FSP0006 – Basic Electricity and Power – Facility Science Podcast #6

By | June 4, 2019

Notes for FSP0006 – Basic Electricity and Power

  • There are quite a few electricity and power related topics I think are relevant to cover on this podcast, might make sense to first cover some of the basic concepts of electricity and power as they are used in the context of facilities management and engineering.
  • Hope to leave you with a good understanding of the concepts of voltage, current, and power
  • Electricity is a form of energy that involves the movement and interaction of charged particles. Charged particles usually means electrons (but not really that important here).

Voltage

  • can be thought of as representing the amount of energy each charged particle has relative to some other place (hopefully I’ll be able to make that clearer). In our context this effectively means how much energy each charged particle will deliver to the elements in a circuit as it travels through. (I’ll define a circuit in a bit)
  • Another name for voltage is electric potential
  • Good analogy is gravitational potential energy
    • If you pick up a rock off the ground, you do work on the rock to move it against gravity (you’re moving it through the gravitational field, almost like pulling on a spring). The rock now has what is called gravitational potential energy proportional to the height you are holding it.
    • If you drop the rock, the rock will fall back to the ground delivering that energy to whatever it hits (so the same amount of energy you spent lifting it up)(kind of not exactly but good enough for this analogy)
    • Voltage is similar, except instead of rocks moving through a gravitational field we have charged particles (in this case, electrons) moving through an electric field, and instead of a (effectively) stationary field like gravity, we create the electric field where we want it and at the strength suitable to our application (with a generator or battery or solar panel or whatever). (Of course there are naturally occurring electric fields, but I’ll call that outside the scope of this podcast).
    • Potential energy (whether that be gravitational (lifting the rock against gravity) or electrical (so voltage)) has to be measured relative to something. An object will move from an area of higher potential to an area of lower potential (the rock always falls down toward the earth). You can’t tell how much energy the rock is going to give to whatever you drop it on without knowing the distance you’re dropping it from. The higher you bring the rock before dropping it, the more energy it will have when it hits the ground. The same can be said for charged particles in an electric field. They are going to move from higher potential (voltage) to lower potential and the amount of energy they deliver will be related to the difference in voltage between those 2 points.
    • Measured in volts, some common values inside building are 120/208/277/480/240/220/600, and also we commonly use 12 and 24 for control systems

Current

  • measured in Amperes, usually shortened to Amps
  • if voltage can be understood to represent the amount of energy each charged particle can expend in the circuit, current represents the number of charged particles flowing through the circuit at any given time (so this is a measurement of the flow of charged particles)
  • The path followed by the current is made up of the wires and devices in the circuit. The amount of current that will flow is determine by the materials of those circuit elements.
    • In the simplest circuits, we can calculate the current that will flow if we know the voltage of the current source and the resistance of the elements in the circuit. Resistance is a property of materials.
    • A material with a low resistance allows a lot of current to flow while a material with a high resistance allows less current to flow.
    • A material that lets current flow easily (like copper wire) is called a conductor
    • A material that doesn’t let current flow very easily (like many types of plastic) is called an insulator.
    • Voltage (V), Resistance (R) and Current (I) are related by a formula called Ohm’s law. V=IR, so given a constant voltage, a circuit with a higher resistance will let less current flow while a circuit with a lower resistance will let more current flow.
    • It actually gets much more complicated than this, but the main point is that the amount of current that flows in a circuit is determined by the devices and materials in the circuit.
  • Direct vs Alternating Current
    • Direct Current (DC) is current as you might think of it. It’s easiest to think of a battery. The battery has a positive and negative terminal. The positive terminal is at a higher voltage than the negative terminal. The charges travel out of the battery from the positive terminal, through the elements of the circuit and back into the battery through the negative terminal. (This isn’t actually true. The charges that actually flow are electrons (negative charges) and they technically from the negative terminal to the positive terminal, but by convention we talk about voltage and current as if there are positive charges flowing from the positive terminal to the negative terminal. It does’t really matter as long as we are consistent).
    • The alternative to direct current is alternating current (AC). In this case the current periodically changes direction because the voltage source periodically changes polarity, as in which side has the higher voltage (it cold also be called alternating voltage). Batteries produce direct current while common electric generators produce alternating current because they create electricity by rotating a a magnet in a coil of wire. The movement of the north ans south poles of the magnet relative to each other causes the current to flow first one way, then the other.
    • DC and AC each have their advantages and disadvantages. We generally choose whichever is more suitable for the application. Generally we get AC from the electric utility and distribute it throughout our building. Various devices convert from AC to DC when DC is more suitable.

