In this article I will be walking through some of the elements of power design and management in a context relevant to IT engineers. Although we, as IT-centric engineers, may only deal with electrical power systems once in a while, it can be very useful to understand the principles at work and come in handy to speak the same language when communicating with electricians.
For supplemental references, check out my Power and Cooling Cheat Sheet. Linked on the right.
Approx Reading Time: 12-15 Minutes
Generation and Types of Current
Electrical energy is [almost always] created by turning a generator. The source of this mechanical energy can be many things (wind power, steam turbine from a nuclear power plant, internal combustion engine, etc) but in the end, it always turns a generator. A generator, in principle, is just an electric motor running in reverse (as in, an electric motor which is being turned by an external device).
NOTE: The only exception to this rule that comes to mind is solar power, which is solid state and does not have a mechanical energy component.
There are two types of electricity which can be used in a circuit: Direct Current (DC) and Alternating Current (AC). Direct current is an electrical flow where electrons consistently flow from one side of the power source to another, never changing direction. Alternating current is, as the name implies, where the direction of electron flow continuously changes. In most countries throughout the world: the frequency of the current reversal is between 50 and 60 times per second. Or 50-60 Hertz (Hz)
You may have heard about AC vs DC electricity and the war between Tesla and Edison. You may also have been told that AC (and Tesla) ended up winning the war because AC power carries better over transmissions lines. This is not exactly true, or perhaps is just a neutered version of the truth. In fact, it turns out that AC current suffers more loss than DC when traversing large distances. The more complete explanation of why AC won over DC is because AC can be easily transformed into higher voltages, and it’s higher voltages that carry more efficiently and suffer less voltage drop over transmission lines; these higher voltages also require smaller wire diameters to provide the same amount of power.
Voltage, Amperage, and Resistance
There are three main measurements of an electrical circuit: Voltage, Amperage, and Resistance. To explain these different components, I’ll use the typical “water hose” comparison.
Think of a wire in an electrical circuit as a water hose and the electricity as the water.
-Voltage (Potential) = Pressure of the water in the hose
-Amperage (Current) = Amount of water flowing through the hose
-Resistance (Impedance measured as Ohms) = Restriction of the free flow of water through the hose
The Voltage of an electrical circuit is similar to the pressure of the water inside a water hose. When pressurized, the water wants to flow from the high pressure source (water pump) to a lower pressure area. This can be equated to a concentration of electrons wanting to flow from the high concentration area (negative pole) to a low concentration area (positive pole). The usable voltage is really just the difference between the voltages of the two poles.
The Amperage, or Current, of an electrical circuit can be thought of as the bulk amount of water flowing through the hose. The more current that can flow through the hose, the more power that can be supplied to the load. Also, the more current required, the bigger the hose needs to be; this is why circuits carrying more amperage require larger gauged wires.
The Resistance, or Impedance of an electrical circuit is measured in Ohms and is the restriction of the flow of power through a circuit. To put it in terms of a water hose: assume you were using the water in the hose to power a small water-powered turbine. The turbine being in the way of the flow of water would cause a restriction on the free flow of that water. This is similar to how it works in an electrical circuit.
Measurement and Calculation
Measurement of electrical properties (Volts, Amps, Ohms) can be performed with different pieces of equipment and the resulting values can be manipulated to derive other, not directly measured, values.
Voltage of a power source can be measured using a voltmeter. A voltmeter measures the electrical potential (how strongly the electrons want to flow) between any two points.
Current flowing through a circuit can be measured using a clamp meter. When electrons flow through a wire, they produce a circular magnetic field around the wire which can be measured by the meter. The larger the current through the wire, the larger the magnetic field.
Resistance (or impedance) on a circuit can be measured in terms of Ohms using an Ohmmeter. An Ohmmeter measures the restriction of free flowing electrons through a conductor. 0 Ohms is a measurement of no resistance in a circuit and this measurement only happens in very special circumstances: on superconductors. Under normal circumstances, at least a slight amount of resistance is always present.
NOTE: Most modern multimeters will measure Voltage (AC & DC), Amperage (usually just AC), and Resistance. I recommend keeping one handy.
Power in a circuit is measured in Watts. Watts are not directly measurable in a circuit but instead are calculated by multiplying Volts with Amps.
NOTE: Watts is the SI unit for [real] electrical power and can be converted to several other units including: Joules, Ergs, Horsepower, dBm, Calories, BTU/hr, and Tons of refrigeration capacity.
