Techcross BWTS Troubleshooting. Complete Operating Logic and TRO Troubleshooting Guide
Greetings! In this article, we'll discuss the Techcross BWTS (ECS) ballast system, its operating logic, and troubleshooting instructions from the Korean manufacturer.
✅ Article➡️ https://www.eto-engineer.com/2026/02/techcross-BWTS-troubleshooting.html
#ballast #ballastsystem #BallastWater #ballasting #BWMS #BWTS #ECS #electrolysis #FlowMeter #Heatsink #HMI #Hydrogen #PLC #PowerRectifierUnit #PRU #PSU #salinity #Techcross #TRO #troubleshooting
Greetings! In this article, we'll discuss the Techcross BWTS (ECS) ballast system, its operating logic, and troubleshooting instructions from the Korean manufacturer.
✅ Article
#ballast #ballastsystem #BallastWater #ballasting #BWMS #BWTS #ECS #electrolysis #FlowMeter #Heatsink #HMI #Hydrogen #PLC #PowerRectifierUnit #PRU #PSU #salinity #Techcross #TRO #troubleshooting
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To check #PNP and #NPN transistors, the easiest way is with a digital multimeter in diode mode.
1. Identify transistor pins
Typical bipolar #transistor has 3 pins:
• B – Base
• C – Collector
• E – Emitter
(Use datasheet or marking to know pin order.)
2. Check an NPN transistor
Meter in diode test mode
1. Put red probe on Base.
2. Touch black probe to Emitter → should read about 0.5–0.7 V.
3. Touch black probe to Collector → should read about 0.5–0.7 V.
4. Reverse probes → should show OL / no conduction.
5. Measure Collector–Emitter both directions → must be OL.
If all OK → NPN is good.
3. Check a PNP transistor
Same idea, but polarity reversed:
1. Put black probe on Base.
2. Touch red probe to Emitter → 0.5–0.7 V.
3. Touch red probe to Collector → 0.5–0.7 V.
4. Reverse probes → OL.
5. Collector–Emitter both ways → OL.
If readings match → PNP is good.
4. Fault indications
• 0 V both ways → shorted transistor.
• OL in all directions → open (dead).
• Conduction between C–E → faulty.
1. Identify transistor pins
Typical bipolar #transistor has 3 pins:
• B – Base
• C – Collector
• E – Emitter
(Use datasheet or marking to know pin order.)
2. Check an NPN transistor
Meter in diode test mode
1. Put red probe on Base.
2. Touch black probe to Emitter → should read about 0.5–0.7 V.
3. Touch black probe to Collector → should read about 0.5–0.7 V.
4. Reverse probes → should show OL / no conduction.
5. Measure Collector–Emitter both directions → must be OL.
If all OK → NPN is good.
3. Check a PNP transistor
Same idea, but polarity reversed:
1. Put black probe on Base.
2. Touch red probe to Emitter → 0.5–0.7 V.
3. Touch red probe to Collector → 0.5–0.7 V.
4. Reverse probes → OL.
5. Collector–Emitter both ways → OL.
If readings match → PNP is good.
4. Fault indications
• 0 V both ways → shorted transistor.
• OL in all directions → open (dead).
• Conduction between C–E → faulty.
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#Relays can differ in several important ways depending on how they operate and where they are used. Here are the main differences explained simply:
1️⃣ Electromechanical Relay (#EMR)
How it works:
A #coil creates a magnetic field → pulls a mechanical contact → opens or closes the circuit.
Features:
• Has moving parts
• Makes clicking sound
• Can switch AC and DC
• Cheap and simple
• Contacts wear out over time
Used for: #Motors, alarms, general switching, automation panels.
2️⃣ Solid State Relay (#SSR)
How it works:
Uses #semiconductors (no moving parts) to switch ON/OFF.
Features:
• Silent operation
• Very fast switching
• Long life (no contacts)
• Produces heat (needs heat sink)
• Usually for #AC loads (#DC types also exist)
Used for: Heaters, temperature control systems, #PLC outputs.
3️⃣ Latching #Relay
How it works:
Stays in last position after power is removed.
Features:
• Saves energy
• No continuous coil power needed
• Can be single-coil or dual-coil
Used for: Battery systems, energy-saving applications.
4️⃣ Time Delay Relay
How it works:
Adds delay before turning ON or OFF.
Types:
• ON-delay
• OFF-delay
• Interval #timer
Used for: Motor starting, sequence control, star-delta starters.
