– The force between the core and the armature is weak because of the distance between the two materials. The farther are the two materials, the weaker is the force. – The magnetic field created by the current passing through the coil must be strong enough to pull the armature down. • When the relay has pulled in , the “com” terminal connection is switched from the “N.C.” terminal to the “N.O.” terminal. • When the relay has pulled in , the current required to keep the armature down is lower than the pick-up current because the relatively large air gap and distance between the core and the armature metal have been minimized.
Relays and Contactors • When the relay has pulled in , its coil current must drop below a certain lower value (called hold-in current or drop-out current ) to cause the relay armature to return to normal. • When the relay has returned to its normal position , the “com” terminal connection is switched from the “N.O.” terminal to the “N.C. terminal. • Relays and contactors naturally provide a differential gap for on-off control because of the hysteresis effect described in the preceding slide..
Relays and Contactors • Below is a diagram showing how a relay is used in an electric heating process . On-off Controller Setting Comparer - + CA Contactor coil Ac signal source CA Temperature Measuring Device Resistive Heating Element Temperature Measuring Device Resistive Heating Elements Three phase high voltage source CA CA CA Relay
Relays and Contactors • The upper portion of the figure in the preceding slide shows a single phase heating element whose power is controlled through a relay . • When the temperature becomes lower than a certain value , the controller produces current at the relay’s coil, thus picking up the relay , and current from the ac signal source flows through the heater (heater is turned on). • When the temperature rises to a certain value , the controller removes the current from the coil of the relay, thus removing the current flowing from the ac signal source (heater is turned off). • The figure at the bottom part of the preceding slide shows a three phase heating system . Power is introduced or removed through the relay as controlled by the controller.
Thyristors • Thyristors are used when the manipulated variable is electric current whose delivery must be continuously variable, because relays and contactors cannot be used for such systems. • Thyristors can also be used in on-off control systems instead of relays and contactors. • The conduction angle of thyristors can be varied in proportion to the error between the controlled variable and the set point, thus they can be used in proportional, proportional plus integral, proportional plus derivative, and proportional plus integral plus derivative controls.
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- Amplifier, Magnetic Field, Electric motor, ac voltage, AC Servo Motor