Voltage drop calculation at starting

[kimseunghee]

The first motor in operation draws big inrush current when generator is started and is on the line to switchboard. The inrush current of second motor will be lower than. In order to calculate voltage drop of second motor, what aspects are to be considered and how the voltage drop can be calculated.

 

[waross]

I am not sure that I agree with your hypothesis.
Why do you think that the second motor will have a lower inrush? It may have, but we need to know why before going any further.
Note that if the inrush is less, it will probably last longer.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Is he thinking the running machine will provide some of the starting current? It will won't it? But how much?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

The only possible scenario I can think of in which the STARTING current might be lower on the second motor would be if they are both connected to a common shaft. This is done all the time with large machines that don't need full torque to accelerate, such as impact rock crushers. Twin motors are connected to a common shaft and the machine is started with only one motor, then the 2nd is put on-line afterwards right before the real load is applied. But that is STARTING current, not INRUSH current, they are different. Inrush current is the very short duration current that flows for a few cycles as the magnetic field is established.

 

[jghrist]

Starting current of a 2nd motor could be less if the starting current of the 1st motor caused a voltage drop. The 2nd motor will have reduced voltage starting.

 

[kimseunghee]

I am sorry that my explanation is very poor. I enclose power distiribution one line diagram. The voltage drops at motor starting and voltage is stablised when motor reaches rated speed. If another motor is started, voltage drops lesss because the first motor is in operation. This means that inductance load of first started motor is engerized. In circuitry, inductance of fisrt started motor is paralleled with inductance of second started motor.
I'd like to calculate voltage drop of second motor at starting.

 

[7anoter4]

hi kimseunnhee
Can you send the attachment as jpg or pdf or word or excel since dwg[autocad] is very difficult to read,do you?
Best Regards

 

[kimseunghee]

Here is attached one line diagram.

 

[7anoter4]

Hi kimseunghee
I understood from your one line diag. that you have a Switchgear with 2 permanent coupled bus bar sections. From that Switchgear by means of 4 feeders 2 remote Panels are supplied.
AFT panel 2 feeders of 3*3*95 sqr.mm each of them supplying one half panel [3 motors]
FWD Panel also subdivided in 2 halves supplied by means of 3*3*120 sqr.mm.
Also I guess that each motor is provided with a star-delta switch for start.
In my opinion if the rated voltage of motors is 400 V [and 50 HZ frequency] according to permitted voltage drop [according to IEC 34-12] all motor M1 up to M10 are 75 kw rated and W1 and W2 are 110 rated.
Let say the switchgear is supplied from 2 transformer [1000 KVA, 420 V ,uk%=6+7.5%maximum error] then, when the M5-M10 will start the voltage on the AFT Panel [all other motors are already in operation] will drop to 385 V [4% drop- maximum permitted is 5%]
When W1 or W2 will start then the voltage on FWD Panel will drop to 358 V [11% >5% permitted.
If no motor is in operation on FWD Panel when W1 start then the voltage will drop only to
373 V [6.5% also >5% but still tolerable]
If you double the feeder that means 6*3*120 you may achieve 6% drop when w1 starts and M motors are in full operation.
My opinion!
See attachment
Best Regards

 

[7anoter4]

Hi kimseunghee
The maximum drop voltage permitted by IEC34-12 for a
limited time operation[if frequency is not less than 95%]may be 10% indeed[Zone B].
Also for M1-M5 starting the voltage drop is 6.25%[the voltage on FWD Panel drops up to 375 volts when all other motor is in operation]
Regards