Background Information

How Induction Machines Work

Cole Ferguson · Oct 13, 2016 · Leave a Comment

There are two main types of induction machines used to transmit electrical energy and perform work: transformers, and AC induction motors. In order to understand how they work, first you need to understand the principle of induction.

Induction

Induction is the process by which a magnetic field can induce current in a wire. The easiest way to test this is to slide a bar magnet through a coil of copper wire and measure the current through the wire. As the magnet passes through the wire, an electric current is generated.

Induction Machines: Transformers

voltage transformers in a landfill of electrical equipment — Old voltage transformers

Transformers use the property of induction to change voltage and current values in an electric circuit without altering the source of the electricity. At the lowest level, a transformer is made of a metal core (usually iron) with two copper windings (wire tightly wrapped around the transformer core). These windings are referred to as primary windings (where the current enters the transformer from the source) and the secondary windings (where the current leaves the transformer to go farther down the power line). As current travels through the wires of the primary winding, it generates a magnetic field that moves through the transformer core. The core is shaped like a hoop: when the magnetic field reaches the part of the core that the secondary winding is wrapped around, the magnetic field induces an electric current in the secondary winding. The magnitude of this current is proportional to the number of turns of wiring in the winding.

Induction Machines: Motors

The most common type of induction machine is a three-phase, squirrel cage induction motor. An induction motor consists of two parts: a stator and a rotor.

Stator

The stator of an induction motor is the non-moving part of the motor. It consists of three pairs of “poles” for the three phases of AC current. The poles are arranged so that when the phases of the oscillating current flow through them, they generate a rotating magnetic field.

Rotor

The rotor of the induction motor is shaped like a squirrel cage, with a number of metal bars joined electrically at either end by two metal rings (which short circuits the rotor).

How It Works

The rotating magnetic field generated by the stator is moving relative to the rotor. The rotor wants to have the same rotating magnetic field as the stator, so a current is induced in the rotor and it is forced to move to try and line up the magnetic field of the rotor with the magnetic field of the stator. If the rotor’s field lined up it would reach “synchronous speed” and stop moving (because the two fields would be in equilibrium). For this reason, the rotor will always move slower than the synchronous speed. The difference between the actual speed of a motor and the synchronous speed of the AC current is called slip.

Torque and Speed

In order for an induction motor to be useful, it has to drive a load at a certain speed. In order to move the load, the motor must generate a certain amount of torque. The torque generated by the motor is proportional to the amount of current that the motor is drawing from its source: more current means more torque. However, the speed of the motor is proportional to the voltage, and voltages at the source are normally fixed. What this means is that if your motor load requires high torque, the high current draw will force the voltage down and this will result in a slow motor.
This is how motor starting works, and most motors are designed to draw a lot of current when they start, in order to generate the torque necessary to move what is initially a static load. As the load starts moving, the torque required to move it is reduced and the motor can speed up. It continues on like this until the motor reaches its operational speed. In the case where for some reason voltage drops, the strain on the motor (from slower speed, higher torque, and higher current) can cause the motor to burn out. You can find out more about how voltage drop affects induction machines here.

When you break them down, induction machines aren’t nearly as complicated as they seem! As always, thanks for reading!
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How Voltage Drop Affects the Operation of Induction Machines

Cole Ferguson · Sep 29, 2016 · Leave a Comment

Voltage drop can negatively affect the performance of your induction machines

Induction machines do not like voltage drop. When talking about induction machines, what most people think about is the 3 phase AC induction motor. For the most part, the power output of an induction machine is directly proportional to voltage and current. If the voltage of a motor drops, the motor must draw more current to compensate. As long as the current being drawn by the motor is within the nameplate limits, this isn’t necessarily something to worry about for short periods of time. However, over longer periods of time or with a significant voltage drop, there will be a negative impact on the performance of your induction machine.

Induction Machines Overheating

If a motor is drawing more current than it is rated for on the nameplate, the wires that make up the windings will start to overheat. Overheating for any extended period of time will lead to a loss of lifespan of the motor. If the amount of current it is drawing is too far outside the nameplate rating, the motor will burn out all together.

Battery Life Reduction caused by Induction Machines

If your motor is supplied power by a battery or battery bank, the drop in voltage and increase in current will drain the batteries faster. This is a critical factor when looking at motors supplied by predominantly solar or wind power, which store energy in batteries when there is a surplus of sun or wind and have loads that draw energy from batteries when there is insufficient sun or wind.

