电气控制与PLC答案.doc

习题答案项目一Problem sets, and there is1. A: (1) used for lighting or heating load, the load switch is equal to or greater than the rated current is the sum of the rated load current control circuit.(2) used in the motor load, the rated current of load switch open general 3 times of the rated current of the motor;Enclosed rated current of load switch are generally 1.5 times the rated current of the motor.2. A: fuse is mainly composed of melt, the installation of the melt fusion tube and fusion of three parts.When the protection circuit of the current more than specified value, and after a certain period of time, heat fusing melt produced by melt themselves, make the circuit disconnect, which have the effect of protection.Fusion tube is used to melt was laid.3. A: (1) according to the use of occasions, and type of option buttons.(2), according to the requirement of the job status indication and situation to choose the color of buttons and indicator lights.(3) according to the need of control circuit, determine the button contact form and contact number of groups.(4) button is used for high temperature, easy to loosen plastic deformation caused by aging, and cause the connection between the screw touching short circuit, can add in the binding screw set of insulation plastic pipe to prevent short circuit.5 buttons with pilot light indication for bulb fever, long-term use of easy to make plastic deformation lampshade, lamp voltage shall be reduced, prolong service life.4. A: ac contactor with electromagnetic mechanism, contact and arc extinguishing system, etc.5. A: intermediate relay: used in relay protection and automatic control system, in order to increase the number of contact and capacity.It is used for intermediate signals in the control circuit.Of the structure and principle of the intermediate relay and ac contactor basic same, with the contactors main difference is that: by large current contactor of the main contact, and intermediate relay contact only by small current.So, it can only be used in control circuit.Generally, it is not the main contact, because overload capacity is small.So it with all of them are auxiliary contact, more quantity.6. A: thermal relay is using the current heating principle, protection appliances to work can be used as a motor overload for a long time, and phase current is not balanced protection operation, but it is due to the thermal inertia, dont make the circuit disconnect immediately when short circuit, it cant as a short circuit protection.7. A: the graphic symbol is as follows:熔断器FU， 复合按钮通电延时型时间继电器：接触器：中间继电器：8. 答：组合开关：额定电压380V；控制电动机则额定电流 IN (1.52.5)IDN，故可取25A或40A。接触器：额定电压380V；接触器的额定电流应大于或等于所控制电路的额定电流。对于电动机负载可按经验公式计算：故额定电流可选20A，吸引线圈工作电压220V或380V。熔断器：额定电压380V；额定电流25A或32A，保护一台异步电动机时，考虑电动机冲击电流的影响，熔体的额定电流可按下式计算IfN(1.52.5)IN，故熔体额定电流为25A热继电器：额定电压380V；额定电流20A，热元件额定电流一般可按下式，故热元件额定电流为16A项目二习题及思考题1. 答：一般可用起动按钮和接触器的常开触点并联,然后再和接触器的线圈串联可得。当按下起动按钮后，可实现长动，它又具有失压保护功能。2.答：将两个接触器的常闭触点分别串联到对方的线圈回路中，即可得到互锁。它用来防止两个接触器同时得电。3.答：热继电器是利用电流的热效应原理来切断电路的保护电器，主要用于电动机或其他负载的过载保护。电机过载导致热继电器动作后，热继电器由于热惯性，其双金属片弯曲变形还没复位，仍处于断开状态，这时马上按下启动按钮，不能让控制电机启动的接触器线圈通电，不能实现开车。4.答：5.答： 6.答：7.答：8：答9. A: as you can see fuse protection characteristic curve, the greater the current through the melt, fusing the shorter the time required to, when circuit overload is not very serious, fuse fusing takes a long time didnt even fusing;The circuit and short circuit or serious overload occurs, it can melt quickly fuse automatically, so the fuse is a kind of short circuit protection components, not for overload protection.Thermal relay is using the current heating principle, protection appliances to work can be used as a motor overload for a long time, and phase current is not balanced protection operation, but it is due to the thermal inertia, dont make the circuit disconnect immediately when short circuit, it cant as a short circuit protection.From the above analysis of fuse and the thermal relay protection function is different, they cannot replace each other, when the electric main circuit no other short circuit protection components must be installed at the same time fuse and the thermal relay.10. A: undervoltage circuit voltage which there is no equipment rated voltage, voltage zero voltage protection refers to when a power outage or severe falls, the contactor coil electricity, under-voltage available under voltage relay to protect, and available contactor to protect pressure loss.When the equipment normal work, under voltage relay armature and core and, normally open contact closure, when the voltage is lower than a certain value, the armature is separated from core, normally open contacts, to cut off the circuit, protection equipment.When the circuit loss of pressure, contactor and armature iron core, self-locking contact reset, realize the protection, the pressure loss have to press the start button to start the equipment.Project threeProblem sets, and there is3. A: relay contactor is connected by wires have rules to reflect the logical relationships between components, and programmable controller is programmed to express the logical relationships between components.For relay contactor, ignore magnetic lag and mechanical lag, all of the same relay contact action is power and its coil electricity or colleagues.But in the PLC, due to the time-sharing scan execution instructions, work by the coil in the same device and its various colleagues contact action does not occur.This is relay contactor system work in parallel mode and PLC serial works of distinction.4. A: mainly include CPU, memory, input and output unit, power supply and programmer.The role of each part are as follows:CPU: under the control of the system program, complete logic, mathematics, internal parts tasks such as coordinate system.Storage: for data storage system program, the user program and operation.I/O modules: used to accept various production process parameters and after send controller of PLC control information.Power supply: provide power for the working unitProgrammer: used for programming, debugging and monitoring7. A: the working process of the PLC by public processing stage, the input sampling stage, program execution stage and output refresh stage.Program execution when the first user instructions from the user program storage area, without interruption or jump control, according to the memory address, way of increasing order to scan the user program, a detailed and order, until the end of the program, to complete a scan cycle.And then execute a program from scratch, and repeat the cycle.8. A: the working process of the PLC internal processing stage, communication processing stage, the input sampling stage, program execution stage and output refresh stage.The five stages in a running cycle execute sequentially, in the input sampling stage, PLC in order to collect all information input terminal, and the collected image register input information in input, and then entered the stage of program execution.The next input sample to be executed until the next cycle, therefore, after entering the program execution stage, during the period the state of the input image register no change this week.9. A: also known as the system response time, input and output the time lag is refers to the external input signal of PLC moment there is a change to the external output signal changes related to its control the time interval between the time check.It