Tag: electricity

Capacitors are traditionally tested with a capacitance meter (commonly found as a function within a multi-meter) with the component taken completely out of the circuit. “Bench testing”, as this method is referred to, is hands down the safest method of checking capacitance in micro-farads. All other methods require the capacitor to be wired into the circuit with an applied load. To bench test, you simply take the meter leads and check across the terminals of the capacitor. For a dual-run capacitor, you would check between the Common terminal and whichever side (Fan or Herm) you wished to test. Another popular test many technicians use is the “Under Load Capacitance Calculation”. This test is performed while the system is in normal operation. A technician would measure voltage across the terminals of the capacitor (again, Common and Motor terminals if dual-run), then current off the start winding of the motor to which the capacitor is attached. Next, you plug those values into a calculation, which uses a mathematical constant: (amps x 2,652) ÷ voltage. Finally, the product of that calculation is compared against the rated capacitance printed on the capacitor. As long as the calculated value is within +/- 6% of the rated value, the capacitor quality is acceptable. Bench testing and capacitance calculations are pretty popular choices when verifying the capacitance of a capacitor against its rating. However, there is yet another way to test a capacitor under load you may not have thought of before. You can use a power quality meter to check the capacitor under load using power factor. In order to explain the validity of this measurement, here is a review of reactive power, inductive loads, and capacitors.   Reactive power is one of three different types of power in an alternating current circuit. True Power is the actual energy in watts dissipated by a circuit. In other words, the real work being done. Then there is Apparent Power, measured in Volts-Amps (VA). Apparent power is the RMS current multiplied by the RMS voltage. Reactive Power is the power dissipated as a result of either inductive or capacitive loads. Reactive Power is measured in Volts-Amps Reactive (VAr). When the current and voltage waveforms are out of phase with each other, that is reactive power.  Inductive loads, such as a condenser fan motor, are inductive by virtue of the fact their alternating current lags behind the alternating voltage as the current flows into the load. Capacitive loads have an alternating current waveform that leads the alternating waveform of the voltage. For the purposes of this tech tip, inductive loads will be exclusively discussed, because they are most common in the field by way of condenser, blower, and compressor motors. Inductive loads use a magnetic field to cause physical movement. The magnetic field is generated as electric current flows through a coil. In other words, this current used to generate a magnetic field is known as reactive power. Notice, however, there is no real work being done. The force of the magnetic field can cause physical movement (work), but it does no real work itself. Inductive loads need reactive power in order to do work, but by using more and more reactive power, the load uses a substantial amount of current (usually from the utility company). Take a look at the “Power Triangle”. Pictured is a power triangle depicting an inductive load. The hypotenuse of this triangle is notated as Apparent Power (the available power in the circuit). The leg on the y-axis is notated as reactive power (magnetic field), and the leg on the x-axis is notated as real power (actual work being done). If you notice the Theta symbol in the left acute hypotenuse angle (𝜭), this is referring to the Power factor of the load. Power factor (cos𝜭) is the ratio of the average Real Power in watts to the Apparent Power in volts-amps . Ideally, the Apparent and Real Power would be the same, as in a resistive load (i.e. a power factor of 1). However, inductive loads need a magnetic field. If the reactive power leg on the y-axis were to increase and rise higher on the y-axis, the hypotenuse (apparent power) would also increase. The power factor, in this case, decreases, and moves closer and closer to the left acute hypotenuse angle, thereby increasing the hypotenuse angle away from the x-axis (real power). This is counter-productive, because as the load uses more current, more heat energy is generated, and the energy used to do the actual work becomes inefficient.   Therefore, the goal of the engineer is to minimize the amount of reactive power the inductive load uses from the apparent power. Basically, the goal is to increase power factor back to as close to unity (1) as possible. This is when capacitors enter the scene. Capacitors are generally accepted as reactive power generators. To understand more about how capacitors work, and some common misconceptions, check out these other tech tips/podcast episodes: Run Capacitor Facts You May Not Know (Podcast) 5 Capacitor Facts You Should Know  Capacitors, when applied to a circuit, decrease the amount of apparent power needed by the inductive load to generate the magnetic field. This effectively increases the power factor. Looking at the power triangle again, as the reactive power on the y axis decreases, the hypotenuse (apparent power) also decreases, moving closer to the real power. This is the endgame of capacitors.    Therefore, it can be inferred from the understanding of inductive loads and capacitors: if a capacitor is attached to an AC circuit in an inductive load (like a PSC motor), but the power factor is low, the capacitor itself is either sized incorrectly or failing/failed. Using a power quality meter on an inductive load, a technician can determine the quality of a capacitor. To do this, Voltage and Current must be measured simultaneously at the load. The Supco Redfish iDVM-550 is a great tool for this application.

