VFDs running off Generator? How clean does power need to be?

mmurray70 · Sep 20, 2021

Hey guys, Been considering buying a backup generator for our house and would like to run our Mitsubishi mini split heat pump and other electronics without damage. The heat pump is VFD driven and ive been told it needs to receive "clean" power but im really not sure whats acceptable. Ideally id like to have 8-10kw.

I've ruled out the cheap box store units, Hondas biggest inverter is only 5.5kw continious. How about Diesels? Yamaha/kubota make a nice 11kw model. Heard older Onans hold frequency and voltage well right to full load.

What about light towers? Can get light towers with high hours for decent price sometime. Nice and quiet with their 1800rpm engines and enclosures, and they have huge fuel tanks as well all in one unit. Would any of the diesels be suitable or do I absolutely need an inverter?

JST · Sep 20, 2021

VFDs and computers etc rectify the AC as the first operation. They really do not need super clean AC, despite "urban legends" to the contrary.

Really dirty AC with huge spikes, etc might cause issues. But most should be fine even with a square wave input as AC. Most common UPSs sold to use with computers have been plain square wave output, and they work fine. Ditto for most any VFDs.

Tony Quiring · Sep 20, 2021

Look for 1800 rpm as the sound level will be greatly reduced.

Diesel would be better as propane just costs too much for energy output.

Have seen some that are connected to the utility provided natural gas, in earthquake they shut that off so no generator.

Anything diesel will likely be well made and power output within needs of equipment.

However, one needs to have enough headroom for starting currents.

We have a client with a very large generator that can easily support the multiple HVAC systems but the new HVAC systems have a heavy surge on startup that the generator does not quite cover so the HVAC control board sees the little dip and refuses to start to protect the compressor.

Inverter HVAC should not have this issue but if possible test before buying.

Do NOT go cheap on the control and switch gear.

Proper wiring to have generator power on required loads only and proper exercise of the engine.

Sent from my SM-G781V using Tapatalk

johansen · Sep 20, 2021

Many but not all hvac systems are power factor corrected input and they should run off of anything above 40hz up to 320 volts. (Which rectifies to 450 volts). Above that voltage something may fail.

The front end pfc circuit boosts the nominal 240vac to about 400 volts dc. That circuitry is not sensitive to surges due to the topology of the circuit, but what i read online is they all require a surge protector installed alongside. But Those surge protectors are just MOVs which start to clamp at well above 240volts. (Sometimes as high as 900)

So i dont know what fails or why a surge protector helps at all.

I would not want to run both a single phase hard start heatpump, and an inverter heat pump at the same time, due to the generators over voltage produced during a load dump senario, especially if the generator is only marginally big enough to start the compressor.

EmGo · Sep 20, 2021

Tony Quiring said:
Diesel would be better as propane just costs too much for energy output.

On the other hand, if you live out in the country and use propane for heat, hot water and cooking, then you already have most everything and the generator is only for the occasional power outage .... might depend on the person's circumstances.

Tony Quiring · Sep 21, 2021

Lots of propane units available but much more complicated.

Few low speed units and fuel system in cold area may require heating.

Sent from my SM-G781V using Tapatalk

wp6529 · Sep 21, 2021

mmurray70 said:
Hey guys, Been considering buying a backup generator for our house and would like to run our Mitsubishi mini split heat pump and other electronics without damage. The heat pump is VFD driven and ive been told it needs to receive "clean" power but im really not sure whats acceptable. Ideally id like to have 8-10kw.

I've ruled out the cheap box store units, Hondas biggest inverter is only 5.5kw continious. How about Diesels? Yamaha/kubota make a nice 11kw model. Heard older Onans hold frequency and voltage well right to full load.

What about light towers? Can get light towers with high hours for decent price sometime. Nice and quiet with their 1800rpm engines and enclosures, and they have huge fuel tanks as well all in one unit. Would any of the diesels be suitable or do I absolutely need an inverter?

