- PMDC Motor Definition: A Permanent Magnet DC Motor (PMDC Motor) is defined as a motor that uses permanent magnets instead of electromagnets to generate the magnetic field needed for its operation.
- Operation Mechanism: In PMDC motors, the armature rotates due to a mechanical force induced by the magnetic field when current flows through its windings, according to Fleming’s left-hand rule.
- Construction Features: The PMDC motor features a simple design with a stator that houses permanent magnets and an armature with windings and a commutator.
- Advantages: PMDC motors are efficient and cost-effective as they require no external power for field excitation and have fewer parts, making them compact.
- Application Areas: These motors are ideal for low-power applications where complex speed controls are unnecessary, such as in toys and automotive components.
What is a Permanent Magnet DC Motor (PMDC Motor)?
In a DC motor, an armature rotates inside a magnetic field. The basic working principle of a DC motor is based on the fact that whenever a current carrying conductor is placed inside a magnetic field, there will be mechanical force experienced by that conductor.
All kinds of DC motors work under this principle. Hence for constructing a DC motor, it is essential to establish a magnetic field. The magnetic field is established by using a magnet. You can use different types of magnets – it may be an electromagnet or it can be a permanent magnet.
A Permanent Magnet DC motor (PMDC motor) is a type of DC motor that uses a permanent magnet to create the magnetic field required for the operation of a DC motor.
Have you ever used a battery operated toy? The motor that drives these toys is nothing but a permanent magnet DC motor or PMDC motor. These types of motors are simple in construction.
They are commonly used as a starter motor in automobiles, windshield wipers, washers, blowers used in heaters and air conditioners, to raise and lower windows – and they are extensively used in toys.
Since the magnetic field strength of a permanent magnet is fixed, external control of this field in a PMDC motor is not possible.
Thus permanent magnet DC motor is used where there is no need to control the speed of the motor (which is usually done by controlling the magnetic field). Small fractional and sub-fractional KW motors are often constructed using a permanent magnet.
Construction of Permanent Magnet DC Motor or PMDC Motor
As it is indicated in name of permanent magnet DC motor, the field poles of this motor are essentially made of permanent magnet.
A PMDC motor mainly consists of two parts. A stator and an armature. Here the stator which is a steel cylinder. The magnets are mounted in the inner periphery of this cylinder.
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 figure below. That means, if N-pole of one magnet is faced towards armature then S-pole of very next magnet is faced towards armature.
In addition to holding the magnet on its inner periphery, the steel cylindrical stator also serves as low reluctance return path for the magnetic flux.
While a field coil is not essential in a PMDC motor, it is sometimes used alongside permanent magnets to enhance magnetic strength if needed.
This is because if permanent magnets lose their strength, these lost magnetic strengths can be compensated by field excitation through these field coils. Generally, rare earth hard magnetic materials are used in these permanent magnets.
The rotor of a PMDC motor is similar to other DC motors. The rotor or armature of a permanent magnet DC motor also consists of a core, windings, and commutator. Armature core is made of a number of varnishes insulated, slotted circular lamination of steel sheets.
By fixing these circular steel sheets one by one, a cylindrical shaped slotted armature core is formed. The varnish insulated laminated steel sheets are used to reduce eddy current loss in armature of permanent magnet DC motor.
Slots on the outer periphery of the armature core house the armature conductors, which are connected to form the armature winding.
The end terminals of the winding are connected to the commutator segments placed on the motor shaft. Like other DC motors, carbon or graphite brushes are placed with spring pressure on the commutator segments to supply current to the armature.
Working Principle of Permanent Magnet DC Motor or PMDC Motor
As we said earlier the working principle of PMDC motor is just similar to the general working principle of DC motor. That is 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 a 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 the length of the conductor in meter 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.
Equivalent Circuit of Permanent Magnet DC Motor or PMDC Motor
As in the PMDC motor, the field is produced by a permanent magnet, there is no need for drawing field coils in the equivalent circuit of a permanent magnet DC motor.
The supply voltage to the armature will have armature resistance drop and the rest of the supply voltage is countered by the back emf of the motor. Hence the voltage equation of the motor is given by,
Where, I is armature current and R is armature resistance of the motor.
Eb is the back emf and V is the supply voltage.
Advantages of Permanent Magnet DC Motor or PMDC Motor
The advantages of a PMDC motor are:
- No need of field excitation arrangement.
- No input power in consumed for excitation which improve efficiency of DC motor.
- No field coil hence space for field coil is saved which reduces the overall size of the motor.
- Cheaper and economical for fractional kW rated applications.
Disadvantages of Permanent Magnet DC Motor or PMDC Motor
The disadvantages of a PMDC motor are:
- The armature reaction of DC motor cannot be compensated hence the magnetic strength of the field may get weak due to the demagnetizing effect of the armature reaction.
- There is a chance of getting the poles permanently demagnetized (partial) due to excessive armature current during the starting, reversal, and overloading conditions of the motor.
- The field in the air gap is fixed and limited – it cannot be controlled externally. This makes it difficult for this type of motor to achieve efficient speed control of DC motor in this type of motor is difficult.
Applications of Permanent Magnet DC Motor or PMDC Motor
PMDC motor is extensively used where small DC motors are required and also very effective control is not required, such as in automobiles starter, toys, wipers, washers, hot blowers, air conditioners, computer disc drives and in many more.
