
▪
▪
▪
▪
Simple and rugged
No brushes
No permanent magnets
No position sensors
No starting mechanism
Easy speed control
Disadvantages
• Induced currents in rotor cause losses
and heat
• Not the lightest and most compact
motor
Induction motors dominate industrial applications.
Induction motors were widely used in the early electric vehicles, but are being replaced more
recently by other types of motors.
Energetic Efficiency of Electric Vehicles
Total Efficiency:
~ 90%
Total Losses:
31 – 35%
www.fueleconomy.gov
Energetic Efficiency of Gasoline Vehicles
Total Efficiency:
~ 20%
www.fueleconomy.gov
Permanent Magnet Motor
Permanent magnet motors are more efficient than induction motor or motors with field windings
for certain high-efficiency and high-performance applications such as electric vehicles.
This type of motor is used in certain electric vehicles, including Chevrolet Bolt and Volt, and the
rear wheel drive of Tesla's Model 3. Recent dual motor Tesla models use a combination of a
permanent magnet motor at the back and traditional induction motor at the front.
Permanent Magnet Motor
Many different permanent magnetic materials are used to drive permanent magnetic motors
and vary based on multiple factors, principally necessary magnetic strength and cost.
The four primary permanent magnetic materials that are found in the vast majority of industrial
applications are neodymium iron boron (NdFeB), samarium cobalt (SmCo), aluminum nickel
cobalt (Alnico), and strontium carbonate-iron oxide (also known as “ceramic magnet”);
furthermore, significant materials science research is ongoing into the development of additional
non-rare earth permanent magnetic materials.
In general, permanent magnet motors are more efficient and lighter than induction motors.
Therefore, they are replacing induction motors in high-performance applications.
Permanent Magnet Motors
Advantages
Disadvantages
▪ Light and Small
▪ Silent
▪ Efficient (especially at lower speeds)
They need:
• Permanent magnets (costly,
environmental impact, can
demagnetize)
• Position sensor
• Starter mechanism
• Electronic Controller
Permanent Magnet Motor – Variable-frequency drive (VFD)
Permanent magnet AC motors are a type of synchronous motors: the rotation of the
shaft is synchronized with the frequency of the supply current; the rotation period is
exactly equal to an integer number of AC cycles.
Most permanent magnet synchronous motors require a
variable-frequency drive to start them. A variablefrequency drive is a type of AC motor drive (a system
incorporating a motor) that controls speed and torque by
varying the frequency of the input AC electricity. It
controls either the associated voltage or current variation.
Synchronous Reluctance Motor
A reluctance motor is similar to a permanent magnet synchronous motor, with the
difference that here no permanent magnets are used. Instead, the rotor is made of steel.
The steel rotor design aligns itself to the flux lines of the magnetic field generated by
windings on the stator.
Reluctance motors can deliver high power density
at low cost, making them attractive for many
applications. Disadvantages include high torque
ripple (the difference between maximum and
minimum torque during one revolution) when
operated at low speed, and noise due to torque
ripple.
Synchronous Reluctance Motors
Advantages
Disadvantages
▪ Torque comparable to permanent
magnet motors
▪ Efficient at higher speeds
▪ Cheap and clean to produce (no
permanent magnets)
• Lower efficiencies at lower speeds
• More noise
• Torque ripple
‘Special Case’ – Stepper Motor
A stepper motor, also known as step motor or stepping motor, is an electric motor that
rotates in a series of small and discrete angular steps.
Stepper motors can be set to any given step position without needing a position sensor
for feedback. Motors vary in size, speed, step resolution, and torque.
Stepper motors effectively have multiple "toothed" electromagnets arranged as a stator
around a central rotor, a gear-shaped piece of iron.
‘Special Case’ – Servomotor
A servomotor (or servo motor or simply servo) is a rotary or linear actuator that allows
for precise control of angular or linear position, velocity, and acceleration in a
mechanical system. It consists of a suitable motor coupled to a sensor for position
feedback, and a controller.
A servomotor is a closed-loop servomechanism that uses position feedback (either
linear or rotational position) to control its motion and final position. The input to its control
is a signal representing the desired position of the output shaft.
Exercise:
An electric motor for a garage door opener lifts a door weighing 50 kg through a distance of 1
m in 10 s while turning at 600 rpm.
Ignore all friction loses in the system.
What are the approximate power and torque requirements for this motor?
If the motor is 80% efficient and is supplied by 120 VAC, how much current will it draw?
Electric Motor Selection
Among the specifications that need to be considered when selecting a motor are:
▪ speed range (rpm),
▪ Torque (N.m),
▪ torque versus speed characteristics,
▪ (mechanical) output power (kW) (P = T . ω),
▪ type of power supply (AC or DC, voltage, current, phases),
▪ efficiency (%),
▪ armature inertia, inductance and resistance,
▪ physical characteristics (motor and shaft dimensions, weight, ventilation requirements, etc.),
▪ cost, etc.