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[Stephen Swaffer]

I will admit up fromt that there may be technical issues that make this impractical, but this thought keeps nagging me, so I will throw it out and see what oponions I get.  It seems that everyone agrees that inverter gennys are the way to go, but cost and power limitations seem to rear there ugly head. Given what I routinely work with, the Honda inverters do seem over priced-probably because they are the only player in the market.

Here's what I am thinking.  I can buy a 7000-9000 watt construction genny for $800 to $1300.  I can buy a 10 kw VFD for $500 to $600 and a 7.5 kw 3 phase y-delta transformer for $200. So cobbled up, I can get 7500 watts for half the price.  Purpose built, you could leave out the transformer, and save money there..

Yes, the construction generators are single phase, but you can and I have used them to power 3 phases off of single phase-it does require derating the drive.  Yes there may be high freq components, but the transformer "should" help filter those out-and you are doing the same thing with essentially the same components as the inverter genny?

The other attraction for sound gear is the Honda tops out at 6500 watts,  if this is at all feasible, you could build an inverter as big as your wallet allows?
 

[David Buckley]

Before you even think about connecting anything other than a motor to a VFD, have a look on a scope how shitty the output of a VFD is.  You would probably be better off just using the builder's genny.

The principle isn't screwed, there are plenty of devices that do exactly what you suggest, but they aren't as nearly as cheap as a VFD.  Lab quality power supplies have the best output, but are expensive.

If you fancy a challenge, then a half-UPS might be the answer.  You can get UPSs in two halves, a charger half and an inverter half, intended to have a battery in between.  With some big caps and a bridge rec, you could get the appropriate DC rails for the inverter half.

[Scott Holtzman]

Before you even think about connecting anything other than a motor to a VFD, have a look on a scope how shitty the output of a VFD is.  You would probably be better off just using the builder's genny.

The principle isn't screwed, there are plenty of devices that do exactly what you suggest, but they aren't as nearly as cheap as a VFD.  Lab quality power supplies have the best output, but are expensive.

If you fancy a challenge, then a half-UPS might be the answer.  You can get UPSs in two halves, a charger half and an inverter half, intended to have a battery in between.  With some big caps and a bridge rec, you could get the appropriate DC rails for the inverter half.

If weight wasn't an object Lorain ConstAC inverters are available from all the telco junkyards.  Designed to invert the -48v back to 110v for Misc. devices in the CO.   Ditto that -48v rectifiers are dirt cheap too.  Toss in 4 marine batteries and run the rectifier off a cheap generator.

It would weigh a ton but work very well.

[Guy Holt]

I will admit up fromt that there may be technical issues that make this impractical, but this thought keeps nagging me, so I will throw it out and see what oponions I get.  It seems that everyone agrees that inverter gennys are the way to go, but cost and power limitations seem to rear there ugly head.

There is a lot more to inverter generators than simply powering a VFD with a construction generator. Unlike the simple two-pole alternators of construction  generators, an inverter generator uses a core that consists of multiple stator coils and multiple rotor magnets. Each full rotation of the engine produces more than 300 three phase ac sine waves at frequencies up to 20 kHz, which is considerably more electrical energy per engine revolution than produced in conventional two pole generators.

Core parts from PWM Inverter Generator. Note the multiple windings of the core stator.

The power generated by the multi-pole core next goes to the inverter module. A basic Pulse Width Modulated (PWM) inverter consists of a converter, DC link, control logic, and an inverter.

Basic wiring schematic of PWM Inverter

The converter section consists of a fixed diode bridge rectifier which converts the more than 300 three phase AC sine waves at frequencies up to 20 kHz to a DC voltage (about 200 V in at least one unit).

Converter and DC Link

AC Output is then generated from the high voltage DC by the inverter section with voltage and frequency set by a PWM control logic. A highspeed microprocessor switches IGBTs (insulated gate bipolar transistors) on and off several thousand times a second according to the PWM control logic to create a variable voltage and frequency.

Control logic and Inverter Section

PWM inverter control logic goes something like this: to generate the positive half cycle of a true AC sine wave, an IGBT connected to the positive value of the DC voltage from the converter is switched on and off by a micro-processor at variable rates and for variable intervals to create current to flow of a variable voltage.

Pulse Width Modulated Current

In other words, the IGBT is switched on for a short period of time, allowing only a small amount of current to build up and then is switched off. The IGBT is switched on and left on for progressively longer periods of time, allowing current to build up to higher levels until the current reaches a peak. The IGBT is then switched on for progressively shorter periods of time, decreasing current. The negative half of the AC sine wave is generated by switching an IGBT connected to the negative value of the converted DC voltage. The fixed DC voltage (200 VDC) is modulated or clipped in this fashion to provide a variable voltage and frequency. Where IGBTs can turn on in less than 400 nanoseconds and off in approximately 500 nanoseconds, they are ideal for the high switching speed necessary to create a true sine wave in this fashion. The fixed DC voltage (200 VDC) is modulated or clipped in this fashion to provide a variable voltage and frequency.

