Commutator

Commutator is made up of specially designed commutator segments, made of copper. These segments are insulated from each other by a thin layer of mica or papersheets. Structure of commutator is likely to cylinderical type. There are two types of commutator. They are

• Mechanical Commutator
• Electronic Commutator

Mechanical Commutator

Here, a carbon brush A is in contact with CS1 and a carbon brush B is in contact with CS7. When a dc supply is connected across A and B, a direct current passes through A, CS1, tapping 1, tapping 7, CS7 and returns via B. This current has two parallel paths in the armature winding. The current passing through the armature conductors set up an mmf along the axis of the tapping 7 and 1, i.e) along the axis of B and A. The commutator rotates along the anti clockwise direction, now the brush A contact with CS2 and Brush B contact with CS8. Thus current passes through brush A, CS2, tapping 2, tapping 8, CS8 and dthe brush B. The current passing through the armature conductors sets up on mmf along the axis os tapping 8 and 2 i.e) along the axis of BA. This process is continued until the supply is given to the motor. Mechanical commutator are of two types, they are

• Slip ring commutator
• Split ring commutator

Electronic Commutator

In a Permanent Magnet BrushLess DC motor, Power semiconductor switching circuit is used instead of mechanical commutator. It is Known as Electronic commutator. In a PMBL DC motor, rotor carries permanent magnet and stator accommodates the armature winding which is connected to the DC supply through a electronic commutator.

Here is the schematic diagram of PMBL DC motor armature circuit along with the switching circuit. Each tappings are connected to the positive of the DC supply i.e) A through 12 switches S1, S2, S3, ... , S12. Rest of each tappings are connected to the negative of the dc supply i.e) B through 12 switches S1', S2', S3', ..., S12'. When S1 and S1' are short circuited i.e) closed and others are in open postion, the dc supply is given to the tappings 1 and 7. Now the current has two parallel paths in armature winging. one is 1, 2, 3, 4, 5, 6, 7 and the other is 1, 12, 11, 10, 9, 8, 7. Thus the current flowing through the armature winding sets up and mmf along the axis 7 and 1. After a small interval of time S1 and S1' are kept open and S2 and S2' are closed. As a result, DC current passes from tapping 2 to 8 sets up an mmf, the direction 8 to 2. Thus by operating the switches in a sequential manner, it is possible to revolve mmf in the airgap. Power semiconductor switching devices used in electronic commutator are SCR's, MOSFET's, IGBT's, Power transistors etc.

Electronic Commutator vs Mechanical Commutator

Electronic Commutator	Mechanical Commutator
Commutator arrangement is located in stator	Commutator arrangement is located in rotor
Number of switching devices is limited to 6	Number of commutator segments are very high
There is no arcing	Arcing occurs
Less reliable	More reliable
No sliding contacts in electronic commutator	In mechanical commutator, there is a sliding contact between commutator and brushes
Electronic commutator requires less maintenance	Mechanical commutator requires more maintenance
Voltage can be controlled by PWM technique	Difficult to control voltage available across tapping