Science universe: Physics articles

Cont rol Circuits TWO-WIRE CIRCUITS Control circuits are divided into two basic types: the two-wire and the three-wire. Figure 19–1 shows a simple two-wire control circuit. In this circuit, a simple switch is used to control the power applied to the coil of a motor starter. If the switch is open, there is no complete path for current flow, and the motor will not operate. If the switch is closed, power is supplied to the motor starter and it closes the M contacts, connecting the three- phase motor to the power line. THREE-WIRE CIRCUITS Three-wire control circuits are used because they are more flexible than two-wire circuits. Three-wire circuits are characterized by the fact that they are operated by a magnetic relay or motor starter. These circuits are generally controlled by one or more pilot devices. ELECTRICAL SYMBOLS FOR PILOT/CONTROL DEVICES When people first begin to learn to read, they learn a set of symbols that are used to represent different sounds. This set of ...

WOUND-ROTOR MOTOR CONTROL An automatic time-operated speed control for a wound-rotor motor is shown in Figure 19–20. In this circuit, the stator of the motor is connected to the line when M load contacts close. Resistors are connected to the rotor of the motor during starting. When the motor is first started, all resistors are connected in series with the rotor. It will be assumed that all timers have been set for a delay of 10 seconds. When the start button is pushed, a circuit is completed through M coil and TR1 coil. When M contacts close, the stator of the motor is connected to the line, and M auxiliary contact is used as a holding contact. Ten seconds after TR1 coil is energized, TR1 contact closes. This energizes S1 coil and TR2 coil. When S1 coil energizes, S1 contacts close and short out the first set of resistors. This permits the motor to increase to the second step of speed. Ten seconds after coil TR2 is energized, contact TR2 closes and energizes coils S2 and TR3 . ...

INTERLOCKING Interlocking is used to prevent some function from happening until some other function or action has occurred. A good example of interlocking is shown in the circuit in Figure 19–12. This circuit is a forward–reverse control. The two normally closed contacts labeled F and R are used to interlock the system. In this circuit, the motor is reversed by changing two of the input lines to the stator. If both motor starters F and R are energized at the same time, there would be a direct short circuit across two of the phases. Forward– reverse starters are generally interlocked mechanically as well as electrically, but this example shows only electrical interlock. It will first be assumed that the motor is to be operated in the forward direction. When the forward push button is pressed, a circuit is completed to F coil. This causes all F con- tacts to change position. The normally open F load contacts close and connect the motor to the line. The normally open auxiliary con...

STEPPING MOTORS Stepping motors are devices that convert electrical impulses into mechanical movement. Stepping motors differ from other types of dc or ac motors in that their output shaft moves through a specific angular rotation each time the motor receives a pulse. Each time a pulse is received, the motor shaft moves by a precise amount. The stepping motor allows a load to be controlled with regard to speed, distance, or position. These motors are very accurate in their control performance. Generally, less than 5% error per angle of rotation exists, and this error is not cumulative regardless of the number of rotations. Stepping motors are operated on dc power but can be used as a two-phase synchronous motor when connected to ac power. Theory of Operation Stepping motors operate on the theory that like magnetic poles repel and unlike magnetic poles attract. Consider the circuit shown in Figure 18–39. In this illustration, the rotor is a permanent magnet and the stator winding ...

CONDUCTIVE AND INDUCTIVE COMPENSATION AC motors larger than 1⁄2 horsepower (hp) are used to drive loads when a high starting torque is required. For these motors, there is excessive armature reaction under load. One method of overcoming the armature reaction is known as conductive compensation . In this method, an additional compensating winding is placed in slots cut in the pole faces. The strength of this field increases with an increase in the load current. Thus, there is a reduc- tion in the distortion of the main field flux by the armature flux. The compensating winding is connected in series with the series field winding and the armature (Figure 18–29). A motor with conductive compensation has a high starting torque and poor speed regulation. However, resistor-type speed controllers can be used to obtain a wide range of speed control. A second method of overcoming armature reaction in ac series motors uses an inductively coupled winding (Figure 18–30). This winding acts lik...