| When my daughter was seven she found out about Ohm’s Law the hard way, although she didn’t know it. She had accidentally bumped into her electric toy train, causing its metal wheels to derail and fall askew of the metal track. This created a short circuit, causing current to flow in an undesirable direction, that is, through the derailed wheels rather than along the track to the electric motor in the locomotive as it should.
What happened during the short circuit is that the bulk of the current began to follow through the path of least resistance, that of the derailed wheels, rather than the higher resistance of the electric motor. Electric current, always opportunistic, will flow along its easiest course, in this case the short, thick metal of the wheels, rather than work its way along the many feet of thin metal wire of the motor’s electromagnetic coils. With its wheels sparking at the site of derailment the train had become an electric toaster within seconds, and the carpet beneath the track began to burn. Needless to say, mom wasn’t very happy.
In this instance Ohm’s Law was at work, with a decidedly negative outcome. The Law’s basic formula concerning the toy train would be written as:
I = V ÷ R
where, I is the current flowing through the metal track, V is the track voltage, and R is the internal resistance of the metal track and locomotive motor, or in the case of a derailment, the metal track and the derailed wheel. So, according to the formula, for a given voltage V, when the R got really small due to the derailment, I got really big.
But enough about toy trains. Let’s see how Ohm’s Law applies to an unregulated power supply circuit. We’ll start with a schematic of the power supply in isolation.
The unregulated power supply shown in Figure 1 has two basic aspects to its operation, contained within a blue dashed line. The dashed line is for the sake of clarity when we connect the power supply up to an external circuit which powers peripheral devices later on. The first aspect of the power supply is a direct current (DC) voltage source, which we’ll call VDC. It’s represented by a series of parallel lines of alternating lengths, a common representation within electrical engineering.
And like all electrical components, the power supply has an internal resistance, such as discussed previously. This resistance, notated RInternal, is the second aspect of the power supply, represented by another standard symbol, that of a zigzag line.
Finally, the unregulated power supply has two output terminals. These will connect to an external supply circuit through which power will be provided to peripheral devices. Next time we’ll connect to this external circuit, which for our purposes will consist of an electric relay, buzzer, and light to see how it all works in accordance with Ohm’s Law.
Posts Tagged ‘electric motor’
| When something is said to be the “heart of the operation,” one usually imagines that it is integral to whatever is being discussed, and it is probably centrally located. The human heart fits this description well. This amazing organ, centrally located within your chest cavity, moves blood, nutrients, oxygen, and carbon dioxide through your body with amazing efficiency. During a twenty four hour period it can pump as much as 2,000 gallons of blood through 6,000 miles of arteries, veins, and capillaries.
At the heart of a local exhaust ventilation system is its fan. Like the human heart, it is a model of efficiency. It first creates a vacuum in the intake hood, which is strategically located at a pollution source, pulling in contaminated air and leading it through ductwork. Sometimes the fan leads the air to a filter or other air cleaning equipment, but eventually the dirty air is exhausted through a stack leading outdoors.
There are two main types of fan, axial and centrifugal. You’re probably most familiar with the axial type, because they’re the type commonly used in tabletop, box, and oscillating fans in your home. These have blades that look like a propeller on an airplane, and they work by drawing air straight through the fan. As helpful as they are within a personal setting, axial fans are not typically used in local exhaust ventilation systems because the electric motor that drives the blades is in the path of airflow. This setup can create a problem if the air flowing over the motor contains dust and flammable vapor. Dust can cause the motor to get dirty and overheat. Flammable vapor can ignite if the motor wiring fails and creates an electrical arc.
Because of the technical difficulties presented by an axial type fan, centrifugal fans are what are most often used in industrial settings. One such fan is shown in Figure 1.
Figure 1 – Centrifugal Fan
The blades of a centrifugal fan are fully enclosed in air tight housing. This housing keeps any dust or fumes from leaking out into the building. The electric motor that drives the fan can be safely located outside of this housing, where it is dust-free and there are no flammable vapors. If you look inside the housing you will see that the moving part, known as the impeller, resembles a squirrel cage. See Figure 2.
