How do you compare the power quality between two gennys? If I were designing a genny for audio, what features/benefits would the industry be willing to pay for?
SMPS rectify the input anyway-they should actually work just fine if fed the correct DC voltage, they would seem to be the least frequency sensitive power supply?
SMPSs are impervious to frequency but very susceptible to the voltage distortion created by the very harmonic currents they draw when powered by conventional generators. Because the capacitors in SMPSs draw current only at the peak of the voltage waveform, SMPSs cause a voltage drop at the peak of the waveform, which leads to “flat topping” of the voltage similar to that in the oscilloscope shot below center.
(http://www.screenlightandgrip.com/images/generators/waveform_elec_ballast.jpg)
Left: A non-linear Load powered by Grid Power. Center: A non-linear Load powered by Conventional AVR Power. Right: A non-linear Load powered by Inverter Power.
And since, according to Ohm’s Law, harmonic currents react with impedance to cause voltage drop, the magnitude of this voltage waveform distortion caused by SMPSs is a function of the source impedance. In the case of generators, source impedance is not an easily defined value as generator reactance varies with time following a load change. However, what is certain is that the generator with the lowest internal reactance to an instantaneous current change at a given load will typically have the lowest value of total harmonic distortion under nonlinear load conditions. This is one of the great benefits to using inverter generators over conventional generators: inverter generators have much lower internal reactance and so they are less prone to voltage waveform distortion caused by the harmonic currents drawn by SMPSs. Most generator manufacturers don’t give specifications for internal reactance, but will give specifications for THD of voltage.
Inverter generators are less prone to voltage waveform distortion because the inverters completely processes the raw power generated by its alternator (converting it to DC before converting it back to AC) –making the AC power it generates completely independent of the engine. In fact, its’ microprocessor controller can vary the engine speed without affecting the voltage or frequency of the power the inverter module puts out. Now that the internal reactance of the engine is separated from the power output, harmonic currents encounter very little impedance; and, as is evident in the oscilloscope shot above right, there is considerably less voltage distortion at the load bus of inverter generators than there are conventional generators. The net benefit is that non-linear loads, like the SMPSs of most electronic equipment, do not adversely affect the power of inverter generators as they do the power of conventional generators. Which means that electronic equipment powered by SMPSs will operate more reliably on inverter generators.
Where there is appreciable voltage waveform distortion created by operating non-linear loads on a conventional generator, other electrical devices operating on the same power are unable to use the distorted waveform effectively. For instance, the SMPSs of electronic equipment depend on the peak value of the voltage waveform to operate effectively. They therefore work sporadically, if at all, on the squared off voltage waveform caused by the harmonic currents they draw encountering the high impedance of power generated by conventional generators. In fact, operating the pseudo square wave of distorted voltage, they will be starved of power.
(http://www.screenlightandgrip.com/images/generators/Ineplicable_Mallfunctions_I.jpg)
Effect of DC Bus Voltage with Flat Topping
Special precautions must be taken with computers and hard drives in particular. The majority of computer based equipment derives its’ internal DC power from AC power switched by a SMPS, or similar power supply, and so it is often here where harmonic problems first arise. As is evident in the illustration above, voltage flat topping from harmonic currents reduces the operating DC bus voltage these power supplies will generate. As a result, the load will be starved of power even though you may read full line voltage with an RMS meter and the power indicator lights light.
Symptoms to look for are hard drives locking up, internal fuses blowing, and inexplicable malfunctions.
For a more detailed explanation of the electrical engineering principles behind these issues and how to resolve them use this link (http://"http://www.screenlightandgrip.com/html/emailnewsletter_generators.html") for a newsletter article on the use of portable generators in motion picture production.
Guy Holt, Gaffer,
ScreenLight & Grip,
www.screenlightandgrip.com
Perhaps I am being too technical-it just doesn't seem to make sense that a power supply happy with 90-240 vac would be overly sensitive to a gennys output voltage (provided it is not pushing the low end) and waveform?
It would be great to understand this apparent inconsistency. I have been in situations of very overloaded line power where SPMS power supplies were shutting down, but my "big iron" supply was still marching on. Obviously there's so much more to this than meets the eye...
More from my white paper on the use of portable generators in motion picture production (available at http://www.screenlightandgrip.com/html/emailnewsletter_generators.html.) To understand what affect a highly distorted voltage waveform with peak voltage drop would have upon SMPSs it is worth reviewing how the diode/capacitor front end of SMPSs operate.
(http://www.screenlightandgrip.com/images/generators/diode-capacitor_sch.jpg)
Step 1: Rectifier Bridge converts line frequency AC power to rectified sine wave.
Step 2: rectified sine wave is flattened to DC by conditioning Capacitor.
The diode/capacitor section converts AC power to DC by first feeding the AC input through a bridge rectifier, which inverts the negative half of the AC sine wave and makes it positive. The rectified current then passes into a conditioning capacitor/s that removes the 60 Hz rise and fall and flattens out the voltage - making it DC.
(http://www.screenlightandgrip.com/images/generators/Working_of_CFLs.jpg)
Yellow Trace: Rectifier Bridge converts AC power to rectified sine wave. Blue Trace: Stored Capacitor Voltage. Red Trace: Current drawn by capacitors once input voltage is greater than voltage stored in the capacitor (Blue trace.)
As shown in the illustration above, the diode-capacitor circuit only draws current during the peaks of the supply voltage waveform as it charges the conditioning capacitor to the peak of the line voltage. Since the conditioning capacitor can only charge when input voltage is greater than its stored voltage, the capacitor charges for a very brief period of the overall cycle time. Since, during this very brief charging period, the capacitor must be fully charged, large pulses of current are drawn for short durations. Consequently, all diode-capacitor circuits draw current in high amplitude short pulses that roughly coincide with the peak of the voltage waveform.
