Tag: electrical

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

 

One of the most common mistakes I hear techs make is confusing Zero ohms with infinite ohms. The fuse above is showing near zero Ohms which means a good electrical path with very little resistance.

If there is a perfect path it would have zero ohms (which isn’t actually possible unless you happen to be testing a superconductor).

If there is no path, the circuit has infinite ohms. This would be shown as Open or OL or something similar.

Often when I ask what ohm reading a tech is getting they will say “none”… None could easily mean zero or infinite so it’s important to clarify.

Once again.

Zero ohms = shorted / closed / directly connected

Infinite ohms = open/ no path

Try to remember to say either Infinite or zero instead of  “no ohms” or “none” to avoid confusion

— Bryan

Erich Vinson is a tech from Colorado and one of the most entertaining people I interact with online. He wrote this quick tech tip on stripping back the outer jacket properly on control wires and it happens to also be something I preach. Thanks Erich.


In the first picture (above), you can see what happens when you try to use a pair of wire strippers to remove the jacket. It damaged the wire underneath. Instead use the pull string when you strip the jacket off of a low voltage (control) cable.

Use your strippers to remove about three inches of the jacket (or cut into the end like I show above – Bryan), and use the pull string to peel away the jacket, as is shown in the below.

Then, cut off the wires just below where you used your strippers.

The result will be low volt wires with no damaged insulation, and no hard to find low voltage short circuits.

— Erich


If you have two extension cords. One nice thick #10 50′ cord with good ends and another crappy #14 25′ cord. Unfortunately you need to connect them both to get to your drill 75′ away.

Which do you connect to the plug and which to the drill.

Come up with what you think…. we will wait…

If you said connect the nice one first (to the plug) you would agree with 95% of people.

The answer is. It MAKES NO DIFFERENCE.

An extension cord creates a full circuit.

From hot 120v down both cords to the load (the drill) and back through both neutrals to the neutral plug terminal.

The resistance (opposition to current) and ampacity (safe current carrying capacity) of the circuit is for the entire circuit, period.

We can often fall into the trap of thinking of electricity in terms of points in the circuit. There are good reasons for that in diagnosis, but the end result is the entire circuit between two points of differing electrical charges (potential difference) and the amps, amapacity, voltage drop, watts and resistance of the entire circuit are really what matter.

An electrical circuit is only as good as its weakest link. Unlike sausage…. because all sausage links are delicious.

— Bryan



Relays can be used for many different control applications including controlling fans, blowers, other relays or contactors, valves, dampers, pumps and much more. A 90-340 is a very common, versatile relay that many techs have on their truck so we will use it as the example.


A relay is just a remotely controlled switch that opens and closes using an electromagnet. The electromagnetic portion that provides the opening and/or closing force of the switch is called the coil. Relay coils can come in many different voltages depending on the application, but in residential and light commercial HVAC 24-volt coils are the most common.

The portion of the relay that opens and closes can be called the switch, contacts or points. These contacts can either be closed meaning there is an electrical path or open meaning there is no electrical path. Often this open or closed circuit will be described as “making” a circuit, meaning the switch is closed or “breaking” a circuit meaning the switch is open.


It is important when connecting a relay to distinguish which two relay points connect the coil. In the case of the 90-340, it is the bottom two terminals of the relay. Even though the coil is unmarked on most 90-340 relays, you can find it easily by locating the terminals with the small strands of wire connected. These two points connect together through the electromagnetic coil. When 24 volts of potential is applied across the coil the switch portion of the relay will switch from open to closed and closed to open depending on the terminal. Keep in mind that in a normal 24v circuit one side of the coil is connected to a 24v switch leg such as the thermostat “G” circuit for blower control, and the OTHER side of the coil is connected back to common.

The other six terminals are switch/contact terminals and the relay has a diagram embossed right on the top for easy reference. The way the circuit is drawn shows the de-energized state of the relay, meaning the state of the switches when no power is applied to the coil. When power is applied to the coil the points that were previously open (broken) now become closed (made) and the ones that were closed become open. When two points are closed when no power is applied to a relay coil we call them “normally closed” when they are open when no power is applied they are called “normally open”.


So based on this embossed diagram on the relay 1 to 3 and 4 to 6 are open (normally open) with no power to the coil and closed when power is applied. 1 to 2 and 4 to 5 are closed (normally closed) with no power and they open when the coil is energized. There is never a path between 2 & 3 or 5 & 6 because between them, at least one of them is always open. There is also no path or circuit between the top three terminals and the bottom three terminals or between the switch and coil portions of the 90-340 relay.

The data tag on a 90-340 shows both the coil voltage as well as the LRA (locked rotor amps) and FLA (full load amps) that the contacts can handle at various voltages for inductive (magnetic) loads like motors. It also lists the amp rating if the relay is controlling a RES (resistive) load like a heater or an incandescent light.


This relay can control a 39.6 LRA and 6.9 FLA Motor or a 15 amp heater at 240 volts based on the data tag.

— Bryan

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