Answer MET Question 21
Question: A. What are the causes of overheating of an induction motor?
B. Preventive measures against damage to an induction motor in installed condition.
Motor protective equipments are fitted to protect motor against damage in the event of any fault. Also periodic maintentance of the devices, its connection and motor itself is important preventive measure.
Overheating can be detected by a rise in line current and by temperature change. Overheating as the result of high ambient temperature or poor cooling due to blocked air passages can only be detected by temperature rise within the windings.
Motor protection Protection of motors is required mainly to prevent overheating which can cause deterioration of winding insulation and burnout, if severe.
Overload protection is required for all motors of more than 0.5 kW although different rules apply to steering gear motors and others essential to safety or propulsion. A conventional electromagnetic overload trip must have a time delay dashpot (similar to those for d.c. switchboards) to allow for high starting current in direct on-line started induction motors. Unfortunately, an electromagnetic overload trip can be reset quickly and a motor restarted repeatedly with the result of excessively high winding temperature, unless a temperature trip is also provided. Each of the three supply phases of the motor is fitted with an overload relay. Such an arrangement should detect single-phasing, where the symptoms are of high current in two supply lines only as well as straight overload. Operation of any of the relays will close the circuit to energise the time-delayed trip.
The thermistor is a thermal device which can be used in conjunction with an electromagnetic overload trip. One of these inserted in each of the three windings would detect overheating from any cause. Therrnistors are available with either a positive or a negative characteristic. The former type are more definite in operation because there is a very sharp rise in resistance at a particular temperature (as opposed to gradual drop in resistance of the other sort). Positive thermistors can be connected simply in series and the very small current which passes through them normally is cut off by the effect of overheating in any one of them. Cessation of the minute checking current is used as the signal to operate the motor trip. Alternative methods of detecting overload current employ directly or indirectly heated hi-metal strips. Excessive current in any of the supply cables will cause deflection of the bi-metal strip through temperature rise. Thickness of the strips is used to delay tripping when a motor starts. (A thick bi-metal strip takes longer to heat.) Short-circuit protection is also a requirement for motors of over 0.5 kW. Fuses of the cartridge/high rupture capacity (HRC) design are employed to provide the necessary rapid interruption of high fault current. Because short-circuit current may be high enough to damage normal motor contacts, the fuses may be arranged to break first in the event of short circuit. The secondary function of fuses is to provide back-up for the other protective devices.
C. Fuses of the cartridge/high rupture capacity (HRC) design are employed to provide the necessary rapid interruption of high fault current. Because short-circuit current may be high enough to damage normal motor contacts, the fuses may be arranged to break first in the event of short circuit. The secondary function of fuses is to provide back-up for the other protective devices.
D. Induction motor development of torque:
The torque is produced by the interaction between the magnetic field of the stator and the magnetic field of the rotor.
The shaft torque produced is given by T x Φ.I where Φ is the flux produced by the series connected stator winding and I is the armature (and supply) current in the rotor. As Φ is produced by the same current the torque is essentially $\displaystyle \small \mathrm{T\ \alpha\ I^2}$ which makes this single phase a.c. motor more powerful than the induction types.
E. Condition to be satisfied for achieving maximum running torque in an induction motor:
Maximum Torque at any time:
from the torque equation we can see $\displaystyle \small \mathrm{ \frac{SR_2}{{R_2^2+(SX_2)^2}}}$ should be maximum.
or $\displaystyle \small \mathrm{ \frac{R_2}{{(\frac{R_2}{\sqrt{s}}-\sqrt{s}X_2)^2+2R_2X_2}}}$ should be maximum.
or $\displaystyle \small \mathrm{\frac{R_2}{\sqrt{s}}-\sqrt{s}X_2 =0}$
or $\displaystyle \small \mathrm{R_2 = sX_2}$
Thus the Torque will be maximum when, $\displaystyle \small \mathrm{R_2 = sX_2}$ which is $\displaystyle \small \mathrm{T_{max}= \frac{KE_2^2}{2X_2} }$
From the above expression, the following conclusions can be drawn
i. Maximum torque is independent of rotor circuit resistance.
ii. Maximum torque varies inversely as standstill reactance of the rotor. Therefore to have maximum torque, standstill reactance (i.e. inductance) of the rotor should be kept as small as possible.
iii. The slip at which the maximum torque occurs depends upon the resistance of the rotor.
