meo 클래스 4 전기 구두 질문 및 답변

퓨즈가 끊어지거나 접촉 불량 또는 와이어가 끊어지는 등 위상의 개방 회로로 인해 모터가 시동되지 않지만 작동 중인 모터는 오류와 함께 계속 작동할 수 있습니다.

나머지 전원 케이블에 과도한 전류가 흐릅니다.

모터 권선에 전류가 불균등하게 분배됩니다.

공급 라인의 과부하 장치 또는 과열을 통해 감지할 수 있습니다.

정지되거나 작동 중인 모터의 과열로 과부하 코일이 소손될 수 있습니다.

경부하 모터에서 전류만 모니터링하는 공급 라인의 전자기 트립에 의해 감지되지 않은 상태로 유지됩니다. 열 효과를 측정하기 위해 권선에 배치된 서미스터에 의해 보호 기능이 향상됩니다.

절연 중립 시스템

이 시스템은 필수 서비스의 손실 위험을 방지합니다. 스티어링 기어

중성선이 접지되고 한 위상의 단락으로 인해 해당 위상의 퓨즈가 끊어지면 시스템은 이제 단일 위상이 되어 모터가 소손될 수 있습니다.

절연된 중성선에서 하나의 접지 오류는 전원을 차단하지 않지만 누전 감지 시스템은 경고를 제공합니다.

절연 시스템의 낮은 지락 전류는 화재 위험이 훨씬 적습니다.

단점

절연 시스템에서 접지 전압은 1.73Vph입니다. 440v 대 250v

선택적인 트리핑이 접지된 회로를 추적할 수 있지만 회로의 실제 위치를 찾기가 여전히 어려울 수 있기 때문에 지락을 추적하는 것은 더 어렵습니다. 절연 중성선이 있는 접촉기 솔레노이드 또는 변압기의 공진 또는 간헐적 결함으로 인해 전압이 접지에 대한 정상 전압의 4배(250v x 4 =1000v)로 확대될 수 있습니다.

참고:큰 전압이 관련되므로 접지되지 않은 중성선을 사용해도 감전이 감소되지 않습니다. 두 시스템 모두 똑같이 위험합니다.

접지된 중립 시스템

백 E.M.F

전기자 도체가 자기장을 가로질러 이동할 때 이 도체는 자력선을 절단하여 E.M.F.를 생성합니다. 이 발생된 E.M.F의 방향은 모터의 인가 전압과 반대입니다. 모터에서 발생하는 E.M.F는 모터에 인가되는 전압과 반대이므로 이 E.M.F를 Back E.M.F라고 합니다.

인가 전압 =전기자 저항에 대한 전압 강하 + Back E.M.F

V =Ir + Eb

자동 전압 조정기(AVR):

카본 파일 레귤레이터.

진동 접점 조절기.

교류기 출력은 전압 기준이 되는 스프링에 대한 자기 코일을 통해 변환, 정류 및 출력 효과가 있으며 직류 여자기 션트 필드와 교류기의 회 전자에 적용됩니다.

정적 자동 전압 조정기.

변압기 및 정류기를 통해 출력되는 교류 발전기에서 파생된 직류는 2개의 암에 고정 저항이 있고 다른 2개에 가변 저항(제너 다이오드 전압 기준)이 있는 브리지에 인가됩니다. 인가 전압이 변경되면 Whetstone 브리지에 의해 오류 신호가 생성됩니다. 오류 신호는 증폭되어 다양한 방식으로 교류 발전기 여기를 제어하는 ​​데 사용할 수 있습니다. (사이리스터, 트랜지스터 직렬)

다이오드

다이오드의 목적은 고주파 및 저주파 발진의 정류기 역할을 하는 것입니다. 수신기 회로에서 검출기로 사용됩니다.

열이온 다이오드는 간접 가열 밸브에 히터, 캐소드 및 애노드로 구성됩니다.

반도체 다이오드는 단결정 게르마늄 또는 2개의 레이어가 다르게 도핑된 실리콘 웨이퍼로 구성됩니다. 정류기 접합 역할을 하는 'pn' 접합.

트랜지스터

이미터, 컬렉터 및 베이스가 있습니다.