It must be mentioned that using a power quality meter to measure power factor on a load is valid only when the load contains run capacitors like compressors and permanent-split capacitor motors. ECMs (electronically commutated motors) use a different type of capacitor altogether, and they are engineered for use with a lower power factor by design. Also, power factor testing is not practical for start capacitors either, since the capacitor is taken out of the circuit too quickly. This measurement is valid and practical only for PSC type blower and condenser motors, and most single-phase, single-stage compressors. 

There are many great diagnostic tools available to the service technician today, but I shaven’t found a tool as versatile than the simple isolation diagnosis.

There are many ways this concept can be applied but let’s start with some examples so you get what I mean.

Low Voltage Short Circuit Isolation Diagnosis

You arrive on a no cooling service call at a home and you find the system is off and doesn’t respond to any thermostat settings. You check the breaker and the condensate switch at the furnace by force of habit and look at the door switch, everything looks in order so far.

You take a look at the 3A low voltage fuse and SURE ENOUGH, visually blown.

Now there are many schools of thought of how to proceed from here but I prefer to use logical possibilities and process of elimination before starting to tear everything apart.

First,  look at any possible rub out points on low voltage wires in the furnace, visually inspect the safeties, go outside and check the control wire both inside and outside the condensing unit and ESPECIALLY wherever wires run across copper tubing. If you find nothing at these common failure points I will pull open the thermostat there and check that it’s wired correctly with no exposed copper.

You will notice that I didn’t just “replace the fuse”, because science and experience has shown me that fuses don’t just fail open on their own.

If I still find nothing then the “logical possibilities” part of the diagnosis has ended and we move into the isolation diagnosis part of our testing.

Some techs prefer disconnecting all the low voltage wires at the furnace and ohming them one at a time to ground and common at this point. Sometimes this works but I prefer to let the system work for me rather than pulling apart all the wires at this stage.

I replace the blown fuse with a 3a re-settable fuse / breaker like the one shown above with the thermostat off in all modes (to keep from wasting fuses). I then close the door switch and see if the fuse blows, if it does not then we can determine that there isn’t a continuous short circuit in R.

I then use a jumper wire and connect each switch leg (G,Y,W,O,W2,Y2 etc…) to the R terminal quickly and see if any of them trip the fuse or throw a spark (remember this is 24v it’s not going to hurt you). Often you will find the circuit with the short just that simply and then you can further isolate until to find the exact part of the circuit causing the issue.

Let’s say the Y circuit is the one blowing the fuse, you can then separate the two wires going to the condenser and the thermostat and tap one side at a time to R, if the conductor going outside is the one throwing the big spark / blowing the fuse then that is the direction to focus. You then go outside and disconnect the Y wire at the condenser wire nut or terminal block. If the short continues you know it is in the wire between the condenser and the furnace and not in the condenser. If it continues then you know it is in the condenser or in the contactor itself.

Shorted Compressor Isolation Diagnosis

Compressors shorted to ground or “grounded” is a pretty common diagnosis in the field and leads to an expensive compressor or system replacement. It’s important that we get that diagnosis correct and take away all the guess work so I use a combination of diagnostic tools and isolation diagnosis to ensure I never get it wrong.

When I walk up to a condensing unit and it has a tripped breaker, the first thing I do is reset it and see what happens…

NO IT ISN’T 

The breaker tripped for a REASON and every time I reset that breaker I run the risk of creating a major arc. If that short is inside the compressor I add carbon and acid to the refrigerant every time I reset that breaker.

So once again we FIRST perform a full visual inspection of all the high voltage wires, terminals, contactor, capacitor, crankcase heater and the breaker itself. If that all looks OK then we pull the top and inspect the compressor leads and terminals themselves. Before you go pulling on the terminals or compressor plug make sure you are wearing gloves and safety glasses, it is possible for one of those terminals to blow out of the fusite at that moment and freeze your face or hands.

Another quick tip is take a photo of all wires and/or tag them before pulling them off. We all have cameras in our pockets nowadays which makes mis-wiring upon reassembly a thing of the past (I wish).

Now measure resistance to ground (In this case the copper stubs on the compressor) from each terminal. It is OK to use a megohmmeter if you want, just keep in mind that some compressors are still considered “good” by the manufacturer all the way down to 0.5 megaohms and some meters say “bad” at 20 megaohms.

Once you are certain as you can be that the compressor short to ground is the culprit or if it is reading between 0.5 and 20 megaohms to GROUND making you unsure I have one more task for you.

Now ISOLATE the compressor by taping and strapping up the plug or terminals so they aren’t touching anything. Now reassemble the unit and reset the breaker.

If it doesn’t trip and everything else (Condenser fan motor etc..)  runs properly then you can feel good about your diagnosis, if it trips again then it’s back to the drawing board.

Other Uses 

Isolation diagnosis can be used in things like finding system noises and even in finding open circuits by using jumper wires. Isolation diagnosis is taking a hypothesis and testing it with one component or conductor at a time allowing you to find the culprit through process of elimination.

