meo 4级电气口语问答
meo 4级电气口语问答
电气
船用电子技术写作
电机打捞
如果你的马达被海水淹没了，你想复活它，你会采取什么行动？
维修受海水影响/清洗的电机
起锚机安全装置：
滑动离合器。（安装在液压马达和齿轮之间）过载跳闸。
梅格：
兆欧表是一种欧姆表，以百万欧姆为单位测量绝缘电阻。 （极对极，极对地）。绝缘好电阻高；绝缘性差，电阻相对较低。实际电阻值可能更高或更低，具体取决于绝缘的温度或水分含量（电阻在温度或水分中降低）等因素。Megger 测试的目的：
验证绝缘电阻。检测任何绝缘故障。
检查 Megger 的正确程序：
用断路器关闭主配电板并取出保险丝，贴上 MEN AT WORK 的标签，断开与定子的连接，用兆欧表连接。热态电阻测量，读数准确。Megger绝缘测试仪本质上是一个内置直流发生器的高量程电阻表（欧姆表）。该仪表采用特殊结构，带有电流和电压线圈，可以直接读取真欧姆，而与实际施加的电压无关。
这种方法是无损的；也就是说，它不会导致绝缘劣化。
怎么知道岸电供电是否正确？
相序指示。（岸接线盒内，顺时针方向）操作 E/R 通风风扇并检查气流方向。
励磁损耗时：
用锤子敲击励磁电机的励磁线圈铁芯。用电池供电。
优惠行程：
如果发生发电机过载情况，其优先过载跳闸将运行以激励定时继电器。然后定时继电器按一定的顺序和一定的时间间隔断开非必要的服务。没有必要（不影响船舶操作。）5秒：
基本（正确运行船舶）5 秒：
顶部极性（推进、导航）15 秒：
静电：
通过物理接触和分离在不同材料上产生的电能出局材料负面，剩余材料正面。
自燃：
由材料本身内部产生放热化学肌动蛋白的热量引起的材料点燃，无需暴露于外部点火源。 （湿油布，湿锯末）电枢反应。
– 当直流发电机或电动机带负载运行时，由于电枢反应，碳刷和换向器之间会产生火花。电枢电流流过电枢时发生电枢反应。– 当电流流入电枢时，电枢铁芯被该电流磁化。电枢电流的磁化作用可分为两种作用：–
交叉磁化或畸变效应。
消磁或弱化作用。
为了补偿电枢反应，现代电机配备了
间极
碳刷摇杆。
中和绕组。
交流电机启动器：
电动机启动器是一种用于控制电动机启动的装置。直接在线启动器。（不带限流元件）
电阻或变阻启动器。（带限流装置）
星三角启动器。
自动变压器启动器。（降压启动55%、60%、70%，全压运行
转子电阻启动器。（启动滑环感应电机）
破折号
机械装置产生延时动作。开关装置、弧光灯、电机启动器、电磁烘烤等的操作。时间延迟根据所用油的粘度进行调整。地球探测灯
接地故障检测器由三个星形连接的白炽灯组成。这三个灯由三个单相降压变压器的次级供电。三个变压器的初级是星形连接的。初级的星点连接到船的框架。这些变压器的初级由船用电源的红相、黄相和蓝相供电。当板载系统发生接地故障时，发出视觉信号和蜂鸣器。
三个星形连接的白炽灯。这三盏灯由三个单相降压变压器的次级供电。
这三个变压器的初级是星形连接的。原色的星点连接到船的框架。这些变压器的初级由船用电源的红相、黄相和蓝相供电。
系统没有接地故障，灯会以相同的亮度发光。
当其中一相发生接地故障时，该相上的灯会变暗，另外两盏灯会以额外的亮度燃烧。
接地故障可以通过将分支断路器一一断开来追踪。当有故障的分支电路被关闭时，接地灯将恢复正常发光。
应急电源：
应急灯。导航灯。
国际通讯设备。
日光信号灯。
船的汽笛。
火灾探测报警装置。
手动火灾报警器。
其他内部紧急信号。
应急消防泵。
舵机。
导航设备和其他设备。
单相：
单相是用来表示三相电路中一相开路时出现的情况的术语。一个相中的开路，通常是由于保险丝熔断、接触不良或断线，会阻止电机启动，但运行中的电机可能会在出现故障时继续运行。
剩余电源线电流过大。
电机绕组中电流分布不均。
可以通过供电线路中的过载装置或通过过热来检测。
停转或运行的电机过热会导致过载线圈烧坏。
在负载较轻的电机中，仅监控电流的电源线上的电磁跳闸不会被检测到。放置在绕组中以测量热效应的热敏电阻可提供更好的保护。
绝缘中性系统
优势该系统避免了失去基本服务的风险，例如舵机
如果中性线接地并且一相短路导致该相中的保险丝熔断，系统现在将单相并可能烧坏电机
在绝缘中性线中，一次接地故障不会中断供电，但漏电检测系统会发出警告。
绝缘系统中的低接地故障电流可大大降低火灾风险。
缺点
在绝缘系统上，对地电压为 1.73 Vph，例如440v 与 250v
追踪接地故障更加困难，因为虽然选择性跳闸可以追踪接地电路，但电路上的实际位置可能仍然难以定位。例如，接触器螺线管或带有绝缘中性线的变压器中的谐振或间歇性故障会导致电压被放大到正常对地电压的 4 倍（250v x 4 =1000v）
注意：使用不接地的中性线不会减少电击，因为涉及到大电压。两种系统同样危险
接地中性线系统
当使用接地的中性发电系统时，接地将通过一个电阻器。电阻器应将接地故障电流限制在不大于配电盘部分最大发电机的满载电流的值，并且不小于使任何设备运行所需的最小电流的三倍反电动势
当电流流过电动机电枢的导体时，将在导体上施加力。这个力会产生一个扭矩，使电机的电枢旋转。当电枢导体在磁场中移动时，这些导体将切断磁力线并因此产生 E.M.F.该产生的 E.M.F 的方向与电机的施加电压相反。由于电机中产生的电动势与施加在电机上的电压相反，因此该电动势称为反电动势
外加电压 =电枢电阻电压降 + 反电动势
V =Ir + Eb
自动电压调节器（AVR）：
发电机上的突然负载电流浪涌（突然大幅增加）（例如由于电机启动）会导致其输出电压发生相应的变化。这是由于发电机交流绕组中的内部电压降造成的，这种效应通常被称为“电压骤降”。同样，甩负荷（以摆脱它）会在母线上产生过电压。由于波动的负载需求引起的电压变化，船上没有调节或非复合的发电机励磁系统是不现实的。需要自动电压调节（AVR）设备来快速纠正这种电压变化。碳堆调节器。
振动接触调节器。
