(Podcast) Using Volts and Ohms in Diagnosis
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- Опубликовано: 18 янв 2018
- (Audio Only) In today's podcast, Bryan talks about voltage (volts) and resistance (ohms), specifically using a voltmeter and an ohmmeter for diagnosis. We also discuss voltage drop.
In many cases, Ohm's law is impractical for field usage because of the additional resistance from inductive reactance. We also don't typically measure impedance and only care about resistance on the windings. However, Ohm's law is still a valuable concept because it teaches technicians the relationship between voltage, amperage, and resistance (ohms).
Ohm's law states that volts equal amps multiplied by ohms (E = I x R). Therefore, if the volts stay constant, ohms will increase as amps decrease and vice versa.
We distinguish lines from loads in circuits; we say that loads are the parts that "do" something due to resistance in a circuit. There are two kinds of loads: inductive and resistive. Inductive loads generate expanding/collapsing magnetic fields, which can also cause rotational force or activate a solenoid. Resistive loads generate light and heat, so heat and resistance are related.
Of course, the diagnostic tools we use (multimeters, voltmeters, ammeters, ohmmeters, etc.) also have their limitations. A voltmeter merely determines a difference in charges between two points. When using a voltmeter on a low-voltage circuit, try to plant one of your leads on the common side and take readings throughout the circuit with your hot lead. Ground is also NOT a reliable reference point for diagnosis.
The point of measurements is to prove what we suspect to be true; we must understand what our data mean for system operation and what our tools' diagnostic limitations are. For example, when we ohm out contactors, we check to see if they're open.
Bryan also discusses:
Fixed wattage or resistance
Reading between wires
Meter lead placement
Amperage (dynamic current/electrons)
Undiagnosed shorted circuits
Contact points
Voltage drop and resistance
Infinite ohms
Wire length
Read all the tech tips, take the quizzes, and find our handy calculators at www.hvacrschool.com/
I enjoy your teaching & very good information thanks.
For me, this included two reminders I haven't heard in a while: 1) dynamic nature of electronic math in a live circuit, and 2) proper test points when taking readings.
Excellent information
Thanks!
I have been backtracking alot of my textbook material to uncover what I've been overlooking and i have came to the conclusion that I'm only really captuturing at best half of what I've been reviewing, your podcast helps me to connect the dots really thank you!
1:01:15: When measuring the voltage across L1 and L2, isn't there a circuit through the voltmeter?
Perfect
Exciting action video! How did you edit all that?
Talent.... pure talent
@@HVACS Modesty is highly overrated!
I think it would be very beneficial to simplify a lot of this, for most beginners new to the trade the majority of this podcast would be hard to digest, especially when you stray off into the deeper scientific analysis of electrical theory etc. (although important), not very practical for common service troubleshooting in the field. Great work nonetheless, just some feedback.
Seems like low voltage due to voltage drop on long wire runs increase amperage draw on compressor motors. What's causing that? Increased slip due to lower voltage??
I'm just learning but that sounds correct to me!
That’s referring to inrush currant.
good work .... sometimes you talk to fast.... which makes it hard to understand
Your not looking for a specific ohm reading across compressor windings ? Yes you are . Ohm out your compressor divide start winding ohm's into run winding ohm's if it falls between 3to 6 times greater within specs , under 3 or over 6 times greater compressor partially shorted.
Voltage is pressure. You are measuring the pressure instead of charge. less confusing for newbies.
"!00 percent voltage drop for a circuit when you measure across two points", what?
When your measuring across two points you are measuring the voltage, pressure, at those two points. The way you explain this, by the time I get to the end of the circuit I won't have any or very little voltage left. Which is untrue.
How does the power company charge you for usage? You being charged for power right? volts times amps, times power factor where your supplied voltage should be plus or minus 10 percent.Not used up.
Why don't you explain it as a liquid header feeding a bunch of systems you measure one end of the header. At a value say, 170.
you measure another end it should be 170. As long as my header is size properly and there are no restrictions. Same with each system coming off that header when that liquid line gets down to the case, 170 should be measured there as well.
If the liquid line solenoid is good and opens fully.
Same with voltage if I have 600v at the distribution panel and at the top of the contractor. Then with contactor pulled in, you should read 600v, on the same legs at the bottom. If not then the pitting and excessive carbon is the resistance of the small series circuit across the contacts. Causing your voltage drop.
I've been if the commercial Refrigeration trade, supermarkets, for 36 years and you confused the fuck outta me.
I was hoping to hear a different explanation to use. My boss and some colleges don't understand why I replace 15yr old pitted contactors when the compressor still runs.
Good post if his tech. is trying to read from yellow to red then he shouldn't be in there and charging for his work. Doug Shakell you can carry my tools!