27-10-2012, 05:43 PM
ELECTRIC DRIVE SYSTEM
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INTRODUCTION TO ELECTRIC DRIVE SYSTEM
An industrial drive plays an important role in today automation system. Drive system basically consists of mechanical working equipment or load, which has to be kept in motion to turn out mechanical work equipment to do this job, called the prime mover, and a transmission to transfer energy from the prime mover to the mechanical load. I.C. engines, steam engine, turbines or electric motors may be used as prime movers.Transmission equipment such as a gearing or belt may be used to match the speeds of the prime mover and the load. The transmission may also be required sometimes to convert rotatory to linear motion and vice versa. Thus, a combination of a prime mover, transmission equipment and mechanical working load is called a drive.
Industrial loads require operation at any one of a wide range of speeds. Such loads are generally termed as variable speed drives. These drives demand precise adjustment of speed in a steeples continuous manner over the complete speed range required. The loads may be constant torque loads, requiring constant torque at all speeds. The torque may also be a function of speed, e.g. in a fan type or pump type loud the torque is proportional to the square of the speed. These loads are driven by hydraulic, pneumatic or electric motors.
An industrial drive has some special features when driven by electric motors. The speed-torque characteristic of the motor can be very easily modified to suit the load characteristic. It has a sufficient overload capacity and can be overloaded for short periods without affecting the life of the motor.
Type Of Electric Drive
1) AC drive
2) DC drive
AC drive mainly consists of AC motor so it called AC drive.
DC drive mainly consists of DC motor so it called DC drive.
The following table shows the comparison of AC drive & DC drive as follow:-
1DC drive
There are mainly two parts of D.C. Motor.
• Armature System
• Field System
• Other mechanical parts.
DC drives, because of their simplicity, ease of application, reliability and favorable cost have long been backbones of industrial applications. The basic principle of a DC motor is if electrical energy is supplied to a conductor lying perpendicular to a magnetic field, the interaction of current flowing in the conductor and the magnetic field will produce mechanical force which creates a torque
Method of speed control of DC motor
The speed can be controlled by varying
(i) Flux/pole, Φ (Flux Control).
(ii) Resistance Ra of armature circuit (Rheostatic Control).
(iii) Applied voltage V (Voltage Control).
Speed Control of Shunt motor:-
(i) Variation of Flux or Flux Control Method: By decreasing the flux, the speed can be increased and vice versa. The flux of a dc motor can be changed by changing shunt current with the help of a shunt field rheostat. Since shunt current is relatively small, shunt field rheostat has to carry only a small current, which means loss is small, so that rheostat is small in size.