交流发电机的输出通过作为电压参考的电磁线圈对弹簧进行变换、整流和输出效应，适用于直流励磁机的分励磁场和交流发电机的转子。
静态自动电压调节器。
通过变压器和整流器从交流发电机输出中获得的直流电被施加到一个桥上，该桥在两个臂上具有固定电阻，而在另外两个臂上具有可变电阻（齐纳二极管电压参考）。当外加电压发生变化时，由惠斯通电桥产生一个误差信号。误差信号可以被放大并用于以不同方式控制交流发电机励磁。 （晶闸管、晶体管串联）
二极管
一种电子设备，它允许电子沿一个方向流动，但对电流沿相反方向流动具有很强的抵抗力。 （同样检查管道系统。）二极管的作用是起到高频和低频振荡的整流作用。在接收电路中用作检测器。
热离子二极管由一个加热器、一个阴极和一个阳极组成，在一个间接加热的阀门中。
半导体二极管由单晶锗或硅晶片构成，两层掺杂不同； “pn”结充当整流器结。
晶体管
三层设备，NPN或PNP它有发射极、集电极和基极
使用来自侧电路的小信号功率来控制其他电路中的较大功率。 （功率增益或放大）
晶闸管
功率控制元件。 P、N结交替的多层器件。例如。可控硅整流器（SCR）~四层器件。
三端双向可控硅开关〜更大的数量。
晶闸管是固态开关，通过被称为栅电极的触发连接施加低电平信号电压来导通
没有可磨损的运动部件，也没有可能被电弧损坏的触点。
远程操作的理想选择。体积小～方便的控制电路元件。可以控制超过 1000 安培的电流和超过 1000 伏的电压。取代大型常规开关。
运行速度更快（每秒 25,000 次）。
激励器：
它是一个小型直流发电机。它为交流发电机的转子提供励磁电流。
励磁电流是产生任何磁场所需的电流。
均衡器
均衡器是并联运行的不同容量发电机之间的连接，使运行负荷在两者之间按比例分配。
直流负载在交流船舶服务系统上：
通用报警系统。
应急照明。
应急无线电电源。
为电池充电进行整流。
电机保养：
保持空气通道清洁。
去除灰尘、油和油脂的沉积物。
取绝缘电阻读数。
防止电刷和换向器产生火花。
轴承、固定螺栓联轴器的机械检查。
定期检查电机启动器和控制器，维护触点、电阻和连接。
检流计：
它是一种测量小电流大小和方向的仪器。
电阻：
它是一种既能导电又能将部分电能转化为热能的装置。
电阻率：
它是一种材料的电阻，以欧姆/单位长度表示。
变阻器
它是一个电阻器，具有易于调节其电阻的装置。
配电盘灭火
首先要断开断路器。然后用CO2手提灭火器随火扑灭。
熄火后，配电盘冷却
所有电线系统都必须进行跟踪和连续性测试。
对所有配件进行跟踪和更新后，总机可以重新投入使用。
防火：
使用正确的保险丝。
断路器和开关各触点接触良好。
所有的电线连接都很好。
起火原因：
接触松散。
保险丝不正确。
绝缘击穿
剩磁：
启动发电机时，必要的磁力由场磁极的铁芯提供。这种磁性称为剩磁。
电池安装及安全措施：
H2 需要良好的通风。出口通风管道需要合适的油漆。通风口应低于电池电量。
电池房内禁止裸灯和吸烟。
膝伤。电枢多并联电路。
波绕电枢中的两个并联电路。
激发失败：
剩磁损失。
励磁回路电阻过高。
万能电机：
系列伤口。
单相。
可以使用交流或直流。
分数（非常小）功率。 （小于 1 HP。）
用于缝纫机、手提钻等。
（当电机连接到直流线路时，电流始终是同一个方向。在交流线路中，电流每个周期都在反转。）
当发电机不同步时，尝试关闭接入者的空气断路器 (ACB) 可能会产生什么后果？
在断路器合闸的瞬间，电压相位差会在机器之间产生很大的循环电流，从而产生很大的磁力来“拉动”发电机同步。这意味着一个转子的快速加速和另一个转子的减速。大的力可能会物理损坏发电机及其原动机，并且大的循环电流可能会使每个发电机断路器跳闸。结果？停电、危险和尴尬！
同步：
同步器
同步灯：
“暗”灯法。 （2盏灯）
“亮”灯法。 （2盏灯）
‘顺序’法。（3盏灯）
不借助同步器或同步灯，如何监测同步的正确时刻？
将一对 500 V 电压表探头连接到进线机器断路器的一个相上。调整发电机速度，直到电压表从零缓慢波动到最大值。电压表过零时合闸。
同步镜针的不同位置是什么意思； 6点钟和12点钟有什么区别？
6点钟表示不同步。我们不能并行传入的生成器。 12点表示完美的同步条件（电压、频率和相序匹配）。
为什么我们在 11 点关闭开关而不是在 12 点关闭？
当同步器接近 12 点钟时，正弦波之间的“滑差”（差）接近最小值（当同步器位于 12 点钟时，滑差为零）并且相位之间的电压差最小（当同步器在 12 点钟方向）。由于关闭同步器开关需要时间，因此通常在 11 点钟完成。由于关门延迟小，通过在 11 点做，我们实现接近 12 点关门。
为什么绝缘测试用兆欧表而不是万用表？
用兆欧表，用不低于500伏直流的电压来测试绕组的绝缘电阻。用万用表，使用的电压不超过 3 伏直流电。 Megger 使用高阻抗测试，因此比万用表更准确
MSB有哪些安全措施？
使用断路器、熔断器和过电流继电器。面板是死前面板，即只有通过断路器切断面板电源才能打开面板进行维修。
什么时候逆潮流？
发电机的情况是通过开关装置为系统供电，该开关装置具有与该发电机并联的多个发电机。当系统正常运行时，电流从发电机流向开关装置。如果一台发电机出现问题并且其端电压低于系统电压，则发电机将充当电动机，就像电动机可以充当发电机一样，电流将从开关装置流向发电机。这是反向功率。如果发生故障的发电机完全机械故障，影响范围从轻微到极端。
逆流有什么危害？
假设发电机正常产生的功率等于 P，并且当作为电动机运行时，它将吸收相似的功率 P。因此，对电网的净影响将与经历了一步增加一样在负载等于 2P。根据电网的大小和剩余发电的强度，2P的步长可能会导致电网的频率发生显着变化。
在某些情况下（尤其是蒸汽轮机）可能会损坏原动机。
逆功率保护是如何给出的？
逆功率保护用于防电机。此功能用于保护原动机而非发电机。它可以切断燃料供应并停止原动机。
如何测试逆功率跳闸？
