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power factor correction capacitors - Resonant condition ?
3

power factor correction capacitors - Resonant condition ?

power factor correction capacitors - Resonant condition ?

(OP)
Has anyone encountered resonant condition where capacitive reactance of the power factor correction capacitor is equal to inductive reactance of the motor ? If so, what were your observations during such resonance ? (excessive current, current oscillation etc.)

RE: power factor correction capacitors - Resonant condition ?

To avoid resonance, the PF-correction capacitive reactance should be sized to offset 80-85% of the inductive reactance of an unloaded motor.


RE: power factor correction capacitors - Resonant condition ?

3
Hello edison123

If you apply enough capacitance to fully correct a motor, or to over correct a motor, then that will be a major problem if the capacitors are permanently connected to the motor terminals. If the capacitors are disconnected from the motor when the supply is disconnected from the motor (i.e. vai a second contactor) this should not pose a problem. When the supply is connected to the motor, the impedance of the supply will swamp the resonant circuit formed by the motor and capacitors and drop the Q very low as well as shifting the resonant frequency up. If the motor is operating at full speed, and then disconnected from the supply, it behaves as a generator, generating voltage at the speed that it is rotating. If the capacitors are still connected across the motor terminals, then resonance will occur and the voltage generated will be very high. The only thing limiting the voltage, is the insulation breakdown. As the insulation breaks down, there will be high current surges and resultant torques. The net result of this situation is insulation damage to the motor windings and terminations, damage to the capaciotrs, and often, damage to the mechanical components in the system due to the transients. This damage can be shattered couplings, broken shafts etc. Definitley undesirable!!

For static correction, only apply 80% of the correction.
Best regards,

Mark Empson
http://www.lmphotonics.com

RE: power factor correction capacitors - Resonant condition ?


The resonant harmonic order of a capacitor set is approximately sqrt(MVASC/MVAR).
  

RE: power factor correction capacitors - Resonant condition ?

(OP)
Thanks shortstub, mark and busbar.

In a parallel LC resonant circuit, since both the impedances (capacitive and inductive) are same, then the total impedance is infnity. That means, the current flowing thru' this circuit is zero (ignoring the resistive components of both the motor and the capacitor). So, theoretically, a resonant condition is ideal from the standpoint of improving pf. So, why we should restrict the capacitive compensation to only 80% ?

RE: power factor correction capacitors - Resonant condition ?

Hello edison123

When the motor is connecte to the supply, there is no problem at all. The problem occurs if the capacitors are permanently connected to the motor terminals and the motor is then disconnected to the supply. At that point, you have a parallel resonant circuit with a generator driving energy into it at the resonant frequency.
Resonant circuits act as amplifiers to any information at their resonant frequency. In this situation, the voltage gnerated by the motor (could be line voltage) will be amplified considerably. The resulting voltage will exceed insulation ratings etc.
Fit a second contactor, one to control the motor, and one to control the capacitors, and the problem goes away.
The 80% rule only applies to situations where the motor is likely to become disconnected from the supply with the capacitors still connected across its terminals.
Best regards,

Mark Empson
http://www.lmphotonics.com

RE: power factor correction capacitors - Resonant condition ?

(OP)
Thanks Mark, that cleared some cobweb.

RE: power factor correction capacitors - Resonant condition ?

Suggestion: A damping resistor may be added in capacitor circuit to the existing resistances of motor winding and conductors to suppress or control the oscillation waves magnitudes.

RE: power factor correction capacitors - Resonant condition ?

(OP)
Another question, after compnesating for 100% pf correction, will the motor pf change from no-load to full load ? Theoritically, it should not since we have fully compensated for the no load lagging current of the motor fully and the balance current is doing only active work. Am I right ?

RE: power factor correction capacitors - Resonant condition ?

Motor requires slightly more vars in the loaded than unloaded condition.... due to the current passing through series leakage reactances.

If you completely compensated the vars in the unloaded condition, you would go slightly lagging in the loaded condition.

RE: power factor correction capacitors - Resonant condition ?

Edison123,
Yes, the theoretical-case parameters you have chosen, i.e., L & C but no R, do result in an infinite impedance.  But, only at the resonance frequency.  No so at line frequency.  The two frequencies are only coincident if all R is ignored.

Thus, line current will be zero at resonance, but the L and C branch currents are equal in magnitude, but opposite in phase.

RE: power factor correction capacitors - Resonant condition ?

(OP)
thanks shortstub.

While the resonance in a parallel LC circuit is created when both the reactances are the same, the frequency at which the resonance occurs need not be the power line frequency. thanks again for making me use my brains.

RE: power factor correction capacitors - Resonant condition ?

I have one more comment on the subject.

If you have corrected to pf=1.0, that means the resonant frequency IS power line frequency.

Now as Mark said if caps are still connected when you cut of the power, the motor continues for a short time to act like a generator initially generating power line frequency.   Draw out the circuit and you see that you now have a SERIES lc loop/circuit being excited by that motor voltage.  Series impedance approaches zero.  That voltage drives the current very high. Voltage accross the terminals can get very high.