Loss of Speed and Torque in Induction Machines

This train is never going to move without the appropriate voltage

Another thing that happens as voltage drops and current increases is a loss of speed and torque at the motor. This loss of speed and torque can negatively impact whatever process the motor is supplying.
If the speed and torque of the motor decreases enough, the motor will go back to its “starting” state. When starting, a motor requires large amounts of torque in order to start moving whatever load is attached to the motor. In order to get this starting torque, you require a large amount of current. So the current value spikes, which causes the voltage to drop even more! All of this results in a motor that doesn’t move and could quickly burn out.

Be Sure to Have Proper Protection!

In order to protect your motors from burning out due to voltage drop, it is important that you size your motor protection relays appropriately so that they will trip when voltage drops and current spikes.
As always, thanks for reading!
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Industrial Power System Configuration – Main Tie Main

Cole Ferguson · Aug 1, 2016 · Leave a Comment

Industrial power systems can become very complex, and the related processes need a reliable source of electricity to keep the process running, both for economic and safety reasons. There are many different kinds of distribution systems. This article will only focus on one of them: the Main-Tie-Main system configuration.

Industrial Power System Main-Tie-Main Configuration

Main-Tie-Main, also formally referred to as a “secondary selective system” consists of two independent circuits connected together at the load buses by a tie breaker. See the figure below for details.

A simple main-tie-main system configuration

Reliability

The biggest advantage that a for an industrial power system that a main-tie-main configuration has over other system configurations is reliability. Usually, the tie breaker is normally open and the system acts as two independent circuits supplied by two independent sources. For example, we will assume that there is a fault on Source 2. This fault trips CB2, cutting off all power to Load 2. Immediately after power is removed from Load 2, the Tie Breaker closes. Source 1 is now providing power to both Load 1 and Load 2, and the system is able to perform its normal functions until the fault at Source 2 is repaired. When normal power is restored and CB2 is closed, the Tie breaker opens and the system resumes normal operation. Operation is only interrupted for a very brief moment, if it is interrupted at all!
The Main-Tie-Main configuration is also good for maintenance for this very same reason: you can open CB2 and perform repairs upstream from Load 2 de-energized, while still supplying Load 2 with power.

Transformer Sizing

One characteristic of the industrial power system main-tie-main configuration is that both transformers must be sized to appropriately handle the load of both buses. In our example, we will assume that both Load 1 and Load 2 are the same size. Transformer 1 and Transformer 2 must each be sized so that in normal operation they are only loaded to 50%. This way when the tie breaker closes, the transformer that is now supplying both loads doesn’t become overloaded and blow up in your face. The downside to main-tie-main is that the added reliability inherently costs more due to the system requiring larger transformers than a system that would not tie both circuits together.

power transformer — Unfortunately, reliability costs money, and you’ll need to oversize your transformers to add reliability to your system in a main-tie-main configuration.

You can also reduce strain on each individual transformer when it is supplying both loads by adding external cooling to the transformers (like a fan cooling system). You can also simply accept that the transformers will have a reduced life in the event of a fault on a transformer or source.
You will not have just two loads in every case. In systems with multiple loads supplied by a single transformer, transformer size (and therefore cost) can be reduced by designing the system so that only essential operational loads (such as emergency lighting) are supplied power when the tie breaker closes.

Summary

For industrial power systems, a main-tie-main configuration is an extremely reliable power system distribution model, able to maintain power during a fault with little to no interruption. Unfortunately, this added reliability has a cost, whether in the form of larger transformers, extra cooling systems, or shorter transformer lifespans. These factors should all be considered when designing an industrial power system with main-tie-main in mind.
As always, thanks for reading! If you like this post but want some actionable advice, tips and information, check out our newsletter. You can sign up here or below. By signing up you get a free report on what an Electrical Safety Program is, and how to go about building one at your facility.

Three Different Types of Grounding

Cole Ferguson · Jul 6, 2016 · Leave a Comment

Today I’m going to give you a brief overview of three different types of grounding systems that are important.

A basic representation of a grounding system

These three systems are:

Ungrounded Systems
Resistance Grounded Systems
Solidly Grounded Systems

I’ve already talked a little bit about what grounding is, including giving a brief overview of why we do it and what it’s used for. If you haven’t read that article yet, go give it a read before continuing.
Finished reading about what grounding is? Alright then, let’s get to the meat and potatoes of today’s topic.

Ungrounded Systems

“Whoa, hold on”, you might be thinking, “We just finished reading about how important grounding is for safety! Why would we have ungrounded systems?” The answer is that we shouldn’t really have ungrounded systems, but they do exist and they do have their purposes.
You see, an ungrounded system isn’t really ungrounded. Electrically, your system is connected to ground through the capacitance between the lines and the earth, so you can say that it’s a capacitance grounded system. We just call it ungrounded because of convention, and because there isn’t a direct physical connection between any of your power lines and the ground.