DIVE DEEPER: Technically speaking, Watts are actually calculated by multiplying Volts, Amps, and something called the “power factor” or PF. The PF is a dimensionless unit which has values between -1 and 1. It’s value depends on the particular load on the circuit and whether it is a resistive (like a computer power supply), inductive (like a motor), or capacitive (like a flash in a camera) load. These different types of loads induce different profiles in an electrical circuit (specifically, the relative time where voltage and current peak during an AC cycle). When the load in a circuit is resistive (as IT equipment usually is), the PF is very close to 1 and for the most part, can be ignored. Watts are a measurement of real power which is Volts x Amps x PF. The measurement for apparent power (Volts x Amps) is the VA, or Volt-Amp. Many UPS’s are rated by their peak VA output. If you are running a purely resistive load on that UPS, you can usually equate VA’s with Watts.
Ohm’s law provides formulas which can be used to calculate electrical measurements when two others are known. Remember that the (P = E x I) formula in the below chart specifies how to calculate single-phase Wattage. To calculate Wattage on a three-phase circuit use the formula P=(E × I)(√3).
As seen above: alternating current is an electrical current which changes its direction of flow many times per second. This change in voltage and direction can be graphed into a sine wave which depicts the voltage of the power source at any instantaneous point in time.
Below are a few examples of [North America based] AC voltages and their graphed sine waves.
This is a single-phase, 120v, 3-wire system; the common household electrical source in North America. Notice that the voltage here is the difference between the neutral power lead (white) and the “hot” power lead (black). The actual voltage of the circuit only reaches 120v for a small amount of time; it reaches +120v once per cycle, -120v once per cycle, and 0v twice per cycle. The neutral lead always stays at 0v.
NOTE: The actual peak voltage of a 120v power source will be ~170v. The 120v rating is the RMS measurement of the voltage. I have labeled the graph as 120v for simplicity’s sake.
This is a split-phase, 120v/240v, 4-wire system; the common North American household panel supply. This system provides two voltage options: hot-to-hot will render 240v, hot-to-neutral will render 120v. The single-phase system described above is actually derived from this one using one hot lead and the neutral lead.
This is a three-phase, 208v, 4-wire system; it consists of three leads, each of which have alternating voltages at 120v but the three voltages are offset in their cycles by 120 degrees, or a third of a full cycle. The result of this system is that the voltage measured between any two leads will be alternating between 208v and 0v. The reason this system does not produce 240v is because no two leads reach +120v and -120v at the same time.
Three-Phase systems are most useful for powering motors, but can also be useful in power distribution for IT equipment. A three phase circuit carries about 70% more wattage than a single phase circuit of the equivalent voltage and amperage (due to the extra hot lead). The calculation for wattage of a single phase circuit is found as [Watts = Amps x Volts]. The calculation of wattage for a three phase circuit is found as [Watts = (Amps x Volts)(√3)].
Three Phase – Balancing Loads
When using a three phase power source, your Power Distribution Units (PDUs) will divide the output plugs into three banks: P1-P2, P1-P3, and P2-P3. These banks are sometimes identified using different colored sockets. The output plugs in each bank have connections to two of the three hot leads. It is important to balance the load of your equipment as evenly as possible across these three banks. An unbalanced three phase system will result in unmatched voltages across the three leads and can have adverse affects on the electrical company’s distribution system (it also tends to really upset electricians).
Power Supply Efficiency
Pretty much all electronic equipment relies on DC power for operation. Since the electricity distributed to most IT equipment comes in the form of AC, power supply units (PSUs) are required to convert AC power into DC power. A modern switching PSU will convert AC to DC using a rectifier, smooth out the voltage using capacitors, and regulate the output voltage using a switching regulator. The result is a consistent DC power source to be used for powering the IT equipment. During this power conversion, some of the input power is lost in the process as heat.
Another way to increase efficiency is to correctly size the PSU for the particular load. A PSU which is under-loaded has a much lower efficiency than one which is correctly loaded.
There are many different power connectors around the world; some are standard to a particular country or region, some are used internationally. For a reference of some of the most common power plugs, use the cheat sheet referenced at the top of this blog post.
Here are a few practical lessons learned during this article.
- Use the highest voltage available (and supported by your PSU) when powering IT equipment
- Size PDUs on your equipment appropriately. Underloaded PSUs are very inefficient
- Max Wattage on a single-phase circuit can be calculated as [P = E x I]. For three phase: [P = (E x I)(√3)
- Use three-phase power and the appropriate PDUs in high density environments. They allow more power to be distributed per unit
- When using three-phase systems, make sure to balance your loads (different pieces of equipment) across all three plug banks
Always consider your safety when working with electrified IT equipment and for the electrical work: always use a licensed electrician!