Tag: electrical

Now is the part where we get specific about Start capacitors and inrush. If you haven’t read the first three parts please do so before reading this one or it may not make sense.

I’m going to come out and say it so you keep reading.

What you were taught about hard start kits decreasing inrush amperage is wrong.

Look at the oscilloscope image above. It’s a 3 ton reciprocation compressor with equalized pressures and 230V applied. Now keep in mind the voltage value shown here is RMS and the amperage is PEAK so if the amps seem high in these charts that is why.

So this compressor starts up at locked rotor (on the run winding remember) and it gets up to speed at around 180 milliseconds of run time. That is pretty typical of a good, equalized compressor under normal conditions.

Sometimes compressors do struggle to start and this can be due to.

  • Low input voltage (usually due to voltage drop)
  • Starting unequalized due to short cycling and or non-bleed (Hard Shut Off) expansion valves
  • Long line lengths
  • Refrigerant migration into the crankcase
  • Compressor wear

Here we show a system with low voltage applied (187v) and it HAS a hard start in place (not necessarily the same compressor as the last so don’t try to compare apples to apples)

In this case, the compressor doesn’t get up to speed until about 550 milliseconds and at that point both the run and start winding amperage drops.

This proves that the hard start kit is working in that case.

You can tell this because the only amperage that can enter the start winding is dictated by the run and start capacitors and like we said before… the capacity of a capacitor is dictated completely by the capacitance (mfd) and the voltage across it.

For the amperage of the start winding to go UP at 580 milliseconds, it can only be due to an increase in back EMF as that motor gets up to speed. Then at 600 milliseconds, the potential relay removes the start capacitor and the amperage drops down to the run capacitor only level.

Conclusion #1

Fixing a voltage drop issue does more good than adding a hard start kit.

A valuable test is measuring the voltage feeding your compressor with it under load and rectifying poor line voltage connections or undersized conductors.

Look at these two identical compressors, one with a hard start kit and one without.

Techs are often taught that measuring inrush on the compressor common wire is a way to show how a hard start kit decreases startup amps.

This isn’t what we are measuring when we see a big difference in inrush amps.

What we are measuring is how QUICKLY the compressor starts not the true inrush amps at start. I’ve looked at the specs on several high-end ammeters that measure inrush and they read at 100 milliseconds.

If you look at the two examples above the compressor with a hard start will read lower at 100 milliseconds because it is already almost at full speed and the LRA has nearly passed.

LRA (locked rotor amps) is essentially the amperage the compressor runs when the run winding is functioning as a heater rather than a motor. The quicker the motor starts turning the quicker it gets out of the LRA range.

Now consider the start winding. Take a look at the start winding with no hard start…. it stays the same even though the compressor is essentially locked all the way up to 600 milliseconds. So even though the compressor is locked the current through the start winding is limited to what the run capacitor can hold and release.

With the hard start in place, the start winding amperage peaks right up until the potential relay takes it out at 100 milliseconds.

The problem is that we use tools that measure at 100 milliseconds when that may be before or after the motor has hit that 80% speed

 

Conclusion #2

Locked rotor amps are what they are… that’s why the manufacturer can publish it on the data tag. It’s the amperage that motor will draw when it’s locked at the rated voltage.

When we measure inrush with a meter we are really just taking a snapshot at a particular point in time that may or may not line up with what the manufacturer published depending on how close the motor to full speed at that time.

Conclusion #3

Hard start kits don’t decrease starting amps at the moment of start, they can’t.

What they can do is reduce the time it takes to get the motor started, So in a time-averaged sense, a hard start kit very well may reduce amperage and wattage.

This is why a hard start is often specified for long line applications and non-bleed TXVs. It’s also why Techs have found that adding a hard start can reduce light dimming complaints by speeding up the amount of time the compressor remains at LRA.

Conclusion #4

Hard start kits do increase the current on the start winding, in fact, that’s essentially all they do differently than a run capacitor. They both provide a phase shifted current to the start winding, a start capacitor just does MORE of it.

