Capacitor Bank

Status
Not open for further replies.
D

derek22r

Member
We have a situation where there is a 22% voltage dip on the primary of a 120Vac cpt during an upstream motor start, causing the control relay to drop out. I do not believe the details of this problem are important because my question is about a discussion that came up while troubleshooting this issue.

A co-worker stated that one option would be to add a capacitor bank at the cpt would remove the voltage dip. He was not seriously considering this a an option, but does believe that it would theoretically work.

To me, a capacitor bank would only improve the power factor by adding reactive power to the circuit. Is he correct in saying that it would increase the voltage during an inductive motor start?
 
080807-1126 EST

derek22r:

A capacitor is an energy storage device as is an inductor in contrast to a resistive element.

In an AC circuit a capacitor will receive energy during a portion of a cycle and return it in another portion. If you have both a capacitor and an inductor in parallel with very little power loss in relationship to the energy stored in the reactive components, then you will have a resonant circuit that will oscillate for a long time without energy added. This is not your situation.

Your have a very high loss condition when starting a motor. Your problem is a voltage drop at the motor from the load current thru the source impedance of the lines and transformers supplying the motor. Since you are down stream from the motor you see this voltage drop. This voltage drop might last for 4 seconds (240 cycles). A capacitor hung across the AC line does nothing for this problem.

Redesign the circuit for the relay that drops out, get a Sola constant voltage transformer (it might work), or get power from some other source.

.
 
coulter said:
He wants to put the cap bank on the 120V side of the cpt?

carl
Click to expand...

Line or load side was not specified. I don't see how it would work either way, but don't know how to explain it.

His argument was that in a vehicle, caps are used when a large sound system causes a voltage dip.
 
derek22r said:
Line or load side was not specified. I don't see how it would work either way, but don't know how to explain it.

His argument was that in a vehicle, caps are used when a large sound system causes a voltage dip.
Click to expand...

He is correct with the car application. "Ultra" or "super" capacitors are used to provide high energy for a very short amount of time to make up for the drop in voltage when the bass hits hard.

In your application switching in a capacitor at the motor during startup has been done before, but not at the sensitive load. In theory, a capacitor with sensing circuitry and feedback loop would work, but it is not economical in this situation to do it that way. Plus you may have other issues with exciting a resonance. What your friend described is a static var compensator and is used at the front end of arc furnaces.

As gar suggested, a better option would be to install a constant voltage transformer (CVT) in place of your CPT. Size it at twice the load requirements to get maximum sag ride through for your 120 V controls.

Or better yet, reduce the voltage sag at the offending motor with a soft start.
 
You have a voltage-drop issue due to the starting current. Unless there is a high-impedance issue in the power supply, the cure here is to supply the offending motor circuit with a feeder and/or branch circuit of much lower impedance than it has now.

To add: high-value caps do store energy and can release it back under peak demand moments, but this only works on DC. A cap on AC charges and discharges twice within a single cycle, while a voltage dip occurs over the duration of many cycles.
 
If the power factor of the motor is low at start, then the capacitor may help the power factor of the system. Because lots of KVARs would flow between the motor and the capacitor, there would be lower current flow through the various feeders and lower voltage drop at the supply transformer. This could help starting.

Of course a capacitor sized to be useful would probably over-correct things once the motor was running, and switching the capacitor out would cause no end of grief from transients.

-Jon
 
derek22r said:
Does anyone know of a 120AC control relay that can withstand a voltage drop up to 30% for 4-5 seconds.
Click to expand...

Yes.
Some manufacturers sell "universal supply" relays can accept an incoming voltage of almost any potential. Siemens has one that accepts 24 - 240VAC or DC, +- 10%
Siemens 3RS18 relay

They essentially run on 24VDC but have a built-in DC switching power supply that will convert AC to DC or just accept DC straight through, then also automatically switch it down to the lowest level. So your AC line voltage can essentially drop from 120V to probably around 34V and it will still work.
 
Thanks very much for all the replies. This site is always a huge help for me. Hopefully I can return the favor someday.
 
Status
Not open for further replies.
Top