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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.

Circuitry


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

L...

external conductor

N...

neutral conductor

F1

Fuses

F2

Thermal cut-out

K1

Main contactor

K2

Delta contactor

K3

Star contactor

M1

Three-phase motor


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

S1; S2

switches

K1...K3

as in Figure 59

K4

time relay

S;

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.

Circuitry


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

K2

Starting contactor

R1

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.

Circuitry


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.

5.4.2. Dahlander pole-changing circuit (speed control)

Mode of operation

Where several separate electrical windings with varying numbers of pole pairs are required for the stator of the asynchronous motor or windings whose pole pair numbers can be varied by switching over the windings then the speed of the rotating field changes and, thus, also the rotor speed. Squirrel cage motors are used for this purpose because, as opposed to slip ring rotor motors, they are not bound by a specific pole pair. The pole-changing winding in the so-called Dahlander pole-changing circuit is thereby the most perceptible practical feature. This pole-changing winding permits a speed change in the ratio two to one. These circuits have been set out in fig. 70.

The coil groups are switched over from series to parallel connection where a smaller pole pair number is selected, that is to say a greater speed is selected.


Figure 70 - Dahlander connection

(1) Delta connection for low speed (p = 2)
(2) Double star connection for higher speed (p = 1)
...U; ...V; ...W Partial windings

The speeds of asynchronous motors can be roughly stipulated by altering the pole pair number.

Where a stator has been executed with two separate windings which are both pole-reversible, then the speed may be established in four stages, for example, by means of the synchronous speeds of 500 - 750 - 1000 and 1500 rpm.

Motors with changeable pole pair numbers are frequently used for controlling machine tools where approximate setting is usually sufficient. Such motors are also used to drive pumps, ventilators, escalators etc.

Circuitry


Figure 71 - Dahlander connection (main circuit)


Figure 72 - Dahlander circuit (control circuit)

Circuitry description

The protection K1 that the series-switched coil halves of each stand of the stator winding are delta-connected.

The pole pair number p1 conditions the rotating field speed n1. The protections K2 and K3 ensure that the coil halves of each strand are parallel connected and the entire stator winding is star connected.

The new pole pair number p2 conditions the rotating field speed n2. Actuating S3 switches on K1 which retains itself by means of its closer in current path two. K3 and K2 are locked by the openers S3 in the current path five whilst K1 is locked in the current path three. Actuating S2 switches on K2 whose closer switches on K3 in current path five. K1 is locked by opener S2 whilst K3 and K2 are locked in current path one.

5.4.3. Rotational reversing circuit

Mode of operation

Cp. section 5.3.4.

Circuitry


Figure 73 - Rotational direction turnover voltage (main circuit)


Figure 74 - Rotational direction turnover voltage (control circuit)

Circuitry description

Rotational direction selection without cut-off compulsion. The K1 is switched clockwise by actuating S3. K2 is locked in current path three by the openers of K1 and S3. K2 drive is switched counterclockwise by dead and simultaneous actuation of S2 and K1.

5.4.4. Braking circuits

Counter-current braking

Mode of operation

Braking by means of counter-current is the simplest way to attain standstill of an asynchronous drive resp. the deceleration of pull-through loads, for instance in pumping stations. Two stator leads are interchanged to this end during motor operation. This changes the rotational direction of the rotating field. The rotor, which is braked, thus runs counter to the rotational direction of the rotating field. This connection can be used both for squirrel cage and slip ring motors. No additional devices are required.

The braking effect during counter-current braking bases on the altered rotational field direction. The motor tries to accelerate in the other rotational direction.

The motor must be disconnected in good time from the mains so that it does not again accelerate in the new rotational field direction. This is mainly made automatically.

Counter-current operation induces pronounced braking reaction. The current impulse on switching over is considerable greater than starting through direct connection. The motor is generally braked in star connection in order to avoid too great a current.


Figure 75 - Counter-current braking (main circuit)


Figure 76 - Counter-current braking (control circuit)

Circuitry description

Protection K1 switches on the three-phase motor. During switching off K2 connects the mains via two series resistors with two interchanged external conductors. The counter field brakes the rotors.

K2 falls off during motor stillstand.

Actuating S2 switches protection K1 which holds itself in the current path 2 through a closer. K2 is locked by the K1 opener in current path 5 whilst the closer in current path 3 switches the locking relay K3. Switching off by means of S1 the K1 opener closes current path 5. K2 is excited. Given standstill (n = 0) the closer of the automatic brake controller interrupts the F3 current path 5. K3 and K2 drop out.

Direct current braking

Mode of operation

During this braking procedure the machine is disconnected from the mains and the stator winding is excited through direct current. Connection to the direct current source ensues acc. to the circuit depicted in Figure 77.

The stator establishes a constant magnetic field. Induction currents are yielded in the rotor winding which is either short-circuited or connected by means of rotor resistors. These induction currents give rise to a braking torque which facilitates impulse-free braking.

The asynchronous machine with direct current braking behaves in the same manner as an external pole synchronous generator.

Direct current braking is suitable for stopping all categories of asynchronous machine drives. The dissipated heat converted through rotor circuit braking is much less than during counter-braking. The minimal exciting power and the admirably controlled speed of slip ring motors are further advantages of this circuitry.


Figure 77 - Direct current braking (main circuit)


Figure 78 - Direct current braking (control circuit)

Circuitry description

K1 switches on the three-phase motor. On switching off K2 connects direct voltage to the stator winding. K2 drops out after commensurate braking.

Actuating S2 switches protection K1 which holds itself via a closer in current path 2. The K1 closer in current path 3 switches on the auxiliary contactor K3 (release delay). K1 openers in current path 5 serve to lock K2. K1 drops out when S1 switches off. Its opener locks current path 5 (braking ensues through K2) whilst its closer in current path 3 switches K3 off with delay.

The closer of K3 in the current path 5 opens with delay whereby K2 drops off.