The argument that hard start kits reduce wear and stress on the start winding is false. With no hard start the load on the start winding is constant and very low. Adding in a hard start adds in more current at start and the possibility of potential relay sticking that can definitely cause start winding stress.

This isn’t to say start kits are a problem when sized properly. They can and do reduce LRA on the run winding by adding more phase shifted current to start.

Conclusion #5

Start capacitor sizing and potential relay voltage ratings are really important and should be selected to do the job of starting the compressor quickly without staying in too long or providing more start winding current than needed.

— Bryan

This tip was created by Jason Pinzak and originally posted on the HVAC Technician’s Facebook group. It is reposted here with permission from Jason. Thanks!


Contactors are useful in commercial and industrial applications, particularly for controlling large lighting loads and motors. One of their hallmarks is reliability. However, like any other device, they are not infallible. In most cases, the contactor does not simply wear out from normal use. Usually, the reason for contactor failure is misapplication. That’s why you need to understand the basics of contactors.

When someone uses a lighting contactor in a motor application, that’s a misapplication. The same is true when someone uses a “normal operation” motor contactor for motor jogging duty. Contactors have specific designs for specific purposes.


When selecting contactors, you’ll use one of two common standards: NEMA or IEC. Both match a contactor with the job it has to do, but they do so in different ways.
The NEMA selection process always results in a choice of a contactor you can use over a broad range of operating conditions. For example, you could use a NEMA Size 5 contactor to run a 50-hp motor operating at 230V or a 200-hp motor at 460V.


Using IEC standards, however, you can size contactors very close to their ultimate capabilities. In many cases, this precision allows you to predict how long they’ll last. For example, an IEC-rated contactor may run a motor that draws 40A at full load. In that duty, it should last for more than two million operations. But, if you used it for consistent jogging and plugging, you’d have to replace it after just a few thousand operations.

Since a contactor should last for years, don’t automatically replace one that fails with an identical unit. Instead, take a few moments to see if there is an obvious problem. A contactor really has only two basic parts: the contacts and the coil. The coil energizes the contactor, moving the contacts into position. The contacts transmit the current from the source to the load. Heat can destroy either of them, so take a good look at both.

Contacts will overheat if they transmit too much current, if they do not close quickly and firmly, or if they open too frequently. Any of these situations will cause significant deterioration of the contact surface and the shape of that surface. Erratic operation and failure will be quick. To check the contacts, just look at them. Some minor pitting (see photos) as well as a black oxide coating is normal, but severe pitting or any melting or deforming of the contact surface is a sure sign of misapplication. Replace contacts with such symptoms.

Coils can overheat if operating voltages are too low or too high; if the contacts fail to open or close because of dirt or misalignment; or if they have suffered physical damage or experienced an electrical short. Coil insulation degrades quickly when it gets too hot. When it degrades, it will short out (and blow a fuse) or just open and stop operating.

To check a coil, measure the ohms across the contactor coil. Infinite resistance means the coil is open. A shorted coil will still often register significant resistance and can be confused with a good coil . If you happen to have a matching contactor nearby, compare the two coils. The shorted coil will usually have significantly lower resistance than the good one but a compromised coil can alos have a higher resistance. If the difference is significant, replace it. Replacing the contacts or coil often means replacing the whole contactor. But no matter what you replace, compare the NEMA or IEC rating with the job the contactor will be doing. If you match it to the application, it should last a long time.

— Jason Pinzak

P.S. – Here is another good article on the difference between IEC and NEMA rated contactors

Grounding is an area of many myths and legends in both the electrical and HVAC fields. This is a short article and we will briefly cover only a few common myths. For a more detailed explanation I advise subscribing to Mike Holt’s YouTube Channel HERE

Myth – Current Goes to Ground

Actually current (electrons) move according to a difference in charges/potential (Voltage). When a potential difference exists and a sufficient path exists there will be current. In a designed electrical system current is always returning to the source, the opposite side of a generator, transformer, battery, Inverter, alternator etc… current only goes to ground when an undesigned condition is present and ground (earth) can be a VERY POOR conductor at times. The only saving grace for the earth as a conductor is all of the parallel paths created with a ground rod because of all the surface area contact to earth.

