Frequent question: Why does a DC motor draws high current at 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 starting current is very high in DC motor How does the starter reduce the starting current to a safe value?

Starters are used to protect DC motors from damage that can be caused by very high current and torque during startup. They do this by providing external resistance to the motor, which is connected in series to the motor’s armature winding and restricts the current to an acceptable level.

How can we reduce the starting current of a DC motor?

To prevent such an incidence from occurring several starting methods of DC motor has been adopted. The main principal of this being the addition of external electrical resistance Rext to the armature winding, so as to increase the effective resistance to Ra + Rext, thus limiting the armature current to the rated value.

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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.

How do you reduce a high starting current?

Some of the techniques used to reduce the starting current of a squirrel induction motor:

  1. Soft starting (voltage control)
  2. Variable Frequency Drives (Voltage and frequency control)
  3. Star/Delta Starting.
  4. Stator impedance and/or resistance starting.
  5. Autotransformer Starting.

What is starting current of DC motor?

Now, when the motor is at rest, obviously, the back emf Eb = 0. 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.

How do you increase the starting torque of a DC motor?

Any DC motor will experience a decrease in speed when a load is applied, which causes the back EMF to decrease. The reduction in back EMF increases the net voltage, since the supply voltage remains constant. The net voltage increase causes the armature current to increase, which increases the motor torque.

Why is the inrush current to large DC motors often limited?

Because DC motor inrush current can be as much as 2-3 times its steady state current, employing Inrush Current Limiters to reduce the inrush current will help to extend the life and efficiency of the DC motor.

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How does the starter reduce the starting current to a safe value?

When the starter arm is brought to stud 1 position, current will flow through the armature winding through the starter resistance. This will reduce the high inrush of starting current as a whole resistance is added to the armature winding.

How does back EMF effect a DC motor?

Back emf is very significant in the working of a dc motor. The presence of back emf makes the d.c. motor a self-regulating machine i.e., it makes the motor to draw as much armature current as is just sufficient to develop the torque required by the load.

How is DC inrush current calculated?

The peak value of the inrush current is determined by the equation, i=C*dv/dt. The capacitance is the total capacitance of the EMI filter and the input capacitance of the DC-DC converter. The dv/dt is the slope of the applied voltage waveform.

What limits the starting current for a shunt DC motor?

As with the shunt motor, under starting conditions the back e.m.f. is zero, and if the full voltage was applied the current would be excessive, being limited only by the armature and field resistances. Hence for all but small motors a starting resistance is required to limit the current to a safe value.