5️⃣ #Thermal Overload Relay
How it works:
Uses heat from current to trip when #overload occurs.
Features:
• Protects motors
• Adjustable current setting
• Works slowly (not for short circuit)
Used with: #Contactors in motor control panels.
• EMR → mechanical contacts, general purpose
• SSR → electronic, fast, silent
• #Latching → remembers position
• Timer relay → adds delay
• Thermal relay → protects motor from overload
1️⃣ Electromechanical Relay (#EMR)
How it works:
A #coil creates a magnetic field → pulls a mechanical contact → opens or closes the circuit.
Features:
• Has moving parts
• Makes clicking sound
• Can switch AC and DC
• Cheap and simple
• Contacts wear out over time
Used for: #Motors, alarms, general switching, automation panels.
2️⃣ Solid State Relay (#SSR)
How it works:
Uses #semiconductors (no moving parts) to switch ON/OFF.
Features:
• Silent operation
• Very fast switching
• Long life (no contacts)
• Produces heat (needs heat sink)
• Usually for #AC loads (#DC types also exist)
Used for: Heaters, temperature control systems, #PLC outputs.
3️⃣ Latching #Relay
How it works:
Stays in last position after power is removed.
Features:
• Saves energy
• No continuous coil power needed
• Can be single-coil or dual-coil
Used for: Battery systems, energy-saving applications.
4️⃣ Time Delay Relay
How it works:
Adds delay before turning ON or OFF.
Types:
• ON-delay
• OFF-delay
• Interval #timer
Used for: Motor starting, sequence control, star-delta starters.
5️⃣ #Thermal Overload Relay
How it works:
Uses heat from current to trip when #overload occurs.
Features:
• Protects motors
• Adjustable current setting
• Works slowly (not for short circuit)
Used with: #Contactors in motor control panels.
• EMR → mechanical contacts, general purpose
• SSR → electronic, fast, silent
• #Latching → remembers position
• Timer relay → adds delay
• Thermal relay → protects motor from overload
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#Heater element resistance depends on the rated #voltage and power of the heater.
For a normal electric heater:
• A 230-volt, 1000-watt heater usually has a #resistance of about 53 ohms.
• A 230-volt, 2000-watt heater usually has a resistance of about 26 ohms.
• A 440-volt, 5-kilowatt heater usually has a resistance of about 39 ohms.
Important points:
• When you measure the heater with a #multimeter while it is cold, the resistance will be slightly lower than the real working value, because resistance increases when the element becomes hot.
• Infinite resistance means the heater element is broken (open circuit).
• Very low resistance close to zero means a short circuit inside the element.
• You should also check #insulation to ground with a #megger. It should normally be higher than 1 megaohm, and in good condition it is usually much higher.
For a normal electric heater:
• A 230-volt, 1000-watt heater usually has a #resistance of about 53 ohms.
• A 230-volt, 2000-watt heater usually has a resistance of about 26 ohms.
• A 440-volt, 5-kilowatt heater usually has a resistance of about 39 ohms.
Important points:
• When you measure the heater with a #multimeter while it is cold, the resistance will be slightly lower than the real working value, because resistance increases when the element becomes hot.
• Infinite resistance means the heater element is broken (open circuit).
• Very low resistance close to zero means a short circuit inside the element.
• You should also check #insulation to ground with a #megger. It should normally be higher than 1 megaohm, and in good condition it is usually much higher.
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An #inductive proximity sensor is a non-contact electronic device used to detect metal objects. It works by creating an electromagnetic field at the sensing face.
When a metal object enters this field, eddy currents are induced in the metal, which changes the sensor’s internal oscillation. The sensor electronics detect this change and switch the output ON or OFF.
Main characteristics
• Detects only metal (steel, aluminum, brass, etc.).
• No physical contact, so no mechanical wear.
• Very fast response and high reliability.
• Short sensing distance, usually 1–20 mm depending on size.
• Commonly powered by #DC (10–30 V) or AC supply.
Typical outputs
• #PNP or #NPN transistor output
• NO (normally open) or NC (normally closed) switching
Common applications
• Position detection in machines
• Counting metal parts on conveyors
• Limit sensing in automation systems
• Speed or rotation detection with metal targets
How to check an inductive #proximitysensor
1. Visual inspection
• Check the #sensor face for damage, oil, or metal dust.
• Look for loose or broken wires.
• Many sensors have an #LED indicator that lights when metal is detected.
2. Verify power supply
• Identify wires (typical DC sensor):
• Brown = +V
• Blue = 0 V
• Black = output
• Measure voltage between brown and blue using a #multimeter.
