Starting Methods of Induction Morters

8/4/2019 Starting Methods of Induction Morters

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Star Delta starter

This is a starting method that reduces thestarting current and the starting torque. Thecomponents normally consist of threecontactors, an overload relay and a timerfor setting the time in the star position. Themotor must be delta connected during anormal run, in order to be able to use thisstarting method.The received starting current is about 30%

of the starting current during direct on linestart and the starting torque is reduced toabout 25% of the torque available at a DOLstart. This starting method only works whenthe application is light loaded.

A two-position switch (manual or automatic) is provided through a timing relay.Starting in star reduces the starting current.When the motor has accelerated up to speed and the current is reduced to its normal value, the

starter is moved to run position with the windings now connected in delta.More complicated than the DOL starter, a motor with a star-delta starter may not producesufficient torque to start against full load, so output is reduced in the start position. The motorsare thus normally started under a light load condition.Switching causes a transient current which may have peak values in excess of those with DOL.

Operation

In operation, the Main Contactor (KM3) and theStar Contactor (KM1) are closed initially, andthen after a period of time, the star contactor is

opened, and then the delta contactor (KM2) isclosed. The control of the contactors is by thetimer (K1T) built into the starter. The Star andDelta are electrically interlocked and preferablymechanically interlocked as well.

In effect, there are four states

1. OFF State. All Contactors are open2. Star State. The Main and the Star contactors are closed and the delta contactor is open. The

motor is connected in star and will produce one third of DOL torque at one third of DOL current.3. Open State. The Main contactor is closed and the Delta and Star contactors are open. There is

voltage on one end of the motor windings, but the other end is open so no current can flow. Themotor has a spinning rotor and behaves like a generator.

4. Delta State. The Main and the Delta contactors are closed. The Star contactor is open. The motoris connected to full line voltage and full power and torque are available.


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This type of operation is called open transition switching because there is an open state between the starstate and the delta state.

Autotransformer starter

motor, which can be connected permanently in delta or in star,is switched first on reduced voltage from a 3-phase tappedauto -transformer and when it has accelerated sufficiently, it isswitched to the running (full voltage) position. The principle issimilar to star/delta starting and has similar limitations. Theadvantage of the method is that the current and torque can beadjusted to the required value, by taking the correct tapping onthe autotransformer. This method is more expensive becauseof the additional autotransformer.

There are two ways of connecting an auto transformer starter, the most obvious way is to applyfull voltage to the transformer via a contactor, and connect the motor to the tap by means of a contactor.When the motor has accelerated to full speed, or has run out of acceleration torque, the tap contactoropens, disconnecting the motor from the transformer and another contactor closes connecting the motorto the supply. The transformer can now be disconnected from the supply. This format is known as anopen transition starter and is less than ideal due to the fact that the motor is disconnected for a shortperiod of time during the start period.

While the motor is connected andaccelerating, there is a rotating magnetic field in thestator which causes flux in the rotor and thus a rotor

current to flow. At the instant the motor isdisconnected, there is a magnetic field in the rotorwhich is spinning with-in the stator winding. The motoracts as a generator until the rotor field decays. Thevoltage generated by the motor is not synchronized tothe supply; and so on reconnection to the supply, thevoltage across the contactor at closure can be as muchas twice the supply voltage resulting in a very highcurrent and torque transient. This open transitionswitching is often known as the auto-reclose effect as ityields similar characteristics to opening and closing abreaker on a supply to one or more motors. Theconsequences of open transition switching can be asbad as broken shafts and stripped gears.


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Rotor resistance starting

The method used for squirrel cage motors can also be employed for starting wound rotor motor ,but it is usually not done because then the advantages of wound rotor motor cannot be realized. Thesimplest and cheapest method of starting wound rotor induction motor is by means of added rotorresistance, with full line voltage across the stator terminals.

The addition of external resistance would,

decrease its starting current increases its starting torque improves its starting power factor

By adding eternal resistance to the rotor circuit any starting torque up to the maximum torque can beachieved; and by gradually cutting out the resistance a high torque can be maintained throughout thestarting period. The added resistance also reduces the starting current, so that a starting torque in therange of 2 to 2.5 times the full load torque can be obtained at a starting current of 1 to 1.5 times thefull load current.

In a slip-ring (wound rotor) induction motor, resistance can be inserted in the rotor circuit via sliprings, so as to increase the starting torque. The starting current in the rotor winding is

Where R ext = Additional resistance per phase in the rotor circuit

The input (stator) current is proportional to the rotor current as shown earlier. The startingcurrent (input) reduces, as resistance is inserted in the rotor circuit. But the starting torque increases, asthe total resistance in the rotor circuit is increased.

Though the starting current decreases, the total resistance increases, thus resulting inincrease of starting torque and also obtained by using the expression given earlier, for increasing valuesof the resistance in the rotor circuit.

If the additional resistance is used only for starting, being rated for intermittent duty, theresistance is to be decreased in steps, as the motor speed increases. Finally, the external resistance isto be completely cut out, i.e. to be made equal to zero (0.0), thus leaving the slip-rings short-circuited.

Here, also the additional cost of the external resistance with intermittent rating is to beincurred, which results in decrease of starting current, along with increase of starting torque, both being

advantageous. Also it may be noted that the cost of a slip-ring induction is higher than that of IM withcage rotor, having same power rating.


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To select the values of the resistors, you need to know the frame voltage and the short circuitcurrent. The maximum torque occurs approximately at the point where the rotor reactance equals thetermination resistance. The final stage of the resistance should always be designed for a maximumtorque close to full speed to prevent a very large step in current when shorting the final stage ofresistance. If a single stage was used and the maximum torque occurred at 50% speed, then motor mayaccelerate to 60% speed, depending on the load. If the rotor was shorted at this speed, the motor woulddraw a very high current (typically around 1400% FLC) and produce very little torque, and would mostprobably stall.

Power electronic starters (soft starters)

The soft starter controls the voltage without steps from a selectable starting value up to 100 percent. This continuously increases the torque and also the current. This means that the soft starterenables loaded motors to be started smoothly, without the steps associated with electro-mechanicalstarters.

The soft starter employs solid state devices to control the current flow and therefore the voltageapplied to the motor. In theory, soft starters can be connected in series with the line voltage applied tothe motor, or can be connected inside the delta loop of a delta connected motor, controlling the voltageapplied to each winding.

1 x Triac per phase

1 x SCR and 1 x Diode reverse parallel connected per phase.

2 x SCRs reverse parallel connected per phase.

The phase control is implemented in all three phases. Twoback-to-back thyristors are used as power semiconductors.This means that the phase voltage is controlled in both halfwaves (full wave control). As a result of the upper harmonicsoccurring during phase control, the motor is nevertheless putunder a higher thermal load than during a direct start.

Advantages of soft starters:

1. Increased acceleration time can be beneficial for motor and machine.2. The starting current is reduced or can be limited.3. The torque is adapted to the corresponding load.4. For pumps, surges during start and stop can be avoided.5. Jerky movements and shocks, which could hamper a process, are avoided.6. The wear and tear of belts, chains, gears and bearings is avoided.7. By means of the different controls, simplified automation is possible.

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Starting Methods of Induction Morters