当两台发电机并联运行，一台发电机可以带载时，可以通过调速器控制负载转移来测试逆功率跳闸。当负载已经充分转移并且卸载发电机承载一小部分负载时，其断路器跳闸并且其原动机的燃料供应切断。这意味着反向功率继电器已动作。可以通过模拟（使用继电器上的测试按钮）对继电器进行测试，看它是否发出跳闸信号。
ACB的全称是什么？
空气断路器。它通常用于400V及更高电流的应用（发电机断路器）。
如果在闲置的发电机上按下 ACB Close 按钮会发生什么？
通常，在您同步之前，断路器不会合闸，因此即使您按下断路器合闸，它也不会合闸。断路器有欠压保护，不会让你合闸。
什么是欠压保护？
它可以防止断路器误合闸，或发电机在并联运行期间带载。当机械连接到配电盘时，它还可以防止电压损失。
优惠行程是什么意思？为什么提供？
优先跳闸是船上的一种电气布置，旨在在主电源部分故障或过载的情况下将非必要电路（即非必要负载）与主母线断开。它让关键负载运行（如舵机）并跳闸非必要负载（如交流电和厨房），是一项安全功能。
配电盘接地故障指示的作用是什么？
它检测并指示电路上的相接地故障。
遇到接地故障报警怎么办？
可以通过首先确定船舶上发生接地故障的可能性很高的区域（例如甲板照明或食品储藏室设备）来调查故障，然后一次关闭其电源以查看警报是否消失。当某个负载解除告警时，我们就知道该电路有接地故障，我们可以去维修它。
什么是直流电？
电流单向流动。
什么是交流电？
电流呈正负峰值的正弦双向流动。
AC中的相位是什么意思？
两个或多个彼此不同步的交流电压或电流。它们的峰值和零点不在同一时间点匹配。这是相位的意思。
DC有相位吗？
没有
为什么DC现在用的不多？
交流发电机是一种更好的技术，电压转换（通过变压器）很容易在交流而不是直流中实现。
您可能会在哪里看到它使用？
电池供电的直流电一般用于导航、控制和安全系统，主要用于导航、雷达、安全系统、天线和通信系统以及应急照明。
三相电流是什么意思？
三个电流正弦波形，幅值相等，相移120度。
什么是有效值？
均方根值是峰值除以 2 的平方根。
What is the meaning of power factor?
Power factor is the cosine of the angle between voltage and current waveform.
What is the usual value you see onboard?
Usually 0.8.
What is the best value possible?
1 is the best value possible. That is possible with a purely resistive load.
What is the benefit of improving power factor?
Power factor close to 1 means for the same real power in kW, the load current is less and hence the I2R losses are less. The system efficiency is higher as the losses are lower. Improved voltage regulation is achieved.
How can it be improved?
Power factor improvement capacitor banks are used if the loads are mainly resistive.
What is the meaning of saying I and V are in phase?
The angle between the I and V waveform is zero which means the load is purely resistive.
Why are motor ratings given in KW and that of alternator and transformer given in KVA?
kW is the output mechanical power of a motor and is expressed in kW.
kVA is the net (apparent) power input to the transformer. This input power is the output + losses.
kW =kVA x system power factor
Where does reactive power go?
Reactive power is not ‘lost’. It is delivered to the motor where it sustains the electric field that enables the motor to convert the real power (electrical) into mechanical torque.
What is a semi conductor?
Substance as germanium or silicon whose electrical conductivity is intermediate between that of a metal and an insulator; its conductivity increases with temperature and in the presence of impurities
What is doping?