No question that a diesel generator, and esp. a mil surp one is best, however it is not required. I have two Mitsubishi mini splits, a 9k and an 18k and they have absolutely no issues running of an inexpensive Champion 7.5KW gas generator. I've periodically run them and everything else including multiple freezers off that generator for several years in both hot and cold weather without any issues at all. I will also note that they are rated for full capacity operation all the way down to 187VAC input.

mmurray70 · Sep 21, 2021

Sounds like diesel is the way to go. Not much military stuff in my area so I think im best going with light tower. Something like this be suitable? https://towabletools.com/portable-e...080kv-8-kw-light-tower-lt1503/?v=3e8d115eb4b3

Has a Marathon Brushless generator. I did see this on the marathon website about one of their light tower generator models:

Application Guidelines: The Marathon Electric capacitor generators are designed to minimize the length of the
generator package. They utilize a capacitor excitation system which succeeds in eliminating the extra generator
length associated with a brushless exciter. Since the capacitor winding is excited from the negative sequence
voltage, there must be some harmonics present to provide this form of generator excitation. Because of the intrinsic
harmonic content of the voltage waveform, this product is not recommended for uses where waveform distortion is of
concern. The motor starting capability of this capacitor generator is also significantly less than a comparable
transformer or electronically regulated generator. Primary use of this product is to power incandescent lighting,
while providing a small amount of auxiliary power (total kW not to exceed rated kW).

Any thoughts on that?

mmurray70 · Sep 21, 2021

thermite said:
Avoid it.

They just told you WHY.

What's not to like about their honesty?

So whats different about this generator that im looking to avoid? Any generator with this "capacitor exciting system" should be avoided? Or does it have to do with some kind of a voltage regulating system on the output of the generator?

Says that this generator has +/-6% capacitor voltage regulation. A larger model has +/-1% AVR regulation. What would be difference here?

thermite said:
That Marathon 1800 RPM head produces true sine-wave "rotating power", no hassle.

Same manufacturer? So how is this model different? Does it have another way of exciting the field or is there some kind of voltage regulator that the other model doesnt have? or both?

Sorry for all the questions. Im not an expert but would like to learn different types and what I should be looking for and what to avoid.

jim rozen · Sep 21, 2021

"Because of the intrinsic harmonic content of the voltage waveform, this product is not recommended for uses where waveform distortion is of concern. "

I think this is what he was objecting to.

It's not clear your application cares about that (his long discussion carefully avoided that question) but it's worth putting it to him direct to see what will happen.

johansen · Sep 21, 2021

mmurray70 said:
So whats different about this generator that im looking to avoid? Any generator with this "capacitor exciting system" should be avoided? Or does it have to do with some kind of a voltage regulating system on the output of the generator?

Says that this generator has +/-6% capacitor voltage regulation. A larger model has +/-1% AVR regulation. What would be difference here?

I don't have any experience looking at the waveform of a large capacitor excited brushless generator head, but i can tell you that an AVR head is going to have a lower idle power consumption, especially if you adjust the avr to drop the voltage a bit to say, 220 instead of 240.

Too many generators achieve regulation by saturation and they suck up a lot of power at no load, just to save a few dollars in copper. I have a couple 5kw generators that suck up 600 watts no load, wasted in the brushed (no avr) generator, and as such they suck up 1/3rd gallon an hour at no load, partly because of the parasitic .6KW load.

JST · Sep 21, 2021

mmurray70 said:
Sounds like diesel is the way to go. Not much military stuff in my area so I think im best going with light tower. Something like this be suitable? https://towabletools.com/portable-e...080kv-8-kw-light-tower-lt1503/?v=3e8d115eb4b3
............

It sounds like a variety of induction generator. Has to have a magnet in it somewhere to get it going. The reference to capacitor excitation is a clue.

Yes they "work". Used lots of places.

Yes, they are bad at starting motors. Problem is that excitation is dependent on load.... the load can essentially steal excitation and cause the output to "collapse". True of any generator that uses the actual output to supply the field. Ones with separate excitation do not have that problem.

