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Sensors and Actuators

Week 4 PE -4030 Electrical Actuators. Sensors and Actuators. Prof. Charlton S. Inao Mechatronics Defence University College of Engineering Bishoftu , Ethiopia. Topics :. Relay. A relay is an electrical switch that opens and closes under the control of another electrical circuit.

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Sensors and Actuators

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  1. Week 4 PE -4030 Electrical Actuators Sensors and Actuators Prof. Charlton S. Inao Mechatronics Defence University College of Engineering Bishoftu, Ethiopia

  2. Topics :

  3. Relay A relay is an electrical switch that opens and closes under the control of another electrical circuit.

  4. RELAYS Relays are electrically operated switches in which changing a current in one electrical circuit switches a current on or off in another circuit.. When there is a current through the solenoid of the relay, a magnetic field is produced which attracts the iron armature, moves the push rod, and so close the normally open(NO) switch contacts and opens the normally closed(NC) switch contacts.

  5. Relay

  6. Example: omron Relay

  7. Simple Relay Circuit

  8. Motor Controller A motor controller is a device or group of devices that serves to govern in some predetermined manner the performance of an electric motor include a manual or automatic means for starting and stopping the motor, selecting forward or reverse rotation, selecting and regulating the speed, regulating or limiting the torque, and protecting against overloads and faults

  9. Contactor • Contactors are used by electrical equipment that is frequently turned off and on (opening and closing the circuit), such as lights, heaters, and motors • to make and break all power supply lines running to a load • to repeatedly establish and interrupt an electrical power circuit (NEMA)

  10. CONTACTORS • contactor is an electrically controlled switch used for switching an electrical power circuit, similar to a relay except with higher current ratings. A magnetic contactor is operated electromechanically without manual intervention Can be operated remotely, without the need for putting a person in a potentially dangerous location Magnetic contactors use a small control current to open and close the circuit.

  11. Magnetic contactors are electromagnetically operated switches that provide a safe and convenient means for connecting and interrupting branch circuits.

  12. Contactors come in many forms with varying capacities and features. • Unlike a circuit breaker, a contactor is not intended to interrupt a short circuit current.. • Contactors are used to control electric motors,  • lighting,  • heating, capacitor banks, • thermal evaporators, • and other electrical loads.

  13. Magnetic Contactor Operation

  14. DOL • The simplest form of motor starter for the induction motor is the Direct On Line starter. The Direct On Line Motor Starter (DOL) consist a MCCB or Circuit Breaker, Contactor and an overload relay for protection. Electromagnetic contactor which can be opened by the thermal overload relay under fault conditions.

  15. Principle of Direct On Line Starter (DOL) • To start, the contactor is closed, applying full line voltage to the motor windings. The motor will draw a very high inrush current for a very short time, the magnetic field in the iron, and then the current will be limited to the Locked Rotor Current of the motor. The motor will develop Locked Rotor Torque and begin to accelerate towards full speed.

  16. As the motor accelerates, the current will begin to drop, but will not drop significantly until the motor is at a high speed, typically about 85% of synchronous speed. The actual starting current curve is a function of the motor design, and the terminal voltage, and is totally independent of the motor load. • The motor load will affect the time taken for the motor to accelerate to full speed and therefore the duration of the high starting current, but not the magnitude of the starting current.

  17. Provided the torque developed by the motor exceeds the load torque at all speeds during the start cycle, the motor will reach full speed. If the torque delivered by the motor is less than the torque of the load at any speed during the start cycle, the motor will stops accelerating. If the starting torque with a DOL starter is insufficient for the load, the motor must be replaced with a motor which can develop a higher starting torque.

  18. Components of DOL Starter

  19. Working Principle

  20. Direct On Line(DOL)

  21. Direct On Line(DOL)

  22. Mini Circuit Breakers(MCB)

  23. There are two arrangement of operation of miniature circuit breaker. One due to thermal effect of over current and other due to electromagnetic effect of over current. The thermal operation of miniature circuit breaker is achieved with a bimetallic strip whenever continuous over current flows through MCB, the bimetallic strip is heated and deflects by bending. This deflection of bimetallic strip releases mechanical latch.

  24. As this mechanical latch is attached with operating mechanism, it causes to open the miniature circuit breaker contacts . But during short circuit condition, sudden rising of electric current, causes electromechanical displacement of plunger associated with tripping coil or solenoid of MCB. The plunger strikes the trip lever causing immediate release of latch mechanism consequently open the circuit breaker contacts.

  25. Operating Mechanism of Miniature Circuit Breaker The Operating Mechanism of Miniature Circuit Breaker provides the means of manual opening and closing operation of miniature circuit breaker. It has three-positions “ON,” “OFF,” and “TRIPPED”. The external switching latch can be in the “TRIPPED” position, if the MCB is tripped due to over-current. When manually switch off the MCB, the switching latch will be in “OFF” position. In close condition of MCB, the switch is positioned at “ON”. By observing the positions of the switching latch one can determine the condition of MCB whether it is closed, tripped or manually switched off.

  26. The MCB has some advantages compared to  fuse.1. It automatically switches off the electrical circuit during abnormal condition of the network means in over load condition as well as faulty condition. The fuse does not sense but Miniature Circuit Breaker does it in more reliable way. MCB is much more sensitive to over current than fuse.2. Another advantage is, as the switch operating knob comes at its off position during tripping, the faulty zone of the electrical circuit can easily be identified. But in case of fuse, fuse wire should be checked by opening fuse grip or cutout from fuse base, for confirming the blow of fuse wire.3. Quick restoration of supply can not be possible in case of fuse as because fuses have to be rewirable or replaced for restoring the supply. But in the case of MCB, quick restoration is possible by just switching on operation.4. Handling MCB is more electrically safe than fuse.Because of to many advantages of MCB over fuse units, in modern low voltage electrical network, Miniature Circuit Breaker is mostly used instead of backdated fuse unit.

  27. Overload Relay(OLR)

  28. Overload (OL) Protection/Overload Relays(OLR) • Overload protection prevents an electric motor from drawing too much current, overheating, and literally “burning out • Most commonly used OL is the overload relay

  29. an overload protection device is required that does not open the circuit while the motor is starting, but opens the circuit if the motor gets overloaded and the fuses do not blow An overload relay consists of: A current sensing unit (connected in the line to the motor). A mechanism to break the circuit, either directly or indirectly.

  30. Mechanism Device • Eutectic (melting alloy) • Bimetallic • Solid State

  31. Characteristics • Many overload protection devices have a trip indicator built into the unit • Overload relays can have either a manual or an automatic reset • Overload relays also have an assigned trip class. The trip class is the maximum time in seconds at which the overload relay will trip when the carrying current is at 600% of its current rating.

  32. Electrical Actuators • AC Motors • DC Motors • Stepper Motors • Servo motors • Linear motors

  33. Motors • DC Motors 1. Series Motor 2. Shunt Motor 3. Compound • AC Motors 1. Induction Motors 2. Synchronous Motors • Servo Motors • Stepper Motors

  34. DC Motor • When current flows in a conductor it produces a magnetic field about it. • when the current-carrying conductor is within an externally generated magnetic field, the fields interact and a force is exerted on the conductor. • Therefore if a conductor lies within a magnetic field: • motion of the conductor produces an electric current • an electric current in the conductor will generate motion • The reciprocal nature of this relationship means that, for example, the DC generator above will function as a DC motor

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