Why Motor Start Current (DOL start) is 4-7 Times of Its Rated Current?

It's the same principle as starting off on a bicycle, hardest to start and accelerate, and easiest while just maintaining speed....Think of the muscle power in your legs as the electricity needed to start the motor...

It's because Power = volts x amps x power factor, and when the motor is just beginning to rotate, the power factor is very low and remains that way until it gets to around 80% speed. So although the current is very high, the motor kW at startup is not that much higher than normal.

To add to what JRaef has said...

When a motor is first energized, it appears to its souce as a purely resistive load. Once current starts flowing, its inductive nature starts to be more apparent to the source then its resistive nature.

Basically... the level of back EMF rises in the core and begins to oppose the current flow, when the back EMF is greatest, the opposition to current flow is greatest and the current draw from the source evens out.

When you load the motor and the amount of energy needed to keep that motor spinning goes up so does the demand from the source as well.

You have two correct answers above. One relates to AC motors and the other to DC motors. Both draw large amounts of current from a dead stop; the reasons are similar but for complete understanding you need two different explanations, one for DC motors and one for AC motors.

All DC machines are both motors and generators. It depends on how they are used. The speed of the machine is directly proportional to the applied VOLTAGE. I built an electric car out of a VW Beetle and I like to use this analogy: if you are cruising along, steady state, at 50 MPH, lets say you are drawing 100 amps out of the battery pack. You come to a hill. The car starts to climb. speed only drops a tiny bit. The motor draws more AMPS out of the battery pack, consuming more power. You crest the hill and start down the other side. Now the motor has become a generator. Speed is slightly above 50 MPH, but now you are shoving AMPS BACK INTO THE BATTERY PACK. The "back EMF" referred to in the gentleman's post refers to this generated voltage. EMF means electromotive force, a fancy way of saying voltage. There is no "back" EMF to opposed the applied EMF (voltage) at motor start. there is a huge inrush of current (and a tremendous amount of torque) as the motor starts to spin up, the COUNTER (back) EMF is created and opposes the applied Voltage until the equilibrium speed is reached. If you add more voltage, a new equilibrium speed will be found.

This simple explanation is from a simple minded mech engr, so I might not be correct:

As other posted noted, stationary motor has only resistance, inductive loads only pick up when motor speed pick up.

A static electric motor has less overall resistance than a running motor. With a somewhat constant voltage, the lower resistance will result in higher currents. As the motor speeds up and gains resistance, the current will decrease.

This is my simplified understanding of the variation of motor current with motor speed, I hope it helps your understanding.

Fegor,

Look at Faraday's work and Lenz' laws. The motor winding is with a fairly low DC resistance, as others have stated. This resistance is independent of the operating status of the motor (off, just energized but not turning, etc.). Once energized, the magnetic fields induced in the iron laminations of the rotor will interact with those in the stator, to give a torque (rotational force) that will (hopefully) cause the rotor to turn. Before it starts to turn, this is called a "locked rotor", and is when the current flow in the stator is greatest.

After the rotor has started turning, its moving magnetic field starts "cutting" across the winding(s) in the stator, inducing a voltage into these windings that is opposite or "counter" to that provided by the power line. As the speed increases, the torque will rise to a peak and then begin to fall some (with a very rapid fall when very close to the synchronous speed of the motor), while the induced voltage increases with increasing speed. At some point, the falling torque matches the torque of the load attached to the motor and its speed holds steady. This induced voltage is usually called the "counter EMF" or the "back EMF", and is always out of phase with the applied voltage. It can be given a calculated value of AC resistance, which we call the inductive reactance. The vector sum of the DC resistance and the inductive reactance is the impedance (AC resistance) of the motor, and is many times the DC resistance of the motor (large enough to reduce the current down to the its given name-plate value).

Did you know that with very efficient motors, the DC resistance is lower, so the inrush current can be up to 10 (or sometimes more) times the running current? This has been reflected in code requirements that starters and circuitry for very efficient motors be capable of handling this higher inrush current.

--John M.

As per Newton's law to over the initial inertia any equipment needs more torque/force like in car first gear ( high torque,low speed) and moves further. Please refer other posting also to clear the concept.

A rotating motor acts as a generator, creating an opposing voltage (back emf) to the applied voltage. For this reason, the motor draws less current when up to speed.

Imagine a transformer with a shorted secondary.It will draw a lot of current.This is the condition when the motor is stopped.The field in the rotor lags the field in the stator,and it appears to be a shorted(almost0 secondary of a transformer.As the current induced magnetic field in the rotor begins to to follow the rotating stator filed, the lag (or speed differential) decreases, and the current on the stator decreases.It will never reach zero, and will always "Slip" a little depending on the design of the motor.The theoretical speed of a synchronous ac motor is Frequency x 120 divided by number of poles.The motor cannot actually achieve this speed, because slip is required to produce work.If there is not slip, there is no relative speed differential between rotor and stator,therefor no induced current or magnetic field in the rotor.

There are,however, true synchronous motors,but they require external excitation.They are frequently used to improve power factor, especially when PF is variable.

Hope this helps.

This explanation coupled with the one with the two diagrams is pretty close to the "whole explanation" However, you are calling an induction motor a synchronous motor and a synchronous motor a "true" synchronous motor. The induction motor is the "squirrel cage" design most of us are familiar with. The alternator in a car with its separately excited field (rotor) would be an example of a synchronous machine. The transformer analogy is excellent. The "shorting bars" of the induction motor have maximum current on initial start up, thus the magnetic field of the shorting bars is maximum. You have maximum coupling of electrical energy from the "primary" of the transformer, which is the stator, to the secondary, which is the rotor. The reason gets back to basic laws of electromagnetics. the speed with which the magnetic field "cuts across" the conductor (shorting bars) is proportional to the current induced. The speed will never be greater than it is at start up. The speed drops to almost zero after the motor has "spun up;" that is assuming no load. The "slip" referred to above.