STARTING METHORDS OF THREE PHASE INDUCTION MOTORS

INSTRUCTED BY: Mr. L.Meegahapola

NAME : H.M.S.L.G.BandaraINDEX NO : 050036JFIELD : EEDATE OF SUBMISSION: 2007.06.12

DISCUSSION:

Construction and working principle of DOL starters

A direct on line starter or DOL starter is used as a starting method of electric motors. In DOL starting method is done by directly connecting the motor to the power supply. Hence, the motor is subjected to the full voltage of the power supply. Consequently, high starting current flows through the motor. At the starting moment the rotating magnetic field generated in the stator winding induces a voltage drop across the rotor. At the short-circuited end there will be a current governed according to the Faradays law. And also that current will create the torque. The motor starter is used with push button switches, relays, isolators, circuit breakers, transformers and interlock switches. The DOL starter circuit is divided into two circuits called as control circuit and power circuit. The motor terminals connect with the power supply by using the power circuit. It is the higher voltage circuit of the DOL. The control circuit is the lower-voltage circuit and it is electrically isolated from the power circuit. The starter is energized by the control circuit using electro-mechanical relays and push button switches. Modern starters employ solid state relays like electronic relays (over load protection relays). The switch may be a manually operated load break switch or circuit breaker. Although more commonly it would be an electromagnetic contactor which can be opened by the thermal overload relay under fault conditions. And also the contactor will be controlled by using different start and stop buttons.

A picture of a DOL switch

Most motors can be run clockwise as well as anti-clockwise. To do that a reversing starter is used which is an electrical or electronic circuit that reverses the speed of a motor automatically. Therefore the circuit is composed of two DOL circuits that are for clockwise operation and the other one for anti-clockwise operation. These types of starters are used to start water pumps, compressors, fans and conveyor belts. The starting current may reach 3-8 times the normal current but the starting torque is tends to be 0.75 to 2 times the full load torque. The motor will draw a very high amount of current for a very short time. That is to establish the magnetic field in the iron and then the current will be limited to the Locked Rotor Current of the motor. The motor will develop Rotor Torque and begin to accelerate towards full speed. As the motor accelerates the current will begin to drop but will not drop significantly until the motor is at a high speed typically about 85% of synchronous speed. This is the most simple and inexpensive method of starting a squirrel cage induction motor. And also by using this method can avoid excessive supply voltage drops because of large starting currents the method is restricted to small motors only.

Shortcomings of the DOL starter.

In DOL method the start time is dependant on the acceleration torque and the load inertia. In this method there is large starting current (That is between 3-8 times the full loads current). And also there is no specific method use to control that current. Because of large starting current used to start the motor there will also be a large voltage drop cross the distribution line. There fore this method is restricted to small motors only. Also due to that will affect on the other electrical consumer’s equipments or some times it may cause a mechanical problems with the driven load. There fore DOL method generally not applied relatively large machines. This method can be use below 7.5 kW. To decrease the starting current cage motors of medium and larger sizes are started at a reduced supply voltage. The reduced supply voltage starting is applied in star-delta and auto transformer methods.

Star-Delta starting

The Star Delta starter method is another type of starting method. But it can only be used with a motor rated for connection in delta operation at the required line voltage. And also it should have both ends each of the three windings available individually. At start the winding is connected in a star connection. When the induction motor comes to its normal operation the switch is thrown quickly to the running position and the star connection is opened. After that open circuiting the motor and the ends of the windings are connected the motor in delta for normal operation. The connection changing is done by manually or automatically. In the manual method the switch changing is done by the relays and the switches which are driven by the relays. Although the tap changing should be done before the motor obtain the speed otherwise the fuses will blow up and the system fail occur. In the Automatic mode the connection changing is done by centrifugal switch which is connected the rotating parts in the motor or the timer circuit will make a certain time delay to speed up the motor and it switch on the relays to made connection changes. The timer delay can be adjusted by the user as required. So the best method is the centrifugal one. And also,

The phase voltages & the phase currents of the motor in star connection are reduced to 1/3 of the DOL values in delta. The starting torque is also reduced to 1/3 of its delta value. Because the phase voltage is proportional to the square of the applied voltage and phase voltage is reduce by 1/3 of the value in delta.

