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Power Factor and Startup Amps

Hedges

I See Electromagnetic Fields!
Joined
Mar 28, 2020
Messages
20,044
Because I have suitable equipment I'm going to put up measurements I've taken, might help someone with planning a system or general education.

Motors draw a startup surge of considerably more amps than they draw while running. With grid power you may see the lights dim briefly, but if you have a long skinny extension cord the motor could fail to start due to excessive voltage droop. With inverter power, there is the added problem of how much current the inverter can supply.

Power factor: For AC loads, a resistive load draws current proportional to voltage and in phase. The current is a sine wave exactly aligned with voltage sine wave. Vrms x Irms = "apparent power" in watts, same as actual power. That was the "dot product" of voltage and current. Actual power is determined by "cross product", which can be computed with an oscilloscope capture of voltage and current, then multiplying the voltage by current at each sample point (including sign as well as magnitude.) Then, take the mean average over time.

For an inductive, capacitive, or motor load, the current waveform isn't in phase with the voltage. Part of the time, power is shoved back into the supply; sign of the current measurement is opposite from sign of the voltage measurement. Driving an inductor, current lags voltage by 90 degrees. Capacitor, current leads by 90 degrees. Resistive load, current is at 0 degrees. Grid-tie inverter, current is at 180 degrees (all power delivered, never consumed.) Motors will be something off zero degrees.

Power Factor is ratio of "Actual" power (which can be computed from math on the waveform) to "Apparent" power (which can be computed as product of Vrms and Irms). Apparent power can be expressed in units of "VA" to distinguish from "Watts". A resistive load has power factor = 1.0, Inductor and Capacitor have power factor 0.0 (they don't consume any power), a motor might have power factor = 0.9

I have set up an oscilloscope with voltage probe and AC current probe connected to an electrical outlet. For now a typical 115V, 15A outlet but I plan to connect a 230V outlet to test larger motors. Pictures and results to follow.
 
Test setup consists of a Tek scope, high voltage differential probe (maximum 2300V from either terminal to ground), and AC current probe (0.333V out for 100A full scale.)
The voltage probe is connected to channel 4 (scale is automatic). Vrms shows AC voltage
The current probe is connected to channel 2 with 300:1 external attenuation (displays 1.0V to represent 1.0A) "Vrms" actually shows current
A math function computes ch4 x ch2 (V x I) and (mean) average is computed. "VV" actually represents watts

The scope:

1592344650877.png

The probes:

1592344694714.png

A 1 kW space heater as resistive load:

1592344773244.png

Scope shot driving space heater:

1592344821238.png

Voltage is 119.6 Vrms, Current is 8.48 Arms, Actual Power is 1012W.
119.6 x 8.48 = 1014 VA or Watts Apparent Power
Power Factor is 1012 / 1014 = 0.997

Observe the current waveform (green) follows shape of voltage waveform (blue) and crosses zero at about the same point. This is as expected for a resistive load.

I did check for a startup surge, and there was nothing at all. That is to be expected when a heating element is kept at a cool temperature, such as this oil-filled radiator. Once it reaches operating temperature after a few minutes the current might drop somewhat. A stove element or a lightbulb should show a greater drop in current when hot. TCR or temperature coefficient of resistance is positive for heating elements; their resistance increases when hot and they find a stable operating points. This is dramatic for the white-hot filament of a light bulb which would draw about 1/10th as much current as when cold.
 
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Next, I tested a Harbor Freight air compressor, which has an induction motor, 1.5 HP 120V 12A:

1592345589648.png

During startup, current showed a peak of 50A for 4 cycles or 1/15th second. 50A peak of a sine wave is 35Arms; the calculation on screen was lower because it was made across the entire waveform displayed:

1592345681751.png

Once the motor was running, current was 12A peak and 7.69 Arms.
Apparent power was 922W but real power was 738W for power factor 0.8:

1592345851994.png

Note that 738W is barely over 1 HP. This was at about 60 PSI in the tank, but didn't change as it reached 150 PSI
Note also the distorted current waveform (green). Also, voltage changes polarity before current, representing a period of time when power gets shoved back into the grid.

What does your battery inverter do with current shoved into it?? Does it store the power in capacitors, or does it get burned up as heat?
 
I'd like to see this topic started in June expanded upon.. :D A Scope does come in handy..

With induction motors, the magnetic field is generated 45 degrees out and it is what creates the power factor. Adding a capacitor to this moves this power requirement closer to 0 and reduces the power factor.

I have a Fortress 27 gallon compressor from harbor freight with 1.6hp motor and 1.0 power factor. Runs great off the generator.
 
Well ,, go ahead and use your second rate test equipment. ??
Looking forward to the results.

Greg
 
@Hedges

I was just searching around the forum for some power factor and start up questions. And what before my wondering eyes should appear? :cool: I wonder if you could help with some PF questions

I just bought a Honda EU2200i generator. The first thing I did was install a propane conversion. I know propane doesn't have quite the power of gasoline. But my plans were to never have gasoline in the tank, unless some zombie apocalypse or chinacrud propane shortages forced me. My intended use was small power around the house when the lights go out for short periods. I have a big generator for when the hurricanes put things out for a week.

My other intended use was to power the A/C in my camper. But I have sold my camper. My dad has the smaller Honda 2000 that will power his A/C on gasoline, with a soft start. So I know I'm at least in the ball park

My new Honda has a cool BT app. It will read amps, or volt amps. VA, as I understand, is watts with the PF applied.