Myth – To Be Safe, Add More Ground Rods

The ground can be an exceptionally poor conductor. The purpose of ground rods is to carry large spikes in current that comes down your electrical distribution lines away from the building. Adding more ground rods can actually EXPOSE the building to current from near ground strikes. Adding more rods isn’t always the solution and often does nothing useful.

Myth – Connecting Neutral and Ground Together In Multiple Places Is a Good Idea

Neutral and equipment ground should be connected in only one location at the main distribution panel to prevent the grounding conductors from carrying neutral current. If the equipment ground is carrying any current there is a problem.

Myth – Electricity (only) Takes The Path of Least Resistance 

If you have ever wired a parallel circuit you know that electrons travel down ALL available paths between to points of differing electrical charge.

Myth – Common, Ground and Neutral are the Same

Not even close. Common and neutral are terms used to describe the one side of a transformer. They are not grounded unless you ground them and when you do you are designating which side of the transformer will have an electrical potential that is equal with EQUIPMENT GROUND. The earth itself simply acts a really poor and erratic conductor between points of electrical potential that we designate and should not be confused with equipment ground.

Myth – Ground Rods Keep Us From Getting Shocked

Nope. Proper bonding connection between appliances, switches and outlets and equipment ground connected back to neutral at the main distribution panel in conjunction with properly sized circuit breakers and GFCI equipment keeps us safe. Grounds rods have little to nothing to do with protecting you from a ground fault.

Here is a great video on the topic and  you can find an article defining grounding and bonding terms HERE

 

— Bryan

Testo 760 Category IV Multimeter

I was standing at a booth at the HVAC Excellence Educators conference and an instructor walks up, grabs a meter and asks me “what’s the difference between a category 3 and a category 4 meter”?

Well, I really wasn’t sure other than that the category 4 is rated for more demanding conditions. So I did some research and dug into IEC 61010-1 and found that category 3 is rated for most uses OTHER than outdoor utility connections and category 4 meters are rated for all uses.

Courtesy of Fluke

There are also some voltage considerations and limitations to the different categories but the primary difference is not the regular duty but the high voltage transients. High voltage transients are often called “surges” or spikes and are most likely when working on outdoor transformers and distribution panels.

Rubber meets the road is that for HVAC use a category 3 meter is likely going to do the job but if you ever work in main panels, or outdoor transformers go for a cat 4 meter.

— Bryan

PS – Fluke has a great info sheet on this HERE

You can see more about the Testo 760 shown HERE

How does a typical motor know how fast to run?

Typical induction motors are slaves of the electrical cycle rate of the entering power (measured in hertz ).

Our power in the US makes one full rotation from positive electrical peak to negative peak 60 times per second or 60hz (50hz in many other countries)

This means that the generators at the power plant would have to run at 3600 RPM if they only had two poles of power 2 poles (60 cycles per second x 60 seconds per minute = 3600 rotations per minute) in reality, power plants generators can run at different speeds depending on the number of magnetic poles within the generator. This phenomenon is replicated in motor design.

The more “poles” you have in a motor the shorter the distance the motor needs to turn per cycle.

In a 2 pole motor it rotates all the way around every cycle, making the no-load speed of 2 pole motor in the US 3600 RPM.

A 4 pole motor only goes half the way around per cycle, this makes the no-load (Syncronous) RPM 1800

6 pole is 1200 no load (no slip)

8 pole is 900 no load (no slip)

So when you see a motor rated at 1075 RPM, it is a 6 pole motor with some allowance for load and slip.