On communicating systems where I wanted to be SURE it was the controller and not the wire I removed the controller from the wall and wired it directly to the fan coil board to make sure it still didn’t work even with no wire between. Sometimes my hypothesis was right and sometimes it was wrong but either way isolation diagnosis has saved me from looking dumb on many occasions.

— Bryan

 

P.S. – Bert Made this video a while back using isolation diagnosis to find a LV short

In a Series circuit (loads connected in a row end to end) it’s easy to calculate total circuit resistance because you simply add up all the resistances and you have the total.

In a Parallel circuit the voltage is the same across all the loads, the amperage is simply added up but the resistance is a bit more tricky.

It gets tricky to imagine because the total circuit resistance of parallel loads goes down the more loads you add.

For example, if you have one light bulb connected to a power source, the total resistance of the circuit is just the resistance of the bulb.

Add in another bulb in PARALLEL and the resistance of the circuit goes DOWN

When you are calculating the total resistance of a parallel circuit you take each individual resistance and divide it into (not by) one. You then add up all the resistances that were divided into one and divide that sum into one. The formula looks like this for the diagram at the top of the article.

1÷Rt (total resistance)= 1÷R1 + 1÷R2 + 1÷R3

For this particular application as shown above it would be.

1÷Rt(total resistance)=1÷120 + 1÷45 + 1÷360

So 1 ÷ 120 = .0083 + 1 ÷ 45 = .022 + 1 ÷ 360 = .0028

Then we add them all up

.0083 + .022 + .0028 = .0331 

Then to find the total you divide one by the total

1 ÷ .0331 = 30.21 Ohms total 

As you will notice, 30.21 Ohms is less than the lowest resistance in the circuit. This makes sense when you think about ohms law.

The lower the resistance the higher the amps. Adding in additional parallel loads INCREASES the amperage in a circuit, and we see this ever day when we notice that compressor amps and condenser fan amps added together equals total condenser amps.

So it stands to reason if lower resistance equals higher amps and adding in more parallel loads increases the amps, then adding in more parallel loads reduces the resistance.

Another myth this busts is the idea that electricity ONLY takes the path of least resistance. Electricity actually takes all paths between positive and negative charges and every additional path (parallel circuit) just decreases the resistance between the two points of potential difference. This increases the total circuit amperage, which is why when you try to run two hair dryers on one 15a circuit the breaker trips. Two hair dryers in parallel = lower  total circuit resistance = higher amps.

Not that I would use two hair dryers….. maybe that’s why I’m almost bald.

— Bryan


Some quick basics –

An Ohmmeter is used to measure the resistance to electrical flow between two points. The Ohmmeter is most commonly used to check continuity. Continuity is not a “measurement” as much as it is a yes / no statement. To say there is continuity is to say that there is a good electrical path between two points. To say there is no continuity means there is not a good electrical path.

In other words, continuity means low or zero ohms and no continuity means very high or infinite ohms. Don’t get the terms zero ohms and infinite ohms confused, they mean opposite things.

 

This type of testing is commonly used to check fuses, Trace wires, check for short and open circuits Etc… Resistance readings are necessary for identifying motor terminals, and checking for a breakdown in insulation. An Ohmmeter continuity can be used to identify normally open, and closed terminals on a relay. Simply place the leads of the meter across the relay points, if there is continuity the relay is normally closed. Now apply power to the magnetic coil of the relay, the contacts that were closed should now open, or vice versa. An Ohmmeter can be used to identify a single wire in a bundle. Go to the opposite end of the wire and expose two separate wires in one sheath. Twist the two wires together and list the colors. Go back to the other end and check for continuity between all wires of that color.

 

Once you find two wires with continuity, you have found the correct wire. If you suspect that a particular wire is shorted to another wire, simply disconnect both wires on each end and check for continuity between the two wires. If continuity is read between the wires you have found a short.

These are only a few examples of ways to utilize an Ohmmeter.  Remember an Ohmmeter should only be used in un-energized circuits, Otherwise the meter could be damaged.

 

— Bryan

wire_rubout

There are a few important things that I suggest checking on every service call to reduce callbacks and increase customer satisfaction. One of them that often gets missed is preventing wire rub outs.

One of my area managers and experienced tech Jesse Claerbout shot a video showing the simple step he takes to prevent major damage.

We also just release a new podcast episode today that you can hear in any podcast app or by listening HERE

Cheers!

–Bryan

electrical-theory

In this episode of HVAC School, Bryan talks to his boys about basic electrical theory and they talk about:

  • Differential Charges
  • Electromotive Force
  • Ohm’s Law
  • Volts, Ohms, Amps and Watts
  • Electrical paths
  • Conductors and insulators
  • Resistive and Inductive loads

And Much more…

As always if you have an iPhone subscribe HEREand if you have an Android phone subscribe HERE

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