Advantages

There are a few advantages to having an ungrounded system. The first is that since your system is never physically connected to the ground, you have negligible ground fault current. For example, in a 3-phase system, because all of the ground fault current is capacitive, when you have a single line-to-ground fault in an ungrounded system, the current and voltage you would lose is negligible, and is instead carried by the other two lines. This allows you to continue operation unimpeded during a single line-to-ground fault.
The other big advantage is that because of the negligible ground fault current, special ungrounded systems can be used to minimize shock risk to people. An excellent example would be medical equipment in a hospital: a patient is directly connected to the machine, and if a fault were to occur, electricity might be able to flow through the patient and into the ground. Because the ground fault current is negligible in an ungrounded system, no power current will pass from the machine through the patient into the ground.

Disadvantages

Of course, the disadvantages of an ungrounded system are obvious. If there is a fault, you are now using two wires to carry an amount of current that was allotted for three wires: the increase in current and voltage will jack up the heat, and the extra heat will wear out your insulation much faster. Worn out insulation could lead to unnecessary damage to your electrical system, particularly at motors.
The other big disadvantage of an ungrounded system is that it is incredibly difficult and time consuming to locate any faults. Each line must be tested individually, which is a very slow process that completely interrupts service. The opportunity cost of a fault in an ungrounded system is very high.
Ungrounded systems were the norm back in the 40’s and 50’s, but because their disadvantages outweigh their advantages in most scenarios, you won’t see too many new ungrounded systems today.

Resistance Grounding

Resistance grounding is when you have a connection between your neutral line and the ground through a resistor. This resistor is used to limit the fault current through your neutral line: if your voltage doesn’t change, then your current is dependent on the size of the resistor according to Ohm’s law (V=IR).

Advantages Over Ungrounded Systems

Because the current in the neutral is controlled instead of negligible, system overvoltages are also controlled. This reduced current and reduced overvoltage means reduced heat, which keeps the wear and tear of your electrical system to a minimum. This is especially important for keeping your motors safe, since the reduced current will not damage the magnetic iron of the motor (which is costly to repair). The reduced currents also reduce the risk of shock and arc flash/blast hazards.
There are two types of resistance grounding: high resistance grounding and low resistance grounding.

High Resistance Grounding

High resistance grounding is typically used to limit ground fault current to < 10 amps. The low ground-fault current also means that, just like an ungrounded system, you can continue to operate the system on a single line-to-ground fault. The low current will typically not trip your protective devices during a single line-to-ground fault.
Overall, you want to use high resistance grounding when you need low fault current and still want to operate with a single fault. High resistance grounding is typically seen in retrofits of previously ungrounded systems in addition to new systems.

Low Resistance Grounding

Low resistance grounding typically limits ground fault current to between 100 and 1000 amps. This offers a similar advantage to high resistance grounding in that you can control the ground fault current, which means you can design your system to withstand the currents without damage.
Low resistance grounding systems have the benefit of tripping your protective devices when there is a fault. Their purpose is to immediately cut the power to the circuit, and so unlike the high resistance grounding systems, a low resistance grounding system will not maintain operation during a single line-to-ground fault.
Low resistance grounding also reduces overvoltage, and is used in medium voltage systems of 15kV or less, typically where big generators/motors are used.

Solid Grounding

Solid grounding is what you get when you connect your system directly to the ground, without any sort of resistance in the way. The ground is typically connected to the system at a neutral point, like the neutral terminal of a generator or transformer.

Pros and Cons

Solid grounding, like resistance grounding, can greatly reduce overvoltages in your electrical system. However, solidly grounded systems have the potential to have huge amounts of ground-fault current. As a result, solidly grounded systems cannot operate with a ground fault (since all of the current in the system is going from fault to ground).
Solid grounding has two main uses:

In systems with voltages of 600V or less, solid grounding can be used if it is not necessary to maintain operation of a faulted circuit.
In systems with voltages of 15kV or greater, solid grounding can be used if high ground fault currents are desirable of any reason, such as quick ground fault detection (since the high current will most definitely trip protective devices).

Recap

You can use ungrounded systems when you want to have negligible ground-fault current.
Resistance grounding offers the advantages of ungrounded systems without the risk of large overvoltages.
Solid grounding reduces overvoltages but has high ground-fault currents.

At the end of the day, the type of grounding you use for your system will depend on which type of grounding best suits your needs and budget.
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What is Grounding?

