Answer / y.v.vsurya teja

as in the initial conditions the motor is at rest,the speed
of the motor is zero.

using KVL equation,
V=Eb+IaRa,Eb=back EMF
Ia=armature current
Ra=armature resistance
(very small),V=input supply(around 220V dc)
Initially speed=0
implies Eb=0
Ia=(V/Ra) which is very high value,since V is high
and Ra is very small.
Hence starting resistance or 3 point starter
are used.

Answer / gajanan

at starting rotor is stationary, due to its inertia it will
require more torque to rotated and torque is proportional
to armature current
so requires more starting current

Answer / electrical

V=Eb+IaRa,Eb=back emf

At starting there is no back Emf
And V (Normal Supply Voltage)=220V(single Phase)
Ia = Armature Current
Ra = Armature resistance(Usually less (.2 to .5 ohms)

From the Above equation,
Ia = (V-Eb)/Ra
Since Eb is Zero, the above eqn will become
Ia = V/Ra (220/0.5 = 440A)
So at starting the current thru the armature is very high
to minimise this we are using the starters.

Answer / rvg

Dear Mr. Gajanan, as you are dealing with more machineries,
i want to stress some points:
During starting of motor:

the armature(rotor) is stationary, and it does not having
any magnetic property to resist the applied rotational
frequency at that time.

so it will act as a conductor fixed at rotating magnetic
field and produce some eddy current loss, and with in short
period of time, it will develope some anti rotional effect
to resist the rotational field, ie: rotating with same
direction to reduce the eddy curent.Thats why after
starting current reduces.

inertia of the rotor also having some major role on the
starting current. The gap between rotor and stator having
role on the starting current and initial power factor.

Answer / ta

What the persons above said is true mathematically.
If you look at it as in theory, we know a DC Machine can act
as a Motor as well as a Generator. So as the Current is
supplied to the conductors present in a magnetic field, a
Force starts to act on the current carrying conductor. This
current forces the Rotor to rotate. At this time the only
Voltage is the supplied Voltage. As soon as the motion
starts there has to be an Emf induced which directly depends
on the speed of rotor. This induced voltage is called
Counter EMF or the Back EMF, which is absent at the instance
the motor is supplied a current. This EMF in effect acts
opposite to the Supplied Voltage. So whatever Drop of
volatge is in The resistance of Rotor wires is due to the
Total Supplied Voltage and Resistance is pretty low so the
current at the start is very high

Answer / vijendra joglekar

There are two reasons

1.At starting the motor will behaves as low resistance circuit.
As we know the fundamental i mean when the
resistance is low the current will be high.

2. At starting the back emf will be zero

As we know the voltage equation of motor
V=Eb+Ia Ra
Where Eb is the back emf which is zero at starting
V is they supply voltage
Ra is they armature resistance which is very small
hence we can prove that by using the equation.

Answer / raul

ARMATURE CURRENT (Ia)= V-E)/Ra, during starting, back emf
E is zero, n supply voltage is practically constant, n
armature resistance is small, so armature current is
initially very high...........

Answer / ajaysinh

THERE IS NO BACK TO OPPOSE THE INRUSH CURRENT ON SWITCHING.

Answer / guest

When voltage is first applied to the motor field winding, only the conductor resistance opposes the flow of current to the motor winding. For example a motor winding rated at 240V with a winding resistance of 0.6 ohms. Using Ohms law we find I (current)= E/R or 240V/0.6 ohms = 400Amps.
DURING MOTOR STARTING = LOW RESISTANCE, HIGH CURRENT
Because the conductor resistance is so low, the motor has a very large inrush current.
Once the rotor begins turning, the rotor bars (winding) are increasingly cut by the stationary magnetic field, resulting in increasing counter electromotive force (CEMF).
CEMF opposes the applied voltage, resulting in an increased opposition to current flow within the conductor, which is also called inductive reactance.
The increase in inductive reactance, because of self induction, causes the impedance of the conductor winding to increase and this results to reduced current flow.
Same motor, R=0.6 ohm, XL=6 ohms, V=240 volts

Z = √(R^2+〖XL〗^2 ) = √(〖0.6〗^2+6^2 ) = 6 ohms
I = E/Z = 240V/6Ω = 40Amps

DURING MOTOR RUNNING = LOW RESISTANCE, HIGH INDUCTANCE, LOW CURRENT

Answer / tawfiq

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current is estimated to be 7*rated current if the motor was
in direct to line connection

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