What happens when there is no back EMF in DC motor?

If back emf of a dc motor vanishes suddenly, motor circuit will try to retain back emf by drawing more current from supply. If supplying unit didn’t trip down by this time, excess current in armature may heat up the armature and it may cause burning of armature winding.

What is no-load current in DC motor?

The no-load current is the current required just to turn the motor shaft with nothing connected. It is the current required to overcome the bearing and brush friction plus the aerodynamic drag caused by the rotor moving in the air and moving air through the motor for cooling purposes.

What is the starting current of a DC motor?

Without a starter, when the power is applied to the motor, the initial current will be 220/0.5=440 A. This is 17.5 times the full-load current! So the need of a starter is imperative. An important feature of DC motors is their ability to maintain the same speed under variable load.

What does no load current mean?

No load current is that current which is flowing when load is disconnected or not present at a time. Lets consider an example of Transformer, when no load is attached on the secondary side of a transformer, then no current flows due to open circuit of secondary side.

How is DC motor DC current calculated?

Calculate the field current for the motor using the formula I = V/R where I is the field current. As an example, assume V is 40 volts and R is 500 ohms: I = 40/500 = 0.08 amps.

Why is starting current high in a motor?

This high current in the rotor will create its own magnetic field which opposes the main stator magnetic field, this weakens the stator magnetic field so the back EMF in the stator will drop and the supply voltage will be much higher than the stator back EMF and so the supply current increases to a high value.

Why is the torque curve for a DC motor greater than zero?

When examining the torque-current curve for a DC motor, notice that the no-load (stall) current is greater than zero. This is because some current is needed to overcome the internal friction of the motor. Torque-current curve for a DC motor.

What is the function of the commutator in a DC motor?

The commutator converts the alternating current generated in armature into the unidirectional current. It is a mechanical rectifier, so the commutator collects induced EMF or current developed in the armature. In the DC motor, the commutator is used to alternate the magnetic field created by the DC current.

What is the counter-electromotive force in a DC motor?

Counter-electromotive force (cemf) in a dc motor. The net voltage acting in the armature circuit in Fig. 5.2 is (Es – Eo) volts. The resulting armature current /is limited only by the armature resistance R, and so

Why current is constant in a DC shunt motor?

Answer 4. Because flux is constant in DC shunt motor. In the case of DC motors Speed ∝ Back emf (E b )/flux (φ). The field winding is connected across the armature. The resistance offered by the field is constant so the current is constant.