5.4.1. Starting connections
Mode of operation
The star-delta connection is mainly used for low and medium powered machines. During starting the stator winding is star-switched and subsequently delta-switched during acceleration.
In order to be switchable from star to delta the stator windings must be laid out for interlinked (conductor) voltage.
1 Conductor voltage
Figure 58 shows that a star connection to a winding strand only receives of the network voltage. The current decreases by the same factor. Moreover as both conductor and strand current in the star connection remain identical (in the delta connection ), a further current reduction by the factor ensues vis-à-vis the star delta connection.
The considerable starting current is effectively restricted by switching the stator winding from the operational delta connection to the star connection. The conductor current of the star connection is one third of the value of the delta connection.
Moreover, the diminished voltage in the star connection not only causes a diminished stator current; the following also applies;
The initial torque in the star connection is but one third of its value in the delta connection.
The advantage of the star-delta connection for limiting the considerable starting current in an effective manner is, however, only possible through a further reduction in the already minimal initial torque. In many cases it will be necessary when employing this starting procedure to start up the motor without load.
Starting up of the squirrel cage motor via K1 and K3 in star connection. Switching the stator winding to delta connection by means of K2. Actuating S2 switched K3 and the timing relay K4 (starting delay). K1 is switched by means of K3 closer. K1 holds itself alone above its closer. Following the adjustment period the opening contact of K4 switches K3 off whilst K2 is switched on by means of the opening contact of K3.
Stator starting resistors
Mode of operation
A further possibility of diminishing stator voltage, thereby reducing motor current whilst starting, is to connect resistors in series to the stator windings (Figure 61). Ohmic resistors are advantageous for lesser powered motors whilst series reactors are more economical for higher powered motors.
Curtailing voltage at the stator windings serves to reduce starting current and starting torque as also applies in other starting procedures.
An effective reduction in starting current is attained by connecting resistors in series within the stator circuit in conjunction with a pronounced decline in starting torque.
This procedure is however only suitable for no-load running motors. In order to ensure a smoother and slower starting (i.e. to exclude torque impulses from impact-switched gears) it is sufficient whilst starting to connect an ohmic resistance or a coil in a lead (Kusa circuit). The significance of this resistance is illustrated in the following for both limit values.
With the help of the resistor Rv in a lead it becomes possible to adjust the possible starting torques between zero and the possible maximum value. Impact-free starting becomes possible. As a result of the circuit asymmetry the conduction currents are distributed unequally in the three leads. An effective reduction of starting current is not possible. Current only declines in the strand with the series connected resistor.
K3 Time relay
Starting ensues via protection K2 and the series resistor R1. Diminish voltage at the stator winding, curtail starting current to ensure smooth starting up. Switching over to network voltage by means of protection K1 without currentless interruption.
Actuating S2 switches on protection K2 and the time relay K2 (initial torque delay). K2 retains itself independently over its closer in the current path two. Following the adjustment spell the K3 closer in the current path switches K1 on whilst K1 switches K2 through its oponer in current path one.
Circuitry of the Kusa circuit
Description of the Kusa circuit
By actuating S2 K1 and the time relay K3 are switched on (initial torque delay). K1 is retained independently above its closer in current path 2. Following the adjustment spell the closer of K3 in current path 3 switches on K2 which maintains itself above its closer in current path 4 and switches K3 off by means of its opener in current path 2.
By actuating S2 K1 and the time relay K2 (initial torque delay) are switched on. Following the adjustment spell R1 is short-circuited by the closer of the time relay (in Figure 65).
Slip ring motor
Mode of operation
The ends of the rotor winding are attached to the slip rings which gave rise to the designation of this rotor (fig. 67).
The torque and rotor current can be aligned in the desired values during the starting operation with the assistance of the additional resistors which may be switched on via the slip rings of the rotor winding. The internal electrical properties of this motor can be undertaken by switching on the resistors from outside. Starting can thus ensue with substantially less current than in the case of squirrel cage motors whilst the initial torque attains substantial values because of the greater ohmic share in rotor current.
Switching on rotor starting resistors ensures that current heat losses through greater resistance generally arise outside the motor and, consequently, the motor is not excessively heated up. The starting resistors dissipate heat quickly.
By and large the starter comprises a fixed resistor with several resistance steps which are progressively switched off during the starting operation.
Figure 68/69 features an automatic starting circuit for ring motors. The starting resistors are switched off by protectors with turn-on delayed closers in three stages.
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