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close this bookElectrical Machines - Basic vocational knowledge (Institut für Berufliche Entwicklung, 144 p.)
close this folder5. Asynchronous motors
close this folder5.4. Circuit engineering
View the document5.4.1. Starting connections
View the document5.4.2. Dahlander pole-changing circuit (speed control)
View the document5.4.3. Rotational reversing circuit
View the document5.4.4. Braking circuits

5.4.1. Starting connections

Star-Delta connection

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.

Figure 58 - Voltages and currents during delta starting

1 Conductor voltage
2 Conductor current
3 Strand voltage (voltage through a winding)
4 Strand current (current in a winding)

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-is the star delta connection.

lIStr = IL

UStr = UL

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.


Figure 59 - Automatic star-delta connection (main circuit)


external conductor


neutral conductor




Thermal cut-out


Main contactor


Delta contactor


Star contactor


Three-phase motor

Figure 60 - Automatic star-delta connection (control circuit)

S1; S2



as in Figure 59


time relay


closers resp. openers of the contactors resp. relay in the commensurate current thread

Circuitry description

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.

Rv ® ¥

The limit current motor is fed from one side only from the stator. Consequently there is no rotating field and the motor does not develop a torque.

Rv = 0

The asynchronous motor is connected directly. The motor develops the maximum possible torque.

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.


Figure 61 - Starting connection by means of series resistors (main circuit)


Starting contactor


Starting resistance

Figure 62 - Starting connection by means of series resistors (control circuit)

K3 Time relay

Circuitry description

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

Figure 63 - Kusa circuit 1 (main circuit)

Figure 64 - Kusa circuit 1 (control circuit)

Figure 65 - Kusa circuit 2 (main circuit)

Figure 66 - Kusa circuit 2 (control circuit)

Description of the Kusa circuit

Circuit 1.

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.

Circuit 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.

Figure 67 - Slip-ring rotor with rotor starting resistance

1 Rotor starting resistance
K; L; M Connecting terminals

Slip ring motors develop a pronounced initial torque notwithstanding minimal current take-up. They can start up under load.

Slip ring motors are suitable for long and repetitive operating spells.

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 - Automatic starting connection for the slip-ring motor (main circuit)

Figure 69 - Automatic starting connection for the slip-ring motor (control current)

Circuitry description

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.