Brushless DC motor was invented by T. G. Wilson and P.H. Trickey in year 1962. It is also called as “A DC machine with solid state commutation”. In 1980 Robert E. Lordo of the powertech industrial introduced BLDC motors into industry. Where as conventional DC motor came in 1856, Ernst Werner Von Siemens. DC motor has many advantages like high starting torque, operates on DC power, excellent speed control and torque variation. However, the limitation of DC motor are high maintenance cost of commutator and brushes.
Permanent Magnet Motors
Normally every electric motor has field coils for producing magnetic flux. Conventional DC motor has field winding on the rotor, which creates required magnetic flux. Pole core and pole shoe act as a required magnetic pole. In synchronous motor field coils are on the rotor side. The current to the field coil is supplied through slip rings.
Brushless dc motor is nothing but a synchronous motor with permanent magnet providing required magnetic field. Permanent magnet synchronous motor is classified based on wave shape of their back EMF. If the shape of back EMF produced is of sinusoidal nature then it is BLAC motor and if the shape of back EMF produced is of trapezoidal nature then it is BLDC motor.
Advantages of Permanent Magnet Synchronous Motor
Permanent magnet produces the require magnetism hence eliminate the requirement of field coils and slip rings. Because of the presence of permanent magnet inside the rotor BLDC motors have low inertia. With the absence of field coils and slip ring the available permanent magnet produce such a strong filed that the size of the motor can be reduced.
Further, since there is no copper loss of the secondary winding the permanent magnet motors have higher efficiency than induction motors. Also, permanent magnet synchronous motor are advantages in incorporating, the reluctance torque in the field- weakening range. Therefore they can be designed to have a wide constant power speed range. As a result, PMSM have higher power density than any other type of Motors.
Structure of Brushless DC motor
Brushless DC motor has three main parts, stator, rotor and hall sensors.
Stator is the outer part of BLDC motor, responsible for producing rotating magnetic field. It has the stacked steel lamination and distributed trapezoidal winding. Due to which trapezoidal shape of back EMF get produced.
Brushless DC motor has a permanent magnet rotor. Starting the brushless DC motor is very crucial task. It is mainly because of magnetic interlocking. For interlocking to take place it becomes very much essential to identify the position of poles of rotor’s permanent magnet. To resolve this difficulty hall sensors are used.
Working of hall sensors
The small experiment can help us to better understand working of hall sensors. In this dc supply is given to a metallic plate. Permanent magnet is placed above the plate which create the necessary magnetic field. Due to the hall effect, when dc supply is provided to plate, electrons deviate from their original path.
Due to which a small potential is developed at both side of the plates. This potential difference can be measured, which comes in micro volt. To sense this small voltage hall gain amplifiers are used. Now if we reverse the magnet then the measured voltage will also get change. By applying this method hall sensors detect the polarity of the magnet.
Hall Sensors in BLDC Motor
Normally, three hall sensors are used to sense the rotor position in BLDC Motor. These hall sensors are either placed 1200 apart from each other or can be 600 apart from each other. Three hall sensors can give eight combinations as shown in the following table. Out of which only six combinations are logically useful. From row number 2 to 7 are the valid combinations. It is mainly because hall sensors sense either north pole or south pole, so it is worthless to send 000 or 111 signals to controller which implies all south or all north poles are available in the rotor which is practically impossible.
|Sr. No||Combination H1 H2 H3||Possibilities|
Inverter in Brushless DC Motor
In DC motor, dc supply is supplied to the armature of the motor. Beside, to rotate the motor alternating flux is required which is given by commutator and brush assembly. In DC motor, the direction of current through armature conductor changes continuously with the help of commutator and brushes.
In Brushless dc motor, instead of brushes and commutator an inverter is used to serve the same purpose of converting direct current to alternating current. Following figure shows the circuit diagram of brushless dc motor. In this Vbatt supplies the dc power to inverter. The six combination of hall sensors give the information of rotor position to the controller. Controller decodes the logic and according to that give switching signal to the inverter.
Application of Brushless DC Motor
- In compressor for Refrigeration
- For conveyor.
- Laser Printer
- Labeling machine.
- Hard disk Drive
- Aerial lift
- Blower drives
- Robotic Equipment
- Chemical /oil /Gas pumping.
- Servo Applications
- Electric Vehicles
- Mining Equipment.
- Pump Drives.
- Exercise Equipment