Ch 4 Motor Control Devices

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1 Ch 4 Motor Control Devices Part 1 Manually Operated Switches 1. List three examples of primary motor control devices. (P 66) Answer: Motor contactor, starter, and controller or anything that control the lines to the load directly. 2. List three examples of pilot motor control devices. (P. 67) Answer: Pushbuttons, flow switches and thermostats or anything that activates a primary motor control. 3. What do the terms Normally Open and Normally Closed refers to when used in defining the switching action of pushbutton switch? Answer: NO and NC represent the state of switch contact when the switch is not activated. 4. The types of enclosures used to house motor control devices have been standardized by NEMA. What criteria are used to classify NEMA enclosure types? (P.68) Answer: The environment in which the equipment is to be installed. 5. Name the three basic part of a standard push button assembly. (P.69) Answer: Operator, legend plate, and control block. 6. Compare the operation of momentary and maintained pushbutton operators. (P 70) Answer: Momentary types return to their normal "on" or "off" state when the operator is released. Maintained types require you to press and release the operator to switch the contacts to their "on" state and to press and release the operator a second time to return the contacts to the "off" state. 7. What is the OSHA requirements for resetting emergency stop switches? (P.70) Answer: The Occupational Safety and Health Administration (OSHA) regulations requires that once the emergency stop switch has activated, the control process cannot be started again until the actuating stop switch has been reset to the ON position. 8. A pilot light is to be connected to indicate when a magnetic starter is energized. Across what component of the circuit should the pilot light be connected? (Fig 4-15) Answer: Starter coil 9. Explain how a push-to-test pilot light operates. (Fig 4-17) Answer: Push-to-test pilot lights can be energized from a normal or test input signal. Pressing the push-to-test pilot light opens the normal signal input to the light while at the same time completing a path to the other side of the line and illuminating the lamp if the unit is not at fault. 10. Compare the way in which pushbutton and selector switch operator actuate contacts. Answer: Pushbuttons operate by a push or pull while selector switches must be rotated.

2 11. When a drum switch is used for starting and reversing a three-phase squirrel-cage motor, how is the reversing action accomplished? (Fig 4-19) Answer: By interchanging two of the three main power lines to the motor. Part 2. Mechanically Operated Switches 1. Define the term mechanically operated switch. (P. 73) Answer: One that is controlled automatically by such factors as pressure, position and temperature. 2. In what way the limit switches normally actuated? (P. 73) Answer: By contact with an object such as a cam. 3. (X) A control application calls for an NC held open limit switch. What type of connection does this imply? Answer: The contact is wired as a N.C. contact, and some part of the machine in its normal off state holds the contact open. 4. List four common types of limit switch operator heads. (P. 73, Figure 4-22) Answer: Lever, fork lever, wobble-stick, and push roller. 5. What is an important operating features of a snap action micro limit switch? (P 74) Answer: They are useful in situations that require a fast opening or closing of a circuit. 6. In what way is the contact configuration of a traditional limit switch different from that of a micro limit switch? (P 74) Answer: Micro switches use a single-pole, double throw contact arrangement that has one terminal connected as a common between the normally open and normally-closed contacts instead of electrically isolated contacts. 7. For what types of machine control applications are rotating cam limit switches best suited? (P 75) Answer: Applications having a repetitive cycle of operation, where motion is correlated to shaft rotation. 8. How does a fluid capillary tube temperature switch actuate its electrical contact block? (P 75, Figure 4-27) Answer: The fluid capillary tube temperature switch is operated by a tube that is filled with a temperature sensitive liquid. As the temperature rises and falls, the pressure in the tube increases and decreases. Bellows motion, in turn, is transmitted through a mechanical linkage to actuate a precision switch at a predetermined setting. 9. In what types of applications are pressure switches used? (P 76) Answer: Pressure switches are used to monitor and control the pressure of liquids and gases.

3 10. Compare the function of a float switch with that of a flow switch. (P 77) Answer: A float switch is used to sense the height of a liquid while a flow switch is used to detect the movement of air or liquid through a duct or pipe. Part 3. Sensors 1. In general, how do sensor pilot devices operate? (P 78) Answer: They basically operate by converting mechanical, magnetic, thermal, optical, and chemical variations into electric voltages and currents. 2. What is the main feature of a proximity sensor? (P 78) Answer: Its ability to sense the presence of an object without physical contact. 3. List the main components of an inductive proximity sensor. (P 79, Fig 4-35) Answer: Coil, oscillator, detector, and output 4. Explain the term hysteresis as it applies to a proximity sensor. (P 79) Answer: Hysteresis is specified as a percentage of the nominal sensing range and is needed to keep proximity sensors from chattering when subjected to shock and vibration, slow-moving targets, or minor disturbances such as electrical noise and temperature drift. 5. How is a two-wire sensor connected relative to the load it controls? (P 79, Fig 4-37 (b)) Answer: In series with the load it controls. 6. In what way is the sensing field of a capacitive proximity sensor different from that of the inductive proximity sensor? (P 80, Fig 4-38) Answer: The capacitive sensor uses an electrostatic field instead of an electromagnetic field for detection purposes. 7. For what type of target would a capacitive proximity sensor be selected over an inductive type? (P 80) Answer: One with no metal available for actuation. 8. Outline the principle of operation of a photoelectric sensor. (P 80) Answer: It is an optical control device that detects a visible or invisible beam of light, and responds to a change in the received light intensity. 9. Name the three most common scan techniques for photoelectric sensors. (P 80-81) Answer: Through-beam (Fig 4-41), Retroreflective (Fig 4-42), diffuse scan (Fig 4-44) 10. What are the advantages of fiber optic sensing systems? (P 82) Answer: Immunity to all forms of electrical interference, can be safely used in hazardous sensing environments, can be routed through extremely tight areas to the sensing location, can be used in locations with high operating temperatures.

