Upstairs E-Lab Final Exam Study Guide
The following study guide has been extracted out of your Lab Text manual. It highlights the main concepts of the lab exercises you have been completing for the past semester.
Also, please make sure you can read, identify and follow the intent and purpose of each component in motor control wiring diagrams. Know how the complete circuit functions and its purpose.
Motor control is a broad term that can apply to anything from a simple toggle switch to a complex system with components such as relays, contactors, and programmable logic controllers (PLCs). The common function of all these controls is to command the operation of an electric motor.
A complete motor circuit is usually divided into control and power sections. The power circuit includes the motor and therefore, operates under higher voltage. On the other hand, the control circuit mostly contains switching devices and typically operates under lower voltage.
Control panel devices, such as push buttons, selectors, or toggle switches, command the operation of electric motors via their open or closed contacts, which relay a control current.
Contactors and control relays are devices that use electromagnetic induction to open and close contacts. Contactors are often part of the motor starter, being power switching devices. Control relays are rather used as control switching devices, because they are designed to withstand lower electrical currents.
Motor starters are systems comprising switching and overload-protection components. They provide a safe, convenient, and economical means of starting and stopping motors.
Circuit-breakers and fuses protect the motor from very high currents. Overload protection devices are safeguards against prolonged, relatively high current levels. The particular application of each motor and control installation must be considered when determining the protective devices required.
Motor controllers command the operation of electric motors through normally open (NO) and normally closed (NC) contacts.
Push button, toggle and selector switches are common manual controllers. The state of their contacts change as a push button is pressed, or the position of a toggle or knob is modified.
Pilot lights are used to show the condition of a circuit. Contactors and overload protection devices are the essential elements of motor starters.
DOL starters are the simplest method of starting a motor. They provide on/off control for small motors. The Manual Starter module is an example of a DOL starter.
Reversing starters reverse the direction of the motor by inverting two motor power lines. The Cam Switch module, with the addition of an overload relay, can be used for that purpose. Solenoids are magnetic devices used to open and close contacts of contactors and control relays.
Contactors are switching devices designed for power circuits, while control relays are built for control circuits and small loads.
Dual contactors with a mechanical interlock enable reversing the rotation direction of a motor, without risking powering both coils at the same time. Circuit-breakers and fuses protect circuits from high current levels. They open the circuit if the current is between their ampere rating and their interrupting capacity. Circuit-breakers can be reset, while fuses have to be replaced after use.
Overload protection is used to prevent burnout of the motor. Overload relays limit the amount of current drawn to a predetermined value. The higher the current, the less time it takes to de-energize the contactor and stop the motor. Thermal overload relays heat up depending on the motor current.
Nameplates are installed on motors to help the purchaser for maintenance purposes and the manufacturer with customer service. These plates display useful information concerning the motor ratings, model and connection.
The NEMA and/or the IEC develop ratings for Industrial control devices, including motor starters, contactors, pilot devices, and control relays. Those specifications are usually located on rating labels. NEMA devices tend to be interchangeable, whereas IEC devices are more specific to the application, and thus require more knowledge.
Symbols are used in diagrams as a shorthand means of illustrating and defining elements and functions of electric circuits. Symbol functions can be defined with abbreviations (designations).
Schematic diagrams show simplified circuit connections and functions and are useful for troubleshooting purposes. Wiring diagrams show the circuits as they physically appear, making circuit construction easier.
Target tables are used to show the state of contacts on control devices. The use of a control voltage transformer is a way to isolate control and power circuits. It provides suitable low voltage for the control circuit.
Motor starters are made of contactors and overload protection devices. When loads are coupled to the motor shaft, the motor accelerates and decelerates slower.
The current level is higher upon starting than during normal operation. This phenomenon, discussed in Exercise 1–5, is called inrush current.
Two-wire control circuits restart a motor automatically when voltage returns, following a power failure. A two-wire controller can be a toggle switch, float switch, limit switch, or any other device with maintained on-off positions.
Three-wire control circuits require an operator present to restart the machine following a power failure. Three-wire controls can be, for example, momentary contact push buttons.
Changing the motor terminal’s line sequence inverts the motor rotation direction. There are only two possible sequences in a three-phase system: one for the forward and one for the reverse direction. Instead of manually modifying the line sequence, a cam switch may be used to simplify the reversal of motor rotation direction.
Reversing magnetic starters are built with two contactors, one per rotation direction. If both contactors are actuated at the same time, a short-circuit can occur. This is why electrical and/or mechanical interlocks are used.
Push button interlocking is an electrical means of disabling two contactors actuation. When a push button is pressed, the circuit controlling the other motor direction is automatically opened.
Mechanical interlocking uses a lever to artificially keep the second contactor de-energized, while the first coil is actuated. This method is more rugged in the way that it prevents short-circuits resulting from a stuck contactor.
Plugging is a method of making the motor brake faster. It is accomplished by reversing phases while the motor is running. A motor can be started or stopped from more than one location by using multiple push buttons. To implement such a circuit, the stop push buttons are connected in series and the start push buttons in parallel.
Emergency buttons are easily accessible and maintain their contact open after it has been pressed. In friction brakes, the movement of a solenoid makes shoes or pads come in contact with a disk mounted on the motor shaft. Fail-safe brakes, which apply automatically when power is turned off, provide an extra level of security to weight-lifting equipment.
Friction brakes are used in applications where a motor has to hold a certain position, and when quick and precise stops are required. Additionally, they provide a smooth braking action that can be useful with high inertia loads.
In jogging control circuits, the motor starter remains energized only as long as the jog push button is pressed. No holding circuit is operating. Because of the repeated switching of high currents, jogging greatly reduces the life expectancy of contactors. Various jogging circuits can be implemented, depending on the requirements of the application:
Control relays make it possible for more specialized circuits. For example, in a jog/run circuit, they can make jogging independent of normal (run) operation.
Friction brakes improve control in jogging operations by reducing stopping time. A jogging control circuit using a reversing starter can be implemented when repeated clockwise and counterclockwise inching is necessary.
Primary resistor starters can be used for starting motors at a reduced voltage. Resistors are inserted in series with the motor terminals and power lines to create a drop in the input voltage.
Reduced voltage is utilized to protect machinery from the shock of sudden acceleration and prevent power line disturbances resulting from high inrush currents. Once the motor reaches a sufficient speed, the starting resistors are bypassed by a set of contactors, allowing the motor to operate at full line voltage.