Using Your Voltmeter As a Voltage Drop Detector
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- Опубликовано: 10 июн 2024
- Bryan Orr explains an interesting way of using your voltmeter as a voltage drop detector.
Ohm's law describes the relationship between volts (electrical pressure), amps (electrons flowing through the circuit), and resistance (ohms or resistance that the circuit must overcome). Ohm's law states that voltage is equal to the product of amps and resistance (E = I x R). As resistance decreases, amperage increases and vice versa. Motors are inductive loads, meaning that applying force against a motor increases the amp draw and reduces the electrical resistance.
Before using a voltmeter, set it on the ohm scale and ohm between the leads to make sure there is a path through them.
We can take a voltmeter and measure the electrical potential (voltage) between any two points. On a simple 120v circuit with a lightbulb and a switch between two legs of power, we would measure 120v between both legs of power. The voltmeter has two leads so that you can measure the potential (or voltage drop) between two points. The total voltage drop is equal to the total resistance of the circuit. In the case of a 120v simple circuit with all the resistance in one lightbulb, the total resistance would be in the lightbulb, and we'd read 120v across it. (However, that's not realistic because wires and switches have a little bit of resistance.) These sorts of tests between points on a circuit are only valuable when the circuit is under load, and a case like an open switch would make an open path and make there be no potential between the two points.
If we read significantly less than 120v across a 120v load, we can determine that we have a voltage drop somewhere in the circuit. We can use our voltmeter as a voltage drop detector to isolate areas of voltage drop (such as 4v across a switch). Our goal is to reduce voltage drops to the best of our abilities, so we need to isolate them within the circuit and address them. Switches and wires should have very little resistance, as those are power-passing devices; a significant voltage drop across a wire or switch is usually a cause for concern, as those components can increase in temperature.
Measuring the applied voltage across a contactor (or open switch) can let us know what the total input is. Measuring the voltage across a closed switch will tell us the voltage drop across the switch. Situations that create resistance, such as insects on the contactor, will also cause current to be drawn. The overall circuit current will decrease, but the localized current will increase and generate heat.
On a compressor in a 240v circuit with the contacts open, we would measure 240v between each leg of power. When the contacts are pulled in, we can begin searching for a voltage drop; if the voltage stays pretty close to 240v at different points across the load and through the contacts, then there is a minimal voltage drop. If the voltage drop is significant, then we can start measuring the voltage drop across each contact point. We don't need to ohm out the individual components; the voltage drop alone can tell us where our points of significant resistance are.
To prevent unwanted voltage drops, you can provide motors and loads with the proper voltage, ensuring that you have tight connections, and using wires of the proper size. When you suspect an area of high resistance, you can use your voltmeter to detect and isolate a voltage drop without ever switching to the ohm scale.
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Good class, as an HVAC/R company owner I wished my guys new as much.
Glenn Sepelak teach them as he teaches his.
That kind of experience is expensive.
@@AlaskanMonroe oh i teach and train them for years, then they open up their own company everytime
I'm a 3rd year HVAC/R apprentice. I'm struggling. You make EXCELLENT instructional/explanatory videos. Thank you!
Brian i love how clear and helpful you are to a tech who was on install for 1yr and now making my way to service tech. I have learned simple things just from watching your videos and the more difficult things being able to use that info in the field is huge. I have so many notes that really help me and i always ask questions to experienced techs when it comes to difficult situations. Thank you and please keep showing us what a service tech should know
Good teachers are like fine wine: hard to find. And you sir, are a good teacher. I've been watching your videos since I began working in OEM refrigeration manufacturing 3 years ago. I just got my first job as a HVACR service tech with my own vehicle and have doubled down on listening to this channel. Please feel free to share anything with me, I love this field and learning new things. I will do the same.
Best explanation so far. Awesome job!
Excellent voltage drop explanation, best I’ve seen.
I deal with 12 volts all day long in the automotive industry but it's totally awesome to see that ac electricity is the same as far as testing
I'm a second year HVAC/R student and just brushing up on the basics. Great videos and thank you!
Hi, Bryan I recently listen to your voltage drop video and wanted to say" Great info". And learned much from it. Thanks for all you do for your trade.
PRODIGY HVAC SCHOOL
Thank you very much for sharing what you know with us. Very helpful information video
Take care and have a great day
PRODIGY HVAC SCHOOL
From Nick Ayivor from London England UK 🇬🇧
That was very helpful! Thanks Bryan!
As always, great video. Thanks
Really informative and very well presented thank you so much.
Thank you very much for all you do for us technicians. I just get the app and is AMAZING!! I am a HVAC company owner and all this classes are making me strong. Thank again!!
I asked you if you please show me the proper way to flush a line set that can't be replace from r22 to 410a.