다른 회로에서 더 큰 전력을 제어하는 ​​측면 회로에서 작은 신호 전력을 사용합니다. (전력 이득 또는 증폭)

사이리스터

예를 들어 실리콘 제어 정류기(SCR) ~ 4층 장치.

트라이액 ~ 더 많은 숫자입니다.

사이리스터는 게이트 전극으로 알려진 트리거 연결을 통해 낮은 수준의 신호 전압을 인가하면 켜지는 솔리드 스테이트 스위치입니다.

마모될 움직이는 부품이나 아크로 인해 손상될 수 있는 접점이 없습니다.

원격 작동에 이상적입니다. 소형~제어회로의 편리한 부품. 1000암페어 이상의 전류와 1000볼트 이상의 전압을 제어할 수 있습니다. 대형 기존 스위치를 교체하십시오.

더 빠른 속도(초당 25,000회)를 작동합니다.

익사이터:

소형 DC 발전기입니다. 교류 발전기의 회 전자에 여자 전류를 공급합니다.

여자 전류는 자기장을 생성하는 데 필요한 전류입니다.

이퀄라이저

이퀄라이저는 서로 다른 용량의 발전기를 병렬로 연결하여 작동 부하가 둘 사이에 비례적으로 분배되도록 합니다.

A.C 선박 서비스 시스템의 D.C 부하:

일반 경보 시스템.

비상 조명.

비상 무선 전원.

배터리 충전을 위해 수정 중입니다.

모터 유지보수:

공기 통로를 깨끗하게 유지하십시오.

먼지, 기름 및 그리스 침전물을 제거하십시오.

절연 저항을 읽습니다.

브러시와 정류자에서 스파크를 방지합니다.

베어링의 기계적 점검, 고정 볼트 커플링

모터 스타터 및 컨트롤러의 정기 점검, 접점 유지, 저항 및 연결.

검류계:

작은 전류의 크기와 방향을 측정하는 기구입니다.

저항기:

전기를 전도하지만 전기 에너지의 일부를 열로 변환하는 장치입니다.

저항:

재료의 저항으로 옴/단위 길이로 표시됩니다.

가변저항

저항을 쉽게 조정할 수 있는 수단이 제공되는 저항입니다.

배전반의 소방

먼저 차단기를 탈거합니다. 그런 다음 CO2 휴대용 소화기로 불을 끕니다.

화재 진압 후 배전반 냉각

모든 배선 시스템을 추적하고 연속성 테스트를 해야 합니다.

모든 부속품을 추적하고 갱신한 후 배전반을 다시 사용할 수 있습니다.

화재 예방:

올바른 퓨즈를 사용하십시오.

차단기 및 스위치의 모든 접점이 잘 접촉됩니다.

모든 전선 연결이 잘 되어 있습니다.

화재 원인:

접촉이 느슨합니다.

잘못된 퓨즈입니다.

절연파괴

잔류 자기:

발전기를 시동할 때 자기장 극의 철심에 의해 필요한 자기가 제공됩니다. 이 자기를 잔류 자기라고 합니다.

배터리 설치 및 안전 조치:

관련된 H2에 대해 환기가 잘 되어야 합니다. 배출구 덕트에 적합한 페인트가 필요합니다. 환기구는 배터리 수준보다 낮아야 합니다.

배터리실 내에서는 적나라한 조명과 흡연을 금지합니다.

무릎 상처. 전기자의 다중 병렬 회로.

전기자에 두 개의 병렬 회로가 감겨 있습니다.

흥분 실패:

잔류 자기 손실.

필드 회로의 저항이 너무 높습니다.

범용 모터:

시리즈 상처.

단상.

A.C 또는 D.C를 사용할 수 있습니다.

분수(매우 작은) 거듭제곱. (1 HP 미만.)

재봉틀, 휴대용 드릴 등에 사용

(모터가 D.C 라인에 연결되면 전류는 항상 같은 방향으로 흐르고 A.C 라인에서는 매 사이클마다 전류가 역전됩니다.)

발전기가 동기화되지 않은 상태에서 소득자의 기류 차단기(ACB)를 닫으려고 시도하면 어떤 결과가 나올 수 있습니까?

