Tag: watts

Most motors are designed to set amount of work, usually rated in either watts or horsepower, which is  746 watts per HP.

Watts law states that Watts = Volts x Amps. If a particular motor needs to do 1 horsepower of work at 120 Volts it will draw about 6.22 amps. And yes in an inductive load like a motor it’s not quite as simple as VxA=P but we are keeping it simple here.

A motor designed to do the same amount of work (1HP) at 240v will draw half the Amps (3.11).

This does not make the second motor “more efficient” because the power company charges by the Kilowatt NOT by the amp.

) If you take a load that is designed for a particular voltage and you DROP the voltage it will also decrease the wattage according to Watts law (Watts = Volts x Amps) as well as decrease the amperage according to Ohm’s law (so long as the resistance remains the same).

Let’s say you take a 5KW heat strip that is rated as 5Kw at 240v and you instead connect it to 120v.

It would then only produce 1.25 kw and draw 1/4 the amps, this is because while we may call it a “5 Kilowatt heater” it is actually just a fixed resistor designed to do 5 kilowatts per hour of work in the form of heat at 240 Volts. Cut the Volts in half you also cut the amps in half and you decrease the amount of work done down to 1/4 because Watts = Volts x Amps.

— Bryan


In a previous article we covered the standard way to check Capacitors under load.

I am now going to give an even easier test.

A properly funcitioning PSC (permanent split capacitor) or CSCR (Capacitor Start Capacitor Run) motor should have a power factor of very close to 1 if they have a properly sized and functional capacitor.

If you have a multimeter that can read power factor directly (like the Testo 770-3) you can measure the power factor by reading the voltage at the contactor and the amperage at the motor common (like usual). If you are at or close to 1 power factor then your capacitor is both functional and the right size.

In the image above I have a compressor that calls for a 35 MFD capacitor and the capacitor is running right at 35 in the under load test as well as the bench test. This is why the power factor is right at 1. I installed a run capacitor of 10 MFD larger and smaller and sure enough… the power factor dropped in both cases.

So not only do I see right away that either my capacitor is failing or improperly sized, we can also see the wattage (power) in real time.

As a side note I was only seeing about a .3 amp difference between Start and run / common together when I oversized the capacitor so I may have missed the issue if I had just used that test alone.

Now, in order to see exactly WHAT is wrong with the capacitor, whether it is failing or improperly sized, you would need to do the under load test (start winding amps x 2652 / capacitor Volts) or bench test the cap and then check against motor data plates.

But as a regular service procedure this power factor feature is a time saver and gives you a unique insight into the operation of the motor.

— Bryan

P.S. – If you are interested in the 770-3 use the offer code “getschooled” at TruTechtools.com for an 8% discount

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