Farm Power And Machinery (1+1)

Direct current motor (D.C Motors)

• The electric motors are classified as Direct current motor (D.C.Motor) and Alternating current motor (A.C.Motor). Electric motors are also classified on the basis of armature designs, rotor design and kilo watt power output.
D.C.Motor

• An electric motor is a machine which when supplied with electric current, can be used for some mechanical work.
  
• Direct current motors are widely used in applications that require control of speed e.g., electric trams, engine starters and elevators etc.
  
• They are made in sizes from 1/100 hp to thousands of hp. D.C. motors are similar to D.C. generators in appearance and construction.
  
• They have a similar armature, field poles and other motors, current is conducted to the armature winding.
  
• This is accomplished by connecting leads from the armature winding to the commutator, and then feeding the commutator with current.
• The commutator is supplied with current by means of carbon brushes.
  
• There are three main types of D.C. motors:
• The series motor,
  
• The shunt motor and
  
• The compound motor.
  
• These motors differ in the construction of their field coils and in the connections between the field coils and armature.
Small D.C. motors of less than 1/2 kW size consume very little current and therefore, can be started by placing full voltage across the motor terminals. But in large D.C. motors, if full voltage is applied, excessive current will flow which damages the motor. To start a large motor, it is necessary to connect a resistance unit in series with the armature, so that the starting current is reduced to a safe value. As the motor starts, the resistance is completely cut off after the motor has reached the desired speed. At high speed the counter emf produced in the armature prevents an excessive current flow. Rotation of D.C. motors can be changed by reversing the direction of the current flow in either the armature or field circuit, but not both. Generally, it is reversed by changing the direction of current flow in the armature circuit.

D.C.Motor Starter

• The operation of the starter of a D.C. motor is usually done manually. When the handle is brought to make the current with the first point, the shunt field is immediately connected directly across the supply line.
• The contact with the point first the shunt field is immediately connected directly across the supply line.
• The contact with the point also connects the entire resistance in series with the armature which begins to rotate and the initial current is limited to a safe value.
• As the handle is moved current up, the resistance is gradually removed from the armature circuit.
• When all the resistance is finally removed, the operating handle rests against a holding magnet mounted along side the last point.
• The holding magnet is connected in series with the main-shunt field of the motor.
• The operating handle is provided with a heavy spring which will instantly return it to the off position if the holding magnet is demagnetized due to failure of the power supply or break in the motor field circuit. In such a case the handle automatically returns to the of position and the motor stops. It will remain so until started again by hand.