A DC Motor whose poles are fabricated from Permanent Magnets is called Permanent Magnet DC (PMDC) Motor. The magnets are radially magnetized and are mounted on the inside periphery of the cylindrical metal stator. The stator of the motor serves as a return path for the magnetic flux. The rotor has a DC armature, with commutator segments and brushes
The cross-sectional view of the two-pole PMDC motor is proven within the determining beneath.
The Everlasting Magnet DC motor usually operates on 6 V, 12 V or 24 Volts DC provide obtained from the batteries or rectifiers. The interplay between the axial present carrying rotor conductors and the magnetic flux produced by the permanent magnet results in the technology of the torque.
The circuit diagram of the PMDC is proven below.
In typical DC motor, the generated or back EMF is given by the equation proven below.
The electromagnetic torque is given as
In Permanent Magnet DC motor, the value of flux ϕ is fixed. Subsequently, the above equation (1) and (2) becomes
The speed control of the PMDC motor can’t be controlled through the use of flux control technique because the flux stays fixed in this kind of motor. Each speed and torque might be controlled by armature voltage control, armature rheostat control, and chopper control methods. These motors have used the place the motor speed below the bottom speed is required as they can’t be operated above the base speed.
Kinds of Permanent Magnet Materials
There are three kinds of Permanent Magnet Materials utilized in PMDC Motor. The detailed data is given below.
Alnico magnet has a low coercive magnetizing depth and high residual flux density. Therefore, it’s used the place low present and excessive voltage is required.
They’re used in price sensitive purposes such as Air conditioners, compressors, and fridges.
Rare earth Magnet
Rare earth magnets are fabricated from Samarium cobalt, neodymium-iron-boron. They have an excessive residual flux and excessive coercive magnetizing intensity. The rare earth magnets are exempted from demagnetizing issues as a result of armature reaction. It’s an expensive material.
The Neodymium iron boron is cheaper as in comparison with Samarium cobalt. However, it could actually withstand higher temperature. Rare earth magnets are used for size-sensitive applications. They’re utilized in cars, servo industrial drives and in large industrial motors.
Applications of the Permanent Magnet DC Motor
The PMDC motors are utilized in varied purposes starting from fractions to a number of horsepowers. They’re developed as much as about 200 kW to be used in various industries. The next applications are given below.
PMDC motors are mainly utilized in cars to operate windshield wipers and washers, to boost the decrease home windows, to drive blowers for heaters and air conditioners and so on.
They’re additionally utilized in computer drives.
These kinds of motors are additionally utilized in toy industries.
PMDC motors are used in electrical toothbrushes, moveable vacuum cleaners, meals mixers.
Utilized in a transportable electrical software reminiscent of drilling machines, hedge trimmers and so on.
Advantages of the Permanent Magnet DC Motor
Following are the advantages of the PMDC Motor.
They’re smaller in size.
For smaller rating Permanent Magnet reduces the manufacturing price and thus PMDC motor is cheaper.
As these motors don’t require field windings, they don’t have subject circuit copper losses. This will increase their efficiency.
Disadvantages of the Permanent Magnet DC Motor
The disadvantages of the PMDC motor are given below.
- Permanent magnets cannot produce a high flux density as that as an externally supplied shunt field does. Therefore, a PMDC motor has a lower induced torque per ampere turns of armature current then a shunt motor of the same rating.
- There is a risk of demagnetization of the poles which may be caused by large armature currents. Demagnetization can also occur due to excessive heating and also when the motor is overloaded for a long period of time.
- The magnetic field of PMDC motor is present at all time, even when the motor is not being used.
- Extra ampere turns cannot be added to reduce the armature reaction.