When the maximum kW rating of an induction motor is reached for direct-on-line starting, it becomes necessary to introduce an alternative method of starting the motor. There are several methods used in the oil industry. The object is to reduce the starting current drawn from the supply during all or part of the run-up period. There are two basic approaches that can be used:-
• Select special-purpose designs for the motor in which the winding arrangements are modified by external switching devices that are matched to the motor, e.g. star-delta motor and starter.
• Select conventional motors but use special external starting devices, e.g. Korndorfer starter, autotransformer starter, ‘soft-starter’ using a controlled rectifier-inverter system.
In all cases of reduced voltage starting, care must be taken to check that the motor will create sufficient torque at the reduced voltage to accelerate the load to the desired speed in as short a time as possible. Excessive run-up times must be avoided as. When the run-up time is expected to be high the manufacturer of the motor should be consulted regarding the possibility of damage and infringement of its guarantees. The following methods are the most commonly used, typically in the order shown below.
• Star-delta method.
• Korndorfer auto-transformer method.
• Soft-start power electronics method.
• Series reactor method.
• Part winding method.
Star-Delta Method Of Starting Induction Motors
A specially designed motor is used. The stator windings are arranged so that the start and finish of each phase winding in the stator is brought out to the main terminal box so that six terminals are available for connection to cables. Usually two three-core or four-core cables are used unless their conductor size becomes too large, in which case single-core cables would be used. The windings are connected externally in star for starting and delta for running. The external connections are made by using a special starter in the motor control centre which also provides control relays and current transformers that determine when the transfer from star to delta should take place.
Disadvantages of Star-Delta Method
• The windings are open-circuited during the transfer and this is not considered good practice, a delay should be incorporated to allow the flux in the motor to decay during the transfer.
• The starting current and torque are reduced to 33% of their value during the run-up period. This reduction may be too much for some applications.
• The running condition requires a delta winding connection and this has the disadvantage that harmonic currents can circulate in the windings.
Figure shows the basic circuit for a star-delta starter.
Korndorfer Auto-Transformer Method Of Starting Induction Motors
A standard design of motor is used. An external auto-transformer is connected between the main circuit breaker, or contactor, and the motor during the starting and run-up period. Figure 5.8 shows the connections that are commonly used in a balanced three-phase arrangement. The voltage ratio of the auto-transformer needs to be carefully selected. If it is too high then the full benefit is not achieved. If too low then insufficient torque will be created. The most effective ratio is usually found between 65% and 80%.
Disadvantage of Korndorfer Auto-Transformer Method
Two extra three-phase circuit breakers or contactors are necessary, thus making three in total for the motor circuit, which require space to be allocated. Retro-fitting a Korndorfer starter may therefore be difficult if space is scarce.
Figure shows the basic circuit for Korndorfer Auto-Transformer Method.
Soft-Start Power Electronics Method Of Starting Induction Motors
A standard design of motor is used. An external rectifier-inverter is connected between the main circuit breaker, or contactor, and the motor during the starting and run-up period. The starter varies the frequency and voltage magnitude of the applied three-phase supply to the motor. Upon starting the frequency and voltage are set to their lowest values, and thereafter they are slowly raised as the shaft speed increases. The intent is to operate the motor in its near-synchronous speed state for each frequency that the motor receives.
The rectifier-inverter equipment is expensive when compared with other switching and transformer methods, but it has several advantages:-
• The starting current can be limited to a value that is equal to or a little higher than the full-load current of the motor.
• The torque created in the motor during the whole run-up period can be in the order of the full-load value, and so the shape of the inherent torque-speed curve of the motor is not a critical issue for most standard designs of motors.
Series Reactor Method Of Starting Induction Motors
A standard design of motor is used. This is a simple method that requires the insertion of a series reactor during starting. The reactor is bypassed once the motor reaches its normal working speed.
Only one extra circuit breaker or contactor is required. The amount of reactance is calculated on the basis of the desired reduction of line current during starting, but the limiting factor is the reduced starting torque. The torque is reduced for two reasons, firstly because the total circuit impedance is increased and secondly the reactance-to-resistance ratio is increased.
Part Winding Method Of Starting Induction Motors
A special design of motor is used. The stator has two three-phase windings that are arranged in parallel and wound in the same slots. If the two windings are the same then on starting and during run-up one winding would provide half of the total torque at any speed. Hence one winding is used for starting and both for running. The method is not suited to small or high speed motors. With two equal windings the starting current and torque are half of their totals. This method is seldom used in the oil industry because of the preference for standard motors, and the availability of satisfactory alternative methods.
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