Answer MET Question 13
Question: Starting methods of an induction motor
Answer: 1. Star-Start and delta-Run Method:
If a motor is direct-on-line started with the stator winding star connected, it will only take one-third of the starting current that it would take if the windings were delta connected. The starting current of a motor which is designed to run delta connected can be reduced in this way. Star-delta starters for small motors may be operated by a manual changeover switch. For large power motors, the phase windings are automatically switched using contractors controlled by a timing relay. A choice of time delay relays are available whose action is governed by thermal, pneumatic, mechanical or electronic control devices. At the instant of starting when the supply has just been switched on and the motor has not yet started to rotate, there is no mechanical output from the motor. The only factors which determine
the current taken by the motor are the supply voltage (V) and the impedance of the motor phase windings ($\displaystyle \small \mathrm{Z_{ph}}$ ). Compare the starting current when star connected to the starting current when delta connected.
$\displaystyle \small \mathrm{\frac{I_{L(Y)}}{I_{L(\Delta )}}=\frac{\frac{V_L}{\sqrt{3}\times Z}}{\frac{\sqrt{3}\times V_L}{Z}}=\frac{1}{3}}$
This shows that the starting current of a delta connected motor can be reduced to one third if the motor is star connected for starting. The shaft torque is also reduced to one-third which reduces the shaft acceleration and increases the run-up time for the drive but this is not
usually a problem. When an induction motor is running on load it is converting electrical energy input to mechanical energy output. The
input current is now determined by the load on the motor shaft.
An induction motor will run at the same speed when it is star connected as when it is delta connected because the flux speed is the same in both cases being set by the supply frequency. This means that the power output from the motor is the same when the motor is star connected as when the motor is delta connected, so the power inputs and line currents must be the same when
running in either connection. If the motor is designed to run in delta but is run as star connected, and on full load, then each stator phase winding will be carrying an over-current of $\displaystyle \small \mathrm{\sqrt{3}}$ x rated phase current. This is because phase and line currents are equal in a star connection. This will cause overheating and eventual burnout unless tripped by the over-current relay. Remember that the motor copper losses are produced by the $\displaystyle \small \mathrm{I^2R}$ heating effect so the motor will run $\displaystyle \small \mathrm{(\sqrt{3})^2}$ = 3 times hotter if left to run in the star connection when designed for delta running. This malfunction may occur if the control timing sequence is not completed or the star contactor remains closed while a mechanical interlock prevents the delta contactor from closing.
For correct over-current protection, the over-current relays must be fitted in the phase connections and not in the line connections.
2. Auto transformer Starting:
Starting a large motor with a long run-up period will demand a very high current surge from the supply generator for a few seconds. This causes a severe voltage dip which affects every load on the system. Reduced voltage starting will limit the starting surge current. One way to reduce the initial voltage supplied to the motor is to step it down using a transformer. Then, when the motor has accelerated up to almost full speed, the reduced voltage is replaced by the full mains voltage. The transformer used in this starter is not the usual type with separate primary and secondary windings. It is an auto transformer which uses only one winding for both input and output. This arrangement is cheaper, smaller and lighter than an equivalent double-wound transformer and it is only in operation during the short starting period. For induction motor starting, the auto transformer is a 3-phase unit, and, because of expense, this method is only used with large motor drives, e.g. electric cargo Pumps the supply voltage is connected across the complete winding and the motor is connected to the reduced voltage tapping. A number of tappings are usually available on the transformer winding, giving voltage outputs ranging from about 50% to 80% of the mains supply voltage. A 60% tap on an auto transformer supplied at 440 V would provide a voltage output of 60% of 440 = 264 V. The auto transformer usually has a few tapping points to give i set of reduced voltages (e.g. 40%, 50% and 65%) which help to match the motor current demand to the supply capability.
As with the star-delta starter, the auto transformer may use what is called an open-transition switching sequence or a closed-transition switching sequence between the start and run conditions. In the former, the reduced voltage is supplied to the motor at start then disconnected and the full supply voltage rapidly reconnected to the motor. The problem with open-transition is that a very large surge current can flow after the transition from reduced to full voltage.
3. "Soft" Starting
This method of supplying a gradually increasing a.c. voltage during start up generally refers to an efficient electronic switching technique A basic method is to use back-to-back connected thyristors or triacs in the supply lines which are "gated" to delay "turn-on, within each a.c. half-cycle. This delayed switching applies a reduced average a.c. voltage to the motor. The applied motor voltage is gradually ramped up by the starter software program until the full voltage level is reached. To achieve maximum efficiency, the electronic switching circuit can now be bypassed for normal running. A "soft" starter may be further adapted to become a voltage controller over the motor operating load range. In this type of efficient "energy manager" application, the controller monitors the motor power factor which is a measure of the motor loading. On light load and full voltage, the power factor is low so the controller reduces the motor voltage which reduces current while improving power factor and efficiency.
