- 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 brushed DC motor, the armature rotates within a stator magnetic field. The working principle of a DC motor follows the force produced when a current carrying conductor lies in a magnetic field. Forces on the armature conductors combine to produce torque.
Every conventional brushed DC motor needs a stator field. When constructing a DC motor, designers may produce this field with wound electromagnets or permanent magnets.
A Permanent Magnet DC motor (PMDC motor) is a brushed type of DC motor whose stator permanent magnets provide the main field instead of a wound field circuit.
A small battery-powered toy may use a brushed permanent magnet DC motor or PMDC motor. These types of motors can provide a compact drive when brush life and commutator maintenance are acceptable.
Brushed PMDC motors also drive small pumps, fans and automotive actuators such as wipers and window lifts. Some geared automotive starter motors use permanent magnets, while many traditional starters use a series-wound field.
The permanent-magnet field is fixed during normal operation, so the motor cannot use wound-field adjustment for speed control or field weakening.
A PMDC motor can still have effective speed control. Varying armature voltage, often with pulse-width modulation and feedback, controls speed below the base operating range. This construction is common from sub-fractional power through larger ratings selected for the required torque, speed and duty cycle.
Construction of Permanent Magnet DC Motor or PMDC Motor
In a permanent-magnet DC motor, magnets attached to the stator provide the field poles.
A brushed PMDC motor has a stator and a rotating armature. The stator yoke is often a steel cylinder with permanent magnets secured to its inner surface.
Adjacent magnet poles face the armature with alternating polarity. An N pole is followed by an S pole around the stator bore, forming the working air-gap field.
The steel stator yoke supports the magnets and provides a low-reluctance return path for magnetic flux.
A standard PMDC motor has no wound field coil. Machines that combine permanent magnets with controllable field windings are hybrid-excited designs and require a separate analysis.
Permanent-magnet material is selected for flux density, temperature range, demagnetization resistance, size and cost. Ferrite, aluminium-nickel-cobalt and rare-earth magnets are all used in different designs.
The rotor of a PMDC motor resembles the armature of other DC motors. It contains a laminated steel core, armature windings and a commutator. Thin insulated laminations reduce eddy-current loss in the rotating core.
The stacked laminations form a cylindrical armature core with axial slots. Electrical insulation between laminations limits circulating eddy currents.
Slots around the armature hold conductors connected as the armature winding.
Winding terminals connect to commutator segments on the shaft. Carbon or graphite brushes press against the commutator and conduct current to the rotating armature while switching coil connections.
Working Principle of Permanent Magnet DC Motor or PMDC Motor
A PMDC motor follows the general working principle of DC motor. A current-carrying armature conductor in the stator field experiences force. The force direction on each conductor follows Fleming’s left hand rule.
The commutator reverses current in each armature coil at the required rotor position. This keeps the electromagnetic torque acting in one direction as the armature turns through successive pole regions.
For a straight conductor perpendicular to the field, the force is F = BIL newtons. B is flux density in teslas, I is conductor current in amperes and L is active conductor length in metres within the magnetic field. At another angle, the force also includes the sine of the angle between current and field.
Forces on the distributed armature conductors combine into shaft torque. With constant permanent-magnet flux, motor torque is approximately proportional to armature current before magnetic or thermal limits are reached.
Equivalent Circuit of Permanent Magnet DC Motor or PMDC Motor
The PMDC equivalent circuit has no field-winding branch because permanent magnets supply the stator flux.
At steady state, applied armature voltage equals back emf plus the armature-resistance drop. During a current transient, the armature-inductance voltage must also be included. The simplified steady-state equation is:
Here, I is armature current and R is armature resistance.
Eb is back emf and V is applied armature voltage.
Advantages of Permanent Magnet DC Motor or PMDC Motor
The advantages of a PMDC motor include:
- No field-excitation winding or field supply is required.
- Eliminating field copper loss can improve efficiency, especially in small motors.
- The absence of a field coil can reduce motor size, mass and circuit complexity.
- For suitable production volumes and ratings, the simple field system can be economical. Magnet material and brush maintenance still affect total cost.
Disadvantages of Permanent Magnet DC Motor or PMDC Motor
The disadvantages of a PMDC motor include:
- Armature reaction opposes part of the permanent-magnet field under load. Pole shape, magnet grade and armature design must limit harmful demagnetizing flux.
- Excess current during stall, reversal or overload can heat the motor and create a demagnetizing field. Excessive magnet temperature or current can cause irreversible loss of flux.
- The fixed air-gap field prevents ordinary field control. Armature-voltage control still provides practical speed control of DC motor, but operation above base speed needs other methods and must respect commutator, voltage and mechanical limits.
Applications of Permanent Magnet DC Motor or PMDC Motor
A brushed PMDC motor suits compact variable-speed or reversing drives where its brush life is acceptable. Applications include toys, small pumps, wipers, washers, window lifts, seat actuators and compact fans. Computer disk drives normally use brushless motors rather than brushed PMDC motors.
