Permanent magnet DC motor

1. 027 Electrical & Electronics Principles
© Seychelles Institutes of Technology Prepared by Mr. Dinesh
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6.0D ROTATING MACHINES
6.D.1 Permanent magnet dc motor (PMDC)
A PM motor does not have a field winding on the stator frame, instead relying on PMs to provide the
magnetic field against which the rotor field interacts to produce torque.
6.D.2 Types of PMDC
There are mainly two types
1. Brushed
2. Brushless
6.D.3 Construction of PMDC
Stator of PMDC
Fig 11: Stator of PMDC
A PMDC motor mainly consists of two parts. A stator and an armature. The permanent magnets are
mounted in such a way that the N – pole and S – pole of each magnet are alternatively faced towards
armature as shown in the Fig 11. The rotor of PMDC motor is similar to other DC motor. The rotor or
armature of permanent magnet dc motor also consists of core, windings and commutator.

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6.0D ROTATING MACHINES
6.D.4 Working principle of PMDC
The working principle of PMDC motor is just similar to the general working principle of DC motor.
When a carrying conductor comes inside a magnetic field, a mechanical force will be experienced by the
conductor and the direction of this force is governed by Fleming’s left hand rule.
As in a permanent magnet dc motor, the armature is placed inside the magnetic field of permanent
magnet; the armature rotates in the direction of the generated force.
Here each conductor of the armature experiences the mechanical force F = B.I.L Newton where B is the
magnetic field strength in Tesla (weber / m2), I is the current in Ampere flowing through that conductor
and L is length of the conductor in metre comes under the magnetic field.
Each conductor of the armature experiences a force and the compilation of those forces produces a
torque, which tends to rotate the armature.
6.D.5 Equivalent circuit of PMDC
The supply voltage to the armature will have armature resistance drop and rest of the supply voltage is
countered by back emf of the motor. Hence voltage equation of the motor is given by,
V = IR + Eb
Where, V : the supply voltage
I : armature current
R : armature resistance of the motor
Eb : back emf

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6.0D ROTATING MACHINES
6.D.6 Advantages of PMDC
1. No need of field excitation arrangement.
2. No input power in consumed for excitation which improves efficiency of dc motor.
3. No field coil hence space for field coil is saved which reduces the overall size of the motor.
4. Cheaper and economical for fractional kW rated applications.
6.D.7 Disadvantages of PMDC
1. The magnetic strength of the field may get weak due to demagnetizing effect armature reaction.
2. There is also a chance of getting the poles permanently demagnetized.
3. The field in the air gap is fixed and limited and it cannot be controlled externally.
6.D.8 Application of PMDC
 Automobiles starter
 Toys
 Wipers
 Washers
 Hot blowers
 Air conditioners
 Computer disc drives and in many more

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6.0D ROTATING MACHINES
6.D.9 Exploded view of a permanent magnet dc motor

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6.0D ROTATING MACHINES
6.D.10 Brushed dc permanent magnet motor

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6.0D ROTATING MACHINES
6.D.12 Circuit diagram of a 2 pole brushed dc permanent magnet motor
6.D.13 Working principle of a 2 pole brushed dc permanent magnet motor (BDC)
The construction of a simple BDC motor is made of the same basic components: a stator, rotor, brushes
and a commutator.
The stator generates a stationary magnetic field that surrounds the rotor. This field is generated by either
permanent magnets or electromagnetic windings.
The rotor, also called the armature, is made up of one or more windings. When these windings are
energized they produce a magnetic field. The magnetic poles of this rotor field will be attracted to the
opposite poles generated by the stator, causing the rotor to turn. As the motor turns, the windings are
constantly being energized in a different sequence so that the magnetic poles generated by the rotor do
not overrun the poles generated in the stator. This switching of the field in the rotor windings is called
commutation.

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6.0D ROTATING MACHINES
6.D.14 Solved questions brushed permanent magnet dc motor
Work example 1
Explain how a brushed dc motor works?
Solution
Circuit diagram of a brushed dc permanent magnet motor
Explanation
The moving part of motor (rotor) turns inside the fixed part the frame or stator. The rotor includes
armature, commutator and shaft.
The current in wires (windings) runs through slots in the armature create magnetic fields that acts
against the fixed magnetic field on the magnetic poles on the stator to turn the armature, and thus the
whole rotor.
The shaft is in the centre is used to apply this rotation externally.
Electricity is fed to the armature via brushes on rotor that rub against the commutator (electrical contact
on the rotor).

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6.0D ROTATING MACHINES
6.D.15 Brushed dc permanent magnet motor design for trains
6.D.16 Application of brushed dc motors
 Toys
 Toothbrushes
 Shavers
 Cordless tools
 Small fans

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6.0D ROTATING MACHINES
6.D.17 Structure of a brushless dc motor
6.D.18 Working principle of brushless dc motor
Brushless dc motor motors are a type of synchronous motor. This means the magnetic field generated by
the stator and the magnetic field generated by the rotor rotates at the same frequency.
The rotor, made of a magnet, is rotated by magnetic fields that are generated by the current flowing
through the stator windings. The current is switched by the sensor and electronic circuit.
6.D.19 Application of brushless dc motor
 Linear actuators
 Servomotors
 Actuators for industrial robots
 Extruder drives motor
 Feed drives for CNC machine tools

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6.0D ROTATING MACHINES
6.D.20 Solved questions brushed permanent magnet dc motor
Work example 1
What is the difference between a brushed and a brushless dc motor?
Solution
Explanation
A Brushed Motor has a rotating set of wound wire coils (armature) which acts as an electromagnet with
two poles. A mechanical rotary switch (commutator) reverses the direction of the electric current twice
every cycle, to flow through the armature so that the poles of the electromagnet push and pull against
the permanent magnets on the outside of the motor. As the poles of the armature electromagnet pass the
poles of the permanent magnets, the commutator reverses the polarity of the armature electromagnet.
During the instant of switching polarity, inertia keeps the motor going in the proper direction.
A Brushless Motor uses a permanent magnet external rotor, three phases of driving coils, one or more
devices to sense the position of the rotor, and the associated drive electronics. The coils are activated,
one phase after the other, by the electronic speed controller as cued by the signals from the rotor
position sensors.