Doping intentionally introduces impurities into an extremely pure (also referred to as intrinsic) semiconductor for the purpose of modulating its electrical properties. The impurities are dependent upon the type of semiconductor. Lightly and moderately doped semiconductors are referred to as extrinsic. A semiconductor doped to such high levels that it acts more like a conductor than a semiconductor is referred to as degenerate.
What happens to insulation when temperature rises?
For insulators, an increase in temperature will cause their resistance to decrease -which is why excessive temperature is often the main reason why insulation fails
What happens to resistance of conductors when temperature rises?
For conductors an increase in temperature will cause their resistance to increase.
What is an induction motor?
An induction motor is an alternating current motor in which the primary winding on one member (usually the stator) is connected to the power source and a secondary winding or a squirrel-cage secondary winding on the other member (usually the rotor) carries the induced current.
What is “slip”?
An AC (Amplitude Current) induction motor consists of two assemblies – a stator and a rotor. The interaction of currents flowing in the rotor bars and the stators’ rotating magnetic field generate a torque. In an actual operation, the rotor speed always lags the magnetic field’s speed, allowing the rotor bars to cut magnetic lines of force and produce useful torque. This speed difference is called the slip.
What is a synchronous motor?
A constant-speed motor, the speed being dependent on the frequency of the ac supply and the number of poles for which it is designed.
Where is it used?
Synchronous motors find applications in all industrial applications where constant speed is necessary. Improving the power factor as Synchronous condensers. Electrical power plants almost always use synchronous generators because it is important to keep the frequency constant at which the generator is connected. Low power applications include positioning machines, where high precision is required, and robot actuators.
How do you use a multi-meter?
What is an AVO meter?
The Avometer was a British brand of multimeter, latterly owned by Megger. It is often called simply an AVO and derives its name from the first letter of the words amperes, volts, ohms
How do you check continuity?
Set the multi-meter in resistance mode and then check with probes the resistance between the points or circuit under check. If the resistance is low or zero, it means continuity. If it is high or infinite, it means open circuit.
