Motor starting current

[jimmy56]

What could be the difference between starting current of a motor (induction) when started on load ( let 100% of full load)and same motor at no load (uncoupled). Is starting current will remain the same and only starting current time will reduce.<br>
I want to test higher rated motor on a motor test bench rated for maximum 90 kw motors.<br>

 

[merbas]

The answer is in the motor model.<br>
<br>
Why the stator takes 5 or 6 times more while starting?<br>
<br>
Because rotor's impedance equals zero in the begining and stator has short circuit current.<br>
<br>
If 100% full load exist, notting will change in electrical diagram, so current will same.<br>
<br>
But, as mechanical view;<br>
<br>
Moment model of motor, related (I) moment of stability. So that to reach desing speed, it takes more time.<br>
<br>
Result: current will same, but time will longer.<br>
<br>
The longer the time, makes to long time for current. <br>
<br>
Therefore wires or isolation can burn. <br>
<br>
All engineers know that if you have heavy load,<br>
<br>
You start first triangle stator coils (delta connection)to improve impedance of stator and less current <br>
(x=y=z)<br>
(a=b=c)<br>
<br>
Then star connection to get efficent from motor.<br>
(x y z)<br>
&quot; &quot; &quot;<br>
(a b c)<br>

 

[umn]

The cause of high starting current: <br>
The starting current will be high (of the order of 4 to 6 times of rated current, depending on the motor rating) if it's on no-load or on load. The current will come down as the motor picks up speed, and settles down at a stable state. When the motor is ON-LOAD, because of the higher inertia more time is needed to attain this state. After that, a steady current will be drawn- the current will be more if the load is more.<br>
<br>
Usually, the no-load current will be 2-10% of the full load current- again, this will depend on the motor-rating.<br>
<br>
I've one suggestion to make in Merbas's posting above:<br>
<br>
Usually, for heavy loads, we use the STAR-DELTA transformation, and not delta-star. If you've three windings A1-A2, B1-B2 and C1-C2 where '1' indicates start of winding and '2' indicates the end,<br>
for Star connection, you connect A2-B2-C2 together. The 3-phase power is applied across A1, B1 andd C1. This will cause less current to flow through the windings.<br>
Once you pick up speed, you quickly change over from star to delta.<br>
<br>
For delta, you connect A2-B1 together, B2-C1 together and C2-A1 together. The 3-phase supply is connected across the three junctions. This allows more current with the same 3-phase voltage.<br>
<br>
Regards.