RE: power factor correction capacitors - Resonant condition ?

(OP)
pete,

in parallel resonant condition, the resonant frequency is 1/[2*PI*sqrt(LC)]. how does this create a resonant frequency of power line frequency ? with 100 mH and 10 microfarad, the resonance will occur at about 159.155 HZ and not at powerline ferquency.

RE: power factor correction capacitors - Resonant condition ?

where did 100 mH and 10 microfarad come from?  It does not correspond to full correction to 1.0.

Let me use a simplified model ignoring series leakage reactance.  Model the motor running at low load as just a parallel combination of:
capacitance (correction cap)
inductance (magnetizing branch)
resistance (rotor/torque branch).

If you correct it to pf=1.0 by adjusting the capacitance, that means you have selected the capacitor which exactly "cancels" the inductance ans satisfies your relation: power frequency is 1/[2*PI*sqrt(LC)]

RE: power factor correction capacitors - Resonant condition ?

One more thing, going back to your previous statement:
"While the resonance in a parallel LC circuit is created when both the reactances are the same, the frequency at which the resonance occurs need not be the power line frequency"

Reactance is a function of frequency.  If you have a parallel LC circuit where reactances are the same at power frequency, then power frequency is the resonant frequency of that circuit.

RE: power factor correction capacitors - Resonant condition ?

Electricpete,
PF is not 1.0 for the parameters chosen by Edison123.

RE: power factor correction capacitors - Resonant condition ?

Shortstub - No kidding.  My point exactly.  My comments:

"where did 100 mH and 10 microfarad come from?  It does not correspond to full correction to 1.0"

What's your point?

RE: power factor correction capacitors - Resonant condition ?

Electricpete,
Are you addressing me? Or Edison123?

RE: power factor correction capacitors - Resonant condition ?

(OP)
pete and shortstub,

You are right. I stand corrected.

RE: power factor correction capacitors - Resonant condition ?

Suggestion: It depends where the capacitors are located. Capacitors may be connected on the load side of the starter, and the capacitor and motor are switched as a unit. This is preferred and most advantageous location from an over-all standpoint. The capacitors may also be connected on the line side of contactor, i.e. permanently connected to the system. Its main advantage is elimination of a separate switching device for the capacitors. Also, the power factor correction of the system takes place no matter whether the motor is on or off.

RE: power factor correction capacitors - Resonant condition ?

Comment: Limitation of Capacitor and Motor when switched as a unit:
1. Transient torques
Transient electrical torques of twenty times full load motor torque have been obtained in tests when too large a capacitor was switched with the motor.
2. Overvoltage due to self-excitation
As a benchmark, the resulting overvoltage with capacitor values to improve the full-load motor power factor to unity will range from 35 to 75 %. Self-excitation can be measured by connecting a voltmeter across the motor terminals and reading the voltage after the motor-starter switch is disconnected from the power source.

RE: power factor correction capacitors - Resonant condition ?

Hello jbartos

The best way of applying static correction, is to use two contactors, one for the motor and one for the capacitors. That way, there is not danger of the capacitors being connected across the motor when disconnected from the supply. The only problem is the additional cost.

Best regards,

Mark Empson
http://www.lmphotonics.com

RE: power factor correction capacitors - Resonant condition ?

Suggestion to the previous posting: It depends. The capacitor connected to the power source permanently can permanently correct the power factor within the upstream power distribution from the motor starter.

RE: power factor correction capacitors - Resonant condition ?

Jbartos,
Yes that is true, but you can end up with excessive capacitance on the supply resulting in a leadsing power factor rather than an almost unity power factor. The idea with static correction is to apply the leading component and the lagging component at the same time so that the influence on the supply is reduced.
Best regards,

Mark Empson
http://www.lmphotonics.com

RE: power factor correction capacitors - Resonant condition ?

Suggestion to the previous posting: If that is the case, the power factor may be monitored and some larger capacitors turned of by the motor-capacitor branch circuit interrupter.

RE: power factor correction capacitors - Resonant condition ?

That is bulk correction as opposed to static correction.
Bulk correction is usually at the point of supply while static correction is controlled with the motor.  Ideally static correction uses a separate contactor, but in practice the same contactor is used.
Best regards,

Mark Empson
http://www.lmphotonics.com

RE: power factor correction capacitors - Resonant condition ?

Suggestion: Reference:
1. Donald Beeman "Industrial Power Systems Handbook," 1st Edition, McGraw-Hill, 1955, page 478 Figure 8.21 shows the capacitor locations, one location is close to the motor, another location is on the contactor load side, and last location is on the fuse/breaker fuse side. The latter one is at a question. It is not called "bulk correction."

RE: power factor correction capacitors - Resonant condition ?

Good comments Mark.

RE: power factor correction capacitors - Resonant condition ?

Suggestion: The third location in my previous posting is on the fuse/breaker load side in the motor branch circuit. It is not referred to as "bulk correction" in the provided reference (Beeman Handbook). Therefore, I found Marke's posting somewhat inappropriate to my posting.

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