The Star Delta starter can only be used with a motor which is rated for connection in delta operation at the required line voltage, and has both ends each of the three windings available individually. The main benefits of the star delta starter are that it puts more money in the pockets of the switchgear supplier, and it is politically correct.

Auto-transformer starting This method also reduces the initial voltage applied to the motor and therefore the starting current and torque. The motor, which can be connected permanently in delta or in star, is switched first on reduced voltage from a 3-phase tapped auto -transformer and when it has accelerated sufficiently, it is switched to the running (full voltage) position. The principle is similar to star/delta starting and has similar limitations. The advantage of the method is that the current and torque can be adjusted to the required value, by taking the correct tapping on the autotransformer. This method is more expensive because of the additional autotransformer.

Star-Delta Starting method

Auto Transformer

Auto Transformer

In Auto Transformer starting, the auto transformer has at least three tapings. If the transformer ratio is 1:k (k<1) at a particular instant, the starting current is reduced to k*IDOL from its full voltage current. This method gives a better flexibility and good starting characteristics compared with the above methods..

The autotransformer starter is more expensive than the two types, so far described and is generally used only for the larger types of motor. It is suitable for motors in which each end of the three phases is not brought out, and which would therefore be unsuitable for star-delta starting.

The auto transformer may have a number of output taps and be set up to provide a single stage starter, or a multi stage charger. Typically the autotransformer would have several taps. Typically there are three taps.

There are two ways of connecting an autotransformer starter. The most obvious way is to apply full voltage to the transformer via a contactor. When the motor gets the full speed the tap connector opens disconnecting the motor from the transformer and another contactor closes connecting the motor to the supply. Then the transformer is disconnected from the supply. This method is known as the open transition starter. The other method is known as the closed transition ‘Korndorfer’ starting method.

An Auto transformer starter uses an auto transformer to reduce the voltage applied to a motor during start. The auto transformer may have a number of output taps and be set-up to provide a single stage starter, or a multistage starter. Typically, the auto transformer would have taps at 50%, 65% and 80% voltage, enabling the motor to be started at one or more of these settings. There are two ways of connecting an auto transformer starter, the most obvious way is to apply full 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 contactor opens, disconnecting the motor from the transformer and another contactor closes connecting the motor to the supply. The transformer can now be disconnected from the supply. This format is known as an open transition starter and is less than ideal due to the fact that the motor is disconnected for a short period of time during the start period. While the motor is connected and accelerating, there is a rotating magnetic field in the stator which causes flux in the rotor and thus a rotor current to flow. At the instant the motor is disconnected, there is a magnetic field in the rotor which is spinning with-in the stator winding. The motor acts as a generator until the rotor field decays. The voltage generated by the motor is not synchronised to the supply, and so on reconnection to the supply, the voltage across the contactor at closure can be as much as twice the supply voltage resulting in a very high current and torque transient. This open transition switching is often known as the auto-reclose effect as it yields similar characteristics to opening and closing a breaker on a supply to one or more motors. The consequences of open transition switching can be as bad as broken shafts and stripped gears.By a rearrangement of the power circuit, it is possible, at no extra cost, to build a closed transition starter and thereby eliminate the current and torque transients. The closed transition auto transformer starter is known as the Korndorffer starter. The open transition switching is achieved by reconnecting the tap contactor between the transformer and motor, to the star connection of the transformer, hard wiring the motor to the tap, and altering the sequence of contactor control. To start the machine, the

