10-30 words
[1] With autotransformer starters what is used to overcome the transition switching problem?
With an auto transformer starter, to overcome the transition switching period, a method called the closed transition “Kornorfor “starting method is used. ( a set of relays and timers are used ).
[2] What kind of starters is now becoming common at sea?
The kind of starters now becoming more common at sea are electronic starters or often referred to as “soft starts”. Thyristors or a combination of thyristors and diodes are used to control the current flow during motor starting.
[3] What is the most common type of starter at sea?
The most common type of starter at sea is the direct on line starter (DOL).
[4] What is the starting current compared with full load current?
The starting current compared to full load current is 5 – 8 times greater.
[5] When connected in star what is the voltage as a percentage of full load voltage?
When connected in Star, the voltage as a percentage of the full load voltage is 58%.
[6] What is the most common reduced voltage starter?
The most common reduced voltage starter is the Star – Delta starter.
[7] With a star-delta starter when in the change-over?
With a star – delta starter it is necessary to change over when the motor has reached its normal running speed.
[8] What are the voltage tappings with an autotransformer?
The voltage tappings with an autotransformer are for different starting conditions, i.e. they normally have three such tappings, such as 40%, 60% and 75% depending on the starting torques.
[9] Where auto transformer starter is used on board ships and why?
Auto transformer starter is used for electric cargo pumps, ballast pumps, FD fan etc, it will reduce starting voltage thus limit the starting surge current by step it down.
30-60 words
[1] With a manual change-over in a star delta starter which is the interlock for?
With a manual change over in a star – delta starter, the interlock is to prevent and lock the handle in place if the operator has left it too late to change over from star to delta, as it would cause current surge as speed would have dropped off. This surge could also cause voltage dip
[2] Why are motor starters fitted with under voltage trips?
Motor starters are fitted with under voltage trips to prevent the generators from tripping when power is restored owing to the total starting current of all connected motors. Some motors have special automatic restarting facilities.
Ø 60-100 words
[1] Comment on the sequence of events following a mains failure?
Control and monitoring equipment of emergency generator on sensing a large drop in voltage or Frequency sends a starting signal to emergency generator. Bus tie breaker between the main Switch board and emergency switch board opens. In 45 secs the emergency generator starts and comes on load during this time the supply for emergency, lighting, alarms etc. is maintained by the emergency batteries. In case emergency generator fails to come on load, these batteries can Supply power to selected lighting, communication and alarm circuits for minimum period of Half an hour.
[2] Explain how the Emergency Generator starts up in the Event of Total Power Failure.
The Start-up of the Emergency Generator is initiated by an Electrical Relay, which monitors the normal Mains Power Supply. Falling Mains Frequency or Voltage causes the “start-up” relay to operate the Generator Starting Equipment. The Prime Mover may be electrically cranked from its own 24V battery and starter motor or air started from its own air reservoir fitted local to the Generator Engine. A manual start-up may also be initiated by push buttons in the Main Control Room and in the Emergency Generator Room.
60-100 words
[1] When would a Star/Delta Starter be used on a Motor?
Where a slow moving high inertia load is involved the starting time must be considered because of the heating effect of the starting current and in this case a Star/Delta Starter would be used.
The Star/Delta Starter first connects the Starter Windings in Star and when running changes over to Delta. The Star connection results in about 58% Line Voltage being applied to each Phase with therefore a reduction in Starting Current. The Starting Torque is also reduced to about 1/3 of its direct on-line valve.
[2] Why star delta starter is preferred with induction motor?
Star delta starter is preferred with induction motor due to following reasons:
• Starting current is reduced 3-4 times of the direct current due to which voltage drops and hence it causes less losses.
- Star delta starter circuit comes in circuit first during starting of motor, which reduces voltage 3 times, that is why current also reduces up to 3 times and hence less motor burning is caused.
- In addition, starting torque is increased and it prevents the damage of motor winding.