Electric Motor Power Factor

I need some education:

I have been following another post, "Motor Amp Draw by Terry G, 7/6/12" and got curious what the power factor would be when running a fractional horse power motor. I have a small bench top grinder and decided to check it using a "Kill A Watt" meter. The label on the grinder indicates 3.2 amp at 120 volt.

The "Kill A Watt" meter readings with the grinder running unloaded are as follows: volts = 119.2, amps = 2.77, hertz = 60, PF = 0.55, watts = 181.

Question:

119.2 volts times 2.77 amps = 330.2 watts. This should be the actual power usage rate as billed by the electric utility....correct? 330.2 watts times 0.55 PF = 181. watts; what I don't understand is why would a meter read in watts based on a PF of 1.0 when the PF is 0.55? What am I missing here?

You did not have the motor under load. Most ratings on motors are with them under maximum load. Take a piece of metal and put the motor under load. Then check the draw.

It sounds like you don't know what power factor is. Power factor is the cosine of the phase angle between the current and voltage of a reactive load. It is measured in volt-amps, not watts.

Worst case, with a purely inductive (or purely capacitive) load, the power factor is zero. Although voltage is applied to the load and current passes through the load, there is no net power because current and voltage are 90 degrees out of phase with each other. Energy is temporarily stored in the inductor (or capacitor), but is returned back the power source 120 times per second.

Your wattmeter reads power, not volt-amps. You get charged for the actual power you use (181 watts), not the apparent power (330 volt-amps). Low power factor is bad, but more so for the power company than for the consumer, as it causes losses in the power company's distribution system.

Motors, being a high inductive load, generALLY HAVE LOUSY power factors i.e. less then unity of one. To balance things out and achieve a nearer unity power factor, correcting capacitors can be used.

If the load is pure resistive like a light or heater then current is in phase with the voltage i.e. theres a zero angle between the two and the cosine of zero is one thence a unity one power factor. If the load is inductive (say a motor) and capacitive (say correction capacitors) and the two can be balanced, such brings the two legs back in phase and again a unity power factor is achieved.

At our facility we payed a huge penalty if our loads were overly inductive (lousy power factor) so we added power factor corrections capacitors in the line.....

If the power facotr is one, then watts = volt amps

John T Too long retired EE and rusty on this but believe this to be correct

That is why there is little difference between no load and full
load current on a single phase induction . In particular the low
end light duty units without a run capacitor.
PF improves as the motor is loaded heavier.
PF is also what messes up bargain minded people who are
purchasing the absolute minimum sized generator as they are
adding up watts of load rather than volt amps of load. I've tried
to explain the fact but then the new generator owner is mad at
me for not making them purchase a larger generator. Go
figure???
Note run and start capacitors are two different applications.

Ken, When you are dealing with fractional hp motors, power factor isn't a big deal.

On the other hand, farmers with large hp motors running irrigation and grain dryers, then the phase angle between current and voltage becomes a concern.

My dad would simply start adding capacitors in parallel with the large motors and use an ammprobe to measure the current going to the motor from the load center. Capacitive reactance is 180 out of phase with inductive reactance. As you start adding capacitors, the total current feeding the load center will decrease up to a point, if you connected too many capacitors, then the current would begin to increase.

My dad had no real idea about phase angles, capacitive reactance, and inductive reactance, he just knew how to get-er-done. He would also make phase converters for the farmers who couldn't afford 3 phase.

The upside to doing this is the farmer could balance things out, not install a larger load center, and cut down on electric bill. Another upside to decreasing the current, the power lost in the wires = I squared R will be at a minimum.

I could give you the formulas for capacitive reactance and inductive reactance, but all you need to know is capacitors, in the right amount will cancel out the inductive reactance and you get maximum power transfered to the motor.

George

In an AC system, volts x amps = volt-amperes.

Volt-amperes x power factor = watts.

If the power factor is unity (i.e, the voltage and current waveforms are aligned in time), then volts x amps = power.

Your electric utility meter measures power, not volt-amperes.

The problem occurs when the current and voltage waveforms are not aligned in time, as a result of a reactive (inductive in the case of your motor) load.

In the limiting case, which occurs with a pure inductive or pure capacitive load, the offset between the voltage and current waveforms is 90 degrees, and no power is transferred even though current is flowing. In this case, the power factor is zero.

The product of volts x amps can be resolved into two components, one representing the in-phase portion of the waveforms (watts) and one representing the portion that is offset by 90 degrees, which is called reactive power, or volt-amperes reactive.

The problem with low power factor loads is that losses in the interconnect wiring is determined by the current, not by the power. In order to accommodate a reactive load, such as your motor, extra current is required which increases losses in wiring. For household loads, the extra loss in the wiring (in the form of heat) can generally be safely ignored, but for a utility having highly reactive loads costs money to run bigger wiring.

If you look along the power distribution system, you'll occasionally see power factor correction capacitors mounted on poles. These capacitors shift the power factor closer to unity by providing a source of reactive current with opposite phase from the reactive current associated with inductive loads such as motors.

Keith

I would like to say thank you to all who posted, for the refresher course in power factor.

As suggested I rechecked the grinder under load. At the full load spec of 3.2 amps, the PF increased to 0.65 from the no load condition of 0.55. I also noticed the watt button on the "Kill A Watt" meter is dual function; the first press reads in watts and pressing again reads out in VA (volt-amps). Had I seen this before I may not have gotten so confused.

Thanks again - Ken

Indiana Ken, Where do you live? I have buckets full of old run capacitors from A/C. These caps have PCB's. If you want to experiment adding caps in parallel with your motor all you have to do is make a trip to Terre Haute. Caps are free. It would be interesting to see your results you measure with your meter.

My email is open, give me a shout if interested.
George

George,

I am familiar with Terra Haute I used to live south of you in Robinson Ill. I am in the north central part of Indiana.

Thank you for your offer on the caps. I plan to be in your area later in the year for the Newport hillclimb. I may give you a shout then.

Thanks, Ken

Any time.