Thread: Electric motors, inverters, generators and power factor.

Where's Gav when you need him?

the p2 is the apparent shaft power, not the power factor.

if the Power factor is included on the appliance plate its usally written as power factor or abreviated to PF.

To the best of my knowledge most single phase capacitor start/run induction motors have a PF of ~.7-.8

my rough rule of thumb with power factor for a single item is this, if its a generator divide or multiply the output by the power factor to make the smallest result. IF its an appliance multiply or divide the power demand by the power factor to get the largest number,.

so long as your generator output is bigger than the appliance demand you'll be ok.

in this situation as the numbers you have line up pretty closely to the power factor corrected values for most 1.5hp motors any 1500w plated magnet swinging generator should run the pump without issues and once the pump is started be able to pick up some other light duty loads (fluro lights, battery chargers), it might struggle if you have it partially loaded and then kick the pump in.

For an inverter, 1500w will run the pump but it may struggle to start it (my 1500w inverter in the car struggles to start my 1.5hp compressor when the input voltage is low or the compressors trying to start against tank pressure) and I'd suggest a 2K inverter.

Originally Posted by Tins

Where's Gav when you need him?

It was Gav or Dave I had in mind😁

But I'm sure there are many other who could help.

Dave got in first, thanks.

Still keen to hear from Gav.

Tony

A 1500watt inverter will certainly not be sufficient. Mind you, those numbers on inverters are usually highly misleading... It's like a 100watt PC speaker with a 12V 1A power supply

"1500watt" means 1500VA or a purely resistive load (I used to say a lightbulb but that is not true anymore :P) and even that is ONLY on under ideal circumstances. Most inverters are rated at 20 or 25c if it get's warmer (in oz the ambient can be much higher than that) you need to de-rate and often by quite a bit!

Take a look at this PDF with data for a proper brand, decent inverter: https://www.victronenergy.com/upload...-2000VA-EN.pdf

Even a 2000VA (or what people would call a 2000watt inverter) unit can only put out 1600watts (resistive) at 25c and derates down to 1000watt when the unit hits 65c. You will however also note that a decent inverter has a fairly high peak power rating. My experience with Victron for example is that it will hit that peak power for a few seconds before it errors. So the startup current should be handled but a soft start would be preferred.

Regarding motor Cos-Phi indeed they are mostly around 0.7 which means that a "1500watt" inverter can actually provide (1500VA x 0.7 = 1050watt) to an electric motor. In other words, a 1500watt electric motor with a Cos-Phi of 0.7 needs (1500/0.7 = 2150watt) of inverter power at the least to run, at the temperature you need it to run on. Inverters get warm when you use them ey?

I reckon that a 3000VA unit is the least you would need to run that water pump for any extended period of time and should be be able to take the hit of in-rush current:

https://www.victronenergy.com/upload...charger-EN.pdf

Mind you, these are multiplus systems so they can also charge battery banks etc but I took these purely as an example, just look at the inverter data and ignore the rest

Regarding a generator. It depends on your generator. If you have a modern inverter type generator like a Honda eu10i or something they can do 1kw peak and 800watt continues, it goes up with the larger models of course. I could not get it to run my 1kw rated A/C unit since the in-rush current was too much, it either tripped the generator or the voltage drop was so heavy that the A/C simply threw an error. An old school generator may fare a lot better in that regard. The generator will struggle when the motor kicks in, causing both a voltage and frequency drop but that will simply force a soft-start of sorts and once the electric motor hits it's stride the generator should stabilize. Still, oversize it as well so go for a 2kw unit.

You do not want to have other stuff connected at the same time however, the voltage drop and frequency change could damage equipment. Most modern computers and devices with switching mode power supplies can handle a lot but just to be safe...

Now, the power tools. There in lies a bit of a challange. A 10" angle grinder has such a high rush-in current that it trips my electric breakers at 16A even, I need to run it off a 25A unit just to use it so it all depends on what power tools you want to use. A large circular saw without soft-start (most with regulated RPM have them, the old school ones with a simple switch do not) also have huge in-rush currents.

Finally the desired function: use when the power fails. If you do not mind switching over gear to the inverter with long extension leads for example a simple unit close to the batteries will do If you plan on having the important bits all run of a single circuit in your house, you might consider a multiplus or equivalent. They have a switchover function like a UPS and everything will simply keep on working. It will also charge your battery bank when needed and it can even have a generator input. The current from the generator can be limited to say 4 amps and if you should have more draw, the multiplus will deliver the extra watts from the batteries and recharge the batteries again when the high load switches off. Quite handy to keep the gennie small and simple (Honda eu10i for example)

Make no mistake btw. Running a fridge also has a fairly high peak in-rush current due to the compressor. I tried to run my fridge and chest freezer of my eu10i and it went well until the chest freezer tried to start up whilst I had other gear connected to the gennie

Hope this helps, if I made it too complex just let me know

Cheers,
-P

I will leave it to Gavin to post a simple explanation. I have been around generators all my working life, including rewinding them. I have worked on big variable speed DC drives and a multitude of every other piece of electrical equipment to be found on ships and oil rigs, from cabin fans up to 6kV generators and motors and their associated control and distribution systems.
All old school, not up to speed with inverters and such modern trickery.

So, I will try and explain about Power Factor. Firstly, do not be ashamed to admit than you cannot understand it, as most of the people that quote about voltage, amps, watts and kilowatts also don’t understand it!

Power Factor is only a factor in AC systems; it is not a factor in DC systems.

The basic meaning is: It is the ratio between real power (kilowatts) and apparent power (kVA)

That is to say: the actual power consumed by a device in watts (or kilowatts), and the apparent power it takes to provide that power expressed in volts and amps, (VA, or kVA)
Or in the case of a generator (alternator), the energy consumed by the prime mover, compared to the what the actual usable power of the generator output is.

This is why all alternators, or inverters, need to be rated in kVA, not watts or kilowatts, and that rating will only be correct at the PF shown on the nameplate, usually 0.8

The stumbling block is “induction” found in every electrical device except something that is pure resistance. It can be referred to as “I2R”; amps squared x resistance. In a pure resistive device, the losses occur as heat, in an AC circuit the losses are called “wattless”, because energy has to be used to produce the amps and volts, but do not do any “work”.

So, to quote how much energy is required to power an AC load we must use a formula that takes into account those losses:

Volts x Amps x Power Factor.

The power Factor number is not a set, or even a known number, it is very much dependent on each individual situation, but in an inductive circuit it always going to be less than one, or unity.

In an AC circuit the voltage can be seen as a sine wave, where it starts at zero, rises to a peak in a positive direction, falls down to zero and then rises again in a negative direction to peak again before falling back to zero. The time it takes to do this is one cycle, (or HZ) and here in Australia this cycle occurs 50 times every second.

Then the amps have to follow the same type of pattern, and in an inductive circuit it can be seen that the amps cannot keep up with the voltage, and so lags the rise and fall of the voltage.

It is the difference in the rise and fall of the amps compared to the voltage that shows us the Power Factor.

To make it more confusing if a suitable capacitor is placed in a circuit, then those lagging amps then advance and lead the voltage.

An old school example is the older florescent lights, wherein there was a ballast, or choke, that is very inductive. So, to improve the efficiency of the light a capacitor was connected across the supply, which then greatly improved the PF.

In a normal household installation the PF is not really a consideration. But across the whole grid it is a major consideration, because all of that “wattless” power is using energy that is not being recorded by any meters.