• Normal value: about 10–30 V DC.
• If no #voltage → wiring or power problem.
3. Check output switching
• Keep the multimeter on DC #voltage.
• Measure between black and blue:
• No metal near sensor: output is either 0 V or supply voltage, depending on PNP/NPN type.
• Bring metal close to sensing face: voltage should change state.
• If voltage does not change, the sensor may be faulty.
4. Quick functional test
• Power the sensor.
• Move a metal object slowly toward the sensing face.
• Watch:
• LED indicator change
• Output voltage change
• No reaction → defective sensor or incorrect wiring.
When a metal object enters this field, eddy currents are induced in the metal, which changes the sensor’s internal oscillation. The sensor electronics detect this change and switch the output ON or OFF.
Main characteristics
• Detects only metal (steel, aluminum, brass, etc.).
• No physical contact, so no mechanical wear.
• Very fast response and high reliability.
• Short sensing distance, usually 1–20 mm depending on size.
• Commonly powered by #DC (10–30 V) or AC supply.
Typical outputs
• #PNP or #NPN transistor output
• NO (normally open) or NC (normally closed) switching
Common applications
• Position detection in machines
• Counting metal parts on conveyors
• Limit sensing in automation systems
• Speed or rotation detection with metal targets
How to check an inductive #proximitysensor
1. Visual inspection
• Check the #sensor face for damage, oil, or metal dust.
• Look for loose or broken wires.
• Many sensors have an #LED indicator that lights when metal is detected.
2. Verify power supply
• Identify wires (typical DC sensor):
• Brown = +V
• Blue = 0 V
• Black = output
• Measure voltage between brown and blue using a #multimeter.
• Normal value: about 10–30 V DC.
• If no #voltage → wiring or power problem.
3. Check output switching
• Keep the multimeter on DC #voltage.
• Measure between black and blue:
• No metal near sensor: output is either 0 V or supply voltage, depending on PNP/NPN type.
• Bring metal close to sensing face: voltage should change state.
• If voltage does not change, the sensor may be faulty.
4. Quick functional test
• Power the sensor.
• Move a metal object slowly toward the sensing face.
• Watch:
• LED indicator change
• Output voltage change
• No reaction → defective sensor or incorrect wiring.
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Phase Loss in Motor (Single #Phasing)
Phase loss (also called single phasing) happens when one of the three supply phases to a 3-phase motor is missing while the motor is running.
What Happens to the Motor?
1. If motor is already running:
• It will continue running.
• Current in the remaining two phases increases sharply.
• Motor overheats very quickly.
• Strong #vibration and humming sound.
• Windings can burn if protection doesn’t trip.
2. If motor tries to start with one phase missing:
• It will not start.
• It will draw very high current.
• Thermal overload or breaker should trip.
Main Causes
• Blown fuse in one phase
• Loose terminal connection
• Faulty contactor (one pole not closing)
• Broken cable
• Upstream supply problem
• Burnt contact inside MCC
Since you work with marine electrical systems, phase loss is often caused by:
• Weak #contactor contacts
• Loose terminals after vibration
• Corroded fuse holders
Symptoms You Can Check
• Motor overheating
• Unequal current on clamp meter
• One phase showing zero or very low voltage
• Overload relay frequently tripping
• Burning smell
How to Check
1. Measure line-to-line voltage
All three readings should be approximately equal.
2. Measure motor current (clamp meter)
• Two phases high current
• One phase low or zero current
3. Check contactor terminals
Measure voltage before and after contactor.
4. Check #fuses individually
Never assume they are good visually.
Why It Is Dangerous
• Current in remaining phases can increase to 1.7 times normal.
• #Motor insulation damage occurs quickly.
• Can destroy winding in minutes under load.
Protection Against #Phase Loss
• Thermal #overload relay (basic protection)
• Phase failure relay (recommended)
• Motor protection relay with phase monitoring
• Proper fuse coordination
On ships, critical motors (pumps, compressors, fans) should always have phase monitoring relay.
Phase loss (also called single phasing) happens when one of the three supply phases to a 3-phase motor is missing while the motor is running.
What Happens to the Motor?
1. If motor is already running:
• It will continue running.
• Current in the remaining two phases increases sharply.
• Motor overheats very quickly.
• Strong #vibration and humming sound.
• Windings can burn if protection doesn’t trip.
2. If motor tries to start with one phase missing:
• It will not start.
• It will draw very high current.