(http://www.screenlightandgrip.com/images/generators/rectifiedpsuedosquarewaveBW.jpg)
A pseudo square wave after being rectified by a full bridge rectifier
Based upon how diode-capacitor circuits operate, what effect would a "flat topped" voltage waveform exhibiting peak voltage drop (like the one pictured above) have upon loads, like lap tops, camera power supplies and battery chargers, that also utilize SMPSs? If we compare one half cycle of a rectified sine wave to one half cycle of the distorted pseudo square wave generated by non-linear loads, we see that one consequence is that the period during which the capacitors of their SMPSs must recharge is appreciably shortened. Given a shorter interval to charge, the capacitor/s will draw current in even higher amplitude shorter bursts. The diode-capacitor circuit therefore works harder, drawing more current during an even briefer charging period, reducing its power factor and increasing its apparent power or load. As a consequence protective circuit breakers may trip or fuses blow.
(http://[img]http://www.screenlightandgrip.com/images/generators/rectified_capacitor_draw_in.jpg)
Left: half cycle of rectified sine wave. Right: half cycle of rectified pseudo square wave. Blue Line: Minimum Capacitor Voltage. Red Lines denote interval during which current will be drawn by capacitors once input voltage is greater than voltage stored in the capacitor.[/img]
Another adverse effect is that more harmonic currents are generated as less of the power waveform is used by the circuit. In fact, a viscous cycle can get started. The more harmonic currents that are generated, the more distorted the power supplied by the generator becomes. The more distorted the power waveform becomes the more harmonic currents are generated. In this fashion, something akin to a feedback loop can get started until the effect of the harmonics is enhanced to the point where equipment stops working all together.
(http://www.screenlightandgrip.com/images/generators/RectifiedSquareWaveLineComp.jpg)
Blue Line: Minimum Capacitor Voltage. Red Lines denote interval during which current will be drawn
by capacitors once input voltage is greater than voltage stored in the capacitor.
To see why this might happen we have only to compare the pseudo square wave created by 1200 watts of non-PFC SMPS load to that created by 2500 watts of non-PFC SMPS load above. Based upon our discussion of how diode-capacitor circuits operate, we can see in the oscilloscope shot on the right that the peak value of the psuedo square wave created by the 2500W load (after it has be rectified) may not reach a sufficient level to charge the capacitor/s of a power supply. Whether the ballast of a light, or the AC power supply of a lap top, the equipment may be starved of power even though its’ power indicator lights up, and a true RMS voltmeter would indicate about 120 volts on the line. Common symptoms of power starvation are computers locking up, breakers tripping, and HMIs not striking or holding their strike.
(http://www.screenlightandgrip.com/images/generators/waveform_hmi_kino_pkg.jpg)
Same as Above Left: Conventional AVR Power w/ Pkg. of non-PFC Elec. Ballasts & Kino Flo Wall-o-Lite. Center: Scope time base adjusted to
bring elongated waveform back on screen. Right: Inverter Power w/ Pkg. of non-PFC Elec. Ballasts & Kino Flo Wall-o-Lite.
The third frame on the right, is the same package of lights but with power factor corrected electronic HMI ballasts on the EU6500is (an inverter generator.) As you can see, the difference between the resulting waveforms is startling. Even though we are running the same overall load in terms of watts, the fact that the ballasts are power factor corrected, that the power generated by the inverter generator has very little inherent harmonic distortion (less than 2.5%), and that the system impedance is very low, results in virtually no voltage waveform distortion of the power running through the distribution system. For this reason, sensitive electronic equipment running on the same power will continue to operate reliably and effectively without damage even though the overall load on the generator has increased.
Guy Holt, Gaffer
ScreenLight & Grip
www.screenlightandgrip.com
The only units they give a THD value are for inverter units-which makes it impossible to compare the power quality of their "portable" "inverter" "mobile" and "home standby " units. No this is not an offshore bargain basement manufacturer.
This might help (from Honda Archives):
(http://www.screenlightandgrip.com/images/generators/sinewave_by_gen_type.jpg)
Guy Holt, Gaffer
ScreenLight & Grip
www.screenlightandgrip.com
Having not thought about this in the past, should I have a grounding connection between two generators in this case?
From my IA Grounding/Bonding Workshop:
(http://www.screenlightandgrip.com/images/generators/ProSound_PP_Image_Bonding_Separately_Derived-Systems.jpg)
Guy Holt, Gaffer
ScreenLight & Grip
www.screenlightandgrip.com
From my IA Grounding/Bonding Workshop:
(http://www.screenlightandgrip.com/images/generators/ProSound_PP_Image_Bonding_Separately_Derived-Systems.jpg)
Guy Holt, Gaffer
ScreenLight & Grip
www.screenlightandgrip.com
It should go without saying, but if the structures have any kind of metallic path between them (snakes, etc) the bonding really ought to be their as well.
From my IA Grounding/Bonding Workshop:
(http://www.screenlightandgrip.com/images/generators/ProSound_PP_Image_Bonding_Separately_Derived-Systems.jpg)
Guy Holt, Gaffer
ScreenLight & Grip
www.screenlightandgrip.com
Are you tying the neutrals together in that diagram or just the ground wires? (small screen and eyes not too good anymore)
I guess that's a moot issue if G-N is connected in the generators
On that note, Do you G-N connect at all generators or only at one? how about ground rods -at each one or centrally?