Why does a DC motor develop a high starting current when 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.

What effect will high starting current in a DC motor have in a circuit?

very high starting torque will be produced (as torque is directly proportional to the armature current), and this high starting torque may cause huge centrifugal force which may throw off the armature winding. other loads connected to the same source may experience a dip in the terminal voltage.

How is the high starting current of a DC motor limited?

Upon starting a DC motor will pull maximum available current limited by the resistance of the current source and the basic resistance of the motor windings. This startup current decreases rapidly as the motor spin and creates back EMF which opposes the incoming current.

Why a DC motor should not be started without a starter?

DC motor will pull large current when started directly without starter. This is because rotor is stationary and there no back emf to oppose inrush of current. Large starting current drawn by the DC motor will dip voltage momentary that affects other potential equipment operating on the same line.

Why starting current is very high in DC motor 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 much larger is the starting current than the normal full load current in DC shunt motor?

The start-up current can be as high as five to eight times the full load current.

Why do motors have inrush current?

When an electrical device, such as an AC induction motor, is switched on, it experiences a very high, momentary surge of current, referred to as inrush current. The interaction of these two magnetic fields produces torque and causes the motor to turn.

How can the starting current of a motor be reduced?

Starting current of three phase induction motor can be reduced to safe value by including:

1. resistor in rotor circuit at instant of starting.
2. capacitor in stator circuit at instant of starting.
3. capacitor in rotor circuit at instant of starting.
4. inductor in rotor circuit at instant of starting.

Can a DC motor start without starter?

Motors below 1 HP (0.7457) is directly connected to the power supply without starter because their armature resistance is very high and they have the ability to afford and pass higher current due to high resistance. So the armature windings are safe from the high starting current while staring a motor.

Why should large DC series motors not be operated without a load?

A series motor should never be started at no load. With no mechanical load on the series motor, the current is low, the counter-EMF produced by the field winding is weak, and so the armature must turn faster to produce sufficient counter-EMF to balance the supply voltage. The motor can be damaged by overspeed.

Why the starting current of a DC motor is high and then the motor current decreases as the motor starts running?

As the speed of the rotor is zero at the start, the rotor conductor will cut the maximum flux and the maximum voltage will be induced in the rotor. As the motor starts accelerating the speed of the rotor will align in the direction of synchronous speed of the motor and the slip will get reduced.

Do DC motors have inrush current?

DC motor inrush is the maximum, instantaneous input current drawn by the DC motor when first turned on and it is important to know how to limit the inrush current to prevent possible damage to the motor.

What happens if the starting current of a DC motor is high?

Such high starting current of DC motor creates two major problems. Firstly, current of the order of 400 A has the potential of damaging the internal circuit of the armature winding of DC motor at the very onset.

How to reduce the high starting current of induction motor?

Therefore different starting of induction motor, methods such as (star delta starter, auto transformer starter and other starting methods) are employed in order to reduce the high starting currents of induction motor. Induction motor can be compared to an electrical transformer with the secondary short circuited.

What are the disadvantages of a DC motor?

As a direct consequence of the two above mentioned facts i.e high starting current and high starting torque of DC motor, the entire motoring system can undergo a total disarray and lead towards into an engineering massacre and non-functionality. To prevent such an incidence from occurring several starting methods of DC motor has been adopted.

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

At this moment to maintain the armature current to its rated value, R ext is progressively decreased unless its made zero, when the back emf produced is at its maximum. This regulation of the external electrical resistance in case of the starting of DC motor is facilitated by means of the starter.