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Voltage drop caused by a motor


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#1 powerboy

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Posted 15 July 2009 - 02:51 PM

I have a motor that caused a 21% voltage drop on the secondary side of a transformer with neg impedence. Would the voltage drop on the primary side be 21% or would if be a function of the turns ratio also.

#2 benbo

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Posted 15 July 2009 - 03:43 PM

QUOTE (powerboy @ Jul 13 2009, 12:51 PM) <{POST_SNAPBACK}>
I have a motor that caused a 21% voltage drop on the secondary side of a transformer with neg impedence. Would the voltage drop on the primary side be 21% or would if be a function of the turns ratio also.

I hate power, so I'm probably wrong, but I think it would also be 21%. I think that's basically what the whole concept of using "per-unit" measurements in power is all about.

But like I said, I'm always wrong when I delve into power issues.

#3 Art

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Posted 30 July 2009 - 01:21 AM

at the motor or transformer?
transient or steady state?
motor HP, xfmr KVA?


#4 powerboy

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Posted 30 July 2009 - 02:06 PM

QUOTE (Art @ Jul 29 2009, 09:21 PM) <{POST_SNAPBACK}>
at the motor or transformer?
transient or steady state?
motor HP, xfmr KVA?

3000hp motor
5000 kva transformer

#5 Art

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Posted 30 July 2009 - 02:33 PM

QUOTE (powerboy @ Jul 28 2009, 01:06 PM) <{POST_SNAPBACK}>
3000hp motor
5000 kva transformer


so the xfmr only serves a 3000 HP motor?
what kind of load, what is the motor driving and how is it started? VFD or across the line?
transient or steady state? does the voltage only dip when starting up, or does it stay low when running?
is the drop at the motor leads or xfmr terminals?
what voltage is the prim/sec ?

with the motor and xfmr that closely sized you're lucky to get only 21% dip when starting...
and without knowing the generating capacity it's impossible to tell whether the prim will dip proportionally...
if transient, probably not
if steady state, probably yes

#6 z06dustin

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Posted 30 July 2009 - 03:26 PM

QUOTE (Art @ Jul 30 2009, 07:33 AM) <{POST_SNAPBACK}>
so the xfmr only serves a 3000 HP motor?
what kind of load, what is the motor driving and how is it started? VFD or across the line?
transient or steady state? does the voltage only dip when starting up, or does it stay low when running?
is the drop at the motor leads or xfmr terminals?
what voltage is the prim/sec ?

with the motor and xfmr that closely sized you're lucky to get only 21% dip when starting...
and without knowing the generating capacity it's impossible to tell whether the prim will dip proportionally...
if transient, probably not
if steady state, probably yes


+1 to what Art said. IMHO, if the transformer is saturating from your motor starting current, you could see voltage drop on the secondary and not the primary. If it's a steady state drop, you could be pulling down the voltage from the feeder, in which case the primary would fall by 21% as well. If you're in a rural system this is more likely to be the case, in a metropolitan/urban area it's more likely you'll have a strong enough feeder..... although 5MVA is pretty large anyways.

Is there any sort of soft-start on the motor?

#7 powerboy

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Posted 03 August 2009 - 04:39 PM

QUOTE (z06dustin @ Jul 30 2009, 11:26 AM) <{POST_SNAPBACK}>
+1 to what Art said. IMHO, if the transformer is saturating from your motor starting current, you could see voltage drop on the secondary and not the primary. If it's a steady state drop, you could be pulling down the voltage from the feeder, in which case the primary would fall by 21% as well. If you're in a rural system this is more likely to be the case, in a metropolitan/urban area it's more likely you'll have a strong enough feeder..... although 5MVA is pretty large anyways.

Is there any sort of soft-start on the motor?

Steady state cross the line stater. I found through monitoring that the voltage drop decreases with the impedences of the system. IE as it gets further away the drop lessens.

#8 Art

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Posted 04 August 2009 - 01:50 AM

QUOTE (powerboy @ Aug 1 2009, 03:39 PM) <{POST_SNAPBACK}>
Steady state cross the line stater. I found through monitoring that the voltage drop decreases with the impedences of the system. IE as it gets further away the drop lessens.


as you get closer to the generating source the voltage increases...that goes without saying...Vd ~ impedence, ie, V=IR

you're not giving us any information:
what are the prim/sec voltages? what wire sizes?
they start a 3000 HP motor across the line? not reduced voltage, delta-wye, etc.?

21% is a large drop...
something isn't sized correctly











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