• Thermal overload or breaker should trip.
Main Causes
• Blown fuse in one phase
• Loose terminal connection
• Faulty contactor (one pole not closing)
• Broken cable
• Upstream supply problem
• Burnt contact inside MCC
Since you work with marine electrical systems, phase loss is often caused by:
• Weak #contactor contacts
• Loose terminals after vibration
• Corroded fuse holders
Symptoms You Can Check
• Motor overheating
• Unequal current on clamp meter
• One phase showing zero or very low voltage
• Overload relay frequently tripping
• Burning smell
How to Check
1. Measure line-to-line voltage
All three readings should be approximately equal.
2. Measure motor current (clamp meter)
• Two phases high current
• One phase low or zero current
3. Check contactor terminals
Measure voltage before and after contactor.
4. Check #fuses individually
Never assume they are good visually.
Why It Is Dangerous
• Current in remaining phases can increase to 1.7 times normal.
• #Motor insulation damage occurs quickly.
• Can destroy winding in minutes under load.
Protection Against #Phase Loss
• Thermal #overload relay (basic protection)
• Phase failure relay (recommended)
• Motor protection relay with phase monitoring
• Proper fuse coordination
On ships, critical motors (pumps, compressors, fans) should always have phase monitoring relay.
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Intermediate Relay (Auxiliary Relay)
An intermediate relay (also called auxiliary relay) is a small control relay used between a control device (PLC, push button, sensor) and a higher-power device (contactor, motor starter, #solenoid).
It works as a signal #amplifier and #isolator.
Why we use intermediate relays
1. Signal isolation
Protects PLC or control circuit from high voltage or current.
2. Increase number of contacts
One PLC output can control several devices using relay contacts.
3. Voltage conversion
Example: PLC output 24V DC → relay switches 230V AC load.
4. Contact multiplication
Relay may have 2, 3 or 4 changeover contacts.
5. Electrical separation
#Coil and #contacts are electrically isolated.
How it works (simple explanation)
• Apply voltage to the coil → magnetic field pulls internal contact arm.
• Contacts change position:
• NO (Normally Open) → closes
• NC (Normally Closed) → opens
• Remove #voltage → contacts return to normal state.
Typical ratings
• Coil voltage: 12V, 24V DC / 110V, 230V AC
• Contact rating: 5A–10A typical
• Types: 2CO, 3CO, 4CO (changeover contacts)
Where you see them (marine / industrial)
Since you work with control systems onboard vessels, you’ll find them:
• Fire alarm interface circuits
• Generator control panels
• Pump automation
• Alarm extension circuits
• Signal repeating circuits
They are often used when #PLC output current is too small to drive a contactor directly.
Difference from contactor
• #Intermediate #relay → for control signals (small current)
• #Contactor → for switching motors / high current loads
An intermediate relay (also called auxiliary relay) is a small control relay used between a control device (PLC, push button, sensor) and a higher-power device (contactor, motor starter, #solenoid).
It works as a signal #amplifier and #isolator.
Why we use intermediate relays
1. Signal isolation
Protects PLC or control circuit from high voltage or current.
2. Increase number of contacts
One PLC output can control several devices using relay contacts.
3. Voltage conversion
Example: PLC output 24V DC → relay switches 230V AC load.
4. Contact multiplication
Relay may have 2, 3 or 4 changeover contacts.
5. Electrical separation
#Coil and #contacts are electrically isolated.
How it works (simple explanation)
• Apply voltage to the coil → magnetic field pulls internal contact arm.
• Contacts change position:
• NO (Normally Open) → closes
• NC (Normally Closed) → opens
• Remove #voltage → contacts return to normal state.
Typical ratings
• Coil voltage: 12V, 24V DC / 110V, 230V AC
• Contact rating: 5A–10A typical
• Types: 2CO, 3CO, 4CO (changeover contacts)
Where you see them (marine / industrial)
Since you work with control systems onboard vessels, you’ll find them:
• Fire alarm interface circuits
• Generator control panels
• Pump automation
• Alarm extension circuits
• Signal repeating circuits
They are often used when #PLC output current is too small to drive a contactor directly.
Difference from contactor
• #Intermediate #relay → for control signals (small current)
• #Contactor → for switching motors / high current loads
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Limit switches in electrical circuits
What is a #limitswitch?
A limit switch is an electromechanical device that detects the physical position or movement of a machine part.
When an object reaches a certain point, the switch changes its contact state (NO/NC), sending a signal to stop, start, or change operation.
