Basic Motor Control and the primary control devices PDF

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Basic Motor Control and the primary control devices The basic functions of a motor controller are to start and stop the motor and to protect the motor, machine, and operator. The controller may also be called upon to provide other functions that could include reversing, jogging or inching, plugging, operation at various speeds or at reduced current levels, and controlling motor torque. The purpose of controller enclosures is to provide protection of operating personnel by preventing accidental contact with energized components. In certain applications, the controller is protected from a variety of environmental conditions including water, rain, snow, sleet, dirt, non-combustible dust, oils, coolants, and lubricants. Motor control centers are designed to meet the requirements of the National Electrical Code (NEC). Types of Controllers Manual Controller A manual controller is one having its operations controlled or performed by hand at the location of the controller, as shown,

A manual starter is used frequently where the only control function needed is to start and stop the motor. The manual starter generally provides overload protection for the apparatus being powered. Manual control, which provides the same functions as those achieved by the manual full-voltage motor starter, can be had by the use of a switch with fusing of the delayed-action type, which provides overload protection for the motor. Examples of this type of control are very common in small metalworking and woodworking shops that use small drill presses, lathes, and pipe-threading machines. Another good example is the exhaust fan generally found in machine shops and other industrial operations. In this installation, the operator or maintenance person generally pushes the START button for the fan in the morning when the plant opens, and it continues to run throughout the day. In the evening, or when the plant is shut down, the operator then pushes the STOP button, and the fan shuts down until needed again. A manual controller is easily identified because it has no automatic functions of control. This type of controller is characterized by the fact that the operator must move a switch or push a button to initiate

any change in the condition of operation of the apparatus being operated. A manual controller must, therefore, have two components: a manually operated switch and a circuit protective device.

Semiautomatic Controller A semiautomatic controller uses a magnetic starter (a switch operated by an electromagnet) and one or more manual devices such as pushbuttons and other similar equipment. The figure shows a simple semiautomatic control scheme for a motor. Semiautomatic control provides flexibility of control by allowing remote and multiple control locations in installations where manual control would otherwise be impractical.

The key to classification as a semiautomatic control system lies in the fact that all the pilot devices are manually operated and that the motor starter is the magnetic-type. There are probably more machines operated by semiautomatic control than by either manual or automatic. This type of control requires the operator to initiate any change in the attitude or operating condition of the machine. Using the magnetic starter, however, this change may be initiated from any convenient location, as contrasted to the manual control requirement that the control point be at the starter. Automatic Controller An automatic controller is a magnetic starter in which functions are controlled by one or more automatic control or pilot devices. The figure shows an automatic control scheme for a motor. As shown, an automatic pilot device is some type of control device, such as a limit switch or float switch, that functions independent of operator action to initiate a change in the operating condition of a motor or machine.

Some systems may use a combination of manual and automatic devices in the control circuit. When a control system uses one or more automatic devices, it is classed as an automatic controller. Consider, for example, a tank that must be kept filled with water between definite limits and a pump to replace the water as it is needed. If we equip the pump motor with a manual starter and station a person at the pump to turn it on and off as needed, we have manual control. Now, let us replace the manual starter with a magnetic starter and put a pushbutton station at the foreman's desk. If we ring a bell to let them know when the water is low and again when it is high, they can do other work and just push the proper button when the bell rings. This would be semiautomatic control. Now, suppose we install a float switch that will close the circuit when the water reaches a predetermined low level and open it when it reaches a predetermined high level. When the water gets low, the float switch will close the circuit and start the motor. The motor will now run until the water reaches the high level, at which time the float will open the circuit and stop the motor. Although the automatic system is more expensive to install, it requires less operator attention and functions more reliably and accurately. Automatic control systems are found in many applications, such as large power plants, where they are used to control many mechanical systems in machine shops, where precision machines, such as drill presses and lathes, are automatically controlled for better accuracy and efficiency, and in the home, where automatic control systems are used to control such common household machines as dishwashers and washing machines.

Control Devices and Symbols Understanding, troubleshooting, and repairing control systems requires a knowledge and understanding of the physical devices that are used in control circuits and the symbols and terminology that are used to designate those devices on wiring diagrams. Most symbols used have been standardized throughout the industry to assure uniformity. Figure 4 shows American National Standard Graphical Symbols for Electrical Diagrams. The chart shown in Figure 5 shows standard symbols used in motor control circuits.

