I'm looking at some wave form of a 4160V, 600 HP, 1200 RPM motor with 175 Kvar Caps. I'm montoring the line side of the motor starter, most of the wave forms look like I'm seeing a ringing form the caps. How ever look at the start on 12_16-2008. No too sure what I'm seeing here. Any help would be gratly appreciated.
Wave form analysis
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jcormack
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- Pennsylvania
Too much capacitance?
Too much capacitance?
175 seems to be high for that HP - I would have expected to see around 125 KVAR or so for that hp. What is the voltage rating of the caps? You can insert a line reactor in series with the caps, reduce size of caps, or insert more wire in conduit (impedence) between the starter and the caps (how close are caps to starter?)
Too much capacitance?
175 seems to be high for that HP - I would have expected to see around 125 KVAR or so for that hp. What is the voltage rating of the caps? You can insert a line reactor in series with the caps, reduce size of caps, or insert more wire in conduit (impedence) between the starter and the caps (how close are caps to starter?)
Not quite sure what you want help with.....
From what you have posted, there is an initial starting transient lasting not more than a couple of cycles.
mayanees
Senior Member
- Location
- Westminster, MD
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- Electrical Engineer and Master Electrician
... what he said..
... what he said..
I'm with jcormack in his assessment that 175kVAR is too high for a 600 HP motor. The most I would recommend is 25% or 150 kVAR, and that would apply to a standard motor, with a 0.8 power factor. Anything above that and the capacitance would be lessened.
I think what's visible in the waveforms is the onset of resonance and the system is susceptible to capacitor fuse pops.
John M
... what he said..
I'm with jcormack in his assessment that 175kVAR is too high for a 600 HP motor. The most I would recommend is 25% or 150 kVAR, and that would apply to a standard motor, with a 0.8 power factor. Anything above that and the capacitance would be lessened.
I think what's visible in the waveforms is the onset of resonance and the system is susceptible to capacitor fuse pops.
John M
gar
Senior Member
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- Ann Arbor, Michigan
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- EE
090130-1525 EST
cornbread:
I have never worked on any comparable experiments and thus I have no experience.
Are the voltages line to line or line to neutral? For my comments below I assume line to line.
Is this a mechanical contact starter? I assume it is.
A capacitor's voltage can not be changed instantaneously. An inductor's current can not be changed instantaneously.
Assume at turn on all capacitor voltages are zero. In other words there was sufficient time since that last power was applied that the capacitors have discharged to zero.
In the left waveform plot it appears all the voltages instantaneously drop to zero. You said the measurements were made on the line side of the contactor and thus when the capacitors with zero charge and zero voltage are connected to the line it is expected that they will define the line voltage just after the contactor closes. That correlates. Then there is a damped oscillation on the waveform from the capacitance and inductance of the circuit. Since the red voltage occurred at the voltage peak it has the largest transient amplitude.
Next the middle waveform, 12-16-08. Here it looks like the two contacts that supply the blue waveform closed at the time of the left transient, and the third contact did not close until about 3.5 to 4 milliseconds later.
The right hand plot looks like impedance was added in series with the capacitors. At least something looks different in the circuit.
.
cornbread:
I have never worked on any comparable experiments and thus I have no experience.
Are the voltages line to line or line to neutral? For my comments below I assume line to line.
Is this a mechanical contact starter? I assume it is.
A capacitor's voltage can not be changed instantaneously. An inductor's current can not be changed instantaneously.
Assume at turn on all capacitor voltages are zero. In other words there was sufficient time since that last power was applied that the capacitors have discharged to zero.
In the left waveform plot it appears all the voltages instantaneously drop to zero. You said the measurements were made on the line side of the contactor and thus when the capacitors with zero charge and zero voltage are connected to the line it is expected that they will define the line voltage just after the contactor closes. That correlates. Then there is a damped oscillation on the waveform from the capacitance and inductance of the circuit. Since the red voltage occurred at the voltage peak it has the largest transient amplitude.
Next the middle waveform, 12-16-08. Here it looks like the two contacts that supply the blue waveform closed at the time of the left transient, and the third contact did not close until about 3.5 to 4 milliseconds later.
The right hand plot looks like impedance was added in series with the capacitors. At least something looks different in the circuit.
.
Depends on the pf you want to attain. If 0.95 pf is the goal and you go with the 0.8pf you suggest for the motor you'd need a little over 200 kVAr pfc.
More likely the motor power factor is a bit better and the 175 kVAr is right for the application.
