Lesson 3: Pushbuttons, Timers,  Pressure, Limit and Sensor  Switches

PUSHBUTTONS 

Pushbutton stations , are spring-controlled switches and , when pushed, are used to complete motor or motor control circuits. ill 5B shows multiple control stations, with pushbuttons, selector switches, and pilot indicating lights. Note the “mushroom” stop button for easy access. This is for convenience and safety.

Pushbutton stations 

Symbols for pushbutton contacts: NORMALLY OPEN; NORMALLY CLOSED; OPEN AND CLOSED

 The symbols used in schematic, drawings to represent momentary pushbutton contacts are given in figure 6. Contacts can either be normally-open or normally-closed. This is the normal condition when there is no mechanical actuation of the contacts. In the pushbuttons illustrated in 6, the contacts are referred to as momentary contacts. This simply means that the contacts change from their normal condition to the opposite condition momentarily when mechanical actuation is applied, and then change back to the normal condition when the actuator is removed. Some contacts are designated as maintained contacts. This means that the contacts will stay as activated (held mechanically) until returned to their original position. See the glossary for the complete set of symbols.

Types of Control Systems 

Motor control systems can be divided into three major types: manual, semiautomatic, and automatic. Manual controls are characterized by the fact that the operator must go to the location of the controller to initiate any change in the state of the control system. Manual controllers are generally very simple devices that connect the motor directly to the line. They may or may not provide overload protection or low-voltage release. Manual control may be accomplished by simply connecting a switch in series with a motor (FIG. 1). 

Semiautomatic control is characterized by the use of push buttons, limit switches, pressure switches, and other sensing devices to control the operation of a magnetic contactor or starter. The starter actually connects the motor to the line, and the push buttons and other pilot devices control the coil of the starter. This permits the actual control panel to be located away from the motor or starter. The operator must still initiate certain actions, such as starting and stopping, but does not have to go to the location of the motor or starter to perform the action. A typical control panel is shown in FIG. 6. A schematic and wiring diagram of a start-stop push button station is shown in FIG. 7. A schematic diagram shows components in their electrical sequence without regard for physical location. A wiring diagram is basically a pictorial representation of the control components with connecting wires. Although the two circuits shown in FIG. 7 look different, electrically they are the same.

Automatic control is very similar to semiautomatic control in that pilot sensing devices are employed to operate a magnetic contactor or starter that actually controls the motor. With automatic control, however, an operator does not have to initiate certain actions. Once the control conditions have been set, the system will continue to operate on its own. A good example of an automatic control system is the heating and cooling system found in many homes. Once the thermostat has been set to the desired temperature, the heating or cooling system operates without further attention from the home owner. The control circuit contains sensing devices that automatically shut the system down in the event of an unsafe condition such as motor overload, excessive current, no pilot light or ignition in gas heating systems, and so on.

Functions of Motor Control 

There are some basic functions that motor control systems perform. The ones listed below are by no means the only ones but are very common. These basic functions are discussed in greater detail in this text. It is important not only to understand these basic functions of a control system but also to know how control components are employed to achieve the desired circuit logic. 

Starting 

Starting the motor is one of the main purposes of a motor control circuit. There are several methods that can be employed, depending on the requirements of the circuit. The simplest method is across the-line starting. This is accomplished by connecting the motor directly to the power line. There may be situations, however, that require the motor to start at a low speed and accelerate to full speed over some period of time. This is often referred to as ramping. 

In other situations, it may be necessary to limit the amount of current or torque during starting. Some of these methods are discussed later in the text. 

Stopping 

Another function of the control system is to stop the motor. The simplest method is to disconnect the motor from the power line and permit it to coast to a stop. Some conditions, however, may re quire that the motor be stopped more quickly or that a brake hold a load when the motor is stopped. 

Jogging or Inching 

Jogging and inching are methods employed to move a motor with short jabs of power. This is generally done to move a motor or load into some desired position. The difference between jogging and inching is that jogging is accomplished by momentarily connecting the motor to full line voltage, and inching is accomplished by momentarily connecting the motor to reduced voltage. 

Speed Control 

Some control systems require variable speed. There are several ways to accomplish this. One of the most common ways is with variable frequency control for alternating-current motors or by controlling the voltage applied to the armature and fields of a direct-current motor. Another method may involve the use of a direct-current clutch. These methods are discussed in more detail later in this text. 

Motor and Circuit  Protection

One of the major functions of most control systems is to provide protection for both the circuit components and the motor. Fuses and circuit breakers are generally employed for circuit protection, and over load relays are used to protect the motor. The different types of overload relays  are discussed later.

Surge Protection 

Another concern in many control circuits is the voltage spikes or surges produced by collapsing magnetic fields when power to the coil of a relay or contactor is turned off. These collapsing magnetic fields can induce voltage spikes that are hundreds of volts (FIG. 8). These high voltage surges can damage electronic components connected to the power line. Voltage spikes are of greatest concern in control systems that employ computer-controlled devices such as programmable logic controllers and measuring instruments used to sense temperature, pressure, and so on. Coils connected to alternating current often have a metal oxide varistor (MOV) connected across the coil (FIG. 9). Metal oxide varistors are voltage-sensitive resistors. They have the ability to change their resistance value in ac cord with the amount of voltage applied to them. 

The MOV has a voltage rating greater than that of the coil it is connected across. An MOV connected across a coil intended to operate on 120 volts, for example, has a rating of about 140 volts. As long as the voltage applied to the MOV is below its voltage rating, it exhibits an extremely high amount of resistance, generally several million ohms. The cur rent flow through the MOV is called leakage current and is so small that it does not affect the operation of the circuit. 

If the voltage across the coil should become greater than the voltage rating of the MOV, the resistance of the MOV suddenly changes to a very low value, generally in the range of 2 or 3 ohms. This effectively short-circuits the coil and prevents the voltage from becoming any higher than the volt age rating of the MOV (FIG. 10). Metal oxide varistors change resistance value very quickly, generally in the range of 3 to 10 nanoseconds. When the circuit voltage drops below the voltage rating of the MOV, it returns to its high resistance value. The energy of the voltage spike is dissipated as heat by the MOV.