Electric Load and Voltage Drop ``` Name: Cyrus-Rhyz Status: student Grade: 9-12 Country: Papua New Guinea Date: Fall 2012 ``` Question: Is it true that if you increase household appliance, it results in voltage drop? For example, if I use a generator as a source of energy that has an output of 220 volts, and I add an additional appliance (t.v, refrigerator, lights and so on) the output voltage from my generator drops from 220 volts to less. Why? Replies: Hi Cyrus-Rhyz Ideally, there should not be a voltage drop when you add another appliance to your generator. But practically, there is a physical limit to the amount of current your generator can provide. There is just so much metal in the generator?s moving parts that can provide the required current. So the result is a voltage drop. The reason you do not see this voltage drop from your household electric service is because there are huge generators, driven by nuclear reactors and large coal, oil or gas fired steam plants. You have so much current available from the household electric service that circuit interrupters (circuit breakers) have to be put in the household wiring to keep from drawing so much current, you burn up the wires and burn down the house. The governing equations (which do not reflect the practical aspects of electric power) are: Voltage = Current x Resistance Power = Voltage x Current Sincere regards, Mike Stewart Hi Cyrus-Rhyz, Yes, it is true. The people who make generators try to make them put out the rated voltage for all loads, which means that they have to supply more current when the load increases. Somehow they have to know how much current to supply. From where is that information going to come? Let us assume resistive loads for simplicity. Adding another appliance reduces the total load resistance, because the loads are all in parallel. The voltage V of the generator is given by V=IR, the current I multiplied by the total resistance R. When the resistance decreases, the voltage decreases. The generator control notices this and increases the generator speed, which results in an increased current, which brings the voltage back up closer to the desired voltage. It is the difference between the desired voltage and the actual voltage that tells the generator how fast to run, so a slight drop in the output voltage is required. Tim Mooney Hi, Cyrus: That is an excellent question. The power source (the generator), the wires which connect your refrigerator to the power line to the power source, and the electrical plugs and sockets all have electrical resistance. Most wiring and electric machinery is made with copper so it has pretty low resistance. Of all metals, copper has the second lowest resistance (only silver is better). But this resistance is more than zero and it can be significant. Whenever an electrical current flows through a resistance, it causes a voltage drop according to V=IR where V is electrical pressure in Volts, I = electrical current flow in Amperes, and R = electrical resistance in Ohms. If your generator is connected to some very long wires but there is no current flow, then V=IR = 0 (even though the R may be relatively high). With no current flow there will be no voltage drop (lost). The voltage at the generator will be the same as that at the far end of the wires. But when you begin to draw electrical current, then current = I in Amperes will increase, and the voltage drop (that is voltage which is lost) in the generator, wiring, and connectors will increase proportionately. The voltage which you can measure at the far end of the wires will decrease accordingly. The voltage drop and current flow in wires will heat the wires. A slight amount of this warming is not a serious problem, but too much heating can cause fires so is very dangerous. For that reason, electricians know to use the correct size of wire for the amount of current which is required. The best approach is to not connect more loads (such as your refrigerator) than you need to. Appliances which are more efficient will draw less current so they should work better. Do not use connection wires (such as extension cords) which are longer than necessary, and use the heaviest wires you can use. In fact there are other reasons why the generator itself will have resistance which is exceeds that of its internal wires, but that will be difficult to explain. I hope this helps. Best wishes, Bob Zwicker This does not have a single answer. Some generator devices have voltage regulators that will keep the voltage constant, e.g. 220 volts. However, these devices have a limited range of operation – you cannot get something for nothing. The output of current, may fall to compensate for the added electrical load. Each appliance has a certain power demand (P = I x E). For a given power, P, the product of the current, I, and the voltage, E, will vary up or down so that the power P remains constant. This of course, assumes that there are no circuit breakers or fuses that will “break” the circuit. Vince Calder Cyrus-Rhyz, The voltage can drop, but by only a small amount. Just as is true with batteries, the internal circuits of a generator are not ideal. If the generator were perfect, the output would always be 220 volts. For every ampere of electric current flowing through the generator, a small amount of the voltage is devoted to waste (such as heat production). As more devices are added, the current through the generator increases. The generator provides more amperes. More volts are lost to waste. With a few devices, each one might receive 219 volts. With many devices, each might receive 217 volts. Amperage is much higher, and power output is much higher, but voltage is a little smaller. Dr. Ken Mellendorf Physics Instructor Illinois Central College Dear Cyrus-Rhyz, The first thing happening is basic Ohm’s law. Ohm’s law is V (Voltage) = I (Current) * R (Resistance). The wires going from the source of energy (the utility company or a generator if you are powering from that) have resistance. These wires, when they draw current, drop voltage according to Ohm’s law. The more you load the connection, the more current you draw, and the more voltage is dropped along the wires to your appliances. You can measure this drop at the appliance as you connect other loads. If you are measuring the voltage at the generator, and it drops when loaded, then you may have a couple of effects going on. The first is that the generator also has windings consisting of wires that have resistance. These wires are internal to the generator, and they can drop voltage when current is drawn through them. Another effect may be that drawing power from the generator loads it mechanically. A generator converts mechanical energy into electrical energy, and the greater the electrical load, the greater the mechanical load. It is much like your car in that when it has to do more work, say by going up a hill, you have to feed it more gas. In the generator case, if you were cranking its shaft by hand in order to create power, you would have to crank harder the greater the appliance load. You would feel this increase in load, and if you had too many appliances connected, you might be pushed to your limit and have to slow down. Similarly, a greater load on a generator can cause its shaft to slow. The output voltage drops in this case. In some generators, this output voltage is monitored, and the generator would compensate by increasing its shaft rotation rate to compensate. More gas (in a gas-driven generator) would be pumped to the generator, and it would increase its rotation rate just enough to bring up the voltage to the required value. Kyle Bunch, PhD, PE Hi Cyrus-Rhyz, Thanks for the question. As more appliances are connected to the generator, the load on the generator increases and more current flows from the generator. The equation P = I*V describes how the power (P) is related to the current (I) and the voltage (V) of a generator. The power output of a generator is constant. So, if the current (I) that the generator puts out increases, the voltage has to decrease. On a gasoline or diesel generator, you will notice that the engine power (and noise!) increases as the load on the generator is increased. This increase in engine speed occurs to compensate for the voltage drop discussed above. I hope this helps answer your question. Jeff Grell Click here to return to the Physics Archives

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