Power Factor and Startup Amps


Photon Sorcerer
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.


Photon Sorcerer
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:


The probes:


A 1 kW space heater as resistive load:


Scope shot driving space heater:


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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Photon Sorcerer
Next, I tested a Harbor Freight air compressor, which has an induction motor, 1.5 HP 120V 12A:


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:


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:


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?


Solar Enthusiast
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.


Flux capacitor builder
Well ,, go ahead and use your second rate test equipment. 😁😂
Looking forward to the results.