gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
111003-2016 EDT
I am working on a set of notes on Electrical Energy Measurement, Conservation, & Methods to Reduce Your Electric Bill. To try to illustrate to readers that power factor correction black boxes are probably useless to the average residential customer I have run the experiment that follows. This is based on the type of demonstration shown in the fraudulent advertizments.
It is probably correct to say that virtually no residential customer in the US pays a penalty for poor power factor. Thus, power factor correction is of no value to the customer.
The experiment uses an old Montgomery Ward Powercraft 1/3 HP 115 V motor. Rated at 60 Hz and 5.9 A . Note: At an output of 1/3 HP this equates to 249 W of output. The input power to the motor is 249 W plus all motor losses.
To make these measurements the supply voltage must be accurately controlled. Initially I tried using a constant voltage transformer as the supply. Note this primarily only regulates relative to incoming line voltage changes. Unfortunately it adds a lot of source impedance. Much being inductance. So I still have to adjust the output voltage manually for load changes. Using a constant voltage transformer actually created more problems than working directly from the line, and so it was not used.
My line voltage is moderately stable for short times so it was workable to just work from the line directly. However, this does not change the fact that load changes require manual voltage adjustment. I used a Variac (actually a Powerstat) and a buck transformer with a maximum output of 9 V for voltage adjustment. I was able to hold closer than 0.1 V. When capacitor power factor correction is performed there is a lot of input current ringing, whether with the buck voltage or directly from the line. The ringing does not appear to have had a negative effect on the results.
A Kill-A-Watt 4460 EZ was used for power, voltage, current, and power factor measurement. I was hoping to show that power input actually increases with the addition of the power factor correction. My measurements tend to show this, but accuracy is not sufficiently good to serve as proof. Above about 0.9 A the power resolution is 1 W.
The motor tested would be described as a split-phase type and has no run capacitor. Therefore unloaded power factor is quite bad.
When the sales people that push power factor correction on residential customers do a presentation they use an unloaded motor and measure current and not power. That is where the Kill-A-Watt meter is to their advantage because its name implies measurement of power, which it can do, but they use the current range, and the measurement being made is in tiny print. The sales pitch shows a large reduction in the displayed number (current) when the power factor correction capacitor is paralleled with the motor.
I used high quality metal film Polypropylene capacitors for my test. The test was in 30 mfd increments and at one point a 12.5 mfd was added and used to get close to 100% correction.
30 mfd at 60 Hz is 88.4 ohms, 60 mfd is 44.2 ohms, 90 mfd is 29.5 ohms, 102.5 mfd is 25.9 ohms, and 120 mfd is 22.1 ohms. I have just used the nominal capacitance and I did not measure the capacitors capacitance. At 117 V the corresponding capacitor currents are calculated to be 1.32, 2,65, 3.97, 4.52, and 5.29 amperes.
Actual measured currents were 1.33, 2.66, 4.00, 4.53, and 5.33 . The capacitor power dissipations were 1, 2, 2, 3, and 3 watts. At these current levels the power is quantized in 1 W increments.
As a first approximation an equivalent circuit for the motor at constant voltage and no external mechanical load can be considered as an ideal inductor in parallel with a resistance.
A no load measurement on the motor at 117 V produced readings of 4.71 A, 140 W, 552 VA, and 0.25 PF.
From this we can calculate the inductive and resistive currents. Iresistor = 140/117 = 1.197 A. Then the inductive current is sq-root of (4.71^2 - 1.197^2) = 4.555 A.
Thus, 90 mfd under corrects, 102.5 mfd almost corrects, and 120 mfd over corrects.
We can assume as the correction capacitance is increased that the motor power should not change because there is no change in the motor voltage or load. Thus, the increase in the input power is a result of the increase in capacitor power dissipation. Measurement error of the Kill-A-Watt prevents us from getting a more accurate correlation between the capacitors alone, and the capacitors plus motor.
We see minimum current and maximum power factor at the near 100% correction point as would be expected.
For a residential customer at this time a power factor correction capacitor at the main panel will not reduce the electric bill, but actually minutely increases the bill. Demonstrable by this data.
Now suppose this motor was fully loaded. I will assume 85% efficiency for the added power at the input from the mechanical load to make full power output, and no power factor correction. This means add 249/0.85 = 293 W to the unloaded motor losses. If this is done and the inductive component is assumed constant, then the input current would be about the combination of 4.555 inductive and (293+140)/117 = 3.701 resistive, or a line current of 5.87 A. That is close to what the motor was rated at. The full load power factor would be about 407/(117*5.73) = 0.59. It would still take the same amount of capacitance to compensate the inductive component.
The four 30 mfd 370 VAC 60 Hz capacitors plus shipping cost $59.30 . These were imports vs US made.
It should be noted that a capacitor run single phase motor has a much better power factor than a single coil induction motor. The unloaded and un-corrected power factor of my capacitor run drill press motor is 0.51 .
.
I am working on a set of notes on Electrical Energy Measurement, Conservation, & Methods to Reduce Your Electric Bill. To try to illustrate to readers that power factor correction black boxes are probably useless to the average residential customer I have run the experiment that follows. This is based on the type of demonstration shown in the fraudulent advertizments.
