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Chapter 20 Motor Control Devices and Circuits Objectives After studying this chapter, you will be able to: • Discuss several types of control devices • Identify the proper usage of different types of control devices • Discuss the different types of controlled devices Objectives (cont’d.) • Identify the proper usage of different types of controlled devices • Describe different methods of starting, controlling speed, reversing, braking, and stopping motors • Develop control circuits to perform the function of starting, controlling speed, reversing, braking and stopping motors Motor Control Devices • Push buttons – Manual control devices – Momentary contact push button – Mechanically held push button – Rated as standard duty or heavy duty • Rotary switches – Switches operated manually with a turning motion, also called selector switches Motor Control Devices (cont’d.) – Used to open or close one or more circuits simultaneously • Limit switches – Automatic devices – Consist of actuator, body and one or more contact blocks – Used to sense presence or absence of an item or to verify position Motor Control Devices (cont’d.) • Proximity switches – Noncontact-type switches used to sense presence or absence of an object without contact – Two types: inductive and capacitive • Inductive senses ferrous and nonferrous metals • Inductive maximum sensing distance is 0.4 inches • Capacitive senses conductive and nonconductive material Motor Control Devices (cont’d.) • Photoelectric switches – Noncontact sensors – Consist of transmitter, receiver and switching device Operating Methods • Thru-beam – Transmitter and receiver are housed separately – Item to be sensed breaks the light beam • Retro-reflective – Transmitter and receiver housed in single unit – Two types: standard and polarized retroreflector Operating Methods (cont’d.) • Diffuse – Uses the reflectivity of the object being sensed to return light from transmitter to receiver – Several different types: standard, long-range, fixed-focus, wide-angle, and background suppression sensor Controlled Devices • Relays – Electromechanical switch – Coil of wire wound around an iron core – Two contacts: one fixed and one moveable – When relay is energized, magnetic field around the coil causes contact to move – Time delay relays provide time delay before contact changes state Motor Starters • Electromechanical switch similar to a relay • Also contains overload protection for the motor • Available for AC or DC voltage • Two different types: standard and reversing motor starter – Reversing allows the motor’s direction of rotation to be changed Annunciators • Signaling apparatus that may be audible or visual • Examples: bell, chime, horn, loudspeaker siren, indicator light, or strobe light • Used as a warning device or to indicate current condition of a machine or process Motor Control Circuits • Two-wire controls – Method of motor control in which manual devices are used to stop and start motor • Three-wire controls – Use some type of electromechanical switch to provide a latching, seal-in, memory or holding function – Power failure will stop the motor Multiple Start/Stop Controls • Control two motors from one location, or one motor from several different locations • Forward/reverse controls – Circuit can allow a motor to run in either the forward or reverse direction – Example: Figure 20-32A Multiple Start/Stop Controls (cont’d.) • Speed control – Controls speed of the motor – Pilot lights can be used to indicate speed • Jog control – Motor may be jogged or bumped slightly – Helpful when using motor to position object – Momentary switch applies power to motor for as long as switch is closed Multiple Start/Stop Controls (cont’d.) • Hand-off-automatic control – Allows circuit to function under the control of either automatic device or manual control Multiple Motor Starter Control • Allows independent control of multiple motors – If first motor does not start, the second and third will not either – If first motor starts and second motor fails to run, third motor will not run Sequential Starting Control • Start a second motor a certain time after starting the first motor • Example: cutting tool with coolant pump Various Starting Methods • Primary resistor starter – Resistors limit the inrush of current to the motor windings • Autotransformer starter – Separate start relay and run relays – Start relay energized first, then after time delay, start relay is de-energized and run relay is energized Braking • Plugging reverses the direction of a running motor – Braking action is very quick – Should be reserved for emergency situations since it will cause motor damage • Dynamic braking – Coasting motor acts as a generator – Braking effect decreases as motor slows Summary • Types of motor control devices include push buttons, rotary switches, limit switches, proximity switches and photoelectric switches • Some operating methods include thrubeam, retro-reflective, and diffuse • Relays are types of electromechanical switches Summary (cont’d.) • Motor control circuit types include two-wire and three-wire, speed control and handoff-automatic control, and those with multiple start/stop capabilities • Various starting methods exist, including the primary resistor starter, and the autotransformer starter • Plugging is a method of braking