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Primary Control and Pilot Control Devices All components used in motor control circuits may be classed as either primary control devices or pilot control devices. A primary control device is one that connects the load to the line, such as a motor starter, whether it is manual or automatic. Pilot control devices are those that control or modulate the primary control devices. Pilot devices are things such as pushbuttons, float switches, pressure switches, and thermostats. An example (Figure 6) would be a magnetic contactor controlled by a toggle switch used to energize and de-energize the contactor, or M coil. To start the motor, the toggle switch is switched ON, which energizes the contactor coil and closes the main line contacts, which energizes the motor. Switching the toggle switch OFF de-energizes the contactor coil, opens the main line contacts, and deenergizes the motor.

In this example, the contactor, in that it connects the motor or load to the line, would be classed as a primary control device. The switch does not connect the load to the line, but is used to energize and de-energize the coil of the starter. Therefore, it would be classed as a pilot control device. For any given controller, there are generally two primary control devices used. These are the disconnecting means, or circuit breaker (usually a manual device), and the magnetic contactor. There may be many pilot devices used in parallel and series combinations to control the function of starting and stopping performed by the primary control device. The overload relays, for instance, which are included in the motor starter, are actually pilot devices used to control the primary device whenever the motor is overloaded. Pilot devices vary greatly with their function and intended use. From manual switches to automatic control devices, pilot devices are what make a motor controller adaptable to fit a multitude of applications.

Contacts Symbols 1 and 2 represent electrical contact devices (see Figure 7). They may represent line contacts on a starter, contacts on a limit switch or relay, or any other type of control device that has electrical contacts. Recall that circuit diagrams are shown in their de-energized condition. Therefore, Symbol 1 is a normally open (NO) contact, and Symbol 2 is a normally closed (NC) contact.

The designations "a" or "b" associated with a set of contacts are used to identify the state of the contacts (open or closed) in reference to the main operating device. An "a" contact will normally be closed when its associated coil is energized and its main contacts are closed. These same "a" contacts will open when the associated coil is de-energized. A "b" contact will normally be open when its associated coil is energized, thus operating the opposite of an "a" contact. Remember, however, that not all electrical drawings will indicate the state of contacts with this designator. If not, the drawing notes should annotate whether the circuit is in the energized or de-energized state. This "a" and "b" notation holds true for auxiliary contactors and relays, as well as the main contactor. Note that all circuits are shown in the de-energized, or shelf, condition unless stated otherwise. Two other terms often used in conjunction with contacts, either relay contacts or switch contacts, are make and break. When a contact goes closed, it is said to "make," and when the contact opens, it " is said to "break. Pushbutton Switches Symbols 3 and 4, shown in Figure 7, represent manually operated pushbutton switches with normally open and normally closed contacts, respectively. This spring-returned type switch will return to its normal position when released by the operator. Because the switch returns to its original position, and its contacts are only closed or open for the moment (however long) the switch is pushed, these contacts are referred to as momentary. Toggle Switches Symbols 5 and 6, shown in Figure 7, represent manual contacts of a toggle type of switch. Symbol 5 contacts are normally open, and Symbol 6 contacts are normally closed. This type of switch has maintaining contacts; that is, once switched to a different position, the switch will stay in that position. When found in a circuit diagram, the switch positions should be labeled as OFF or ON, FAST or

SLOW, or other appropriate labeling. Symbol 7 is a toggle switch of the single-pole, double-throw (SPDT) type, where one contact is normally open and the other normally closed. When more than one set of contacts are operated by moving one handle or pushbutton, they are generally connected by dotted lines, as in symbols 8 and 9. The dotted lines represent any form of mechanical linkage that will make the two contacts operate together. One other method that is used frequently to show pushbuttons that have two sets of contacts is shown in symbols 10 and 11. Symbol 10 has two normally open contacts, and symbol 11 has one normally open and one normally closed contact. Switches can be designed to operate in one of two ways. The first, and most common, is referred to as break-before-make contact arrangement. In an arrangement of this type, one set of contacts opens before the next set of contacts closes....