Cole Ferguson · Jul 4, 2016 · Leave a Comment

What is grounding in an electrical sense? Anyone with a little bit of a background in electronics will tell you that grounding is something you need to keep your circuit safe and it keeps the electrical parts of your circuit from blowing up. But what is it, exactly?

Grounding is a connection between an electrical circuit and an arbitrary reference point.
Grounding is a method of protecting your electrical system and the people using it from harm by setting a known “reference” point for voltage. When you hear someone mention ground, you should ask them to clarify: is it the reference voltage, or is it earth itself? Or is that both? In order to explain grounding, we need to know a little bit about voltage and current. Before you continue, read our article “What is electricity?” It has a bunch of the background knowledge that I’m going to assume you know when I talk about grounding.

Grounding: A History

The earliest instance of grounding that I can think of is when Benjamin Franklin invented the lightning rod.

A lightning rod is a metal rod attached to the top of the house, connected by a wire to another rod stuck in the ground. When lightning would strike, it would seek out the rod (since the rod has a lower resistance than the rest of the house) and electricity would flow into the ground. This prevented houses from burning down when struck by lightning, which can generally be considered a good thing.
If a lightning rod can be seen as the first real instance of grounding, then we can look at grounding as one part of the overall electrical protection system for your circuit. So in order to keep things safe, you need to figure out a way to create a pathway for the electricity to travel that has less resistance than your body, fragile circuit components, and building materials.
In the event that something goes wrong in your circuit for whatever reason, you need to direct the electric current along a very low resistance pathway. We’ve already talked about how to figure out what resistance is, but what kind of wire would have a low enough resistance to draw all the current?

Why not use the ground itself?

Think about it: use the planet earth as a conductor. Not only does the electricity not have very far to go to get in to the ground, but it also has a HUUUUUUUUUGE cross sectional area, meaning it has very, very little resistance. Electrons always want to go to the place of least resistance, which makes going into the ground their favourite place.
The lightning rod used this concept to great effect, and modern day power companies do the exact same thing. This also doubles as a useful way to complete your circuit: by using the earth itself as your ground wire, you can be sure that every point in your circuit (which in this case is the entire power grid) is connected to the same reference point. This means that when we now talk about voltage (for example, the 120V wires in your house) we know that we’re talking about it with reference to the voltage of the ground itself. There are 120V between the wires in your house and the ground you’re standing on, there are 1000kV between the overhead power lines and the ground, etc. What this also means is that the current in your circuit now has a clear and easy return path to its source: the current can just flow into the ground near, for example, your house, and travel back through the ground to the generators in the power plant.

But isn’t the system still unsafe?

You might see the new problem with that: if electrons want to go in to the ground, how do we get them there safely? If you are standing on the ground and you touch a wire with electric current moving through it, won’t the current just go through you and into the ground, instead of going through the wire?
Current flows through all available pathways at once. This means that even if the ground wire path is the least resistive path for current to flow through, you could still harm yourself by adding yourself to the circuit when you touch it (making a path from the wires through you to the earth). Initially, I said that grounding was to keep your circuit safe, and I wasn’t lying to you. The reason grounding keeps the circuit safe is that grounding makes up a part of your circuit’s protection system.
Most circuits are designed so that when there is a fault to ground in the circuit, the majority of the current will flow through this fault, into the ground wire, and into the ground. Because we know where the current will flow, we can put protective devices along the ground path in such a way that as soon as the fault occurs, the protective devices (such as a fuse or a breaker) are tripped very abruptly. When this happens, the complete pathway for the circuit is broken, and current cannot flow. If current can’t flow, we have no voltage difference, and if we have no voltage difference, we have no real danger. This is the most common way that grounding protects circuits: it forces protective devices to turn the circuit off ASAP.

A fuse is a protective device that gets destroyed when too much current goes through it, cutting off power by breaking the circuit

In order for grounding to work in tandem with your protection system, you need to know how much current can be expected to flow through your ground wires in the event of a fault. You can figure out what amount of current to expect by performing a protection and coordination study. This will let you pick the appropriately sized fuses or breakers.

Wrap up

In summary, we can think of grounding as a circuit protection technique that doubles as an easy way to measure voltages. Electrical grounding can be:

A common point of reference in your circuit that is used to measure all voltages
The earth itself, when the common point of reference is stuck into the ground that you walk on
A technique used to direct fault current along a low resistance path to protective devices that will immediately turn off your circuit
All of the above

Hopefully you’ve enjoyed reading this article, and now know a little bit more about grounding.
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