It is widely used in industrial automation, cranes, hoists, elevators, conveyors, valves, and marine equipment — so you’ll often see them in cargo and machinery systems onboard vessels.
How a limit switch works
Inside the switch:
• There is a mechanical actuator (lever, roller, plunger, etc.)
• When pressed, it moves internal electrical contacts
• Contacts change from:
• Normally Open (NO) → becomes closed
• Normally Closed (NC) → becomes open
This change is used in control #circuits (not usually in power circuits directly).
Main parts
1. Actuator (roller / lever / plunger)
2. Spring mechanism
3. Contact block (NO / NC)
4. Housing (often IP rated for dust/water)
Types of #limitswitches
1. Roller lever type
Used when moving parts slide past the switch.
2. Plunger type
Activated by direct pushing.
3. Adjustable rod type
Used where position may vary.
4. Heavy-duty industrial type
For #cranes, hatch covers, winches (very common in marine use).
Contact configuration
Typical markings:
• COM – Common
• NO – Normally Open
• NC – Normally Closed
Example use:
• NC contact in series with motor control → when limit reached, circuit opens → motor stops.
• NO contact → sends signal to #PLC or alarm.
Where they are used (practical examples)
Since you work with ship systems, you probably see them in:
• Hatch cover open/close end position
• #Crane boom angle limits
• Cargo #winch travel limits
• Engine room dampers
• Fire doors
• Steering gear mechanical stop feedback
They are very important for safety interlocks.
Advantages
• Simple
• Reliable
• No electronics required
• Works in harsh environments
• Easy to #troubleshoot
How to check a limit switch
1. Isolate power.
2. Use a #multimeter in continuity mode.
3. Check COM–NC (should have continuity normally).
4. Press actuator:
• COM–NC should open
• COM–NO should close
If readings do not change → switch may be faulty.
What is a #limitswitch?
A limit switch is an electromechanical device that detects the physical position or movement of a machine part.
When an object reaches a certain point, the switch changes its contact state (NO/NC), sending a signal to stop, start, or change operation.
It is widely used in industrial automation, cranes, hoists, elevators, conveyors, valves, and marine equipment — so you’ll often see them in cargo and machinery systems onboard vessels.
How a limit switch works
Inside the switch:
• There is a mechanical actuator (lever, roller, plunger, etc.)
• When pressed, it moves internal electrical contacts
• Contacts change from:
• Normally Open (NO) → becomes closed
• Normally Closed (NC) → becomes open
This change is used in control #circuits (not usually in power circuits directly).
Main parts
1. Actuator (roller / lever / plunger)
2. Spring mechanism
3. Contact block (NO / NC)
4. Housing (often IP rated for dust/water)
Types of #limitswitches
1. Roller lever type
Used when moving parts slide past the switch.
2. Plunger type
Activated by direct pushing.
3. Adjustable rod type
Used where position may vary.
4. Heavy-duty industrial type
For #cranes, hatch covers, winches (very common in marine use).
Contact configuration
Typical markings:
• COM – Common
• NO – Normally Open
• NC – Normally Closed
Example use:
• NC contact in series with motor control → when limit reached, circuit opens → motor stops.
• NO contact → sends signal to #PLC or alarm.
Where they are used (practical examples)
Since you work with ship systems, you probably see them in:
• Hatch cover open/close end position
• #Crane boom angle limits
• Cargo #winch travel limits
• Engine room dampers
• Fire doors
• Steering gear mechanical stop feedback
They are very important for safety interlocks.
Advantages
• Simple
• Reliable
• No electronics required
• Works in harsh environments
• Easy to #troubleshoot
How to check a limit switch
1. Isolate power.
2. Use a #multimeter in continuity mode.
3. Check COM–NC (should have continuity normally).
4. Press actuator:
• COM–NC should open
• COM–NO should close
If readings do not change → switch may be faulty.
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To understand a basic motor control circuit, you need to break it into simple parts. Almost every standard motor starter (especially for 3-phase motors on vessels or industrial systems) follows the same logic.
There are two main circuits:
1. Power Circuit – carries motor current
2. Control Circuit – controls the contactor (low current)
1️⃣ Power Circuit (Main Circuit)
Components:
• Main Supply (L1, L2, L3)
• Circuit #breaker or fuses
• Contactor (main contacts)
• Overload relay
• Motor
How it works:
When the contactor closes, the 3-phase supply goes through:
Supply → Contactor → Overload #relay → Motor
If overload trips, it opens the circuit and protects the motor.
