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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.
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 the starting torque of a dc series motor is high?
load is heavier so starting torque is requiredAnswerBecause the same current is passing through both the armature and field windings, the torque is proportional to the squareof the current. Since the starting current is alway high (no back emf), the torque will be very high indeed.
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 is a starter required for a DC motor?
Large DC motors with field windings instead of permanent magnets present a very heavy load when starting. To prevent fuses blowing, a series of ever smaller resistors are sequentially switched in series with the motor. As the motor picks up speed, a back EMF in opposition to the applied voltage limits the maximum current. When the motor reaches it's running speed the 'starter' is out of circuit.
Why the no load current is more in an induction motor because of high reluctance airgap?
due to high reluctance air gap magnetisation current is very high as a result no load current is more
Why dc series motor should start with load?
In DC Series Motors the field is in series with Armature. When the motor starts at no load, the armature current is very low and so is the field current. As the speed of DC Motor is inversely proportional to field current, the motor will try to shoot up to very high speed at no load and it can be dangerous for the motor itself and for this reason, any DC Motor should always be started on load.
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.
Why 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.
Why the series motor is started with some initial load?
In dc motors Speed is directly proportional to back emf and inversely proportional to flux i.e N∝(Eb/φ) In series motor back emf is practically constant. So if we start motor without any load Flux will be small and so speed will be very high. if we start motor without any load it will gain very high speed and may get damaged due to centrifugal forces. Series motor have very high starting torque and used where high starting torque is required.
Why can we start a shunt motor under no load but why can't a series motor?
The torque on a series motor is determined by the square of the armature current and, so, is very large -much larger than that of a shunt motor. Consequently, on starting, with no back-emf being generated, the torque on a series motor can be dangerously high if the machine has no mechanical load.
Why starting current of the motor is high?
The simplest way to start a three-phase induction motor is to connect its terminals to the line. In an induction motor, the magnitude of the induced EMF in the rotor circuit is proportional to the stator field and the slip speed (the difference between synchronous and rotor speeds) of the motor, and the rotor current depends on this EMF. When the motor is started, the slip speed is equal to the synchronous speed, as the rotor speed is zero (slip equal to 1), so the induced EMF in the rotor is large. As a result, a very high current flows through the rotor. This is similar to a transformer with the secondary coil short circuited, which causes the primary coil to draw a high current from the mains. When an induction motor starts DOL, a very high current is drawn by the stator, on the order of 5 to 9 times the full load current. This high current can, in some motors, damage the windings; in addition, because it causes heavy line voltage drop, other appliances connected to the same line may be affected by the voltage fluctuation. To avoid such effects, several other strategies are employed for starting motors.