An 825 RPM motor is an 8 pole motor with some allowance for slip.

A multi-tap / multi-speed single phase motor may have three or more “speed taps” on the motor. These taps just add additional winding resistance between run and common to increase the motor slip and slow the motor.

This means  a 1075, 6 pole motor will run at 1075 RPM under rated load at high speed. Medium speed will have greater winding resistance than the high speed and therefore greater slip. Low speed will have a greater winding resistance than medium and have an even greater slip.

Variable speed ECM (Electronically commutated motor) are motors that are powered by a variable frequency. In essence the motor control takes the incoming electrical frequency and converts it to a new frequency (cycle rate) that no longer needs to be 60hz. This control over the actual frequency is what makes ECM motors so much more variable in ten speeds they can run.

So in summary. There are three way you can change a motor speed.

  • Change the # of poles (more = slower)
  • Increase slip to make it slower, decrease slip to bring it closer to synchronous speed
  • Alter the frequency (cycle rate)

— Bryan

First off, the correct acronym for a GFI (Ground Fault Interrupter) is a GFCI (Ground Fault Circuit Interrupter) and the purpose is to act as a safety device to protect from electrical shock.

GFCIs can be built into outlets, circuit breakers and even extension cords and are generally used for safety in wet environments like bathrooms, kitchens and outside.

A GFCI measures the difference in current between the line (hot) and the neutral. When even a small difference exists between neutral and hot the GFCI trips. This happens because a difference between neutral and hot means that some of the current is “leaking” to ground instead of being carried properly on neutral.

An example would be an electric drill plugged into an outlet outside and the cord plug falls into a mud puddle. If there is no GFCI some of the current will go out of the plug to ground through the puddle, causing hot to carry more current than neutral and making the puddle a potential shock hazard. If the circuit were protected with a GFCI it would trip immediately when the imbalance was detected.

Another nice thing about a GFCI is that it can help protect a circuit that does not have an equipment ground such as tools and appliances with two prong cords or two conductor outlets.

— Bryan

I was talking about dry contacts with one of my techs and he was looking at me like I had three heads and one of them was on fire.

So I figured it would be good to cover the difference…

Basically “dry” contacts is a switch that has no shared power source or supply integral to the control. A common example would be the contacts in a compressor contactor. The contactor has a 24v coil (in residential) but the power supply through the contacts doesn’t have any connection to the coil.

We see wet contacts every day when we connect a residential thermostat. A thermostat uses the same voltage/power source to power the control that it passed to the contacts from the “R” terminal.

This is especially important to differentiate when working on commercial equipment that may have different and varied control. The Danfoss ERC 213 shown above is an example where the compressor (terminals 1 & 2) may be of a different voltage than the wet contacts on 5&6 which must be 120V.

Here is a video where I describe this in more detail –

—  Bryan

 

 

carrier

We keep 2 pole 40 amp 24v coil contactors on all of our vans. They are versatile, reliable and you can replace most residential A/C contactors with them.

There are a few things to watch for though, especially when you have a crankcase heater. Many brands power the crankcase heater constantly and shut it on and off with a thermostat, often mounted on the discharge line (here’s looking at you Trane).  When you replace a single pole with a two pole contactor in this type you need to make sure you connect BOTH sides of the crankcase circuit across the L1 and L2 line side of the  contactor to ensure the heater can function when the compressor is off.

Even more confusing that that…. Look at the diagram at the top and focus on the top left part of the diagram where the crankcase heater is located…

How does that work do you think?….. I will wait while you think it through…. Don’t cheat… Look at it.

This is a common Carrier Heat Pump crankcase heater configuration.

You notice that one side of the heater is going to L1 line side Terminal 1 and the other side is going to L1 load side terminal 2.

So the crankcase heater ONLY functions when the compressor contactor is OPEN and even then it does so by back feeding through the compressor common and back through the run winding of the compressor to the constant powered L2 side of the contactor.