What limits the electrical load?
The available power is fixed and the voltage is fixed, so the setting of the breaker or fuse is the normal current of the load. If the load exceeds the rated current, then the breaker or fuse will operate and limit the load.
What is the voltage used in a meggar?
500V DC.
How does current flow during welding?
The current flows through the electrode and to the clamp. That’s how it flows in MIG, stick, flux cored. On Tig, the electricity flows through the electrode also except that now that’s the only function of it, the rod is held in your hand and electrode in the other.
What do we use for welding on ships? AC or DC?
What is the voltage?
What is the voltage used for ignition in boilers/incinerators etc.?
Normally 10,000V (10kV).
Why is it different from that for welding?
It is higher as the ignition requires break down of air gap for park to occur. The air breaks down at this high voltage of 10kV. Therefore, it is higher than normal welding voltage.
Does current flow through hull during welding?
Yes I think.
If so why don’t you get a shock?
. Because the whole hull is at the same potential, so there is no potential difference to get a shock.
What is safe voltage to prevent shock?
Normally 50V. It depends on the body resistance and how conducting the path is (example use of insulating boots or mat may allow slightly higher voltage to be safe)
What is the meaning of earthing in a ship?
What is the meaning of Star winding/ Delta winding?
Refer Dennis T Hall
Is the ship’s alternator Star wound or Delta wound?
Refer Dennis T Hall
What is the meaning of ACB, MCCB, NFB?
Air circuit breaker for 400V higher current ratings.
Moulded case circuit breaker for 400V medium current ratings.
No Fuse Breaker – 75 A capacity
What is a magnetic contractor? Where is it used?
A contractor is an electrically controlled switch used for switching a power circuit, similar to a relay except with higher current ratings.
Contractors are used to control electric motors, lighting, heating, capacitor banks, and other electrical loads.
What is a relay? How is it different from a magnetic contractor?
Contractor is a heavy duty switch whereas the relay is light duty switch. Relays can be as simple as 12 V DC solenoids with 12 V, 0.5 A rated contacts. They are used to detect faults on the power system and signal a local or remote switching device (breaker) to trip the circuit.
Can you draw a DOL starter circuit?
Refer to DOL diagram in Dennis T Hall
What is the meaning of overload?
An overload is a current over and above the normal load current (over the load).
In other words greater than the original design current. This can be a momentary overload such as the starting current for a motor which is usually ignored or it can be a sustained overload such as plugging in too many appliances at once or a motor that has faulty bearings or gets jammed
What is meaning of short circuit?
A short circuit occurs when conductors are connected (shorted) together by a fault. This is normally phase to phase fault due to failure of insulation by something cutting the wires, fires, etc.
How is protection provided for OL and SC?
Damage from short circuits and overloads can be reduced or prevented by employing fuses, circuit breakers, or other overload protection, which disconnect the power in reaction to excessive current. The tripping time is much less (in milli seconds) for short circuit due to very high currents but can higher (a few seconds) due to currents say 1.1 to 1.5 times the normal current.
What is the function of a fuse?
Fuse places a limit on the amount of current that can be drawn by an electric circuit by opening (blowing or melting) when the current exceeds a preset limit. This protects the circuit and the surroundings from fire or damage in the case of an overload or short circuit.
What is the normal setting of overload relays?
Normally 90-100% of rated normal current. Sometimes can be set to 105% of rated current.
How does a thermal overload relay work?
The bi-metallic strip in the relay heats up by the over load current and the movement in the bi-metallic strip causes the breaker to trip. The breaker has to be reset (turn off) before turning on again. Sometimes it takes a time delay for the bi-metallic strip to cool down after which the breaker can be reset and switched on.
What is single phasing?
Single phasing is a condition in three phase motors and transformers wherein the supply to one of the phases is cut off.
How can it happen?
Single phasing is caused by the use of single-phase protection devices such as fuses. Three phase loads should be protected by devices which cause the interruption of power to all three phases simultaneously when a fault occurs. Defective contacts or loose connections in three phase breakers can also cause single phasing.
What is the harm caused by this?
Single phasing causes higher than rated currents in the healthy phases of loads such as motors causing over heating of the motor and consequently motor failure.
Single phasing can sometimes cause excessive noise and vibration in motors.
What protection is provided against this?
Single phasing can be identified by special protective relays which can identify and isolate the connected loads. Smaller motors rely on over current and negative phase sequence relays. Motor protection relays for larger motors come readily fitted with protection against single phasing.
Why is it necessary that incoming alternator frequency is more than bus bar?
Prior to paralleling alternators together, the frequency of the incoming alternator is made slightly higher. This is to ensure that the incoming generator will take some of the bus load. If the incoming generator is slow it will become motorized and will add load onto the bus. If this happens it is possible to cause other generators to trip off line.
