DC motor are those motor which tuns on the dc supply only. the motors which runs of ac suppu called induction motor wherever motor runs on dc supply called dc motors. dc motors are generally available in three different forms: dc series motor, dc shunt motor and dc compound motors. here we will see all the details about all these three induction motors. SO read this article completely to find which dc motor used in where placed efficiently.
by the different applications of dc series, shunt, and compound motor, we can easily understand the characteristics of different dc motors. So let see all the motor characteristics in easy understandable language.
DC SERIES MOTOR
- DC series motor is having high-starting torque and having the capability of handling overloads that is essential for traction drives.
- These motors are having simple and robust construction.
- The speed control of the series motor is easy by series-parallel control.
- Sparkless commutation is possible because the increase in armature current increases the load torque and decreases the speed so that the emf induced in the coils undergoing commutation.
- Series motor flux is proportional to armature current and torque. But armature current is independent of voltage fluctuations. Hence, the motor is unaffected by the variations in supply voltage.
DC SHUNT MOTOR
¨That is motor input drawn from the source is proportional to the square root of the torque. Hence, the series motor is having self-retaining property.
From the characteristics of DC shunt motor, it is not suitable for traction purpose, due to the following reasons:
- DC shunt motor is a constant speed motor but for traction purpose, the speed of the motor should vary with service conditions.
- In case of DC shunt motor, the power output is independent of speed and is proportional to torque. In case of DC series motor, the power output is proportional to torque, So that, for a given load torque, the shunt motor has to draw more power from the supply than series motor.
(T ∝ Ia).
¨The flux developed by shunt motor is proportional to shunt field current and hence supply voltage. But the torque developed is proportional to φsh and Ia. Hence, the torque developed by the shunt motor is affected by small variations in supply voltage.
5. If two shunt motors are running in parallel, the currents drawn by the motor from the supply mains will be different and cause to unequal sharing of load.
what happens in AC motor compared to DC?
¨If DC series motor is fed from AC supply, both the field and the armature currents reverse for every half cycle. Hence, unidirectional torque is developed at double frequency.
¨Alternating flux developed by the field winding causes excessive eddy current loss, which will cause the heating of the motor. Hence, the operating efficiency of the motor will decrease.
¨ Field winding inductance will result abnormal voltage drop and low power factor that leads to the poor performance of the motor.
¨ Induced emf and currents flowing through the armature coils undergoing commutation will cause sparking at the brushes and commutator segments.
WHich DC MOTOR suitable for traction
¨No single motor can have all the electrical operating features required for traction.
¨In earlier days, DC motor is suited for traction because of the high-starting torque and having the capability of handling overloads.
¨DC compound motors are also preferred for traction applications since it is having advantageous features than series and separately excited motors.
But nowadays squirrel cage induction and synchronous motors are widely used for traction because of the availability of reliable variable frequency semiconductor inverters.
The squirrel cage induction motor has several advantages over the DC motors.
- Robust construction.
- Highly reliable.
- Low maintenance and low cost.
- High efficiency.