A HANDBOOK

OF

INDUCTION
MOTORS

ERICK MUANDA
July 2024

By ERICK MUANDA TULUME

 INTRODUCTION
Motors are widely used these days for many purposes, they are
used almost everywhere even at home but especially in
industrial places. And they are used to drive: pumps, fans,
household appliances, machine tools, power tools, vehicles,
disk drivers, blowers, boilers... so knowing how they work is
vital especially for electricians.
There are many types of motors but in this handbook, we are
going to see induction motors only. How they work and how
to start them.
There are also many starting methods: electronics… but we
are going to see magnetic starters, how they work, their
advantages, disadvantages, applications and their circuits. For
control circuits we are going to use 24V DC circuits.
We aimed to write this present handbook to share knowledge
with everyone but especially electrical technicians and
engineers. We did our best to share accurate and upbuilding
information. we hope that you find this book helpful and
enjoyable.

By ERICK MUANDA TULUME

 INDUCTION MOTORS
1. INTRODUCTION
Induction motors known as asynchronous motors are 3 phase
AC motors. They are called induction because electric current
in the rotor that produces torque is obtained by
electromagnetic induction from the magnetic field of the stator
winding. They can be either wound type or squirrel-cage type.
They are asynchronous because the motor operates at less than
its synchronous speed.
The synchronous speed is the speed of rotation of the magnetic
field in the rotor. And is dependent on the frequency and
number of poles.

2. PRINCIPLE
They operate on the principle of electromagnetic induction,
where the rotor (rotating part) turns at a slightly slower speed
than the synchronous speed of the magnetic field created by
the stator (stationary part).

3. TYPES
Based on their construction we can categorized them into 2
different types:
- Squirrel-cage; and
- Wound rotor known also as slip ring.
Three-phase squirrel-cage induction motors are widely used as
industrial drives because they are self-starting, reliable, and
economical.

By ERICK MUANDA TULUME

Big differences are on their rotors but their stators are almost
the same with just few differences.

3.1. SQUIRREL-CAGE
The squirrel-cage is a cylindrical shaped cage that fits around
the shaft with bars extending between its two ends. At either
end of the squirrel-cage, end rings are attached to create a
short-circuit that induced current will flow trough.

Squirrel-cage rotor (Wikipedia)

3.2. WOUND ROTOR (SLIP RING)

A wound rotor known as slip ring consist of rotor windings,
slip rings, and brushes, allowing variable speed and torque
control.

By ERICK MUANDA TULUME

 wound rotor (slip ring)

4. STARTING METHODS
There are 5 common starting methods:
- Direct on-line (D.O.L.) Starter;
- Stator resistance starter;
- Rotor resistance starter;
- Autotransformer starter; and
- Star-delta starter.
4.1. DIRECT-ON-LINE (D.O.L.) STARTER
In this method the induction motor is started by connecting it
directly to three-phase supply.

4.1.1. ADVANTAGES
- Simple and most economical starter;
- More comfortable to design, operate, and control;
- Provides nearly full starting torque at start.
4.1.2. DISADVANTAGE
High starting current about 4 to 7 times of the rated
current and at low power factor

By ERICK MUANDA TULUME

 4.1.3. APPLICATIONS
This method is suitable for small motors (up to 10 kW)
Commonly used for:
- Small water pumps;
- Conveyor belts;
- Fans; and
- Compressors.
4.1.4. POWER CIRCUIT

power circuit

By ERICK MUANDA TULUME

 4.1.5. CONTROL CIRCUIT

Control circuit

4.2. STATOR RESISTANCE STARTER

In this method external resistance is connected in series with
each phase of the stator winding during starting. To drop the
voltage across the motor terminals, consequently the starting
current is reduced too. The resistances are gradually cut out
and completely out and full line voltage is applied across the
motor terminals.

4.2.1. ADVANTAGE
Suitable for use in speed control application

By ERICK MUANDA TULUME

 4.2.2. DISADVANTAGES
- The reduced voltage reduces the starting torque and hence
increases acceleration time;
- A lot power is wasted in the starting resistances;
- Expensive.
4.2.3. APPLICATIONS
- Conveyor belts;
- Crushers; and
- Hoists.
4.2.4. POWER CIRCUIT

By ERICK MUANDA TULUME

 4.2.5. CONTROL CIRCUIT

Control circuit

4.3.ROTOR RESISTANCE STARTER

In this method the resistance is connected in the rotor circuit
trough slip-rings. The full voltage is applied to the stator
windings. During starting the maximum resistance is in series
with each phase of the rotor circuit.
As the motor accelerates, the external resistance is gradually
removed from the rotor circuit. and the whole external
resistance is removed from the circuit as the motor attains
rated speed.