차단기를 닫는 순간 전압 위상 차이는 발전기를 동기화하기 위해 큰 자기력을 생성하는 기계 사이에 큰 순환 전류를 발생시킵니다. 이것은 하나의 로터의 빠른 가속과 다른 로터의 감속을 의미합니다. 큰 힘은 발전기와 그 원동기를 물리적으로 손상시킬 수 있으며 큰 순환 전류는 각 발전기 차단기를 트립할 수 있습니다. 결과? 정전, 위험 및 당혹감!

동기화 중:

싱크로스코프

동기화 램프:

'다크' 램프 방식. (램프 2개)

'밝은' 램프 방식. (램프 2개)

'시퀀스' 방식.(램프 3개)

싱크로스코프 또는 동기화 램프의 도움 없이 동기화를 위한 정확한 순간을 어떻게 모니터링할 수 있습니까?

들어오는 기계 회로 차단기의 한 단계에 걸쳐 500V 전압계 프로브 쌍을 연결합니다. 전압계가 0에서 최대로 천천히 변동할 때까지 발전기 속도를 조정합니다. 전압계가 0을 통과하면 차단기를 닫습니다.

싱크로 스코프 바늘의 다른 위치는 무엇을 의미합니까? 6시와 12시 사이의 차이점은 무엇인가요?

6시는 동기화되지 않음을 의미합니다. 들어오는 발전기를 병렬로 연결할 수 없습니다. 12시 방향은 완벽한 동기화 상태(전압, 주파수 및 위상 시퀀스 일치)를 의미합니다.

스위치를 12시가 아니라 11시에 닫는 이유는 무엇입니까?

싱크로스코프가 12시에 접근할 때 사인파 사이의 "슬립"(차동)은 최소에 접근하고(싱크로스코프가 12시에 있을 때 슬립은 0임) 위상 간의 전압 차는 최소(0일 때 0임) 싱크로 스코프는 12시에 있습니다). 싱크로스코프 스위치를 닫는 데 걸리는 시간으로 인해 일반적으로 11시에 완료됩니다. 마감이 약간 지연되어 11시에 하여 12시에 가까운 마감을 달성하고 있습니다.

절연 테스트에 멀티미터가 아닌 메거를 사용하는 이유는 무엇인가요?

메가를 사용하면 권선의 절연 저항을 테스트하는 데 500V DC 이상의 전압이 사용됩니다. 멀티미터의 경우 사용되는 전압은 3볼트 DC 이하입니다. Megger는 고임피던스 테스트를 사용하므로 멀티미터보다 정확합니다.

MSB의 안전은 무엇입니까?

회로 차단기, 퓨즈 및 과전류 계전기가 사용됩니다. 패널은 전면 패널이 아니므로 회로 차단기로 패널의 전원을 끌 때까지 유지 관리를 위해 패널을 열 수 없습니다.

역전력은 언제 흐르나요?

발전기의 상황은 이 발전기와 병렬로 연결된 여러 발전기를 갖는 스위치 기어를 통해 시스템에 급전하는 것입니다. 시스템이 정상적으로 작동할 때 전류의 흐름은 발전기에서 스위치 기어로 흐릅니다. 한 발전기에 문제가 발생하고 그 단자 전압이 시스템 전압 아래로 떨어지면 모터가 발전기로 작동할 수 있는 것처럼 발전기가 모터로 작동하고 전류가 스위치 기어에서 발전기로 흐릅니다. 이것은 역전력입니다. 발전기의 완전한 기계적 고장이 발생한 경우 그 영향은 경미한 것에서 극단적인 것까지 다양합니다.

역 전력이 흐르면 ​​어떤 피해가 있습니까?

발전기가 일반적으로 P와 동일한 양의 전력을 생산하고 모터로 작동할 때 유사한 양 P를 흡수한다고 가정합니다. 따라서 그리드에 대한 순 효과는 단계 증가를 경험한 것과 동일합니다 2P와 동일한 부하에서. 그리드의 크기와 남은 세대의 세기에 따라 2P씩 증가하면 그리드의 주파수가 크게 변할 수 있습니다.

어떤 경우에는 원동기 손상이 있을 수 있습니다(특히 증기 터빈).