main contactor and the star contactors are closed applying reduced voltage to the motor. When the motor has reached full speed, (or run out of acceleration torque) the star contactor is opened effectively converting the auto transformer starter into a primary reactance starter. Next the primary reactance is bridged by a contactor applying full voltage to the motor. At no time does the motor become disconnected from the supply.The transformer is generally only intermittent rated for the starting duty, and so the frequency and duration of the starts is limited. With a transformer starter, it is relatively easy to change taps and thereby increase the starting voltage if a higher torque is required. The auto transformer starter is a constant voltage starter, so the torque is reduced by the voltage reduction squared over the entire speed range, unlike the primary resistance or primary reactance starters which are constant impedance starters and where the start voltage is dependant on the ratio of the motor impedance to the motor plus starter impedance. As the motor accelerates, it's impedance rises and consequently, the terminal voltage of the motor also rises, giving a small torque increase at higher speeds.Unlike the primary resistance and primary reactance starter, the current flowing into the motor is different from that flowing from the supply. The supply current flows into the primary circuit of a transformer, and the secondary current is applied to the motor. The transformer reduces the primary current by the same ratio as the voltage reduction. If the motor is connected to the 50% tap of the transformer, the voltage across the motor terminals will be 50%. Assuming an LRC of 600%, there will be 300% current flowing into the motor. If 300% current flows into the motor, then the current into the transformer will be 150%. This would suggest that the lowest starting current will be achieved by the use of an auto transformer starter. In most instances, the load will require an increasing torque as it accelerates, and so often a higher tap must be selected in order to accelerate the load to full speed before the step to full voltage occurs. If a multistage transformer starter is employed, then the primary current will certainly be lower than other forms of induction motor starter.

Reversing the motor The motors are expected to run on both directions. To achieve this, we should change the direction of the rotating magnetic field and by changing the direction of the current flow, we can do this. Changing any two of the supply lines is enough in this process.

Reversing: Reversing the connections to any two of the three motor terminals can reverse the direction of rotation of 3-phase induction motor Procedure 1. Study the construction and the various parts of the 3-phase induction motor. 2. For rotor resistance starting, connect the slip-ring motor as shown in FIG.1. Start the motor with full starting resistance and then decrease the resistancein steps down to zero. Take observations of the stator & rotor currents 3. For direct-on -line starting , connect the cage motor as shown in FIG.2 4. For star-delta starting , connect the cage motor to the terminals of the stardelta switch (FIG.3) 5. For autotransformer starting, connect the cage motor as shown in FIG.4. Take care at starting that the "Run" switch is open and that it is not closed

before the "Start" switch is opened. 6. In each case observe the starting currents by quickly reading the maximum indication of the ammeters in the stator circuit. 7. Reverse the direction of rotation of the motor by reversing of two phases at the terminal box. The reversal has to be made when the motor is stopped and the supply switched off.

The speed reversing starter of 3-Phase Squirrel-Cage Motor is shown below. The circuit uses two contactors. The method of reversing the motor is simple which is switching any two phases.

Shaded-pole motor

A common single-phase motor is the shaded-pole motor, which is used in devices requiring low torque, such as electric fans or other small household appliances. In this motor, small single-turn copper "shading coils" create the moving magnetic field. Part of each pole is encircled by a copper coil or strap; the induced current in the strap opposes the change of flux through the coil (Lenz's Law), so that the maximum field intensity moves across the pole face on each cycle, thus producing the required rotating magnetic field.

A Shaded Pole AC Motor

Here is a photo of a typical shaded pole motor.  See the close-up of the notch in the laminations and the extra heavy winding of two turns creating the phase difference between the two sections of the laminations, giving the magnetic field a directional motion.   The rotor spins CW as seen from the end with the screw on the shaft.  Motors like this are used in thousands of applications.

Another Shaded Pole AC Motor

Here is a photo of a ceiling fan motor, also shaded pole, but with six windings instead of only one as seen above.  The rotor laminations are skewed to provide smoother torque.   The pole pieces with the windings have a slot in them to create a delayed flux, creating a direction for rotation.

Applications A very well-known motor starter is the DOL Starter of 3-Phase Squirrel-Cage Motor. This starter is sometimes used to start water pumps, compressors, fans and conveyor belts.

Applications of 3 phase:

changing/ adjusting the wind pressure in windblown organs. Motor controllers for lathes, large saws etc... 3-phase current generator Brushless DC motor drives

References

Internet (from www.yahoo.com)

ELECTRIC MACHINARY, By A.E.Fitzgerald and Charles Kingsley.

Lecture Notes