4 11. Outline the principle of operation of a Hall effect sensor. (P 82) Answer: When a current-carrying conductor is placed into a magnetic field, a voltage will be generated perpendicular to both the current and the field. 12. Outline the principle of operation of an ultrasonic sensor. (P 83) Answer: It operates by sending sound waves toward the target and measuring the time it takes for the pulses to bounce back. The time taken for this echo to return is directly proportional to the distance. 13. List the four basic types of temperature sensors and describe the principle of operation of each. (P ) Answer: Thermocouple: Two dissimilar metals that generate a small DC voltage when there is a difference in temperature between the measuring and reference junctions. RTDs: Metals (usually platinum) whose resistance varies proportionally with temperature. The hotter they become the higher the value of their resistance. Thermistor: Temperature sensitive resistor whose resistance varies with temperature. Depending on the type of thermistor, this change of resistance with temperature can result in a increase or decrease in resistance. IC Temperature Sensor: Depending on the configuration used, will provide a voltage or current which varies proportionally with temperature. 14. Compare the way in which a tachometer and magnetic pick-up are used in speed measurement. (P 86) Answer: Tachometer Generator: Converts rotational speed into an analog signal whose value is directly proportional to the rotor speed. Magnetic Pickup: As the target passes through the probes magnetic field the flux density is modulated causing an AC voltage to be induced in the coil of the sensor. By measuring the frequency of this signal voltage, the shaft speed can be determined. 15. Outline the principle of operation of an optical encoder. ( P 86) Answer: An electronic circuit counts the number of interruptions of a beam of light that shines on an optical disk with lines or slots; this generates the encoder s digital output pulses. 16. What approach is usually taken to measurement of fluid flow? (P 87) Answer: The kinetic energy of the fluid is converted to some other measurable form. 17. List three common types of flowmeters. (P 87) Answer: Turbine Flow meter, Target Flow meter, Magnetic Flow meter.

5 Part 4. Actuators 1. Define the term actuator as it applies to an electric circuit. (P 88) Answer: Any device that converts an electrical signal into mechanical movement. 2. In what ways are electromagnetic relays employed in motor control systems? (P 88) Answer: Switching multiple control circuits and controlling light loads such as motor starter coils and pilot lights. 3. What are the two main parts of an electromagnetic relay? (P. 89, Fig 4-63) Answer: The coil with the iron core, and the moveable contacts on the armature. 4. Describe how an electric solenoid operates. (P. 90) Answer: When the coil of the solenoid is energized it produces a magnetic field that attracts the plunger, pulling it into the frame and thus creating mechanical motion. 5. Which type of solenoid (AC or DC) is constructed with laminated instead of solid pieces of steel? Why? (P 90) Answer: AC types in order to limit the amount of eddy currents induced by the magnetic field. 6. Why are AC solenoid coils likely to overheat if the plunger sticks in the open position when energized? (P. 90) Answer: Because of the inrush current that will be present, which can be ten times the closed or operating current. 7. In what way is the design of a rotary solenoid different from that of a linear solenoid? (P 90) Answer: The rotary solenoid incorporates a mechanical design that converts linear motion to rotary motion. 8. A solenoid valve is a combination of what two elements? (P 91) Answer: Solenoid coil operator and valve. 9. Explain how rotation is achieved in a stepper motor. (P.92) Answer: Every revolution is divided into a number of steps and the motor must be sent a voltage pulse for each step. The amount of rotation is directly proportional to the number of pulses and the speed of rotation is relative to the frequency of those pulses. 10.What is the basic difference between an open-loop and closed-loop positioning or motor speed control system. (P 93) Answer: An open loop system operates without feedback while a closed system compares speed or position feedback with the commanded speed or position and generates a modified command to make the error smaller. 11.What do all servo motor have in common? (P 93) Answer: They operate using a closed loop control system.

6 12.What replaces brushes in a brushless DC motor? Answer: Electronically communicated. Page 95 Troubleshooting Scenarios 1. (p.67) 10 A DC => 10A AC 2. 2A at 120V => Rs = Vs/Is (ohm s law) = 120V/2A = 60 ohms DMM => Ohm meter => 2 wires (1 ohm) Shorted circuit 4.A 12 V pilot light <= 120V (dim, no light) 5.mv

MOTOR TERMINAL CONNECTIONS

MOTOR TERMINAL CONNECTIONS Motor Classification Most of the industrial machines in use today are driven by electric motors Motors are classified according to the type of power used (AC or DC) and the motors