STAR DELTA STARTER

Starting Electric Motors:

Due to their simplicity, robustness and cost-effectiveness, squirrel-cage motors are the

preferred choice of industry. During start-up, they develop currents of up to

approximately eight times the rated current and the high starting torque linked to this.

The high starting currents often lead to unwelcome voltage drops in the supply network

and the high starting torque put the mechanical elements under considerable strain.

Therefore, the electricity companies determine limiting values for the motor starting

currents in relation to the rated operational currents. The permissible values vary from

network to network and depend on its load-bearing capacity. With regard to mechanics,

methods are required which reduce starting torque.

Various starters and methods can be used to reduce currents and torque:

• Star-Delta-Starting

• Auto-transformer-Starting

• Starting via chokes or resistors

• Multi-stage starting

• Starting using electronic soft starters

• Starting using frequency inverters

In the following passages, the main starting methods used in practice are explained

further.

1 Traditional motor starting

1.1 Star-delta starting

A difference is made between:

• Normal Star-Delta Starters

• Enhanced Star-Delta Starters

• Star-Delta Starters with uninterrupted switchover (closed transition)

1.1.1 Normal star-delta starters

To enable the motor to start, the motor windings are configured in a star formation to the

supply voltage. The voltage applied to the individual motor windings is therefore

reduced by a factor of 1√

_

3 = 0.58 this connection amounts to approximately 30% of the

delta values. The starting current is reduced to one third of the direct starting current, i.e.

typically to 2...2.5 Ie.

Due to the reduced starting torque, the star-delta-connection is suitable for drives with a

high inertia mass but a resistance torque which is low or only increases with increased

speed. It is preferably used for applications where the drive is only put under a load after

run-up, i.e. for presses, centrifuges, pumps, ventilators, etc.

Motor Starting

After motor run-up, in most cases an automatic timing relay controls the switch-over

from star to delta. The run-up using star connection should last until the motor has

reached the approximate operational speed, so that after switching to delta, as little postacceleration

as possible is required. Post-acceleration in delta connection will instigate

high currents as seen with direct-on-line starting. The duration of start in star connection

depends on the motor load. During delta connection, the full mains voltage is applied to

the motor windings.

To enable a switch-over from star to delta, the six ends of the motor winding are

connected onto terminals. The contactors of a star-delta starter switch over the windings

accordingly.

Starting in star, the main contactor connects the mains to winding endings U1, V1, W1.

The star contactor shorts winding endings U2, V2, W2. After successful run-up, the star

contactor switches itself off and the delta contactor connects terminals U1/V2, V1/W2,

W1/U2.

When changing from star to delta, attention has to be paid to the correct phase sequence,

i.e. the correct connection of the conductors to motor and starter. Incorrect phase

sequence can lead to very high current peaks during the cold switch-over pause, due to

the easy torque reduction following re-start. These peaks can damage the motor

windings and stress the controlgear unnecessarily. The rotation of the motor has to be

considered as well.

Motor Starting

A sufficient time period has to be maintained between the star contactor’s deenergisation

and the energisation of the delta contactor, in order to safely extinguish the

star contactor’s disconnecting arc before the delta contactor is energised. During a

switch-over which is too fast, a short circuit may develop via the disconnecting arc. The

switch over time period, however, should be just long enough for an arc disconnection,

so that the speed decreases as little as possible. Special timing relays for a star-delta

switch over fulfil these requirements.

Motor Protection and Contactor Sizing

The overload relay is situated in the motor line. Therefore, the current to be adjusted is

lower than the motor’s rated current by a factor of 1√

_

3 = 0.58. Due to the third

harmonics currents circulating in the motor windings, a higher setting of the overload

relay may be required. This may only be carried out on the basis of utilising a

measuring device which records the correct r.m.s. value. Conductor cross-sectional areas

must be of a suitable size in order that they will be protected against temperature rises

resulting from overload conditions. Therefore, the conductor size selected must be in

accordance with the protective device(s) rating.

For motor protection by means of power circuit breakers with motor protection

characteristics, the power circuit breaker is switched into the network supply lines, as it

also carries out short circuit protection of starter and lines. In this case, the current is set

to the rated motor current. A correction of the set value because of the third harmonics is

irrelevant under these circumstances. The lines are to be thermally proportioned

depending on the power circuit breakers setting.

For normal star-delta starting, the controlgear must be sized in accordance with the

following currents:

• Main contactor K1M 0.58 Ie

• Delta contactor K2M 0.58 Ie

• Star contactor K3M 0.34 Ie

For starting times exceeding approximately 15 seconds, a bigger star contactor has to be

selected. If the star contactor is equal to the main contactor, start times of up to

approximately one minute are permissible.

2.  Soft starters:

2.1 General

Depending on the network supply quality, rapid load current changes which occur

during motor start up can cause voltage drop which may affect devices fed by the same

network:

• brightness fluctuations affecting lighting

• interference with computer systems

• contactor and relay drop outs

Mechanical machine or plant components are put under severe stress by torque surges

which occur during starting.

Traditional solutions like

• Star-delta connection

• Autotransformer

• Chokes or resistors

can influence the voltage applied to the motor terminals and, hence, the current, only

step by step.

The soft starter controls the voltage without steps from a selectable starting value up to

100 per cent. This continuously increases the torque and also the current. This means

that the soft starter enables loaded motors to be started smoothly, without the steps

associated with electro-mechanical starters.

2.2 Soft starter types

The differences between the different soft starter types lie, above all, in the structure of

the power component and the control characteristics.

As already mentioned, the soft starter is based on the phase angle principle. By means

of thyristors it is possible to switch at different points the sine half wave and supply only

part of the mains voltage to the motor.

The thyristor permits the current to flow in one direction only. This requires a second

semiconductor with opposite polarity which supplies the negative current (back-to-back

switched semiconductors).

2.1   Single-phase full-wave controlled soft starter

In case of the single-phase controlled soft starter, a phase angle (Phase L2) is

implemented in a phase by means of two back-to-back thyristors. The phases L1 and L3

are directly connected to the motor.

During start, approximately the 6 x rated motor current still flows in phase L1 and L3. It

is only possible to reduce the current to the 3 x rated current during the controlled

phase.

2.3    Three-phase half-wave controlled soft starter

For a three-phase half-wave controlled soft starter, the phase cut is implemented in all

three phases. A thyristor with an anti-parallel diode serves as a power semiconductor.

This means that the phase control is only implemented in one half-wave (half-wave

controlled). This means that the voltage is only reduced during the half-wave when the

thyristor conducts. During the second half-wave, when the diode conducts, the full

mains voltage is applied to the motor.

2.4. Three-phase full-wave controlled soft starter

For this soft starter type, the phase control is implemented in all three phases. Two backto-

back thyristors are used as power semiconductors. This means that the phase voltage

is controlled in both half waves (full wave control). As a result of the upper harmonics

occurring during phase control, the motor is nevertheless put under a higher thermal

load than during a direct start.