Hysteresis Motor
A Report on Construction, Principle of Operation,
Characteristics, and Applications
Prepared for:
Special Electrical Machines
Prepared by:
Ahmed Abdelkareem Elsayed Abdelmageed
Under supervision:
Dr. Hussein M. Waly
Department:
Electrical Power and Control Engineering
Code:
20210435
Institute:
The Higher Institute of Engineering and Technology
Contents
Abstract ........................................................................................................................................... 4
1. Introduction ................................................................................................................................ 4
2. Definition of Hysteresis Motor .................................................................................................... 5
3. Construction of Hysteresis Motor ............................................................................................... 5
3.1 Stator ..................................................................................................................................... 5
3.2 Rotor...................................................................................................................................... 5
4. Principle of Operation ................................................................................................................. 6
4.1 Rotating Magnetic Field ........................................................................................................ 6
4.2 Rotor Magnetization Lag ....................................................................................................... 7
4.3 Starting and Acceleration ...................................................................................................... 7
4.4 Synchronous Running ........................................................................................................... 7
5. Torque Production in Hysteresis Motor ...................................................................................... 7
5.1 Hysteresis Torque .................................................................................................................. 7
5.2 Eddy Current Torque ............................................................................................................. 8
6. Speed of Hysteresis Motor .......................................................................................................... 8
7. Characteristics of Hysteresis Motor ............................................................................................ 9
7.1 Self-Starting ........................................................................................................................... 9
7.2 Smooth Torque ...................................................................................................................... 9
7.3 Quiet Operation .................................................................................................................... 9
7.4 Constant Speed ..................................................................................................................... 9
7.5 Low Power Rating.................................................................................................................. 9
7.6 Low Maintenance.................................................................................................................. 9
8. Advantages of Hysteresis Motor ............................................................................................... 10
9. Disadvantages of Hysteresis Motor .......................................................................................... 10
10. Comparison with Other Motors .............................................................................................. 11
10.1 Hysteresis Motor vs Induction Motor ............................................................................... 11
10.2 Hysteresis Motor vs Reluctance Motor ............................................................................. 11
11. Applications of Hysteresis Motor ............................................................................................ 11
12. Industrial and Practical Importance ........................................................................................ 12
13. Conclusion ............................................................................................................................... 13
References ..................................................................................................................................... 13
Abstract
The hysteresis motor is a special type of single-phase synchronous motor that is widely known
for its quiet operation, smooth torque production, and constant-speed performance. Unlike
conventional induction and synchronous motors, the hysteresis motor operates based on the
magnetic hysteresis property of the rotor material. This unique principle enables the motor to
start automatically and accelerate smoothly until it reaches synchronous speed, where it
continues to run with high stability. Due to its simplicity, reliability, and precision, the hysteresis
motor has historically been used in timing devices, audio equipment, measuring instruments,
and other low-power applications requiring constant speed. This report presents a detailed
study of the hysteresis motor, including its construction, operating principle, torque production,
advantages, disadvantages, performance characteristics, and practical applications.
1. Introduction
Electric motors are essential components in industrial, domestic, and commercial applications.
While most commonly used motors include induction motors, synchronous motors, and DC
motors, there are several special electrical machines designed for specific operating conditions
and performance requirements. One of these machines is the hysteresis motor.
The hysteresis motor belongs to the category of single-phase synchronous motors. It is unique
because it combines two desirable characteristics: self-starting ability and constant
synchronous speed operation. These features make it highly suitable for precision applications
where smooth and quiet motion is more important than high torque or large power output.
The motor derives its name from the phenomenon of magnetic hysteresis, which refers to the
lagging behavior of magnetic flux in a ferromagnetic material when subjected to a changing
magnetic field. This magnetic lag is directly responsible for the production of torque in the
motor rotor.
This report aims to provide a comprehensive explanation of the hysteresis motor from both
theoretical and practical perspectives.
2. Definition of Hysteresis Motor
A hysteresis motor is a single-phase synchronous motor in which the torque is developed due
to the hysteresis loss occurring in the rotor material when subjected to a rotating magnetic
field.
In simple terms, the stator produces a rotating magnetic field, and the rotor—made of a
magnetically hard material—responds to this field with a time lag due to hysteresis. This lag
creates a continuous torque that accelerates the rotor. Eventually, the rotor locks into
synchronism with the stator field and continues to rotate at synchronous speed.