The three phases of the inverter generator process: high frequency AC converted to DC; DC inverted to stable clean 120V, 60 Hz AC.

To summarize this complex process: the generator's multi-pole core produces high voltage multiphase AC power. The AC power is then converted to DC. Finally the DC power is converted back to AC by an inverter. Since the inverter completely processes the raw power generated by the alternator, the voltage and frequency of the power it generates is no longer linked to engine speed (RPM) as is the case with conventional generators. Rather, using microprocessor controlled IGBTs the inverter module switches the high voltage DC according to PWM control logic to provide AC power with a voltage stability within ± 1%, and frequency stability within ± 0.01 HZ. The end result is a nearly pure sine wave with a wave distortion of only 2.5%; which is as clean or cleaner than commercial power.

… I routinely work with, the Honda inverters do seem over priced-probably because they are the only player in the market…,  if this is at all feasible, you could build an inverter as big as your wallet allows?

Honda is not the only player in the inverter market.  They are the only ones you ever hear about because they are by far the best.  Subaru-Robins also manufactures a complete line of inverter generators. What makes inverter generators so expensive are two things. First, as you can see from my description above, they are a very sophisticated piece of electronics. Second, they use very expensive Neodymium permanent magnets in their rotors instead of the electromagnets of a construction generator.

As described above, inverter generators take a radically different approach to generating power than do conventional generators. That difference extends also to how voltage is regulated. Voltage and Frequency in conventional generators are intractably linked to their Engine/Alternator. According to the basic principles under which conventional generators operate voltage can be regulated either by engine speed or field strength in it’s Rotor electromagnets. Conventional portable generators use AVR systems to regulate the field strength through a DC excitation current because, in order to provide a constant AC Frequency, the motor must run at a constant speed. For this reason, portable generators with simple two-pole cores require Barber Coleman governors to govern their engines to run at a constant 3600 RPM to produce stable 60 hertz (cycle) power. Regulation of voltage through engine speed is simply not an option for this reason. 



Inverter generators by comparison do not have to run at a constant speed because the AC power they output is generated from high voltage DC power that is micro-processor switched according to a PWM control logic with a voltage stability of ± 1%, and Frequency stability of ± 0.01 HZ. This simple fact, that the voltage and frequency of inverter generators are no longer linked to their engine, offers a number of benefits. To understand these benefits, we need to first take a closer look at how the generator portion of inverter generators like the Honda EU6500is operate.

Since they adjust their speed to accommodate the load placed on the generator, the motors used in inverter generators have to have a wide torque range to be able to accelerate rapidly to accept an increase in load put on it. For this reason they use a high torque ‘stepper motor’ capable of responding rapidly to signals from the inverter microprocessor control section. And, because the speed of the motor is always changing, inverter generators cannot maintain voltage output by the conventional means of regulating the excitation current in Rotor electromagnets. Instead, inverter generators use permanent magnets in place of electromagnets. 



A permanent Magnet is an object made from a material that is magnetized (Neodymium in this case) and creates its own persistent magnetic field. As such, permanent magnets do not require a DC supply for the excitation circuit, nor do they have slip rings and contact brushes. And, unlike an electromagnet, a permanent magnet is not dependent on input power to the Exciter (voltage regulator) to produce and maintain the magnetic field in the Rotor. For this reason, permanent magnets offer tremendous benefits in this application. 
In conventional generators there is a power feedback loop from the generator output (via the Sensor Coil) back through the Exciter (Voltage Regulator) to an electromagnet in the Rotor. If the generator output is subjected to a sudden heavy load the engine bogs down, and the voltage in the feedback loop is greatly reduced, which, in turn, results in a reduction of the AVR’s capability to recover from the application of the heavy load. When a permanent magnet is used in place of this closed loop excitation circuit, the persistent magnetic field in the Rotor is independent of the generator output and consequently of sudden heavy loads. Thus the permanent magnet in an inverter generator provides stable continuous power to the inverter, enabling voltage to be regulated by engine speed rather than field excitation. The result is that the generator is much better able to sustain output current against transient loads and, therefore, it has a much lower internal reactance compared to conventional AVR machines.