By ERICK MUANDA TULUME

This starting method in this case is the only method used for
wound rotor (slip-rings) induction motors.

4.3.1. ADVANTAGES
- Starting current reduced, at the same time starting torque
is increased due to external rotor resistance;
- Smooth acceleration and precise speed control.
4.3.2. DISADVANTAGE
- Lower efficiency due to energy losses; and
- Higher maintenance requirements.
4.3.3. APPLICATIONS
Requiring high-starting torque such as:
- Loaded belt conveyors in the mining industry; and
- Boiler.

By ERICK MUANDA TULUME

4.3.4. POWER CIRCUIT

Power circuit(www.electricaltechnology.org)

By ERICK MUANDA TULUME

 4.3.5. CONTROL CIRCUIT

Control circuit

4.4.AUTOTRANSFORMER STARTER
For this method an autotransformer is used to reduce the
stating voltage of the motor. At the start the autotransformer
reduces the applied voltage (around 60 to 80%) of the line
voltage, consequently, the starting current is limited to safe
value.
The autotransformer is removed from the circuit and full line
voltage is applied across the motor terminals.

4.4.1. ADVANTAGES
- Low power loss;
- Low current;
- Highest starting torque.

By ERICK MUANDA TULUME

 4.4.2. DISADVANTAGES
- Low power factor;
- Higher cost;
- Space limitations due to its larger size;
- Higher short-circuit fault level for the system.
4.4.3. APPLICATION
This method is used for large motors over 25 HP

4.4.4. POWER CIRCUIT

Power circuit (EEP)

By ERICK MUANDA TULUME

 4.4.5. CONTROL CIRCUIT

Control circuit

4.5.STAR-DELTA STARTER
In this method the motor started in star and run in delta. This
means the winding of the motor is designed for delta operation
and is connected in star just during starting. So, when the
motor attains sufficient speed, the connections are charged to
delta.

In star each phage gets 𝑉⁄√3 volts. V = phase line voltage.

Thus, the starting current is reduced and when the motor
attains around 80% of rated speed, the connections are
changed to delta as a result each phase gets full line voltage.

By ERICK MUANDA TULUME

 4.5.1. ADVANTGES
- Inexpensive;
- Less inrush current when starting
4.5.2. DISADVANTES
- Large reduction in starting torque due to reduced voltage
in star connection at the instant of starting.
- Possible break in supply
4.5.3. APPLICATIONS
This method is used only for squirrel-cage induction
motors of medium size up to 25HP for:
- Conveyor belts;
- Compressors; and
- Pumps

By ERICK MUANDA TULUME

4.5.4. POWER CIRCUIT

Power circuit(www.electricaltechnology.org)

By ERICK MUANDA TULUME

4.5.5. CONTROL CIRCUIT

Control circuit

By ERICK MUANDA TULUME

 CONCLUSION
We have seen how induction motos work, their types and we
have seen also the starting methods in details.
We have used Fluid SIM-P Software to create and run the
control circuits used in this handbook. And We made sure that
they are working perfectly.

We thank you all

For more information or questions please contact us on

Erickmuanda12@gmail.com

By ERICK MUANDA TULUME

 APPENDIX Ⅰ
ABBREVIATION AND SYMBOLS
M: Motor
3~: three-phase
AC: Alternating current
DC: direct current
KW: Kilowatt
MCB: Miniature circuit breaker
MCCB: molded-case circuit breaker
O/L: Thermal overload
KM (K): contactor (contactor or relay)
KT: Time Relay
V: voltage
HP: horsepower =745.7 watts
F: fuse.

By ERICK MUANDA TULUME

 APPENDIX 2
FORWARD/REVERSE
To reverse the direction of induction motors, we need to
interchange any two of the three supply phases. By doing so
the field rotates counterclockwise rather than clockwise. But
the number of poles and the speed at which the magnetic field
rotates remain unchanged.
To accomplish that we need to add a second contactor to the
supply lines as you can see on this picture.

Power and control circuit

By ERICK MUANDA TULUME

 1. A PRACTICAL EXAMPLE OF
FORWARD/REVERSE

Power circuit (www.electricaltechnology.org)

By ERICK MUANDA TULUME

 2. A PRACTICAL EXAMPLE OF WIRING
DIAGRAM

Wiring diagram (www.electricaltechnology.org)

By ERICK MUANDA TULUME