Capacitors, Dirty Power, Power Ripple

[Sahib]

Bes:
I do not know why you are diverting the subject, because the answer to your question

What if the motor was running at half rated load - or even no load?
How then would "improving" the voltage affect the current? Up? Down? Stay the same?

is fairly clear for fan motor loads: the motor current follows the voltage variation. But this is not what the OP is seeking.

 

[mivey]

Bes:
I do not know why you are diverting the subject, because the answer to your questionis fairly clear for fan motor loads: the motor current follows the voltage variation. But this is not what the OP is seeking.

They did not say fan loads in particular. They claimed:

Motor life is improved with PACS because the reduction in motor current has a direct benefit in lower motor temperature. Heat is the main cause of failure, or shortened service life, of a motor. If the heat can be reduced 10%, it is possible a motor could run for a longer period before needing replacement.

PACS reduces the reactive power, an interference and distortion of the incoming electrical power ?waveform?. Power factor of 0.90 or above tells us the motor will require fewer amps and therefore run cooler and last for a longer life.

 

[Sahib]

mivey:
Perhaps they meant constant torque motors which have increase in full load current when voltage is reduced.

 

[mivey]

mivey:
Perhaps they meant constant torque motors which have increase in full load current when voltage is reduced.

I think they removed any specifics in order to compile a list of all possible positive end results for the sales pitch, even if they do not apply to the targeted customer.

 

[gar]

130418-0839 EDT

If you have an ideal voltage source, meaning zero internal impedance, and a motor connected in parallel with that source, then any other devices connected in parallel with the voltage source will not in any way modify the current in the motor.

The claim in mivey's quote

Motor life is improved with PACS because the reduction in motor current ....

is obviously false.

.

 

[Girl Engineer]

It sounds like what you guys are talking about 2 things: Applying reactors to a capacitor bank to avoid resonance and voltage distortion.
At the frequency where the capacitive reactance of the capacitor and supply (inductive) reactance are equal (resonant), large harmonic currents may circulate if a harmonic source of the same frequency is present. The capacitor acts as a sink for these currents, the resulting heat breaking down the dielectric. The harmonic voltages also ride on the fundamental voltage, possibly exceeding the rating of the capacitor. There are several solutions for this situation, reactors being one of them. They can be used to detune the resonant frequency of the capacitor bank to another frequency that is not present in the system, avoiding this situation (ref EC&M Calc Book pg 107, http://www.alcorn-energy.com/tech4.htm)

 

[Girl Engineer]

For a 100hp motor, the capacitor eliminated 17.82kVA of reactive power draw, which is about 21 Amps (FLA on the motor being 124A)

We know that real power is the power that does work in a system and reactive power contributes to the apparent power. If we can reduce the reactive power drawn from the utility, we will reduce the overall apparent power.

Please see my previous post. #Sahib #gar

 

[Besoeker]

is fairly clear for fan motor loads: the motor current follows the voltage variation.

Half the voltage results in half the current? Is that what you are suggesting? For a fan load?

 

[Besoeker]

They can be used to detune the resonant frequency of the capacitor bank to another frequency that is not present in the system, avoiding this situation

And that's pretty much how I go about it.
My background is power electronics - VSDs, rectifiers...mostly industrial 3-phase stuff.
These take non-sinusoidal input currents that have a well-defined spectrum. The non-sinusoidal currents distort the supply voltage to a greater or lesser extent. But distort it they do. And at well-defined harmonic frequencies of the fundamental.
Higher frequencies result in higher currents in capacitors. Undesirably so.

For 3-phase systems the lowest order harmonic is the fifth. For that reason it is quite common to set the resonant frequency at lower than fifth. That way, the X increases with increasing harmonic frequencies.
As you rightly mention, supply reactance needs to be considered to avoid parallel resonance and it can sometimes be a bit difficult to get good information on that. With a good solid relatively local supply it may not be a big factor but I've had to deal with relatively low fault levels at remote sites.

 

[Sahib]

I think they removed any specifics in order to compile a list of all possible positive end results for the sales pitch, even if they do not apply to the targeted customer.

Yes You are correct. I think one of our objectives in this thread is to find any truths in their statements with conditions applied.

 

[Sahib]

130418-0839 EDT

If you have an ideal voltage source, meaning zero internal impedance, and a motor connected in parallel with that source, then any other devices connected in parallel with the voltage source will not in any way modify the current in the motor.

The claim in mivey's quote is obviously false.

.

gar:
For what you state to be true, the impedance of the connecting wires between the ideal source and the motor also to be zero. In practice neither is true and improvement of voltage drop by capacitor depends on the system fault level. Please see
http://www.samwha.com/fc/tech/tech4.pdf
in this regard.

 

[motormuff]

Snake oil

Snake oil

Capacitors do not clean up power, nor do they save you any money on your electric bill. (one exception is if your power company has a low PF charge) Remember you are buying watts (E x I x cos of pf) from your power company. Look at your PF triangle, a capacitor changes the the vars resulting in apparent power changing. But the watts do not change your electric bill doesn't change.

Ran across statements of capacitors cleaning up "dirty power" which they describe as a reactive power ripple. I would also like to discuss some of their other claims.

They mix a lot of fact with fiction. Some of the facts, at least for the commercial PACS product say that a capacitor should be installed near the reactive load. But then they go on to discuss huge savings and that power companies bill for kVA.

Another treat is reading about dirty power. Essentially they call reactive power the dirty power because it causes power ripples. I'm looking now for their take on harmonics.

They parade the staff engineers and certifieds to add credibility to what they are saying. I'm wading through now trying to separate fact from fiction in their write-ups. They seem to be pushing these products fast & furious and I would like to compile a reality check-list.

Anyone care to join in?