Left: Conventional AVR Generator w/1200W non-pfc electronic ballast. Right: Inverter Generator w/1200W non-pfc electronic ballast

A second benefit in using permanent magnets in place of electromagnets in the generator’s Rotor is that inverter generators consequently have very low sub-transient impedance. The Impedance of a generator is a function of the internal reactance of the engine to changes in load. Since permanent magnet generators are less susceptible to changes in load, the power fed to the inverter module is more stable. Add the fact that the inverter completely processes the raw power (converting it to DC before converting it back to AC), the AC power it generates is completely independent of the engine. In fact, the microprocessor controller can vary the engine speed without affecting the voltage or frequency of the power the inverter module puts out. Now that the combination of a permanent magnet generator/inverter module separates the internal reactance of the engine from the power output, harmonic currents encounter very little impedance; and, as is evident in the oscilloscope shots above, there is considerably less voltage distortion at the load bus. The net benefit is that non-linear loads do not adversely affect the power of inverter generators as they do the power of conventional AVR generators.

…. The other attraction for sound gear is the Honda tops out at 6500 watts…

Not true. We get 7650W from our modified Honda EU6500 and EU7000 generators. 

The information above is excerpted from my white paper on the use of portable generators in motion picture production available at http://www.screenlightandgrip.com/html/emailnewsletter_generators.html. For more detailed information about inverter generators use this link: http://www.screenlightandgrip.com/html/emailnewsletter_generators.html#anchorTrue%20Sine%20Wave%20Generators. At this link you will also find a very informative video available online titled “Inside an Inverter Generator, Car Alternator, AC." Produced by Green Power Science, whose mission is “Free Power For The World Through Creative Thinking”, the video surveys alternate types of generators that can be used with wind turbines, ranging from car alternators to fan motors, but begins with a very good “under the hood” look at the difference between conventional generators and Inverter generators.

Guy Holt, Gaffer
ScreenLight & Grip

[Stephen Swaffer]

Guy,

I was hoping you would chime in-I find the information very interesting.  However, I have worked on and with VFDs for years (and inverter welders)-and fundamentally, the electronics is the same in all of them and the same as what you show.  Many VFDs include a capacitor bank that helps smooth out load variations.  Theoretically, once the power is converted to a DC bus, as long as the bus voltage does not droop, it should not matter what type or how many phases of power were rectified-that is why a 3 phase VFD can be run on single phase power.  I recently installed a VFD in an application that has extreme load variations (easily from no load to 120% instantaneously), yet the VFD allows me to run on a circuit that otherwise could not handle the load.  This would seem to indicate this method would make it easier on the mechanics of the generator.

The output waveform should be dependent primarily on software.  Motors, while they can be forgiving, in demanding applications are affected by waveform accuracy because distortion creates extra heat-ultimately what determines a motors lifetime-so an accurate sine wave is very desirable on a VFD.

I can see an engineering choice being made to use a multipole, high freq AC to alleviate the need for a capacitor bank resulting in a lighter, perhaps even more efficient generator.  But a capacitor bank designed to be adequate on 208 volt supply should have a little extra reserve if it is being fed 240 volts.

Frankly, I see little difference in complexity of the electronics in a VFD vs an inverter generator.  If anything, the VFD is probably more costly-bus voltage runs closer to 300 volts (600 on 480 drives) and you are limited to 60 hz incoming with no way to get around a capacitor bank.  Obviously you have that cost-the mutipole armature would be more expensive-and the market is perhaps smaller.

As for power ceiling, I was thinking the capability to go significantly above the 7kw tier of gennys.

Obviously not a simple plug and play solution and some interesting things to consider.  Perhaps it might make more sense to look into a PV inverter vs a VFD

[Frank DeWitt]

Honda doesn't seem to be the only player in the market. Good quality, good track record, but not alone. Among many others, both Yamaha and Briggs & Stratton are in the game.

Inverter generators have many benefits including low noise, low weight and the ability to parallel them but there is no reason not to run electronics off a good quality conventional generator.  My home backup power is a 1960 something Onan 7kw generator and it runs everything I have without any issues at all.  During a long power outage I hooked up my 1947 Kohler because it is automatic on load demand start.  Its main job was to keep the refrigerator running but I didn't hesitate to run the TV and computers with it.  Neither of these are throw away big box generators.  They have a lot of copper in them and they run nice and smooth at low RPM but they don't have a inverter.

[TJ (Tom) Cornish]

Honda doesn't seem to be the only player in the market. Good quality, good track record, but not alone. Among many others, both Yamaha and Briggs & Stratton are in the game.

Inverter generators have many benefits including low noise, low weight and the ability to parallel them but there is no reason not to run electronics off a good quality conventional generator.  My home backup power is a 1960 something Onan 7kw generator and it runs everything I have without any issues at all.  During a long power outage I hooked up my 1947 Kohler because it is automatic on load demand start.  Its main job was to keep the refrigerator running but I didn't hesitate to run the TV and computers with it.  Neither of these are throw away big box generators.  They have a lot of copper in them and they run nice and smooth at low RPM but they don't have a inverter.

Non-inverter generators can work - in fact I'm not aware of an inverter generator over 10KW, so all of the large entertainment generators are "conventional".

