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Practical Marine Electrical Knowledge: Program 4. Motors and Starters
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Practical Marine Electrical Knowledge: Program 4. Motors and Starters

Marine Electrical Knowledge: Three-Phase Induction Motors

Overview of Program Four

  • This program focuses on three-phase cage induction motors, covering testing for continuity, phase faults, and insulation resistance.
  • It includes practical demonstrations on drying out motors that have been exposed to water.

Motor Starters and Safety Protocols

  • Discussion includes various motor starters used on ships: direct online starters, star-delta starters, and auto transformer starters. Familiarity with these components is crucial upon joining a ship.
  • Emphasizes the importance of electrical safety; always isolate circuits before work by removing fuses or locking circuit breakers in the open position. Post warning signs to alert others.

Testing Procedures for Electric Motors

  • The most common electric motors onboard are three-phase squirrel cage rotor induction types with specific winding configurations (star or delta). These motors operate under diverse atmospheric conditions and may be enclosed for protection against elements.
  • Maintenance practices increasingly rely on vibration monitoring and insulation resistance trends rather than traditional scheduled maintenance to detect issues early without withdrawing motors from service unnecessarily.

Fault Diagnosis Techniques

  • Common motor faults include overheating, failure to start, low-speed operation, earth faults between phases, and single phasing faults. Initial checks should ensure the motor shaft is free to move and terminals are dead even after isolation.
  • Testing involves checking feeder cables for continuity and insulation faults while ensuring proper access to starter locations near the motor for efficient troubleshooting procedures.

Insulation Resistance Testing

  • Each phase winding can be tested separately for insulation integrity; high meter readings indicate healthy conditions while low readings suggest further investigation is needed. Proper separation of connections is essential during testing processes.

Motor Maintenance and Control Techniques

Current Injection and Insulation Resistance

  • Perform current injection below the motor's full load current, monitoring temperature to prevent overheating of both the motor and welding set. Adjust current as necessary during a 15-30 minute run.
  • After running, switch off and recheck insulation resistance readings to Earth and between phases; multiple heating cycles may be required for readings to exceed 1 megaohm.

Direction of Rotation

  • Always verify the direction of rotation before coupling a motor to its load. If incorrect, interchange any two supply lines at the motor terminal box to change the rotation direction.

Special Application Motors

  • Discusses special application motors used for auxiliary deck machinery, which can be DC motors or specially designed squirrel cage rotor induction motors that operate at varying speeds.
  • Types include wound rotor types with external resistor control for speed variation or multi-pole stator constructions allowing pole changes (e.g., from four poles to eight).

Single-phase Motor Maintenance

  • Two types of single-phase motors are discussed: commutator and capacitor start types. Testing procedures similar to three-phase machines apply here.
  • Check centrifugal switch contacts in single-phase motors for proper operation; clean mechanisms with approved cleaners and lubricate sparingly.

Capacitor Checks and Cleaning Procedures

  • Disconnect starting capacitors, using a multimeter set to read resistance; gradual charging indicates good condition. Ensure all windings are cleaned from dust, grease, and moisture.
  • For incorrect rotation after an overhaul in capacitor starting motors, swap leads connected to the start winding.

Commutator Motor Care

  • Maintain cleanliness of commutators by removing carbon dust, oil, and grease. Inspect brushes for length ensuring they make proper contact with the commutator surface.

Starters and Controls Overview

Types of Starters Used on Deck Machinery

  • Common starters include Direct Online starters, star-delta starters, and autotransformer starters designed for heavy-duty drives while providing protection against overcurrent conditions.

Direct Online Starter Functionality

  • A direct online starter consists of one contactor with an overcurrent relay; it connects power supply directly when activated by push buttons while maintaining circuit integrity through auxiliary contacts.

Protection Features in Starters

  • The closing coil circuitry is protected by a no volt coil feature that interrupts power during significant voltage drops preventing automatic restart after interruptions.

Maintenance Considerations

  • Regular maintenance includes cleaning components periodically; older starter versions require main contact point cleaning due to wear over time.