The hysteresis motor is therefore considered a special-purpose motor designed primarily for
constant-speed, low-power, and quiet-operation applications.
3. Construction of Hysteresis Motor
The hysteresis motor consists of two main parts:
3.1 Stator
The stator of a hysteresis motor is very similar to that of a single-phase induction motor. It
generally consists of:
•
Laminated steel core
•
Distributed stator winding
•
Main winding
•
Auxiliary winding (for starting)
•
AC power supply connection
The stator is designed to produce a rotating magnetic field when connected to an alternating
current supply.
3.2 Rotor
The rotor is the most distinctive part of the hysteresis motor. It is typically made of a smooth
cylindrical core constructed from hard magnetic material, such as:
•
chrome steel
•
cobalt steel
•
tungsten steel
•
other high-retentivity magnetic alloys
Rotor characteristics
The rotor generally has:
•
No rotor winding
•
No commutator
•
No slip rings
•
A smooth surface
•
High coercivity and high retentivity magnetic material
The choice of rotor material is critical because the motor depends on the hysteresis loop
characteristics of this material to generate torque.
4. Principle of Operation
The operation of the hysteresis motor is based on the principle of magnetic hysteresis.
4.1 Rotating Magnetic Field
When a single-phase AC supply is applied to the stator windings (along with the auxiliary
winding arrangement), a rotating magnetic field is produced in the air gap.
This rotating magnetic field revolves at the synchronous speed, given by:
𝑁𝑠 =
Where:
•
𝑁𝑠 = synchronous speed (rpm)
•
𝑓= supply frequency (Hz)
•
𝑃= number of poles
120𝑓
𝑃
4.2 Rotor Magnetization Lag
As the stator field rotates, it magnetizes the rotor material continuously. However, because the
rotor is made of a magnetically hard material, its magnetic polarity does not change instantly.
Instead, it lags behind the rotating stator field.
This lag between the rotor magnetization and the stator field produces a dragging action,
resulting in hysteresis torque.
4.3 Starting and Acceleration
At standstill, the rotating stator field begins to exert torque on the rotor. The rotor starts moving
in the same direction as the rotating field. As speed increases:
•
the rotor continues to experience hysteresis torque
•
acceleration remains smooth and continuous
•
no sudden jerks or pulsations occur
4.4 Synchronous Running
Once the rotor reaches the synchronous speed, it becomes magnetically locked with the stator
field and continues to rotate at exactly the same speed as the rotating magnetic field.
Thus, the motor runs as a synchronous motor under normal operation.
5. Torque Production in Hysteresis Motor
The hysteresis motor develops torque mainly due to magnetic hysteresis in the rotor.
5.1 Hysteresis Torque
The major component of torque in this motor is called hysteresis torque. It is produced because
the rotor flux does not follow the stator field instantaneously.
Important feature:
One of the most remarkable properties of the hysteresis motor is that the hysteresis torque
remains nearly constant from standstill to synchronous speed.
This means the motor develops a relatively uniform torque throughout the acceleration process.
5.2 Eddy Current Torque
In addition to hysteresis torque, some designs may also produce a small amount of eddy
current torque due to induced currents in the rotor body.
Thus, the total starting torque may be considered as:
•
Hysteresis torque
•
Eddy current torque
However, the defining operating mechanism remains the hysteresis effect.
6. Speed of Hysteresis Motor
The speed of a hysteresis motor is determined by the synchronous speed of the stator field:
𝑁𝑠 =
120𝑓
𝑃
Example:
If:
•
frequency = 50 Hz
•
number of poles = 4
Then:
𝑁𝑠 =
120 × 50
= 1500 rpm
4
Thus, the motor runs at 1500 rpm once synchronism is achieved.
Significance:
This constant-speed behavior is one of the most important reasons why hysteresis motors are
used in:
•
clocks
•
timers
•
recording equipment
•
precision instruments
7. Characteristics of Hysteresis Motor
The hysteresis motor has several distinctive characteristics that separate it from ordinary
motors.
7.1 Self-Starting
Unlike many synchronous motors, the hysteresis motor can start on its own without any
external starting mechanism.
7.2 Smooth Torque
Because the torque is developed magnetically and not through mechanical commutation or
saliency effects, the torque is very smooth and uniform.
7.3 Quiet Operation
The motor produces very little noise and vibration during operation. This is especially important
in sound-sensitive or precision applications.