That said, the typical set of eyes in this forum is weighing the choice between the $1500 10KW generator Menards has for sale against the $4500 Honda EU7000.  The inverter is a big part of the deal, but not all.  The noise level difference is deal killer #1.   The other major difference is the inverter can mask some of the problems of sudden load changes as voltage output and especially frequency stability of the inverter generator aren't directly tied to engine RPM.

[Guy Holt]

... there is no reason not to run electronics off a good quality conventional generator.  My home backup power is a 1960 something Onan 7kw generator and it runs everything I have without any issues at all.

While that may be true of the requirements of home standby power applications, event production poses very different challenges.  LED lights can draw a very distorted current that is rich in harmonics (see PQM readings for a Chauvet Slim Par Pro RGBA below.) 

(The Chauvet Slim Par Pro RGBA has a pf of .61 and Total Harmonic Distortion of 81%)

Operating a number of these on a conventional generator will distort the voltage waveform of the generator.  According to Ohm’s Law, as each harmonic current encounters the high impedance of conventional generators it creates a voltage drop at a corresponding harmonic voltage. Since The SMPSs of LED lights draw current only at the peak of the supply voltage, the net result is voltage flat topping like that seen in the oscilloscope shots below.

Left: Conventional AVR Generator w/1200W of non-pfc SWMPs. Right: Inverter Generator w/1200W non-pfc SWMPs.

The adverse effects of the harmonic distortion exhibited here, can take the form of overheating and failing equipment, efficiency losses, circuit breaker trips, excessive current on the neutral wire, and instability of the generator voltage and frequency. Harmonic noise of this magnitude can also damage HD equipment, create ground loops, and possibly create radio frequency (RF) interference.

Guy Holt, Gaffer
ScreenLight & Grip

[Jonathan Johnson]

My home backup power is a 1960 something Onan 7kw generator and it runs everything I have without any issues at all.  During a long power outage I hooked up my 1947 Kohler because it is automatic on load demand start.  Its main job was to keep the refrigerator running but I didn't hesitate to run the TV and computers with it.  Neither of these are throw away big box generators.  They have a lot of copper in them and they run nice and smooth at low RPM but they don't have a inverter.

Non-inverter generators can work - in fact I'm not aware of an inverter generator over 10KW, so all of the large entertainment generators are "conventional".

That said, the typical set of eyes in this forum is weighing the choice between the $1500 10KW generator Menards has for sale against the $4500 Honda EU7000.  The inverter is a big part of the deal, but not all.  The noise level difference is deal killer #1.   The other major difference is the inverter can mask some of the problems of sudden load changes as voltage output and especially frequency stability of the inverter generator aren't directly tied to engine RPM.

Another factor between a home-center 7-10kW generator and some of those older generators, or even a 7kW WhisperWatt is the amount of rotating mass. The better generators have larger flywheels and larger rotors. This greater mass means greater inertia, giving better response to sudden current changes, therefore reducing the severity of voltage and frequency variations.

As to the noise issue, I've heard of people fitting stock motorcycle ("sport bike" or "crotch rocket") mufflers on those small construction generators to quiet them down. (Many people replace the stock mufflers on their brand new motorcycles because they want the bike to be louder.) Supposedly it quiets the exhaust of a Briggs & Stratton motor so much that you can hear the mechanical noise of the engine.

[Guy Holt]

Non-inverter generators can work - in fact I'm not aware of an inverter generator over 10KW, so all of the large entertainment generators are "conventional".

Yes they can, if precautions are taken to remediate the adverse effects of the harmonic distortion exhibited in the scope shots above.  I can give you countless examples of problems caused by harmonics on large entertainment generators.

The means by which the entertainment industry has more or less successfully dealt with harmonics - namely the over-sizing of generators, the over-sizing of neutrals, the incorporation of power factor correction circuitry in large lights, and finally the use of generators with 2/3 pitch windings – are generally not available to users of small portable generators as their primary source of power. It is generally not an option for small independent productions using portable gas generators by necessity to upscale to larger generators; and, given that there is not much that the end user can do to alter the power output panel of a portable gas generator, all they can do to remediate the adverse effects of harmonic currents is downsize their total load by 50% when it consists predominantly of non-linear equipment.

If there is one conclusion to be drawn from the tests I conducted for my white paper, it is essential to use power factor corrected equipment and to operate it on an inverter generator. The combination of improved current waveform and the nearly pure power waveform of the inverter generator makes it possible to power larger loads, or more smaller loads, on a small portable gas generator. My white paper on the use of portable generators in motion picture production is available at http://www.screenlightandgrip.com/html/emailnewsletter_generators.html.

Guy Holt, Gaffer
ScreenLight & Grip

[ProSoundWeb Community]