Electromechanical Education

A three-phase motor may be run from a single-phase power source. However, it will not self-start. It may be hand started in either direction, coming up to speed in a few seconds. It will only develop 2/3 of the 3-φ power rating because one winding is not used. 3-φ motor runs from 1-φ power but does not start Single Coil of a Single Phase Motor The single coil of a single-phase

Charles Proteus Steinmetz’s first job after arriving in America was to investigate problems encountered in the design of the alternating current version of the brushed commutator motor. The situation was so bad that motors could not be designed ahead of the actual construction. The success or failure of a motor design is not known until it is actually built and tested. He formulated the laws of magnetic hysteresis in

All About Diodes A diode is an electrical device allowing current to move through it in one direction with far greater ease than in the other. The most common kind of diode in modern circuit design is the semiconductor diode, although other diode technologies exist. Semiconductor diodes are symbolized in schematic diagrams such as the figure below. The term “diode” is customarily reserved for small signal devices, I ≤ 1 A. The

PARTS AND MATERIALS Steel flat bar, 4 pieces Miscellaneous bolts, nuts, washers 28 gauge “magnet” wire Low-voltage AC power supply “Magnet wire” is small-gauge wire insulated with a thin enamel coating. It is intended to be used to make electromagnets, because many “turns” of wire may be wrapped in a relatively small-diameter coil. Any gauge of wire will work, but 28 gauge is recommended so as to make a coil with as many

As versatile as electromechanical relays can be, they do suffer many limitations. They can be expensive to build, have a limited contact cycle life, take up a lot of room, and switch slowly, compared to modern semiconductor devices. These limitations are especially true for large power contactor relays. To address these limitations, many relay manufacturers offer “solid-state” relays, which use an SCR, TRIAC, or transistor output instead of mechanical contacts

A wound rotor induction motor has a stator like a squirrel cage induction motor, but a rotor with insulated windings brought out via slip rings and brushes. However, no power is applied to the slip rings. Their sole purpose is to allow resistance to be placed in series with the rotor windings while starting (figure below). This resistance is shorted out once the motor is started to make the rotor

PARTS AND MATERIALS 4001 quad NOR gate (Radio Shack catalog # 276-2401) 4070 quad XOR gate (Radio Shack catalog # 900-6906) Two, eight-position DIP switches (Radio Shack catalog # 275-1301) Two light-emitting diodes (Radio Shack catalog # 276-026 or equivalent) Four 1N914 “switching” diodes (Radio Shack catalog # 276-1122) Ten 10 kΩ resistors Two 470 Ω resistors Pushbutton switch, normally open (Radio Shack catalog # 275-1556) Two 6 volt batteries Caution! Both the 4001 and 4070 ICs are CMOS, and therefore sensitive to static

PARTS AND MATERIALS 6 volt battery Power transformer, 120VAC step-down to 12VAC (Radio Shack catalog # 273-1365, 273-1352, or 273-1511). One 1N4001 rectifying diode (Radio Shack catalog # 276-1101) One neon lamp (Radio Shack catalog # 272-1102) Two toggle switches, SPST (“Single-Pole, Single-Throw”) A power transformer is specified, but any iron-core inductor will suffice, even the home-made inductor or transformer from the AC experiments chapter! The diode need not be an exact model 1N4001.

Logic circuits, whether comprised of electromechanical relays or solid-state gates, can be built in many different ways to perform the same functions. There is usually no one “correct” way to design a complex logic circuit, but there are usually ways that are better than others. In control systems, safety is (or at least should be) an important design priority. If there are multiple ways in which a digital control circuit

Pulse Width Modulation (PWM) uses digital signals to control power applications, as well as being fairly easy to convert back to analog with a minimum of hardware. Analog systems, such as linear power supplies, tend to generate a lot of heat since they are basically variable resistors carrying a lot of current. Digital systems don’t generally generate as much heat. Almost all the heat generated by a switching device

PARTS AND MATERIALS 6-volt battery Low-voltage incandescent lamp (Radio Shack catalog # 272-1130 or equivalent) Long lengths of wire, 22-gauge or larger Household light switch (these are readily available at any hardware store) Household light switches are a bargain for students of basic electricity. They are readily available, very inexpensive, and almost impossible to damage with battery power. Do not get “dimmer” switches, just the simple on-off “toggle” variety used for

Normally, the rotor windings of a wound rotor induction motor are shorted out after starting. During starting, resistance may be placed in series with the rotor windings to limit starting current. If these windings are connected to a common starting resistance, the two rotors will remain synchronized during starting. This is useful for printing presses and drawbridges, where two motors need to be synchronized during starting. Once started, and the

Synchronous motors load the power line with a leading power factor. This is often useful in canceling out the more commonly encountered lagging power factor caused by induction motors and other inductive loads. Originally, large industrial synchronous motors came into wide use because of this ability to correct the lagging power factor of induction motors. This leading power factor can be exaggerated by removing the mechanical load and overexciting the

Don’t just sit there! Build something!! Learning to analyze relay circuits requires much study and practice. Typically, students practice by working through lots of sample problems and checking their answers against those provided by the textbook or the instructor. While this is good, there is a much better way. You will learn much more by actually building and analyzing real circuits, letting your test equipment provide the “answers” instead

Though it may seem strange to cover the elementary topic of electrical switches at such a late stage in this book series, I do so because the chapters that follow explore an older realm of digital technology based on mechanical switch contacts rather than solid-state gate circuits, and a thorough understanding of switch types is necessary for the undertaking. Learning the function of switch-based circuits at the same time that

AC Motors Perhaps the most important benefit of polyphase AC power over single-phase is the design and operation of AC motors. As we studied in the first chapter of this book, some types of AC motors are virtually identical in construction to their alternator (generator) counterparts, consisting of stationary wire windings and a rotating magnet assembly. (Other AC motor designs are not quite this simple, but we will leave those

After the introduction of the DC electrical distribution system by Edison in the United States, a gradual transition to the more economical AC system commenced. The lighting worked as well on AC as on DC. Transmission of electrical energy covered longer distances at a lower loss with alternating current. However, motors were a problem with alternating current. Initially, AC motors were constructed like DC motors, but numerous problems were encountered

All About Contactors When a relay is used to switch a large amount of electrical power through its contacts, it is designated by a special name: contactor. Contactors typically have multiple contacts, and those contacts are usually (but not always) normally-open, so that power to the load is shut off when the coil is de-energized. Perhaps the most common industrial use for contactors is the control of electric motors. The

The variable reluctance motor is based on the principle that an unrestrained piece of iron will move to complete a magnetic flux path with minimum reluctance, the magnetic analog of electrical resistance. Synchronous Reluctance If the rotating field of a large synchronous motor with salient poles is de-energized, it will still develop 10 or 15% of synchronous torque. This is due to variable reluctance throughout a rotor revolution. There is no practical

In ladder logic symbolism, an electromechanical relay coil is shown as a circle, and the contact(s) actuated by the coil as two parallel lines, almost like a capacitor symbol. Given this knowledge, interpret the following ladder logic diagram: How do we know which relay contact is actuated by which relay coil? How does this convention differ from that of standard electrical/electronic schematic diagrams, where the relay coil is