Help with reducing AC Mains voltage sag at large chiller motor startup

  • #1
A 4160v chiller motor is drawing too much power at start-up. Utility company only lets them run it once or twice a year because of it. It dims all the lights etc.. It was originally a primary reactor starter (2 contactors). The PR was replaced with an auto-xfmr and wired the same. So now it’s a series auto-xfmr with no shorting contactor. The xfmr is fed, the secondary output supplies the motor at start-up, then a bypass or run contactor pulls in. So the xfmr is not shorted or disconnected from the circuit.

What could be causing the voltage dip on the source, or why is it causing it?

And why does a 3-phase auto xfmr need a shorting contactor? What does connecting the windings do besides dissipate the energy in the winding, and what does leaving them open do?

Thanks!
 
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Answers and Replies

  • #2
davenn
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A 4160v chiller motor is drawing too much power at start-up.

4000+ V motor ??
surely a typo
 
  • #3
No sir. It’s a 4160v 3-phase wye motor. They are common
 
  • #4
jim hardy
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what could be causing the voltage dip on the source, or why is it causing it?

Motors draw several time rated current during startup. See
https://www.engineeringtoolbox.com/locked-rotor-code-d_917.html

upload_2018-3-15_17-2-19.png


The source is not "stout" enough to hold up under your chiller's starting current.

What is the KVA code of your motor on its nameplate? And its horsepower ? How does that compare to the nameplate of your building's supply transformer ?

Somebody should have specified a motor with KVA code that's early in the alphabet.
In my Nuke plant Westinghouse did that for motors that have to start from our diesel generator. Little details make a big difference.

And why does a 3-phase auto xfmr need a shorting contactor?
Without knowing how it's connected one can only speculate - to take it out of the circuit once motor has reached running speed, perhaps ?

old jim
 

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  • #5
davenn
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No sir. It’s a 4160v 3-phase wye motor. They are common


WOW, OK .... way outside my field of expertise :eek::smile:
 
  • #6
jim hardy
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WOW, OK .... way outside my field of expertise :eek::smile:
You don't run across them at Home Depot.

Our boiler feed pumps were 4160 volt 7,000 horsepower. Size of a big storage shed.
 
  • #7
jrmichler
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Sounds like a good application for a VFD. The utility would also like it if the motor was decelerated gradually when stopping instead of merely cutting power.
 
  • #8
berkeman
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Our boiler feed pumps were 4160 volt 7,000 horsepower. Size of a big storage shed.
Pictures? :smile:
 
  • #9
anorlunda
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Pictures? :smile:

Something like this. I used to work next door to GE's Large Generator & Motor Factory. There are lots of those babies around.

smBRD12-097-1.jpg
 

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  • #10
jim hardy
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Something like this.
Thanks ! I was looking in my archives... i don't have any of them opened up like that..

The roar standing next to one has to be experienced in person.... as you well know.
 
  • #11
Tom.G
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Sounds like this was the initial configuration. The Primary Reactors limited the inrush current and after speed was obtained the Shorting Contactors close to supply full line voltage to the motor. I can conceive of a similiar function with autotransformers but best get a schematic of what exists before further speculation.


Pri Reactor.png
 

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  • #12
Motors draw several time rated current during startup. See
https://www.engineeringtoolbox.com/locked-rotor-code-d_917.html

View attachment 222064

The source is not "stout" enough to hold up under your chiller's starting current.

What is the KVA code of your motor on its nameplate? And its horsepower ? How does that compare to the nameplate of your building's supply transformer ?

Somebody should have specified a motor with KVA code that's early in the alphabet.
In my Nuke plant Westinghouse did that for motors that have to start from our diesel generator. Little details make a big difference.


Without knowing how it's connected one can only speculate - to take it out of the circuit once motor has reached running speed, perhaps ?

old jim
Thanks Jim. I couldn’t find a good nameplate on the motor but it’s not an overly big motor. Main power fuses are around 100-120 amp fuses. I know I’m not giving
 
  • #13
Thanks Jim. I couldn’t find a good nameplate on the motor but it’s not an overly big motor. Main power fuses are around 100-120 amp fuses. I know I’m not giving
^Sorry I’m a newbie. Here’s my circuit. Only the reactor has been replaced with an auto-xfmr tapped at 80%
 

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  • #14
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Many years ago, I worked in a steel rolling mill. I'm pretty sure the drive motors for the mill were 4160 v, and pulled several thousand amps when starting. They were truly awe-inspiring.

The current draw was enough that it was measured by simply measuring the voltage difference between two pins about 1-1/2 inches apart embedded in the copper buss bars feeding the motors.
 

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