Responding to RJ_Make’s question:
Typically, heat causes resistance in a conductor, electrical connection (switch or contact) or an inductor (winding). In your question, your winding is cooling down - so resistance will decrease as a result of its cooling down.
For EVERYONE ELSE:
Voltage drop is a HORRIBLY difficult topic to explain to most folks. Some GET IT. Most…DON’T.
Brian's reference to Ohm's Law is REALLY the best possible way to explain it, but the math must be applied while actually seeing it on a meter.
AGAIN: Affirmation of what he's explaining can be seen by using a voltmeter and seeing it as a reading...using your brain's ingrained memory of Ohm's Law.
DESPITE what Brian explains (and Brian has my greatest admiration for what he does), I've always impressed upon techs to remember Ohm Law like you'd remember how to brush your teeth. Make it an instinct. Make it something that's in your head EVERY TIME you put your test leads into a circuit. THEN - factor in the inductive aspect of that circuit (motors, relay coils, etc) when it wasn't what you’d expected. You may not know the math involving THAT, but at least it's something to be considered before calling a motor, solenoid, relay or contactor as BAD.
With that, you might want to consider studying up on inductive loads.
THE MOST IMPORTANT thing to remember is that a voltmeter, when you’ve introduced into a circuit, will only read a DIFFERENCE in potential between the two points being read. Applying what that "view" of what you're seeing in the circuit's performance, when compared to the OVERALL circuit's design...will give you a clue as to what's the problem.
I've MOSTLY learned to use voltage drop to determine bad switches, contacts, fuses, circuit breakers, etc...as the ONLY way to find their failure. On that note…
The NEXT MOST IMPORTANT thing to remember is to TRY to do your troubleshooting under the conditions which the problem occurs. That will inevitably be by using a voltmeter reading live voltages of 120v, 208v, 240v…or even 480v.
I say this because an ohmmeter’s 9v battery usually won’t LABOR a circuit’s components adequately enough to duplicate the conditions under which a problem occurs. There’s not enough push from a 9vdc battery to duplicate the high-amp conditions demanded by a compressor, heating element…or whatever…to make it “fizzle” into the problem which occurred and you’re looking for.
So, an ohmmeter…as handy as it is, often won’t tell all. A voltmeter will ALWAYS tell the true story - by looking for the VOLTAGE DROP.
LASTLY: when troubleshooting…VERIFY!
@EC Tofix. Thanks for the great explanation. I'm a bit confused because of 5:38. As the **physical resistance** (say spinning up a blower wheel) increases so does heat right? So if heat is increasing why is my winding resistance decreasing according to 5:38? I missing something here..
Or as the Samurai says... "ohms can lie. Verify!"
@@RJMaker might want to understand conductive heat vs intended heat in a circuit, ie running a load generates heat, BUT stalling a fan motor (outside of design operation condition) the current is flowing through just one of the windings on the stator, creating inducted heat/ and eventually hopefully the fan will cut out on thermal overload before permanent damage occurs. This situation is where you will see resistance drop as it cools, vs normal operation as the wires would be sized to suit this.
Excellent video. Voltage drop can be utilized as another way to diagnose a systems components.
great video amigo...
Good free info
Good stuff...
10:27 great tip ..
Good!
I’m missing something… where did it pass me? So how exactly do you perform a voltage drop test? By probing two ends and getting the voltage? I’d like to know how you determine the cause of the voltage drop. If I read 120 at an outlet but then get 86v once under load…aside from what’s plugged in, why is the voltage no longer 120 no matter what I plug in?
I have a question: On a hot winding why does my resistance continue to drop as the winding cools down?
As heat decreases, resistance decreases? Sounds normal to me...but in all honesty, how big a change are we talking, as I imagine that conductor will not last for long...
Perfect instructions thanks!
I dont get it. At 9:07 with the switch OPEN you measured 120v at the top two points. Then at 9:30, you closed the switch and still measured 120v at the top points. How do you measure 120v at the top two points whether the switch is opened or closed??🤔🤔🤔🤔
The top two points are your source voltage.Your source should stay the same always in the circuit.The voltage across the switch is your voltage drop.If you add the voltage drops through out a circuit it should equal your source.
A light bulb is an inductive load not a resistive load.I am confused.I have seen 120 volts on a coil and it would not pull in because it was not true power because of a bad switch that measured 0 volts across the switch.Thanks
HVAC SCHOOL, The DVM meter can measure Back EMF voltage from the compressor motor but how can a technician know if the measured voltage is Forward Voltage or Back EMF Voltage? because the DVM meter won't tell you if its forward voltage or back EMF voltage. I'm not sure why when compressor motors start aging and slower RPMs the back EMF voltage will "increase" any reasons why the back EMF voltage increases?
