There are three modes of operation: Start mode Transition mode Run mode
Disadvantages The cost of a written pole motor is comparitively higher than that of a conventional motor owing to extra features such as the excitation windings and the ferrite magnetic layer. The design of exciter windings is complicated.
Ride through characteristics
Golf course irrigation Small municipal water systems Oil pumps Grain handling
written pole electric motor
Under the guidance of:
•INTRODUCTION TO WRITTEN POLE
•LIMITATIONS OF CONVENTIONAL
This is a hybrid concept, which utilizes
basic concepts from induction,
hysteresis and permanent magnet
motors, with several important
The wp technology allows the machines
to operate in which the speed and the
frequency are independent of each
other, because the number of poles can
be a controlled variable.
Poles are changed by a “writing” coil
that can change the polarity of a
normally “permanent” magnetic
1.Dependence of speed on no. of poles of
speed = 120 X frequency
no. of poles
2.The machine must have even no. of
poles and deviating from this condition
will cause instability at its rated speed.
have a fixed, even
number of discrete
field pole structures,
and therefore have a
between shaft speed
and the frequency of
the electrical input.
as if it has a
of poles. The
motor consist of
on the rotor and
The rotor is made up of a high resistance
The high resistance cage is a key factor in
limiting the starting current of Written-
Pole motors and provides considerable
induction torque during the initial stage of
A continuous layer of permanent magnetic
ferrite material covers the rotor lamination
The stator of a written-pole motor is
very similar to a typical induction motor.
The stator lamination stack is
constructed using low loss electrical
The windings are similar in design and
function to those used in a conventional
induction or synchronous motor.
Written-pole motor makes use of a
concentrated excitation winding located at
one or two points on the stator.
The excitation windings are contained
within the stator structure and are located
between the main stator windings.
The excitation winding is designed to
produce a magnetic field powerful enough to
fully magnetize the portion of the rotor’s
magnetic layer that is immediately across the
air gap from it.
The unique feature:
Since the power supplied to the excitation
winding is a constant frequency 50 Hz, AC
sinusoidal supply, the actual size and quantity
of poles generated on the surface of the rotor
are dependent on the rotational speed of the
Lower speeds result in a larger number of
smaller poles with shorter spans.
While higher speeds result in a smaller
number of larger poles with longer spans.
Writing of poles
There are three modes
In the start mode, a written-pole motor produces
large amounts of hysteresis and induction torque,
which begin to accelerate the motor to its rated
As the written-pole motor accelerates toward its
rated speed, it enters the transition mode, during
which the excitation winding begins to influence the
magnetic geometry of the rotor.
The rotational speed at which the motor switches
to transition mode is generally in the range of 80 to
90% of normal synchronous speed.
A written-pole motor enters run mode upon
reaching its rated synchronous speed.
Since operation of the excitation winding is not
required in this mode, it is turned off and the motor
continues to operate as a permanent magnet
synchronous motor until power is turned off.
(50 hp unit)
Feature Conventional WP
Speed 3,560rpm 3600 rpm
Efficiency 85% 93%
Power factor 90.5% 100%
Full-load power 53kW 40kW
Full-load torque 99.92N-m 98.97N-m
Starting torque 119.31N-m 111.17N-m
Full-load current 219 amps 173 amps
Locked rotor current 1450 amps 312amps
Cost (p.u.) 1.0 1.2
1. Low starting current requirements
2. Energy efficient operation
3. Unity power factor operation
4. Simple construction
5. Low temperature rise
6. High inertia starting
7. Instantaneous restart capability
8. Ride through characteristics
The cost of a written pole motor is
comparitively higher than that of a
conventional motor owing to extra features
such as the excitation windings and the
ferrite magnetic layer.
Its cost is about 20% higher than that of a
The design of exciter windings is
Most Written-Pole motors feature an
external rotor that spins around an
internal stator, opposite to that of
conventional motors. This inverted
structure creates a flywheel effect that
allows the machine to ride through 99%
of power-quality disturbances. At full
load, Written-Pole motors can provide
full power to driven equipment through
power interruptions as long as 15 sec.
• The Written-Pole motor is designed to
operate in ambient temperatures
ranging from 32ºF to 104ºF.
• However, test results show that Written-
Pole motors have reliably operated at
temperatures ranging upto 122ºF.
• They may be operated safely in the open
air if there is adequate protection from
direct rain. The motor base must be
mounted on a raised concrete pad or
steel frame to avoid exposure to
The main application is as irrigation
pumps at remote places where 3 phase
supply is not available.
Other applications include,
• Golf course irrigation
• Oil pumps
• Small municipal water systems
A WRITTEN POLE RIDE
THROUGH MOTOR USED IN A
The written-pole motor is a motor design that
allows for the operation of very large single-
This unique product has applications virtually
anywhere that three-phase power is either not
available, or is not economical to provide.
The written-pole motor also has performance
advantages over a conventional induction
motor including low starting current, very high
operating efficiency, and excellent power
This motor technology has the additional
benefit of being able to ride through brief
service interruptions on single-phase power