Figure 2 – Centrifugal Fan Impeller
This impeller is made up of many blades, set up within a wheel configuration. When an electric motor causes the wheel to rotate, air is made to move off the blades and out of the impeller due to centrifugal force. This air is sent crashing into the fan housing, shown in Figure 1, which is curved like a spiral to direct the air into an outlet duct which is connected to ductwork that leads to the exhaust stack. As air leaves the impeller, more air rushes into its center from the inlet duct to occupy the empty space that’s been created. Hence, as long as the motor keeps spinning the impeller, air will flow through the fan.
In order for all this to work effectively, the centrifugal fan must be the right size, one that is capable of providing enough suction to capture contaminated air at the hood source, then overcoming the resistance to air flow that is presented by ductwork, filters, and other air cleaning devices. Because air resistance factors such as these impede the fan’s ability to move air through the system, the fan must be of sufficient strength make up for these factors. To size up the right centrifugal fan for the job, engineers must calculate the resistance to airflow that is expected to be encountered, and to do this they use data supplied by manufacturers of component parts, as well as tabulated data that is readily available in engineering handbooks. Just as a lawn mower engine won’t provide sufficient energy to power a car, an undersized fan won’t be able to move air through a system which is beyond its capacity limit.
Next time, we’ll finish our series on local exhaust ventilations systems by looking at the last component in the system: the exhaust stack.
Last week we looked at how a mechanical brake stopped a rotating wheel by converting its mechanical energy, namely kinetic energy, into heat energy. This week, we’ll see how a dynamic brake works.
Chances are you have directly benefited by a dynamic braking system the last time you rode in an elevator. But, to understand the basic principle behind an elevator’s dynamic brake system, let’s first take a look at the electric braking system in Figure 1 below.
Figure 1 – A Simple Electric Braking System
Here the brake consists of an electric generator wired via an open switch to an electrical component called a resistor. The weight is attached to a cable that is wound around a pulley on the generator’s shaft. As the weight freefalls, the cable unwinds on the pulley, causing the pulley to turn the generator’s shaft.
Unlike last week’s mechanical brake which required a good deal of effort to employ, a dynamic braking system requires very little. All that needs to be done is to close a switch as shown in Figure 2 below. When the switch is closed, an electrical circuit is created where the resistor gets connected to the generator. The resistor does as its name implies: it resists (but doesn’t stop) the electrical current flowing through it from the generator. As the electrical current fights its way through the resistor to get back to the generator, the resistor gets hot like an electric heater. This heat is dissipated to the cooler surrounding air. At the same time, the weight begins to slow down in its descent. But how is this happening?
The electric braking system can be thought of as an energy conversion process. We start out with the kinetic, or motion energy, of the freefalling weight. This kinetic energy is transmitted to the electrical generator by the cable, which spins the generator’s shaft as the cable unwinds. Electrical generators are machines that convert kinetic energy into electrical energy. This energy travels from the electric generator through wires and a closed switch to the resistor. In the process the resistor converts the electrical energy into heat energy. So, kinetic energy is drawn from the falling weight through the conversion process and leaves the process in the form of heat. As the falling weight is drained of kinetic energy, it slows down.
Figure 2 – Applying the Electric Brake
Okay, now let’s get back to dynamic brakes on elevators. An elevator is attached by a cable to a hoist that is powered by an electric motor. When it’s time to stop at the desired floor, the automatic control system disconnects the elevator’s electric motor from its power source and turns the motor into a generator. The generator is then automatically connected to a resistor like the one shown in the electric brake above. The kinetic energy of the moving elevator is converted by the generator into electrical energy. The resistor converts the electrical energy into heat energy which is then dissipated into the surrounding environment. The elevator slows down in the process because it’s being robbed of kinetic energy. When the dynamic brake slows the elevator down enough, a mechanical brake is introduced, taking over to bring the elevator to a complete stop. This two-fold process serves to reduce wear and tear on the mechanical brake’s parts, lengthening the operational lifespan of the system as a whole.
Next time, we’ll tie everything together and show how mechanical and dynamic brakes work together in a diesel locomotive.