Why does a DC motor draw heavy current at the moment of starting?

Answer: The starting current is high as there is no back EMF or counter EMF present in the armature circuit because atstarting counter EMF is zero. … It’s armature has very less resistance due to this it need more current at starting time. Hence DC starters are used to limit the starting current ofmotor.

Why motor takes heavy current at starting?

At start, the reactance of the rotor is high because the slip of the motor is equal to unity. The value of Rr/s increase as the slip gets decrease.When the induction motor is started the rotor reactance is more than the rotor resistance and because of the large Xr/Rr ratio the motor takes large inductive current.

Why is the starting current of a DC motor much greater than the running current of the same motor at rated speed?

The starting current of a dc motor is much higher than the running current while the motor is operating at its rated Speed. At the instant power is applied, the armature is motion less and the armature current is limited only by the very low armature circuit resistance.

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What is starting current of a DC motor?

Hence, armature current at the moment of starting can be given as Ia = E / Ra. In practical DC machines, armature resistance is basically very low, generally about 0.5 Ω. Therefore, a large current flows through the armature during starting. This current is large enough to damage the armature circuit.

What are the damages that can be caused by high starting current in DC motors?

the starting current will be enormously high. Direct-on-line (DOL) starting are not used for large DC motor because the starting current will be enormously high which will cause large voltage drop in the supply mains and armature may get damaged due to excessive heating.

What is the difference between inrush current and starting current?

The inrush current is the current transient that flows in the first cycle or so that establishes the magnetic field in the iron. … The inrush current can be significantly higher than the locked rotor current with 2 – 2.5 times the start current being quoted by some.

Why is DC motor current so high?

Answer: The starting current is high as there is no back EMF or counter EMF present in the armature circuit because atstarting counter EMF is zero. … It’s armature has very less resistance due to this it need more current at starting time. Hence DC starters are used to limit the starting current ofmotor.

Does current affect motor speed?

The current through the motor times the resistance of the motor creates a small voltage that must be subtracted from the applied voltage. As the load and current increase, this IR drop also increases causing the motor to lose a little speed.

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What does current do to a motor?

Increasing Voltage, Increases the Current Pulled, which Increases the Strength of the coil, which increases the RPM AND Torque of the motor. They are all connected. … Current and Voltage are related, especially with a fix Resistance like the motor. Except that the motor is only a fixed resistance depending on it’s load.

Why is DOL starter used up 5 hp?

For a large rating motor, ranging from 5 HP to 25 HP, oil immersed DOL starters are used which provides insulation against sparking on contact points and hence increases the life of starter. DOL starter consists of MCCB, contactor, and overload relay.

How can a DC motor limit current?

4 Answers

  1. Make sure the load is never so high that the motor stalls or overloads.
  2. Make sure that the voltage is so low that the current through the windings will never be higher than rated.
  3. Use a current controller to drive the motor that can limit the current at the given maximum.

6.10.2013

What are the applications of DC motor?

DC machines are also used in applications where high starting torque and accurate speed control over a wide range are important. Major applications for DC motors are: elevators, steel mills, rolling mills, locomotives, and excavators. Like other rotating machines, DC motors result from the interaction of two magnets.

Is dc series motor self starting?

DC motors are self-starting motors. The armature is stationary, and the magnetic field rotates in the AC motor but in DC motor the armature rotates, and the magnetic field is stationary.

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What is used to adjust the speed of a DC shunt motor?

In this method, speed variation is accomplished by means of a variable resistance inserted in series with the shunt field. An increase in controlling resistances reduces the field current with a reduction in flux and an increase in speed. This method of speed control is independent of load on the motor.

Is the most common cause of motor failure?

Low Resistance. The most common cause of motor failure, and arguably the most difficult to overcome, is low resistance. Low resistance is caused by the degradation of the insulation of the windings due to conditions such as overheating, corrosion, or physical damage.

What are the disadvantages of DC motor?

Disadvantages of DC motors

  • High initial cost.
  • Increased operation and maintenance cost due to the presence of commutator and brush gear.
  • Cannot operate in explosive and hazard conditions due to sparking occur at brush (risk in commutation failure)

20.10.2017

Motorization