Marine Electrical Oral Questions & Practical Answers (Motors, Starters & Load Control)

⚡ Marine Electrical

Oral Questions & Practical Answers

Part 4 – Motors, Starters & Load Control

Motors explained the way you see them running onboard — not just on paper.

1. What is an electric motor?

An electric motor converts:

Electrical energy
into
Mechanical energy

This is achieved by interaction between:

Magnetic field
Current carrying conductors

2. What are the most common motors used onboard ships?

Three-phase induction motors (most common)
Synchronous motors (limited use)
DC motors (control and special applications)

3. Why are three-phase induction motors mostly used onboard?

Because they are:

Simple in construction
Robust and reliable
Low maintenance
Suitable for marine environment

4. What is an induction motor?

An induction motor is an AC motor where:

Stator is supplied with AC
Rotor current is induced
No direct electrical connection to rotor

5. What is slip in an induction motor?

Slip is the difference between:

Synchronous speed
Actual rotor speed

Slip is necessary to:

Induce rotor current
Produce torque

6. Why does the rotor never reach synchronous speed?

If rotor reaches synchronous speed:

No relative speed
No induced current
No torque

Hence rotor always runs slightly slower.

7. What is starting current and why is it high?

Starting current is high because:

Rotor impedance is low at standstill
No back EMF initially

High starting current can cause:

Voltage drop
Overload

8. What is a DOL starter?

DOL – Direct On Line starter

Motor connected directly to supply
Full voltage applied at start
High starting current
High starting torque

9. Where is DOL starter commonly used?

Small and medium motors
Pumps
Fans
Compressors

10. What are the disadvantages of DOL starting?

High inrush current
Mechanical stress
Voltage dip on switchboard

11. What is star-delta starting?

A starting method where:

Motor starts in star connection
Reduced starting current
After acceleration, changed to delta

12. Why is starting done in star connection?

Star connection:

Reduces phase voltage
Reduces starting current
Reduces starting torque

13. What is the disadvantage of star-delta starting?

Reduced starting torque
Not suitable for high load at start

14. What is a soft starter?

A soft starter:

Gradually increases voltage
Limits starting current
Reduces mechanical stress

15. What is a VFD?

VFD – Variable Frequency Drive

Controls motor speed
Varies supply frequency
Provides smooth starting and stopping

16. Why are VFDs increasingly used onboard?

Because they:

Save energy
Improve control
Reduce mechanical wear
Improve power factor

17. How is motor direction reversed?

By changing:

Phase sequence
(e.g. R-Y-B → R-B-Y)

18. What is overload in a motor?

Overload is:

Current above rated value
Sustained over time

Caused by:

Mechanical load
Bearing issues
Misalignment

19. What protection is provided against overload?

Thermal overload relay
Motor protection relay
Circuit breaker

20. How does a thermal overload relay work?

Uses bimetallic strip
Heats up due to overcurrent
Bends and trips circuit
Requires manual reset after cooling

21. What is single phasing?

Single phasing occurs when:

One phase supply is lost
Motor continues on two phases

22. Why is single phasing dangerous?

Excess current in remaining phases
Rapid overheating
Motor burnout

23. What protection is used against single phasing?

Negative phase sequence relay
Motor protection relay
Overcurrent protection

24. Why are motors rated in kW?

Because:

kW represents mechanical output power
Motor output is shaft power

25. Why are generators and transformers rated in kVA?