What is dark lamp and bright lamp method for synchronizing? How is the connection made?
Formerly, three light bulbs were connected between the generator terminals and the system terminals. As the generator speed changes, the lights will rise and fall in intensity at a rate proportional to the difference between generator frequency and system frequency. When the voltage at the generator is opposite to the system voltage (either ahead or behind in phase), the lamps will be bright. When the voltage at the generator matches the system voltage, the lights will be dark. At that instant, the circuit breaker connecting the generator to the system may be closed and the generator will then stay in synchronism with the system.
Apart from renewing bearings is there any reason for opening up motors for maintenance?
Insulation checks and renewal (example using insulation spray on stator winding insulation).
Why is this reverse power used instead of reverse current in alternators?
It is extremely difficult to detect reverse current with an alternating current system, reverse power can be detected and protection can be provided by reverse power relay.
What is the meaning of excitation in an alternator?
An electric generator or electric motor consists of a rotor spinning in a magnetic field. The magnetic field may be produced by permanent magnets or by field coils. In the case of a machine with field coils, a current must flow in the coils to generate the field, otherwise no power is transferred to or from the rotor. The process of generating a magnetic field by means of an electric current is called excitation.
Is the field current in an alternator AC or DC ?
DC. To confirm check Dennis T Hall.
Is the field rotating or stationary?
Rotating. To confirm check Dennis T Hall.
How is it supplied?
The rotor’s magnetic field is supplied by a rotor winding energized with direct current through slip rings and brushes.
What is a brush less alternator?
A brushless alternator is composed of two sections:main alternator and the smaller exciter. The exciter has stationary field coils and a rotating armature (power coils). The main alternator uses the opposite configuration with a rotating field and stationary armature. A bridge rectifier, called the rotating rectifier assembly, is mounted on a plate attached to the rotor. Neither brushes nor slip rings are used, which reduces the number of wearing parts. The main alternator has a rotating field as described above and a stationary armature (power generation windings).
How is the excitation achieved in this?
Varying the amount of current through the stationary exciter field coils varies the 3-phase output from the exciter. This output is rectified by a rotating rectifier assembly, mounted on the rotor, and the resultant DC supplies the rotating field of the main alternator and hence alternator output. The result of all this is that a small DC exciter current indirectly controls the output of the main alternator.
What is the meaning of residual magnetism?
Residual magnetism is a property in which certain amount of excitation remains back in the conductor even after the removal of the magnets.
Why is an air compressor started unloaded?
The air pressure inside the compressor cylinder offers resistance to the movement of the piston. Hence started unloaded. Ask 4/E or 2/E in ship.
How is the speed of a 3-phase induction motor varied?
The speed of a normal 3-phase induction motor is a function of the frequency of the supply voltage. Changing the speed of such a motor hence requires building a 3-phase power frequency convertor. This can be realised by using power MOSFETs (or IGBTs) capable of handling high voltages and fast switching speeds.
How is the direction of rotation varied?
By changing the phase sequence (from say R-Y-B to R-B-Y)
How does this cause a rotation to change?
Due to the change in phase sequence, the rotating magnetic field changes direction and the rotor rotates in the opposite direction.
How is the speed of a DC motor varied?
To vary the speed of a DC motor we need a constant voltage power supply with a series POT resistor of higher watt rating. A Pulse Width Modulator (PWM) can be used.
How is the direction varied?
A general purpose DC motor can be reversed by changing the polarity of either the armature or the field but not both.
What is a diode?
In electronics, a diode is a two-terminal electronic component with asymmetric transfer characteristic, with low (ideally zero) resistance to current flow in one direction, and high (ideally infinite) resistance in the other. A semiconductor diode, the most common type today, is acrystalline piece of semiconductor material with a p-n junction connected to two electrical terminals
What is a zener diode?
A zener diode is a special kind of diode which allows current to flow in the forward direction in the same manner as an ideal diode, but will also permit it to flow in the reverse direction when the voltage is above a certain value known as the breakdown voltage, “zener knee voltage” or “zener voltage.”
What is a thyristor? What is an SCR?
A thyristor is a solid-state semiconductor device with four layers of alternating N and P-type material. They act as bistable switches, conducting when their gate receives a current trigger, and continue to conduct while they are forward biased (that is, while the voltage across the device is not reversed).
A silicon-controlled rectifier (or semiconductor-controlled rectifier) is a four-layer solid state device that controls current. It is a type of thyristor.
How does an inverter work?
An inverter converts DC supply into AC by use of electronic devices such as diodes and thyristors.
How does a tube light work?
It is well explained in Dennis T Hall
When voltage is applied to the fluorescent lamp, here’s what happens:
The starter (which is simply a timed switch) allows current to flow through the filaments at the ends of the tube.