I think the waveforms show damped resonance - after a couple of cycles its gone.
gar
Senior Member
- Location
- Ann Arbor, Michigan
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- EE
090130-1944 EST
To understand the circuit that is causing the oscillation create the following series path:
A battery of voltage V. The negative end will be common (ground if you want). From the positive terminal create the series circuit --- inductor, measurement point, a switch, a capacitor and connect the capacitor to common. Connect an oscilloscope between the measurement point and common.
Discharge the capacitor with the switch open. Before the switch is closed the scope reads the battery voltage. Close the switch, the voltage drops to 0 (common), and you get a damped oscillation superimposed on the battery voltage. After the oscillation damps out the oscilloscope trace is at the battery voltage.
If you have the equipment this is a fairly easy experiment to perform.
.
To understand the circuit that is causing the oscillation create the following series path:
A battery of voltage V. The negative end will be common (ground if you want). From the positive terminal create the series circuit --- inductor, measurement point, a switch, a capacitor and connect the capacitor to common. Connect an oscilloscope between the measurement point and common.
Discharge the capacitor with the switch open. Before the switch is closed the scope reads the battery voltage. Close the switch, the voltage drops to 0 (common), and you get a damped oscillation superimposed on the battery voltage. After the oscillation damps out the oscilloscope trace is at the battery voltage.
If you have the equipment this is a fairly easy experiment to perform.
.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
090130-2207 EST
I setup the experiment I mentioned above.
The components used were:
15 V DC power supply.
1 mfd capacitor.
The primary of a Stancor P8668 transformer with no connection to the secondary as the inductor. Approx inductance is 1/2 henry.
A Tektronix scope with 2 MS/DIV horizontal sweep.
This produced a damped sine wave as I had described.
.
I setup the experiment I mentioned above.
The components used were:
15 V DC power supply.
1 mfd capacitor.
The primary of a Stancor P8668 transformer with no connection to the secondary as the inductor. Approx inductance is 1/2 henry.
A Tektronix scope with 2 MS/DIV horizontal sweep.
This produced a damped sine wave as I had described.
.
mayanees
Senior Member
- Location
- Westminster, MD
- Occupation
- Electrical Engineer and Master Electrician
nope
nope
I don't disagree that a theoretical calculation of the capacitiance required to bring that motor from 0.8 to 0.95 is in excess of 200 kVAR.
But it is not a recommended practice to install that much pf correction capacitance on a motor of that size, as evidenced by the chart located at the following link:
http://www.abb-control.com/pdf/catalog/lv023/20.6.pdf
It's a link that shows the maximum size pf correction for a 500 HP motor is 120 kVAR for an 1800 rpm motor. That extrapolates out to a maximum of 150 kVAR for a 600 HP motor. Numerous other references support a maximum level of 150kVAR.
John M
nope
I don't disagree that a theoretical calculation of the capacitiance required to bring that motor from 0.8 to 0.95 is in excess of 200 kVAR.
But it is not a recommended practice to install that much pf correction capacitance on a motor of that size, as evidenced by the chart located at the following link:
http://www.abb-control.com/pdf/catalog/lv023/20.6.pdf
It's a link that shows the maximum size pf correction for a 500 HP motor is 120 kVAR for an 1800 rpm motor. That extrapolates out to a maximum of 150 kVAR for a 600 HP motor. Numerous other references support a maximum level of 150kVAR.
John M
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Hi JC - no relent to your study. Although the steeters suck, we steel love you . . . . . 140 is my answer
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"An Alternate Selection Method"
Alternate to what, one wonders.
And see the note at the bottom of the table.
"..when switched with a capacitor as a single unit."
As a matter of good practice, we (our company) do not switch motor and PFC as a single unit. There are a few reasons why.
Protection requirements are different.
- Contactor ratings are different (for us Brits it's AC3 for motors and AC4 for capacitors).
- Self excitation of the induction motor with connected capacitors can be an issue. At a guess, that's the limitation ABB are using for their suggested (not recommended) maximum kVAr.
- Customers specify that the motor and the PFC shall have their own contactors.
Horses for courses.....
ghostbuster
Senior Member
- Location
- Great White North
It looks like typical grid switching by the utility.Frequencies generated are usually 5-15 khz.:smile:
wirenut1980
Senior Member
- Location
- Plainfield, IN
The only difference I see with the 12-16 cap switch is that the three phases do not close in at the same time like the other two switches. Are all three waveforms from the same capacitor? The 12-16 switch looks like a typical switching of transmission capacitors (69 KV and above). Do you have a dedicated substation at your facility with transmission capacitor bank on the high side of your transformer.
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