It is probably correct to say that virtually no residential customer in the US pays a penalty for poor power factor. Thus, power factor correction is of no value to the customer.
The experiment uses an old Montgomery Ward Powercraft 1/3 HP 115 V motor. Rated at 60 Hz and 5.9 A . Note: At an output of 1/3 HP this equates to 249 W of output. The input power to the motor is 249 W plus all motor losses.
To make these measurements the supply voltage must be accurately controlled. Initially I tried using a constant voltage transformer as the supply. Note this primarily only regulates relative to incoming line voltage changes. Unfortunately it adds a lot of source impedance. Much being inductance. So I still have to adjust the output voltage manually for load changes. Using a constant voltage transformer actually created more problems than working directly from the line, and so it was not used.
My line voltage is moderately stable for short times so it was workable to just work from the line directly. However, this does not change the fact that load changes require manual voltage adjustment. I used a Variac (actually a Powerstat) and a buck transformer with a maximum output of 9 V for voltage adjustment. I was able to hold closer than 0.1 V. When capacitor power factor correction is performed there is a lot of input current ringing, whether with the buck voltage or directly from the line. The ringing does not appear to have had a negative effect on the results.
A Kill-A-Watt 4460 EZ was used for power, voltage, current, and power factor measurement. I was hoping to show that power input actually increases with the addition of the power factor correction. My measurements tend to show this, but accuracy is not sufficiently good to serve as proof. Above about 0.9 A the power resolution is 1 W.
The motor tested would be described as a split-phase type and has no run capacitor. Therefore unloaded power factor is quite bad.
When the sales people that push power factor correction on residential customers do a presentation they use an unloaded motor and measure current and not power. That is where the Kill-A-Watt meter is to their advantage because its name implies measurement of power, which it can do, but they use the current range, and the measurement being made is in tiny print. The sales pitch shows a large reduction in the displayed number (current) when the power factor correction capacitor is paralleled with the motor.
I used high quality metal film Polypropylene capacitors for my test. The test was in 30 mfd increments and at one point a 12.5 mfd was added and used to get close to 100% correction.
30 mfd at 60 Hz is 88.4 ohms, 60 mfd is 44.2 ohms, 90 mfd is 29.5 ohms, 102.5 mfd is 25.9 ohms, and 120 mfd is 22.1 ohms. I have just used the nominal capacitance and I did not measure the capacitors capacitance. At 117 V the corresponding capacitor currents are calculated to be 1.32, 2,65, 3.97, 4.52, and 5.29 amperes.
Actual measured currents were 1.33, 2.66, 4.00, 4.53, and 5.33 . The capacitor power dissipations were 1, 2, 2, 3, and 3 watts. At these current levels the power is quantized in 1 W increments.
As a first approximation an equivalent circuit for the motor at constant voltage and no external mechanical load can be considered as an ideal inductor in parallel with a resistance.
A no load measurement on the motor at 117 V produced readings of 4.71 A, 140 W, 552 VA, and 0.25 PF.
From this we can calculate the inductive and resistive currents. Iresistor = 140/117 = 1.197 A. Then the inductive current is sq-root of (4.71^2 - 1.197^2) = 4.555 A.
Thus, 90 mfd under corrects, 102.5 mfd almost corrects, and 120 mfd over corrects.
Code:
[FONT=Courier New]The results are:
Voltage Capacitance Input Current Input Watts Input VoltAmp Power Factor
117.0 0 4.71 140 552 0.25
117.0 30 3.44 141 400 0.34
117.0 60 2.30 141 270 0.52
117.0 90 1.45 142 168 0.83
117.0 102.5 1.33 142 158 0.90
117.0 120 1.58 143 185 0.78
[/FONT]We can assume as the correction capacitance is increased that the motor power should not change because there is no change in the motor voltage or load. Thus, the increase in the input power is a result of the increase in capacitor power dissipation. Measurement error of the Kill-A-Watt prevents us from getting a more accurate correlation between the capacitors alone, and the capacitors plus motor.
We see minimum current and maximum power factor at the near 100% correction point as would be expected.
For a residential customer at this time a power factor correction capacitor at the main panel will not reduce the electric bill, but actually minutely increases the bill. Demonstrable by this data.
Now suppose this motor was fully loaded. I will assume 85% efficiency for the added power at the input from the mechanical load to make full power output, and no power factor correction. This means add 249/0.85 = 293 W to the unloaded motor losses. If this is done and the inductive component is assumed constant, then the input current would be about the combination of 4.555 inductive and (293+140)/117 = 3.701 resistive, or a line current of 5.87 A. That is close to what the motor was rated at. The full load power factor would be about 407/(117*5.73) = 0.59. It would still take the same amount of capacitance to compensate the inductive component.
The four 30 mfd 370 VAC 60 Hz capacitors plus shipping cost $59.30 . These were imports vs US made.
It should be noted that a capacitor run single phase motor has a much better power factor than a single coil induction motor. The unloaded and un-corrected power factor of my capacitor run drill press motor is 0.51 .
.