This means if you replace this contactor wire for wire with a 2 pole contactor the crankcase heater will never work. You must put the compressor run wire (yellow) to the bottom of the contactor (L2 line side) instead of the top like it was if you want the crankcase heater to function in this situation…

All of this to remind you, DON’T BE A PARTS CHANGER! Know what you are replacing, why you are replacing it and what each wire and component actually does.

— Bryan


The term “short” has become a meaningless phrase in common culture to mean “anything that is wrong with an electrical device”.

A short circuit is a particular fault that can mean one of two things in technical lingo.

Any two circuits that are connecting in an undesigned manner. This would be the case if a control wire had two conductors connected together due to abrasion. Like a Y and G circuit “shorted” in a thermostat wire between the furnace and the thermostat. This would result in the condenser running whenever the blower is energized.

A short can also be described as a no load path between two points of differing charges. This would be a traditional “short to ground” low voltage hot to common connection or a connection between legs of power without first going through a load of appropriate resistance.

Both of these conditions will result in something occurring that should not be occurring. Either something being energized when it shouldn’t be or fuses and breakers tripping or blowing or damaged components.

This is different than an Open circuit which is no path at all. So if a load has power applied and NOTHING is happening it is an open. If power is applied and breakers or fuses trip or blow or something comes on at the wrong time or order, that is a short

— Bryan

Grounded_240v

One of the most common questions we get from techs is about using a voltmeter to diagnose a high voltage circuit. It’s especially tricky when a tech is used to working on Low voltage or 120V circuit where there is a clear “hot” side of the circuit and a clear “grounded” side of the circuit. In 120V you have one hot leg and the other side is neutral which is actually connected to ground back in the panel. Most (but not all) 24v transformers have one hot leg and the other leg is grounded. A car has one 12VDC Hot and the other side is grounded to the chassis.

All of these cases cause techs to get used to putting one meter lead to ground and “walking” the other lead through the circuit, looking for where the voltage is lost. While this is still not the best idea even on these circuits, it usually works.

However…

in 240v or 3 phase diagnosis it doesn’t work. Here is why –

The other “side” of the completed circuit is not grounded at all. So when you check to ground, you are checking to a point that has literally NOTHING to do with the completed circuit you are diagnosing. Even more important is the fact that you will often read “120v to ground” even when the leg you of power you are attempting to diagnose is open.

Here’s an example

Simple_Schematic_240v

Let’s say you are trying to see if the IFR contact is open. So you put your meter from L1 to ground. Good news you have 120v. So now you are feeling confident and you read from IFR terminal 2 to ground and you still have 120v. So now you think, “The IFR terminals are closed because I have 120v on each side”…

WRONG!

You will have 120V to ground on IFR terminal 2 regardless of whether the contacts are open or closed. If they are open you will be reading 120v backfed through the motor from L2, if they are closed then you will read L1.

In other words, it’s a pointless test.

Take a deep breath…

This next part is gonna take some focus to understand. If you don’t intend to pay careful attention to these next paragraphs you won’t benefit.

Instead read from L1 to L2 and confirm 240V then read from IFR1 to L2 and from IFR2 to L2. If you have 240v on IFR1 and not IFR2 then you know IFR is open…

An alternate method if you are DEAD SET on reading to ground is to check IFR1 to ground. If you have 120V then check from IFR1 to IFR2. If you read anything across the contacts you would then know they were open.

Remember…

You will read potential (voltage) so long as a path and difference in charges exist, across a load and across an open switch. You will not read potential (voltage) across a closed switch because a closed switch has no potential difference across it.

Final notes –

You are encouraged to check both legs to the ground for safety purposes to confirm the disconnect is actually off and open.

Checking to the equipment ground can be a way to check the ground itself, although in that case, a de-energized ohm or megohm test can often be a better test.

–Bryan

Scroll to top
Translate »

Daily Tech Tip

Get the (near) daily Tech Tip email right in your inbox!
Email address
Name