The current causes the starter’s contacts to heat up and open, thus interrupting the flow of current. The tube lights.
Since the lighted fluorescent tube has a low resistance, the ballast now serves as a current limiter.
When you turn on a fluorescent tube, the starter is a closed switch. The filaments at the ends of the tube are heated by electricity, and they create a cloud of electrons inside the tube. The fluorescent starter is a time-delay switch that opens after a second or two. When it opens, the voltage across the tube allows a stream of electrons to flow across the tube and ionize the mercury vapor.
Without the starter, a steady stream of electrons is never created between the two filaments, and the lamp flickers. Without the ballast, the arc is a short circuit between the filaments, and this short circuit contains a lot of current. The current either vaporizes the filaments or causes the bulb to explode.
What is the function of the choke?
The choke has two main functions. In conjunction with the starterit causes the tube to ignite by using the back emf to create a plasma in the tube and it controls the current through the tube when it is ignited.
Where is a slip ring used and where is a commutator used?
Slip rings are commonly found in slip ring motors, electrical generators for alternating current systems and alternators. They can be used on any rotating object to transfer power.
A commutator is a common feature of direct current rotating machines. By reversing the current direction in the moving coil of a motor’s armature, a steady rotating force (torque) is produced.
What is capacitance?
Capacitance is the ability of a body to store an electrical charge. Any body or structure that is capable of being charged, either with static electricity or by an electric current exhibits capacitance. C =Q/V where Q is the charge and V is the voltage
Does current flow though a capacitor?
No. Current (or more specifically, charge) flows into one plate, and an opposing current (charge) flows out of the other plate, but the current (except for leakage current) does not flow across the dielectric. The result is that there is a charge differential between the plates.
What is the unit for capacitance ?
Farad
What is a neutral?
In multiphase circuits, the conductor used to carry unbalanced current. In single-phase systems, the conductor used for a return current path.
What is the difference between neutral and earth?
The ground or “earth” wire is a circuit’s safety protective wire that normally carries no current.
What is a synchronous condenser?
A synchronous condenser is a device identical to a synchronous motor, whose shaft is not connected to anything but spins freely. Its purpose is not to convert electric power to mechanical power or vice versa, but to adjust conditions on the electric power transmission grid. Its field is controlled by a voltage regulator to either generate or absorb reactive power as needed to adjust the grid’s voltage, or to improve power factor.
Where and why is it used?
Used for power factor correction as a synchronous capacitor.
If a motor is wound star and you want to change to delta what should you do?
Open the terminal connection box. Remove the shorting link connecting the common connection (star) point of A2, B2 and C2. With the 3 phase windings marked A1A2, B1B2, C1C2, connect A2 to B1, B2 to C1 and C2 to A1.
If the AVR, is defective, when additional loads come on will the generator be able to take it?
No. and refer to Dennis T Hall.
How do you test reverse power trip, high current trip, preferential trip?
Reverse power trip – refer answer for question 11..
High current (over load trip) – Set your overload relay high current trip setting to say 75% of the rated current. Run the motor. The relay will trip the breaker. Another way of testing it is pushing the trip button on the relay which trips the breaker.
Preferential trip – With one generator running, try to switch on additional loads to overload the generator. Preferential trip will trip all the non-critical loads..
What is residual magnetism? Where is it important?
Residual magnetism is a property in which certain amount of excitation remains back in the conductor even after the removal of the magnets.
It is required as we need some magnetism to start off the generation before the field winding has power to produce the full field.
Sometimes, when you change the running direction of a E/R blower (from supply to exhaust) it trips. Why?
If the flaps of the blower are not set or opened properly or the filters are clogged, the air supply is affected and the blower may be overloaded causing it to trip. Ask E/O for more information.
What is the function of the friction clutch mechanism used in purifiers?
It is a mechanical question.
What is the regular maintenance done on batteries?
Every week the cell voltage is checked. The specific gravity is monitored using the battery hydrometer. The terminals are lubricated to prevent corrosion. Sometimes load tests are done on the batteries.
What safety precautions need to be taken during maintenance on batteries?
The room needs to be ventilated to prevent explosions due to gases generate by the battery. Protective clothing, gloves and eye protection should be used to prevent acid splash over. No flames or ignition sources should be brought near batteries. Never short both positive and negative terminals as this will cause short circuit and high energy flashover. When disconnecting batteries, always disconnect negative terminals first and vice versa when reconnecting.
Why is special lighting used in battery rooms?
Spark proof lighting is used as explosive gases may be generated during battery charging and so there should be no sparks in the room.
What is the voltage available from each cell in a lead acid battery?
2 V DC.
What is the full battery voltage ?
24 V DC.
How is this voltage achieved?