I attached a heat gun and a hair drier to my Honda. They were not enough to kill the gen. It will power those inductive/resistive combo loads to 2130VA. But no clue as to what the PF would be

IMG_7347.JPG

On a camper forum, a guy installed a power meter on the camper I would like to buy. I had him post a picture of the meter with the A/C running. That A/C unit has a pretty crappy PF. Do the two of these images give me enough info that I can make an assumption either way, that my new gen will run or not run the A/C system I hope to have?

148181734_5289758924382657_4442022560606562623_o.jpg
 
Heat gun is almost purely resistive load, so 1.0 PF

His meter shows 1.34 kW running load.

My test of a small window air conditioner showed running load (mild thermal conditions) was 1/2 of nameplate,
but starting current was 5x nameplate for less than 0.2 seconds. That would be 10x my measured running current.

Even taking 5x his running load, 6500W to start it.

Generator specs say 2200W max, don't mention starting surge.


Rotating mass tends to deliver a surge, but inverter means limited by transistors and inductors.
I've read that A/C which first starts a smaller fan load (causing generator to ramp up to full speed) before starting compressor can do better.

Maybe propane reduces max power from 2200W to 1800W, same as continuous? But you'd still have the rotating mass for surge at least to 2200W.

I wouldn't be surprised if it couldn't start. Running load 2/3 of generator max rating, 3/4 of continuous rating.
Maybe a soft-start kit would help.

A field test with your generator and someone's AC would be most meaningful.
Maybe seller of RV has access to same model generator.

Hey, if you used generator to feed an inverter with good surge capability, that would start the AC!
(So long as inverter and generator were happy playing together.)

What I don't think I've seen anyone mention is feeding PV into an inverter generator's DC bus. Doesn't help your starting surge, but would be interesting project (likely dealing with 170VDC)
 
Take the sum of all your loads except the largest motor in KVA. Now add three times the KVA of the largest motor.

That is all other loads PLUS 3 times the KVA of the largest motor.
That is the minimum reliable starting size generator one should use.
If you have voltage sensitive equipment you may have to increase the motor factor from 3 times to 5 or 6 times.

*********************************************
Here it is in recipe form:
1. Add up your total load excluding the largest motor in Amps.
2. Multiply this by the voltage they will use and divide by 1000.(could be 120 and 240V loads) The answer will be in KVA.
3. Multiply the Amps for the largest motor by 3.
4. Multiply this amperage by the voltage you will use on it and divide by 1000. The answer will be in KVA.

Add these two KVAs together.
That is the size of generator you need in KVA.

Example:
Total load without largest motor = 18 Amps @ 120 Volts = 2.6 KVA
The largest motor draws 7 Amps @ 240 Volts.

7 Amps x 3 = 21 Amps

21 Amps times 240 Volts = 5 KVA
2.6 KVA + 5 KVA = 7.6 KVA

Round this KVA value up to the next available sized generator. In this case probably you'd be looking
for an 8 KVA Generator. (Or an 8 kW generator. In that size KVA often equals kW.)
********************************************************

That's the rule of thumb for home generator sets if you don't want them failing quickly or failing to reliably start motors. Note that failing to fully start a motor will often result in its immediate destruction.
 
Heat gun is almost purely resistive load, so 1.0 PF

His meter shows 1.34 kW running load.

My test of a small window air conditioner showed running load (mild thermal conditions) was 1/2 of nameplate,
but starting current was 5x nameplate for less than 0.2 seconds. That would be 10x my measured running current.

Even taking 5x his running load, 6500W to start it.

Generator specs say 2200W max, don't mention starting surge.


Rotating mass tends to deliver a surge, but inverter means limited by transistors and inductors.
I've read that A/C which first starts a smaller fan load (causing generator to ramp up to full speed) before starting compressor can do better.

Maybe propane reduces max power from 2200W to 1800W, same as continuous? But you'd still have the rotating mass for surge at least to 2200W.

I wouldn't be surprised if it couldn't start. Running load 2/3 of generator max rating, 3/4 of continuous rating.
Maybe a soft-start kit would help.

A field test with your generator and someone's AC would be most meaningful.
Maybe seller of RV has access to same model generator.

Hey, if you used generator to feed an inverter with good surge capability, that would start the AC!
(So long as inverter and generator were happy playing together.)

What I don't think I've seen anyone mention is feeding PV into an inverter generator's DC bus. Doesn't help your starting surge, but would be interesting project (likely dealing with 170VDC)
The A/C is rated at 1,500w. A soft start smooths out the A/C start load, no issues on the tiny gens. Yes the gen is 2200w. But look at my BT app shot. It rated in VA, which is also 2200.

Help me out with VA, as related to watts and PF
 
2200W with 1.0PF is 2200VA

900W with 0.9PF is 1000VA or something similar.

1340W / 0.91PF = 1473VA

VA is the apparent power, volts x amps without regard to phase. Could be a perfect sine wave shifted in phase (e.g. capacitive or inductive load draws current 90 degrees out of phase for zero watts, zero PF) Could be distorted wave like diode rectifying AC to DC on a capacitor. Could be distortion as motors seem to draw from AC, with a couple peaks per cycle or a shoulder like my scope image.
 
2200W with 1.0PF is 2200VA

900W with 0.9PF is 1000VA or something similar.

1340W / 0.91PF = 1473VA

VA is the apparent power, volts x amps without regard to phase. Could be a perfect sine wave shifted in phase (e.g. capacitive or inductive load draws current 90 degrees out of phase for zero watts, zero PF) Could be distorted wave like diode rectifying AC to DC on a capacitor. Could be distortion as motors seem to draw from AC, with a couple peaks per cycle or a shoulder like my scope image.
Make sense to my lizard brain. Thanks!
 
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