Fuses

Question 1

What kind of fuse would you use for a big motor and discuss the starting arrangement you would expect on a big motor?

Answer 1

A high voltage/ high current fuses such as a HRC ( High Rupting Capacity Fuse)

Starting arrangements could be DOL, Star-Delta, Soft starters or VFDs

Question 2

Star delta starter and why do we need reduced torque for start up?

Answer 2

Connecting the motor windings in a star configuration during starting to reduce the voltage applied to each winding, thereby limiting the starting current and torque, and then switching to a delta configuration for normal operation.

A motor experiences a high torque requirement to overcome inertia and accelerate the load to operating speed. However, excessive starting torque can cause mechanical stress on the motor shaft, bearings, coupling, and connected equipment. By gradual starting torque allows for a smoother acceleration of the motor and load, reducing mechanical stress and wear on components.

Question 3

Why does starting in star configuration produce more heat?

Question 4

Why do we have different fuses for different jobs, with different inbuilt time to blow characteristics?

Answer 4

• Depending on the equipment, it will operating at different currents so a fuses rated for that equipment is needed
• The load characteristic vary, high start current but low operating or vice versa
• Some equipment might be designed for momentary surges so an immediate fuse tripping is not required

Question 5

What type of protection would you expect to find on a big motor i.e. Bow thruster?

Answer 5

• Overload protection
• Short circuit protection
• Phase failure protection
• Overcurrent protection
• Under voltage protection
• Emergency stop
• Temperature monitoring protection
• Ground fault protection