If it is an induction type, then it is cheaper, because it does not have a wound rotor. Cheaper is not the same as better.

johansen · Sep 21, 2021

No JST, they are synchronous generators. the rotor has a diode in series with its shorted winding, and it picks up power via the third harmonic. as such there has to be some third harmonic in the output.

there is also another generator out there that has a diode shorted 4 pole rotor winding, a 4 pole stator, and a 6 pole stator winding which has a capacitor in series with it. never seen one in the wild though.

JST · Sep 21, 2021

Some third harmonic will not bother rectified supplies, as long as the peak is high enough.

But if the genset does not like to be overloaded at all, it may not be the best for a rectified supply. I think I'd opt for something that had an exciter, so that it will hold up under load. Even the ones that are brushless, with the diodes right on the rotor.

The capacitor may be resonating with some inductance. Resonance works fine, but could be "damped" by heavy loads, so as to reduce excitation.

I've seen some gensets with a resonated stator winding that provided excitation, with the eventual output coming off another stator winding, IIRC. Similar load problem existed with those.

mmurray70 · Sep 21, 2021

I watched a few videos on AVR generators tonight and its pretty neat how they work. Nice and simple too. Seems like this would be the way to go for any generator. I dont totally understand the capacitor setup but seems like its a compromise.

Looks like this capacitor setup is used on a lot of smaller, cheaper, portable generators right? The ones that are known for poor quality output. But I see that a lot of the larger "cheap" box store generators are using an AVR now. Maybe the output from them might not be as bad as I was thinking? Ive heard voltage and frequency can drop at high load but other then that might not be too bad?

Still would prefer Diesel. Ill keep an eye out local for anything that might come up and probably will post back to this thread if I see anything that might be suitable. Thanks for the help so far.

JST · Sep 22, 2021

mmurray70 said:
........................ But I see that a lot of the larger "cheap" box store generators are using an AVR now. Maybe the output from them might not be as bad as I was thinking? Ive heard voltage and frequency can drop at high load but other then that might not be too bad?

..............................

What does a generator provide? What is a "high load"? How long is a string.....?

Voltage and frequency control is what a generator DOES. "Load" is current flow.

So they may not be too bad, but a higher loads (more current) they may fail at the two prime features of a generator?

Yeah, that sounds like a compromise. Any standard generator is going to slow, reducing voltage and frequency at loads above maximum. But most exciter based ones do pretty well.

The "big box" ones often use a "saturating inductor" built into the stator to regulate the output. That uses the capacitor, similar to those voltage regulating transformers that used to be available. That's fine, but at higher loads, the regulation capability of the saturating inductor can be exceeded, so that the output drops.

Problem is that same output runs both the load and the field, so the dropoff is much more abrupt than for a generator with a separate exciter, and a field voltage regulator. More like a "square law" than a straight-line dropoff.

I'd forgotten about those saturating inductor types. They are clever, but not perfect. Generally cheaper, which is why they are pushed by the big box stores.

wp6529 · Sep 22, 2021

mmurray70 said:
Still would prefer Diesel. Ill keep an eye out local for anything that might come up and probably will post back to this thread if I see anything that might be suitable. Thanks for the help so far.

Big box store larger gen will run what, $600-or so? Rather a no brainer to get one to cover you now while you look for the good deal on a used diesel set. The cheap portable gas generator will still be plenty handy for other uses, loaning to a friend, etc. A friend had somewhere around $40k of losses in the spring TX freeze that could have been largely prevented with such a $600 investment.

jim rozen · Sep 22, 2021

thermite said:
Cheap.
Good.
Pick any ONE.

Cheap
Good
Fast

Pick two.

(fixed it for ya..)

jim rozen · Sep 22, 2021

You can get your genset cheap and fast. It won't be good.

You can get it good, and RIGHT now, but it won't be cheap...

Or if you wait around a *long* time, you may luck onto a good one, cheap.

Even *you* can figger that one out.

VFDs running off Generator? How clean does power need to be?

Stainless

Diamond

Titanium

Stainless

Diamond

Titanium

Aluminum

Stainless

Stainless

Diamond

Stainless

Diamond

Stainless

Diamond

Stainless

Diamond

Aluminum

Diamond

Diamond

Similar threads