7.4 Constant Speed
After synchronization, the motor runs at a constant synchronous speed, regardless of small load
variations.
7.5 Low Power Rating
The hysteresis motor is usually manufactured for small power outputs and is not suitable for
heavy industrial loads.
7.6 Low Maintenance
Since the rotor has no winding, no brushes, and no commutator, maintenance requirements are
relatively low.
8. Advantages of Hysteresis Motor
The hysteresis motor offers several advantages:
1. Self-starting capability
2. Smooth and silent operation
3. Constant synchronous speed
4. Simple rotor construction
5. Low maintenance
6. Good reliability
7. No hunting during synchronization
8. Suitable for precision timing applications
These advantages make the motor very useful in low-power precision systems.
9. Disadvantages of Hysteresis Motor
Despite its benefits, the hysteresis motor also has some limitations:
1. Low efficiency
2. Low output power
3. Low torque compared to larger industrial motors
4. Expensive rotor material
5. Not suitable for high-load applications
6. Limited use in modern large-scale industrial systems
As a result, hysteresis motors are generally restricted to specialized low-power applications.
10. Comparison with Other Motors
10.1 Hysteresis Motor vs Induction Motor
Feature
Hysteresis Motor
Induction Motor
Starting
Self-starting
Self-starting
Running Speed Synchronous speed
Less than synchronous speed
Torque
Smooth and nearly constant Varies with slip
Noise
Very low
Moderate
Rotor
Hard magnetic material
Squirrel cage or wound rotor
Applications
Precision devices
General industrial use
10.2 Hysteresis Motor vs Reluctance Motor
Feature
Hysteresis Motor Reluctance Motor
Rotor Type
Smooth cylindrical Salient pole rotor
Torque Production Hysteresis effect
Reluctance effect
Noise
Lower
Higher
Operation
Smoother
Less smooth
Application
Precision timing
Light synchronous loads
This comparison shows that the hysteresis motor is preferred where smoothness and silence
are more important than high power output.
11. Applications of Hysteresis Motor
Because of its unique characteristics, the hysteresis motor is used in several low-power and
precision applications.
Common applications include:
•
Electric clocks
•
Timers
•
Record players
•
Tape recorders
•
Turntables
•
Audio devices
•
Precision measuring instruments
•
Control devices
•
Scientific instruments
•
Synchronous timing systems
Why these applications?
These devices require:
•
accurate speed
•
quiet operation
•
low vibration
•
stable performance
The hysteresis motor is particularly suitable in such cases.
12. Industrial and Practical Importance
Although the hysteresis motor is not commonly used in heavy industrial machinery, it has
important practical value in applications requiring speed precision and smooth motion.
Historically, it played an essential role in electromechanical timing systems and recording
devices. Even though modern electronics and servo systems have replaced it in many
applications, the hysteresis motor remains an important machine in electrical engineering
education because it demonstrates the practical use of magnetic material properties in torque
production. Studying this motor also helps engineering students understand:
•
magnetic hysteresis
•
synchronous operation
•
special machine design
•
constant-speed applications
Thus, it has both educational and practical significance.
13. Conclusion
The hysteresis motor is a remarkable example of a special electrical machine that utilizes the
magnetic hysteresis property of rotor material to produce torque. It is classified as a singlephase synchronous motor and is distinguished by its ability to start automatically and operate
at constant synchronous speed.
Its construction is simple, especially on the rotor side, and its performance is characterized by
quietness, smooth torque, and excellent speed stability. These features make it highly suitable
for precision and low-power applications such as clocks, timers, record players, and measuring
instruments.
However, the motor is limited by its relatively low efficiency, low output power, and special
rotor material requirements. Therefore, its use is generally restricted to specialized applications
rather than heavy-duty industrial work.
In conclusion, the hysteresis motor remains an important topic in the study of special electrical
machines because it combines magnetic theory with practical motor operation in a unique and
elegant way.
References
1. Dr. Hussein M. Waly, Special Electrical Machines Lecture Notes, Mechatronics
Engineering Program, The Pyramids Higher Institute for Engineering and Technology.
2. A. E. Fitzgerald, Charles Kingsley Jr., and Stephen D. Umans, Electric Machinery.
3. I. J. Nagrath and D. P. Kothari, Electric Machines.
4. P. S. Bimbhra, Electrical Machinery.
5. B. L. Theraja and A. K. Theraja, A Textbook of Electrical Technology.