역전력에 대한 보호는 어떻게 제공됩니까?

역전력 보호는 모터링 방지에 사용됩니다. 이 기능은 발전기가 아닌 원동기 보호에 사용됩니다. 연료 공급을 차단하고 원동기를 정지시킬 수 있습니다.

역 전원 트립을 어떻게 테스트합니까?

두 개의 발전기가 병렬로 실행되고 하나의 발전기가 부하를 전달할 수 있는 경우, 역방향 전원 트립은 거버너 제어를 사용하여 부하를 이동하여 테스트할 수 있습니다. 부하가 충분히 이동하고 무부하 발전기가 적은 비율의 부하를 전달하면 차단기가 트립되고 원동기에 대한 연료 공급이 차단됩니다. 이는 역전원 릴레이가 작동했음을 의미합니다. 릴레이가 트립 신호를 시작하는지 확인하기 위해 시뮬레이션(릴레이의 테스트 푸시 버튼 사용)으로 릴레이를 테스트할 수 있습니다.

ACB의 전체 형식은 무엇입니까?

공기 차단기. 일반적으로 400V 이상의 전류 애플리케이션(발전기 차단기)에서 사용됩니다.

유휴 발전기에서 ACB 닫기 버튼을 누르면 어떻게 됩니까?

일반적으로 차단기는 동기화하기 전까지 닫히지 않으므로 차단기 닫기를 눌러도 차단기가 닫히지 않습니다. 차단기에는 저전압 보호 기능이 있어 차단할 수 없습니다.

저전압 보호란 무엇입니까?

실수로 차단기가 닫히거나 병렬 운전 중에 부하가 걸리는 발전기를 방지합니다. 또한 기계가 배전반에 연결되어 있는 동안 전압 손실을 방지합니다.

특혜 여행의 의미는 무엇입니까? 제공되는 이유는 무엇입니까?

우선 트립은 비필수 회로, 즉 주 전원의 부분적 고장 또는 과부하가 발생한 경우 주 버스 바에서 비필수 부하를 차단하도록 설계된 선박의 일종의 전기 장치입니다. 이는 중요한 부하(예:조향 장치)를 작동시키고 비필수 부하(예:AC 및 조리실)를 작동하도록 하며 안전 기능입니다.

배전반의 접지 오류 표시의 목적은 무엇입니까?

회로의 위상 대 지락을 감지하고 표시합니다.

지락 경보를 받으면 어떻게 하시겠습니까?

결함은 먼저 지락 가능성이 높은 선박 영역(예:갑판 조명 또는 식료품 저장실 장비)을 식별한 다음 알람이 꺼졌는지 확인하기 위해 한 번에 하나씩 전원을 꺼서 조사할 수 있습니다. 특정 부하가 경보를 해제하면 해당 회로에 지락이 있다는 것을 알고 수리할 수 있습니다.

직류란 무엇입니까?

전류의 단방향 흐름.

교류란 무엇입니까?

양수 및 음수 피크가 있는 정현파 형태의 양방향 전류 흐름입니다.

AC의 위상이란 무엇을 의미합니까?

서로 일치하지 않는 둘 이상의 AC 전압 또는 전류. 피크와 제로 포인트는 동일한 시점에서 일치하지 않습니다. 이것은 단계를 의미합니다.

DC에 위상이 있습니까?

아니요

DC가 현재 많이 사용되지 않는 이유는 무엇입니까?

AC 발전기는 더 나은 기술이며 DC가 아닌 AC에서 전압 변환(트랜스포머에 의한)이 용이합니다.

어디에서 사용되는 것을 볼 수 있습니까?

배터리 구동 DC는 일반적으로 주로 항법, 레이더, 안전 시스템, 안테나 및 통신 시스템 및 비상 조명의 항법, 제어 및 안전 시스템에 사용됩니다.

3상 전류는 무엇을 의미합니까?

3개의 전류 정현파는 크기는 같지만 위상 변이가 120도입니다.

실효값이란 무엇입니까?

제곱 평균 제곱근 값은 피크 값을 2의 제곱근으로 나눈 값입니다.

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?

회전. 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 ? 동작