For advanced techs ,you should do a video on the topic of Inductive reactance and power factor. Most techs get the dear in the head lights look if you ask them , why is an RMS meter better than others?
If we are going to compare two different ac motors, can we always say that the one with higher amp (current) reading consumes more power? Or are there cases where the motor with lower amp reading actually uses more power?
When it comes to amps readings, you always have to be aware of the voltage level, since power (w/kw) is P=V*I*p.f. you can have a motor with higher power consumption at lower amp readings.
Example:
Assuming P.F. 0.9
Motor 1: Reads 5 Amps @220V ---> P=220[V]*5[A]*0.9 = 990 [W]
Motor 2: Reads 3 Amps @480V----> P=480[V]*3[A]*0.9= 1296[W]
So you see, you can have a motor with a lower reading of current but still have a higher power consumption.
How about motors with same voltage?
@@ramos_dt At the same fixed voltage, lower amp readings should always mean lower power consumption, unless there's something wrong with the motor.
Remember also that for three phase motors, power is calculated by P=V*I*√3*p.f.
Thank you.
But the 480v system is 3 phase, so you have 3 contactors and circuit breakers, yes you have lower current on one phase but if you group all the neutrals you should get a standard higher amp reading, identical to the phases in... if I’m not mistaken...but I’ll check on the next unit.. inductive motors give me more grief since it’s not straight forward to me yet...
I'm only in my first year but isn't your 'close on fall" switch for your compressor wrong? Shouldn't it be a "close on rise"?
15:52 great work cameraman lol
so intuitively if I measure a volt or more of potential across a closed contacter (under load), I would classify that as an issue. That being said, I also think a motor can typically operate +/- 10% of its rated voltage rule of thumb. Who's to say that volt of potential is really an issue until the small amount of excess heat it produces further deteriorates the contact 3 years later? I guess I'm approaching the question from the angle of a customer paying for parts that may or may not make a difference for the longevity of the system so long as the load is receiving nameplate voltage. Do best practices sometimes deviate from the reality of the situation? I'm sure there're plenty of things I'm not considering i.e sacrificing a contactor before sacrificing a compressor. By the way, HVAC school is a great resource so thank you.
Most of this is just for you the tech to put into memory, for each job, (the ones where everything’s fine, but you can see it’s getting tired) where you can quietly consider the next call from them... obviously you replace things that you must, but don’t go losing sleep over it for the client.
Slow clap. Well done.
If you show the actual contact or with electricity may be more understand able
So new apprentice can relate to what they see in the field
Instead of using the term of voltage drop it’s much easier to understand and use the term differential your any gauge or meter only measures the differential between the two points including pressure gauges measure differential
At first, I thought he had a cigarette
I don’t get that neither. Physical resistance is inverse to electrical resistance. I always thought that when there is low resistance, there is low heat, but if you hold a fan motor and stop it from spinning and start to get hot, it’s not that motor getting hot due to the resistance increasing, just like a resistor gets hot, or a heating strip gets hot due to the high resistance in it. Let’s say that you have a fan and instead of you stopping it from spinning with your hand, you replace the wires to it to a size small enough so that wires start to get hot and burn. Wouldn’t you say that the wires burn because there is too much resistance in the wires? And like you said, resistance and current are inversely proportional of each other, given the voltage stays the same. Just like you mention in your video, your contactor contacts get pitted, causing high resistance, which also causes high amperage, which causes heat. Even the voltage drops a little bit. Anyway, I would really appreciate if you can educate me a bit more on this subject. I’m by no mean a collage or university graduate, I’m barely a practitioner of the trade that got a bit confused. Thanks.
My thinking is same as your rationale. I am confused of Brian's explanation.
@@thembones12 I am disappointed in the general lack of follow up in these comments, whether from fellow viewers or from the experts or the channel owners. But late as I may be, I will try to give you my take.
What makes the wires hot is *too much current* . This is why a fuse melts. It is NOT resistance. Low resistance=high current. If you have a short circuit you have essentially zero resistance, which gives you infinite current. I=V/R so if R is 0 you are dividing by 0, which is undefined or infinity. Infinite current will quickly heat the wire and trip the breaker.
You contradict yourself at 3:35 and 4:04. So which is it?
I agree it’s confusing. And I think he’s a bit loose with the term “work”. But I think his intentions are both correct, as they are two different examples with two different results.
In the first case, with longer grass (more resistance) if you use the SAME amount of effort, he’s saying less work (ball distance) gets done. The ball doesn’t go as far compared to short grass.
In the second case with long grass (more resistance), if you INCREASE effort, more work is done. Then the ball to goes the same distance compared to short grass.
wrong. you're conflating voltage drop with difference in potential. difference in potential is a broader notion whereas voltage drop refers to a decrease in voltage due to circuit components.