Answer MET Question 19
Question: The direct on line start of squirrel cage motor is used for most electrical drives on a.c. powered ships.
Describe with sketches as necessary one method of overcoming each of the following Problems:
A. High starting current; B. Low starting torque.
Answer: Reason of High starting current at startup:
When an induction motor is connected directly to its three-phase a.c. supply voltage, a very large stator current of 5-8 x full-load current (FLC) is taken. This is due to the maximum rate of flux cutting (s = 100%) in the rotor, creating large induced rotor currents.
The corrosponding supply power factor at startup is very low (0.2 lagging) which rises to about 0.5 (lagging) on no-load and then to 0.85 (lagging) on full-load. This starting surge current reduces as motor accelerates up to its running speed. Operating on light loads at low power factor is inefficient as the supply current is relatively high causing significant $\displaystyle \small \mathrm{I^2R}$ resistive (copper) losses.
When very large motors are started DOL they cause a significant disturbance of voltage (Voltage dip) on the supply lines due to the large starting current surge. This voltage disturbance may result in the malfunction of other electrical equipment connected to the supply e.g.
lighting dip and flickering effects.
A. Method of overcoming High starting current:
To limit the starting current some large induction motors are started at reduced voltage and then have the full supply voltage reconnected when they have accelerated close to their rated speeds. Two methods of reduced voltage starting by switching are called star-delta starting and autotransformer starting but an electrortic "soft" starting option is also used. Contactors perform the switching action in starters to-connect and disconnect the power supply to the motor.
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 contactors 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 overcurrent 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 overcurrent 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 overcurrent protection, the overcurrent relays must be fitted in the phase connections and not in the line connections.
When an induction motor is connected directly to its three-phase a.c. supply voltage, a very large stator current of 5-8 x full-load current (FLC) is taken. This is due to the maximum rate of flux cutting (s = 100%) in the rotor, creating large induced rotor currents.
The corrosponding supply power factor at startup is very low (0.2 lagging) which rises to about 0.5 (lagging) on no-load and then to 0.85 (lagging) on full-load. This starting surge current reduces as motor accelerates up to its running speed. Operating on light loads at low power factor is inefficient as the supply current is relatively high causing significant $\displaystyle \small \mathrm{I^2R}$ resistive (copper) losses.
When very large motors are started DOL they cause a significant disturbance of voltage (Voltage dip) on the supply lines due to the large starting current surge. This voltage disturbance may result in the malfunction of other electrical equipment connected to the supply e.g.
lighting dip and flickering effects.
A. Method of overcoming High starting current:
To limit the starting current some large induction motors are started at reduced voltage and then have the full supply voltage reconnected when they have accelerated close to their rated speeds. Two methods of reduced voltage starting by switching are called star-delta starting and autotransformer starting but an electrortic "soft" starting option is also used. Contactors perform the switching action in starters to-connect and disconnect the power supply to the motor.
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 contactors 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 overcurrent 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 overcurrent 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 overcurrent protection, the overcurrent relays must be fitted in the phase connections and not in the line connections.
see question 119 for more methods.
B. Method of overcoming low starting torque:
Starting torque is given by: $\displaystyle \small \mathrm{T_{st}=K\frac{E^2R_2}{R_2^2+X_2^2}}$
The problem of low starting toque can be solved by increasing rotor resistance.
The slip ring motor can produce more torque than the squirrel cage induction motor. The additional resistance can be added to the rotor winding of the slip ring induction motor. The starting current gets reduced and starting torque of the slip ring induction motor improves. When motor attains its rated speed the rotor resistance is short-circuited and motor functions like a squirrel cage induction motor.
The starting torque of the squirrel cage induction motor can be improved if the rotor resistance is increased. The double cage squirrel cage induction motor is used for the application which requires higher starting torque. At start, the current flows through the outer cage winding of the motor which has higher resistance than inner cage winding resistance.The current starts shifting from the upper or outer cage to inner cage when motor starts accelerating towards its base speed. The most of the current flows through the inner cage when motor is running at its rated speed.
Starting torque is given by: $\displaystyle \small \mathrm{T_{st}=K\frac{E^2R_2}{R_2^2+X_2^2}}$
The problem of low starting toque can be solved by increasing rotor resistance.
The slip ring motor can produce more torque than the squirrel cage induction motor. The additional resistance can be added to the rotor winding of the slip ring induction motor. The starting current gets reduced and starting torque of the slip ring induction motor improves. When motor attains its rated speed the rotor resistance is short-circuited and motor functions like a squirrel cage induction motor.
The starting torque of the squirrel cage induction motor can be improved if the rotor resistance is increased. The double cage squirrel cage induction motor is used for the application which requires higher starting torque. At start, the current flows through the outer cage winding of the motor which has higher resistance than inner cage winding resistance.The current starts shifting from the upper or outer cage to inner cage when motor starts accelerating towards its base speed. The most of the current flows through the inner cage when motor is running at its rated speed.
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