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The flip-book animation beginning in Appendix [animation_blinking_lights] beginning on the page shows how reversing two of the three lines has the effect of reversing phase sequence. pumps, shop grinders, drill presses, etc. All rights reserved. Preplanned Post-Installation Inspections: Do you need periodic testing of water features after the work is complete. The fact that all AC induction motors must start as polyphase machines even though they can run as single-phase machines means that an AC motor designed to run on three-phase power may actually continue to run if one or more of its phases are “lost” due to an open wire connection or a blown fuse. As such, it is important to understand the functioning of motor control circuits. This is why all modern motor contactors are equipped with arc shield covers. So long as the stator coil insulation is not damaged by excessive moisture, heat, or chemical exposure, these motors will continue to operate indefinitely. [90-7 ¶ 2, 300-1(b), 450-1 Exc.No.2] [250-20(b)] This means the conductive object would begin to rotate in the same direction as the stator’s rotating magnetic field, always trying to “catch up” to the rotating magnetic field. This electrical motor controls Training (MCTrainer) Software also includes the following units of instruction. Section 370-28 provides specific instructions for calculating the dimensions of pull or junction... © 2003-2020. The sizing of overload heaters is unrelated to wire ampacity, and therefore unrelated to the ratings of the fuses or circuit breakers delivering line power to the motor. They show the relative location of the components. The zero-sequence CT is a special current transformer encircling all three motor phase conductors, providing an indication of a ground fault within the motor. The rotor has been removed, held by my fingers for inspection: Instead of a capacitor creating a leading phase-shift for current through a special stator winding, this “shaded-pole” induction motor uses a pair of copper loops wrapped around the corners of the magnetic poles to produce a lagging phase shift in the magnetic field at those corners. This “amplifying” action enables relatively small control switches, PLCs, and relay circuits to start and stop relatively large (high-current) electric motors. An interesting experiment you can try yourself is to obtain one of these small shaded-pole AC motors and make it rotate by applying pulsed DC power to it, from a battery. No induction would mean no electric currents induced in the rotor, which would mean no reactive magnetic field, which would mean no torque to motivate the rotor. Aluminum, of course, is electrically conductive but non-magnetic. Once a PLC is included in the motor control circuit, a great many automatic control features may be added to enhance the system’s capabilities. Thus, induction machines always spin a bit slower than synchronous speed. The network wiring may simply be paralleled (“daisy-chained”) between units, such that several buckets reside on the same physical network, each one programmed with a unique address. Published under the terms and conditions of the, Creative Commons Attribution 4.0 International Public License. A diagram showing how a modern (digital) protective relay would monitor various parameters on a medium-voltage (4160 volts AC, three-phase) industrial electric motor is shown here: In this example, line voltage (4160 volts AC) and line current are both too great to be directly connected to the protective relay, and so the relay senses line voltage and line current via potential transformers (PTs) and current transformers (CTs), respectively. De-energizing the coil causes it to de-magnetize, releasing the armature and enabling a return spring inside the contactor to snap all three contacts to the open (off) position. A modern trend in motor control is the use of digital networks to both command the contactor as well as monitor the motor’s operating status remotely. If, however, a ground fault develops within the motor where some AC current “leaks” from a stator winding to earth ground to return to the 4160 VAC power source’s neutral connection, that imbalance of phase currents will be sensed by the zero-sequence CT, since that ground-fault current represents the fourth path for current not accounted for by the three power conductors passing through to the motor. An alternative way to detect overloading conditions is to monitor the temperature of the stator windings directly, using thermocouples or (more commonly) RTDs, which report winding temperatures to an electronic “trip” unit with the same control responsibilities as an overload heater assembly. Basic wiring for motor control – Technical data. Starting off with wiring diagrams vs. ladder diagrams and motor control theory, coving the entire AC motor control system.Solenoids, … The most common variant of the induction motor is the so-called squirrel-cage design, where the rotor is made up of aluminum bars joining two aluminum “shorting rings,” one at either end of the rotor. ): One of the major principles to grasp about AC induction motors is that they must start as polyphase machines, although they may continue to run as single-phase machines. Control wiring is vastly simplified with digital networks, as a single network cable is able to address multiple motor buckets. 14 copper is permitted to bond one conductor of the secondary to the grounded switchboard frame or transformer enclosure. As with a transformer, where a reduction in secondary current (from a load change) results in a reduction in primary current, the reduction in induced rotor current (from reduced slip speed) results in a reduction in stator winding current. So long as the object is electrically conductive, electromagnetic induction will ensure the creation of electric currents in the rotor, and these currents will produce their own magnetic fields that react against the stator’s rotating magnetic field to produce a torque on the rotor. They can be used as a guide when wiring the controller. A very simple and common overload protective device is known as an overload heater, consisting of resistive elements connected in series with the three lines of a 3-phase AC motor, designed to heat and to cool at rates modeling the thermal characteristics of the motor itself. A schematic diagram of a three-phase contactor connected to a three-phase motor (with fuses for overcurrent protection) is shown here: Energizing terminals A1 and A2 magnetizes the electromagnet coil, causing all three switch contacts to simultaneously close, sending three-phase AC power to the motor. Recall the principle that swapping any two phases in a three-phase power system reverses the phase rotation, which in this case make the electric motor spin the other direction. This next photograph shows a digitally monitored and controlled “bucket,” using DeviceNet as the control network: Using a digital network standard such as Ethernet, DeviceNet, Modbus, Profibus, or any number of others to monitor and control a motor brings a host of benefits for maintenance and operations. The motor cannot deliver full-rated mechanical power in this condition, but if the mechanical load is light enough the motor will continue to spin even though it no longer has multiple phases powering it! Fuses and circuit breakers also protect against overcurrent, but for different reasons and for different parts of the motor circuit. Now, if it has been determined that the control circuit must be grounded, how is this accomplished? However, several … The motor runs when the switch is in the closed state, and stops when the switch opens. Instead of individual wires running between the PLC and the motor starter to command each motor to run and stop, the PLC simply transmits “start” and “stop” commands over the network to individually addressed digital starter modules. As such, it is important to understand the functioning of motor control circuits. The purpose of an “interlock” is to prevent incompatible events from happening, in this case preventing the actuation of the “reverse” contactor when the “forward” contactor is already actuated, and vice-versa. Similarly, a typical “four-pole” induction motor with a synchronous speed of 1800 RPM may only attain a rotor speed of approximately 1760 RPM. A simple latching motor control circuit is shown here: In this circuit, an auxiliary contact actuated by the motor contactor is wired in parallel with the “Start” pushbutton switch, so that the motor contactor continues to receive power after the operator releases the switch. An alternative to this design is to build a latching circuit allowing the use of momentary contact switches (one to start, and one to stop).

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