Star-Delta Starter Operation

Transformer Starter and Motor Control

Transformer Starter Overview

  • The transformer starter utilizes a supply transformer with reduced starting voltage tappings, typically at 50%, 60%, and 70%.
  • A timer is employed to switch the motor to full line voltage after it has reached operational speed.
  • This system prevents surge current during the changeover by ensuring that the motor is switched to direct on-line before releasing the tapping contactor.

Variable Speed Motors

  • Control and protection circuitry for variable speed motors operates differently, adjusting either rotor winding resistance or pole states during connection.

Multipole Motor Control Unit

  • The control unit of multipole motors features multiple extra contact linkages that frequently engage as the motor transitions between speeds.
  • Regular maintenance is crucial; contacts must be checked and cleaned often, with any badly burned contactors needing replacement.

Motor Speed Control and Maintenance Techniques

DC Current Switching

  • The control unit switches DC current in both directions, which is essential for managing motor speed effectively.

Electromagnetic Brakes

  • Deck machines typically include electromagnetic brakes, which are highlighted as part of routine servicing practices.

Summary of Program Content

Key Takeaways from Program Four

  • General points about three-phase motor maintenance were discussed alongside various test procedures for fault tracing and insulation resistance testing.
  • Demonstrations included drying out a drenched motor using current injection from a welding set and maintenance techniques specific to single-phase motors.

Recommended Resources

Video description

This is Program 4 of VIDEOTEL series on Practical Marine Electrical Knowledge. Designed to assist sea-going personnel in their understanding of the safe operation, testing and maintenance of ships electrical equipment and services. Examines practical electrical maintenance and fault-finding procedures on board various ship types. The series is made up of 8 programs. Program 4 deals with three cage induction motors showing a testing of each electrical component for continuity between phase faults and insulation resistance testing. You can watch how to dry out the motor which had been flooded of water. This is followed by single phase motors and motor starters used on board ship. We discuss direct online starters, star-delta starters and auto-transformer starters. There are many system variations around. So, it's most important that you become familiar with the components of the electrical system and the layout of the main switchboard immediately you join a ship. Pay particular attention to the layout of emergency switchboard. This study will pay dividends during a blackout or when troubleshooting in cause of major breakdown. Motors and Starters Electrical Safety Golden Rule is: Before any work is done on the electrical installation, first, isolate the circuit by removing the supply fuses or locking the circuit-breaker in the open position. So, the circuit cannot be energized accidentally. Then, post a warning sign to alert others that the circuit is being worked down. Then prove the circuit dead with voltmeter or approved line tester. ______________________________ This video series explains a ship's electrical system, including its generation plant, switchboards and distribution network. Electrical safety and safe working practice are stressed throughout with the significance of circuit faults and the various forms of protective switchgear. Based on book "Practical Marine Electrical Knowledge" which covers every aspect of a ships electrical system and includes correct operation, safety, testing, UMS requirements and standards for Classification Society Surveys. ______________________________ Timeline of Program 4. Motors and Starters: 0:45 Content 1:15 When you joined a ship 1:38 Electrical Safety Golden Rule 2:25 The most used electric motor on board ships 2:30 Three-phase squirrel-cage rotor induction type 3:06 Maintenance of electric motor on board ship 3:43 Test techniques 3:54 Common faults 4:08 What must be checked first 4:26 Test of cable 4:54 Insulation resistance test 5:20 Testing for continuity 5:48 Winding testing 8:16 Typical example of testing 8:42 Removing the rotor 8:56 Bearings check 9:07 Washing out the motor 9:14 Electrical cleaner 9:30 Drying up by current 10:53 Direction of rotation 11:18 Re-vanishing 11:35 Special application motors 12:10 Speed control 12:44 Single phase motors 13:11 Typical switch contact 13:28 Switching mechanism 14:05 Starting capacitor testing 14:36 Winding insulation 15:08 After overhaul 15:22 Commutator 15:53 Brushes 16:09 Deck machinery starters 16:37 Schematic view of the simplest starter 17:47 Components 18:18 Older version of the starter 19:20 Star-delta starter 20:20 Auto-transformer starter 21:24 Control for variable speed motors 22:07 Electromagnetic brakes 22:18 Conclusion ______________________________________ Made by VIDEOTEL 1992 archive #marine #electrical knowledge