Circuit

  • Now that we have voltage and current, i’ll try to define a circuit.
  • In simple terms, a circuit is the path that the charged particles take from the higher voltage to the lower voltage.
  • A circuit is generally a closed loop for 2 reasons
    • first you have actual electrons flowing from somewhere to somewhere else the first place doesn’t have an unlimited number of electrons, so it can’t supply much energy if all of its electrons leave and none ever come back.
    • The second reason is that in order for the charged particles to move, there has to be a difference in potential. The device that is generating the electricity creates a difference in potential inside itself so we can be sure that the two points on the generating device definitely have the desired potential difference (voltage). We can’t be sure that any other point anywhere will have any particular difference in potential compared to our electricity generating device.
  • Open circuit – a condition where there is no path for charge to flow from an area of higher voltage to an area of lower voltage. In practical terms this means that some device or wire has been disconnected or broken or a switch had been opened. No current will flow in a open circuit.
  • Short circuit – a condition where two points in a circuit that shouldn’t be connect are connected. Basically you have an unintended path for current to travel through the circuit. In practical terms this usually results in too much current going through some part of the circuit which will break something or start a fire or open a fuse or circuit breaker.
  • Parallel vs Series circuits –
    • In a series circuit, the devices (well say 3 devices) are connected to each other so that the current travels from the high voltage side of the voltage source through the first device, then through the second device then through the 3rd devices then to the low voltage side of the source. This means that each device will consume some fraction of the total voltage from the source. So if we have a 24 V source and 3 devices that each have the same “resistance” then there will be a drop of 8 V across each device.
    • When we are talking about power distribution in buildings, we might want to connect multiple devices to the same circuit, but real devices are designed to all use the same voltage which is the voltage at the wall plug. We can’t connect them in series. Instead we connect them in parallel, which means we connect one end of each device to the high voltage side of the source and the other end of each device to the lower voltage side of the source. It might help to picture a ladder where the sides of the voltage source are the vertical sides of the ladder and the devices we want to power are the rungs of the ladder that each cross from one side to the other. Each device gets the same voltage and you can remove any device without affecting any other device.

Power

  • Now that we have current and voltage (volts and amps) we can combine them together to get power. We said voltage is energy per charged particle, current is charged particles per time, now power is energy per time. So that means that power = voltage multiplied times current.
  • Power is measured in Watts.
  • Horsepower – another unit of measurement for power is horsepower. This is often given for engines and motors. 1 horsepower is somewhere around 740 Watts depending on which definition of horsepower you use (there are a few). For something like an electric motor, you can use the rated horsepower to approximate the current required by the motor at a given voltage.
  • Device ratings – when choosing and installing devices in our buildings, it is usually important to know the voltage, current, and power requirements. Most devices will have a sticker and a plate on the that tells you which voltage the device is designed for and also the current and or power requirements of the device (sometimes both and sometimes only one or the other).
  • Volt-Amps – Some devices will give their power consumption in Volt-Amps, instead of Watts.
    • I said Watts is volts times amps, and volt-amps is also volts times amps, so why do we have both?
    • A Watt is a measure of what is called “real power.” Multiplying voltage times current only gives you watts for certain types of electrical loads.
    • For other types of loads you don’t get quite watts, so we call that volt-amps instead of watts.
    • Volt amps is similar to watts except that it measures something called apparent power (or sometimes reactive power if we’re talking about volt-amp-reactive or VAR.
    • This is complicated so I don’t want to get into it here. I might cover this if I do a podcast about power factor correction later on.
    • It’s generally safe to treat watts and VA the same for the purposes of loose estimates and back-of-the-envelope calculations and for now we can leave the difference to the engineers.

What do you buy from the electric utility?

  • The electric utility sells you kilowatt-hours.
  • The kilo- part means x1000 which is just a convenient multiplier for billing purposes, so a a kilowatt-hour is 1000 watt-hours.
  • A watt-hour is watts times hours, remember from before that watts is energy per time or energy divided by time, so watt-hours is energy divided by time multiplied by time which is just energy.
  • The electric utility is selling you units of energy. So they basically count the number of electrons that go through your building and multiply by the supply voltage.
  • You can approximate how much the electricity to run something will cost if you know it’s power consumption (which is usually on the sticker…they either give you W or VA or V and A).
    • Say you have a 100 Watt light bulb that you run for 10 hours a day. 100×10 gives you 1000 watt-hours per day.
    • So that 100 watt light bulb consumes 1 kilowatt-hour per day.
    • If you’re paying $0.10 per kilowatt-hour to the utility, you will be paying 10 cents per day to run that light bulb.
    • Annually, assuming you run the light bulb every day, you will spend $36.50 to run that light bulb.

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