By connecting 12 cells in series. 12 x 2 V =24 V is total battery bank voltage.
What is the meaning of a normally open contact?
The contact is open in the normal (de-energised condition).
How does a Salinometer function?
It detects the salinity in parts per million (ppm) and provides a alarm signal and also initiates a command to the bypass valve of a fresh water generator. Ask E/O and look in your ship Fresh water generator manual for more details.
How does the air compressor start and stop automatically?
The air reservoir has pressure switch which is set for cut-off and cut-in depending on the pressure desired. When the pressure drops below the cut-in setting, the switch energises the contactor coil and the compressor starts. When the pressure reaches the cut-off setting, the switch opens its closed contact in series with the power circuit to contactor and cuts supply to the contactor.
How is the cold room temperature maintained?
The cold room has temperature switch (thermostat) which has a cut-in and cut-off temperature setting. Depending on the setting, the compressor cuts when the temperature rises and cuts-off when the temperature cools down to the set value.
What is a self-monitoring alarm circuit?
It self monitors the health of the alarm circuit. That is, it senses whether the power supply to the alarm circuit is healthy and all the relays and contacts are functioning normally. Check with E/O for more details.
Out at sea, if there is a black out during your watch, what action will you take?
After a black out the emergency generator comes on; On restoring the main supply we are closing the circuit breaker of the main generator without bothering about synchronizing. How is this possible?
There is a power link connection breaker between the main generator and the emergency generator. During black out, this breaker trips. The emergency generator starts and its breaker closes and feeds only the emergency loads. During restoration, the main generator is started again and its breaker is closed. As the link breaker is still open, there is no paralleling between main and emergency generator.
How does the emergency generator start automatically?
It is activated by a under voltage relay. When there is blackout, the under voltage relay senses loss of voltage and starts up the emergency generator. Similarly when the power is restored, the relay stops the emergency generator.
If all your air bottles are at low pressure and there is a blackout how do you start the generators?
If main air bottles are empty, the emergency air bottle can be used to start the main generator. If even the emergency air bottle is empty, then it can be filled by running emergency air compressor (powered by emergency generator). Once the emergency air bottle is filled, this air can be used to start the main generator.
What is a shaft generator?
A shaft generator is coupled to the main engine. It uses the main engine as its prime mover and has a frequency converter (thyristor controlled) that converts the variable engine speed to near constant speed and produces electrical power. It can only be employed at sea speed (full speed) and not at manoeuvring speed.
Even though the main engine runs at varying speeds, how does the alternator maintain constant frequency?
By use of thyristor controlled frequency (rpm) converters.
Is the alternator connected star or delta
Look up in Dennis T Hall. Normally star.
Why is 440 V used for motors and 110/220 V used for lighting?
Motor load currents are large. Motors are 3-phase loads. A higher 440V voltage means lesser current for the same power and hence losses and size of cable wires is lower. Lighting is single phase loads and its load currents are small, hence lower voltage means less insulation in the cable wires.
For a given line voltage, four heating coils will produce maximum heat when connected in-
Parallel
The unit of absolute permittivity of a medium is-
Farad/ metre
Explain 3 phase motor working principle ?
How emergency generator comes on load after blackout ?
Explain induction &synchronus , difference between the two construction and operational ?
Explain testing of emergency generator ?
Induction motor working principle ?
How static charge is produced ?
Emergency steering procedure ?
Explain 3 phase motor working principle ?
What is slip ring motor ? Where it is used ?
Emergency generator where it supplies ?
What is the significance of Air gap ?
Transformer in your ship , what is its type and working ?
Parts of brushless alternator ?
What are the advantages of earthed neutral system ?
Explain parallel operation of alternator ?
What are the types of fuses ?
Parts of brushless alternator ?
What are the advantages of earthed neutral system ?
what is AVR ? and its function , how does it works ?
Explain parallel operation of alternator ?
Difference between ME/MC ?
What are the types of earthing system ?
What is mutual induction and which machine works on mutual induction ?
What are the voltages in your ship ?
Advantages of earthed neutral system ?
What is mutual induction and which machine works on mutual induction ?
what is AVR ? and its function , how does it works ?
Parts of brushless alternator ?
What are the advantages of earthed neutral system ?
Explain parallel operation of alternator ?
What are the types of earthing system ?
What place you take insulation test in motor ?
What is avr function ?
What is steering gear electrical safety in dudes and cables ?
What is essential load ?
What is the value of motor insulation ?
Explain AE preferential trip?
Explain Pv breaker and pv valve ?
Explain Earth fault ? Action
Explain Generator mounting and connection ?
Explain Power calculation ?
What is Rms value ?
What is insulation resistance ?
Explain Power calculation ?
What is insulation resistance ?
What are MSB safeties ?
What is kw and kva
Explain Earth fault ?行动
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