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A rotating d.c. motor generates a back-emf which opposes the supply voltage and reduces the current drawn by the motor. When the motor is stationary, it cannot generate this back emf and, so, the only opposition to current is the resistance of its windings which is relatively low. So, on startup, the current is large; as the machine starts to run, the resulting back emf, acts to reduce the current.
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Why you use DOL starter in starting of motor?
because of starting current of induction motor is very high and it damages the insulation of motor
Is torque and current proportional?
Sometimes, for example in a separately excited dc motor. But in a series-wound dc motor the torque is proportional to current-squared, which gives the motor a very high starting torque and this makes series motors very popular for traction.
Why a 750 kw motor takes 17 amps starting current?
The starting current is high because the motor's rotor winding has very low resistance. It's similar to a transformer with a shorted secondary windings. As the motor accelerates,the back emf increases which resist the flow of current in the rotor winding. Hence,the current drop to the rated full-load value.
Why it is dangerous to supply power to an unloaded series connected DC motor?
Starting torque becomes very high due to which motor can attain very high speed and can damage its body and connected equipments.
What advantage and disadvantage of DC motor?
Advantage over?DC motors used to be preferred where precise control and variability of rotation speed were desired, but modern motor design and electronic controls have very largely negated that advantage.A series motor is useful mainly for driving a tram or bus because the torque is proportional to the square of the current. That means that a very large torque is produced for starting up and no gearbox is necessary. The motor is started with a current-limiting resistor which is switched down by the driver or automatically as the speed builds up.