2️⃣ Control #Circuit (Start/Stop Logic)
Components:
• Stop push button (NC – Normally Closed)
• Start push button (NO – Normally Open)
• Contactor coil (A1–A2)
• Auxiliary contact (NO – holding contact)
• Overload NC contact
🔎 Step-by-Step Operation
1️⃣ At rest:
• Stop button = closed
• Start button = open
• Contactor coil = OFF
• Motor = OFF
2️⃣ When you press START:
• Current flows through:
• Stop (NC)
• Overload (NC)
• Start (NO – now closed)
• #Contactor coil
• The coil energizes.
• Main contacts close → motor starts.
• Auxiliary contact closes (creates holding circuit).
Now you can release the start button and the motor keeps running.
3️⃣ When you press STOP:
• Stop button opens.
• Coil loses power.
• Contactor drops out.
• #Motor stops.
4️⃣ If Overload trips:
• #Overload NC contact opens.
• Coil de-energizes.
• Motor stops automatically.
💡 How To Read Any Motor Control Circuit
Follow this method:
Step 1 – Separate Power and Control
Always mentally divide them.
Step 2 – Find the Contactor Coil
Everything in control circuit exists to energize or de-energize this coil.
Step 3 – Check Normally Open (NO) vs Normally Closed (NC)
Remember:
• NC = closed in normal state
• NO = open in normal state
Step 4 – Follow Current Path
Ask yourself:
“From supply, where does the current go step by step?”
If you can trace the path to the coil → motor will run.
If path is broken → motor will not run.
⚡ Simple Real Example (DOL Starter Logic)
Control supply → Stop (NC) → Overload (NC) → Start (NO) → Coil → Neutral
Auxiliary NO contact is connected parallel to the Start button (self-holding).
🚢 Since you work with ship systems
On vessels you will often see:
• Emergency stop in series (NC)
• Pressure switches (NC)
• Temperature switches (NC)
• Interlocks from other equipment
All of them are usually in series with the coil.
If any protection opens → motor stops.
There are two main circuits:
1. Power Circuit – carries motor current
2. Control Circuit – controls the contactor (low current)
1️⃣ Power Circuit (Main Circuit)
Components:
• Main Supply (L1, L2, L3)
• Circuit #breaker or fuses
• Contactor (main contacts)
• Overload relay
• Motor
How it works:
When the contactor closes, the 3-phase supply goes through:
Supply → Contactor → Overload #relay → Motor
If overload trips, it opens the circuit and protects the motor.
2️⃣ Control #Circuit (Start/Stop Logic)
Components:
• Stop push button (NC – Normally Closed)
• Start push button (NO – Normally Open)
• Contactor coil (A1–A2)
• Auxiliary contact (NO – holding contact)
• Overload NC contact
🔎 Step-by-Step Operation
1️⃣ At rest:
• Stop button = closed
• Start button = open
• Contactor coil = OFF
• Motor = OFF
2️⃣ When you press START:
• Current flows through:
• Stop (NC)
• Overload (NC)
• Start (NO – now closed)
• #Contactor coil
• The coil energizes.
• Main contacts close → motor starts.
• Auxiliary contact closes (creates holding circuit).
Now you can release the start button and the motor keeps running.
3️⃣ When you press STOP:
• Stop button opens.
• Coil loses power.
• Contactor drops out.
• #Motor stops.
4️⃣ If Overload trips:
• #Overload NC contact opens.
• Coil de-energizes.
• Motor stops automatically.
💡 How To Read Any Motor Control Circuit
Follow this method:
Step 1 – Separate Power and Control
Always mentally divide them.
Step 2 – Find the Contactor Coil
Everything in control circuit exists to energize or de-energize this coil.
Step 3 – Check Normally Open (NO) vs Normally Closed (NC)
Remember:
• NC = closed in normal state
• NO = open in normal state
Step 4 – Follow Current Path
Ask yourself:
“From supply, where does the current go step by step?”
If you can trace the path to the coil → motor will run.
If path is broken → motor will not run.
⚡ Simple Real Example (DOL Starter Logic)
Control supply → Stop (NC) → Overload (NC) → Start (NO) → Coil → Neutral
Auxiliary NO contact is connected parallel to the Start button (self-holding).
🚢 Since you work with ship systems
On vessels you will often see:
• Emergency stop in series (NC)
• Pressure switches (NC)
• Temperature switches (NC)
• Interlocks from other equipment
All of them are usually in series with the coil.
If any protection opens → motor stops.
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