Starting Methods and control
.The end of the stator coil ( 6 ends ) are terminated in the stator terminal box , where they are connected to the incoming cable from the 3-phase supply
( 1 ) - Direct On-Line Starting ( D. O. L )
-The Stator Coil sets ( 3-phase ) are connected in delta or star connection (Fig. 5.5) to supplying directly with 3-ph supply ,
The device for direct on-line starting consists essentially of three contacts, which are closed to connect the three-phase supply from the switchboard to the stator windings of the motor(shown on the left of Fig. 5.6)
However, rapid operation is beneficial and to achieve this a closing coil is used, which is energized from a low voltage( d.c or a.c.) control circuit (shown on the right of Fig. 5.6).
- Closing the isolating switch makes main power available to the main contacts and, via the transformer and rectifier, to the operating circuit.
- When the start button is pushed the closing coil is energized, and as the main contacts close so also does the contact (S).
- Release of the start button will not interrupt the closing coil circuit because continuity is maintained through the retaining contact (S).
- The main contacts are closed against a spring and de-energizing of the closing coil by opening the control circuit with the top button, or operation of safety trips, will cause the contacts to open and the motor to stop.
The closing coil acts as a no-volts trip to prevent involuntary restarting of the motor after a power loss (except for steering gear motors).
Disadvantages :
- Simple direct on-line start, squirrel-cage induction motors have three disadvantages:
(1) high starting current, ( 6-8 x full load current )
(2) low starting torque, and
(3) single speed operation (apart from slight slip with increase of torque). Characteristics are shown in Fig. 5.7.
- When very large motors are started Direct On-Line they cause a disturbance of voltage ( voltage dip ) on the supply lines due to the high starting current surge and it is unacceptable ,
- To Limit the starting current some large induction motors are started at reduced voltage (initial torque is also reduced) and then have full supply voltage reconnected when they have run up to near rated speed .
- Reduced voltage starting (Low-Current Starting ) is used for large motors driving loads like that ( Cargo pump – Bow thruster - Hyd, pump )
- TOW methods of reduced voltage starting are :
STAR – DELTA Starting and AUTOTRANSFORMER starting
( 2 ) - Star - Delta Starting
- The three sets of stator windings have end connections which are
brought out to a starter box. Changeover contacts in the starter enable the six ends to be star-connected for starting (Fig. 5.8) and then as the rotor comes up to speed ( 15- 20 sec), to be reconnected in delta.
- Star starting has the effect of reducing the voltage per phase to 57.7% of the line voltage. Starting current and torque are only a third ( 1/3 ) of what they would be with direct on-line starting.
- The low current start is obtained at the expense of torque and star-delta motors can only be used with light starting loads.
- Automatic switching to the delta running condition is preferred to manual changeover which may be made too soon or too slowly and cause a current surge
In the delta running condition, phase voltage is equal to line voltage and the motor behaves as a straightforward squirrel-cage type.
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The starter is also designed so that star contacts have to be made before it is possible to change to the run position.
موضوع: Starting Methods and control 
الجمعة 30 نوفمبر - 20:35