If a motor is direct-on-line started with the stator winding star connected, it will only take one-third of the starting current that it would take if the windings were delta connected. The starting current of a motor which is designed to run delta connected can be reduced in this way. Star-delta starters for small motors may be operated by a manual changeover switch. For large power motors, the phase windings are automatically switched using contractors controlled by a timing relay. A choice of time delay relays are available whose action is governed by thermal, pneumatic, mechanical or electronic control devices. At the instant of starting when the supply has just been switched on and the motor has not yet started to rotate, there is no mechanical output from the motor. The only factors which determine
the current taken by the motor are the supply voltage (V) and the impedance of the motor phase windings ($\displaystyle \small \mathrm{Z_{ph}}$ ). Compare the starting current when star connected to the starting current when delta connected.
$\displaystyle \small \mathrm{\frac{I_{L(Y)}}{I_{L(\Delta )}}=\frac{\frac{V_L}{\sqrt{3}\times Z}}{\frac{\sqrt{3}\times V_L}{Z}}=\frac{1}{3}}$
This shows that the starting current of a delta connected motor can be reduced to one third if the motor is star connected for starting. The shaft torque is also reduced to one-third which reduces the shaft acceleration and increases the run-up time for the drive but this is not
usually a problem. When an induction motor is running on load it is converting electrical energy input to mechanical energy output. The
input current is now determined by the load on the motor shaft.
An induction motor will run at the same speed when it is star connected as when it is delta connected because the flux speed is the same in both cases being set by the supply frequency. This means that the power output from the motor is the same when the motor is star connected as when the motor is delta connected, so the power inputs and line currents must be the same when
running in either connection. If the motor is designed to run in delta but is run as star connected, and on full load, then each stator phase winding will be carrying an over-current of $\displaystyle \small \mathrm{\sqrt{3}}$ x rated phase current. This is because phase and line currents are equal in a star connection. This will cause overheating and eventual burnout unless tripped by the over-current relay. Remember that the motor copper losses are produced by the $\displaystyle \small \mathrm{I^2R}$ heating effect so the motor will run $\displaystyle \small \mathrm{(\sqrt{3})^2}$ = 3 times hotter if left to run in the star connection when designed for delta running. This malfunction may occur if the control timing sequence is not completed or the star contactor remains closed while a mechanical interlock prevents the delta contactor from closing.
For correct over-current protection, the over-current relays must be fitted in the phase connections and not in the line connections.
2. Auto transformer Starting:
Starting a large motor with a long run-up period will demand a very high current surge from the supply generator for a few seconds. This causes a severe voltage dip which affects every load on the system. Reduced voltage starting will limit the starting surge current. One way to reduce the initial voltage supplied to the motor is to step it down using a transformer. Then, when the motor has accelerated up to almost full speed, the reduced voltage is replaced by the full mains voltage. The transformer used in this starter is not the usual type with separate primary and secondary windings. It is an auto transformer which uses only one winding for both input and output. This arrangement is cheaper, smaller and lighter than an equivalent double-wound transformer and it is only in operation during the short starting period. For induction motor starting, the auto transformer is a 3-phase unit, and, because of expense, this method is only used with large motor drives, e.g. electric cargo Pumps the supply voltage is connected across the complete winding and the motor is connected to the reduced voltage tapping. A number of tappings are usually available on the transformer winding, giving voltage outputs ranging from about 50% to 80% of the mains supply voltage. A 60% tap on an auto transformer supplied at 440 V would provide a voltage output of 60% of 440 = 264 V. The auto transformer usually has a few tapping points to give i set of reduced voltages (e.g. 40%, 50% and 65%) which help to match the motor current demand to the supply capability.
As with the star-delta starter, the auto transformer may use what is called an open-transition switching sequence or a closed-transition switching sequence between the start and run conditions. In the former, the reduced voltage is supplied to the motor at start then disconnected and the full supply voltage rapidly reconnected to the motor. The problem with open-transition is that a very large surge current can flow after the transition from reduced to full voltage.
3. "Soft" Starting
This method of supplying a gradually increasing a.c. voltage during start up generally refers to an efficient electronic switching technique A basic method is to use back-to-back connected thyristors or triacs in the supply lines which are "gated" to delay "turn-on, within each a.c. half-cycle. This delayed switching applies a reduced average a.c. voltage to the motor. The applied motor voltage is gradually ramped up by the starter software program until the full voltage level is reached. To achieve maximum efficiency, the electronic switching circuit can now be bypassed for normal running. A "soft" starter may be further adapted to become a voltage controller over the motor operating load range. In this type of efficient "energy manager" application, the controller monitors the motor power factor which is a measure of the motor loading. On light load and full voltage, the power factor is low so the controller reduces the motor voltage which reduces current while improving power factor and efficiency.
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