Starting torque (S=1):
$\displaystyle \small \mathrm{T\ =K_1 \times \frac{E_2^2R_2}{{R_2^2+(X_2)^2}}}$
$\displaystyle \small \mathrm{T_{st}\ =K_2 \times \frac{R_2}{{R_2^2+(X_2)^2}}}$
where, $\displaystyle \small \mathrm{K_2= K_1 \times E_2^2 }$
Maximum Starting Torque:
$\displaystyle \small \mathrm{\frac{dT_{st}}{dR_2}\ =K_2 \times\left ( \frac{1}{{R_2^2+(X_2)^2}}-\frac{R_2(2R_2)}{{(R_2^2+X_2^2)^2}} \right )=0 }$
$\displaystyle \small \mathrm{R_2=X_2}$
Thus it is clear the maximum starting torque will be when, $\displaystyle \small \mathrm{R_2=X_2}$
Starting torque of a squirrel-cage motor.
The squirrel cage rotor resistance is fixed and small as compared to its reactance which is very large especially at start (because at standstill the frequency of rotor current is equal to that of supply frequency). Hence, the starting current $\displaystyle \small \mathrm{I_2 }$ of the rotor, though very large in magnitude, lags by a very large angle behind $\displaystyle \small \mathrm{E_2 }$ ; consequently the starting torque per ampere is very poor. It is roughly 1.5 times the full-load torque although the starting current is 5 to 7 times the full-load current. Thus such motors are not suitable for applications where these have to be started against heavy loads. Starting torque of a slip ring motor.
In a slip ring motor the torque is increased by improving its power factor by adding external resistance in the rotor circuit from the star. connected rheostat ; as the motor gains speed the rheostat resistance is gradually cut out. This additional resistance, however, increases the rotor impedance and so reduces the rotor current. At first, the effect of improved power factor predominates the current-decreasing effect of impedance, hence starting torque is increased. But after a certain point, the effect of increased impedance predominates the effect of improved power factor and so the torque starts decreasing.
B. What preventive measures are provided against damage to an induction motor in installed condition?
C. What is the purpose of ‘fuse back up protection’ provided to an induction motor?
D. How does an induction motor develop torque?
E. What is the condition to be satisfied for achieving maximum running torque in an induction motor?
Answer:
A. Overheating is the result of overload, stalling, single-phasing,
mechanical obstruction, moiture ingress, frequent start-stop or
prolonged starting period. B. Preventive measures against damage to an induction motor in installed condition.
Motor protective equipments are fitted to protect motor against damage in the event of any fault. Also periodic maintentance of the devices, its connection and motor itself is important preventive measure.
Overheating can be detected by a rise in line current and by temperature change. Overheating as the result of high ambient temperature or poor cooling due to blocked air passages can only be detected by temperature rise within the windings.
Motor protection Protection of motors is required mainly to prevent overheating which can cause deterioration of winding insulation and burnout, if severe.
Overload protection is required for all motors of more than 0.5 kW although different rules apply to steering gear motors and others essential to safety or propulsion. A conventional electromagnetic overload trip must have a time delay dashpot (similar to those for d.c. switchboards) to allow for high starting current in direct on-line started induction motors. Unfortunately, an electromagnetic overload trip can be reset quickly and a motor restarted repeatedly with the result of excessively high winding temperature, unless a temperature trip is also provided. Each of the three supply phases of the motor is fitted with an overload relay. Such an arrangement should detect single-phasing, where the symptoms are of high current in two supply lines only as well as straight overload. Operation of any of the relays will close the circuit to energise the time-delayed trip.
The thermistor is a thermal device which can be used in conjunction with an electromagnetic overload trip. One of these inserted in each of the three windings would detect overheating from any cause. Therrnistors are available with either a positive or a negative characteristic. The former type are more definite in operation because there is a very sharp rise in resistance at a particular temperature (as opposed to gradual drop in resistance of the other sort). Positive thermistors can be connected simply in series and the very small current which passes through them normally is cut off by the effect of overheating in any one of them. Cessation of the minute checking current is used as the signal to operate the motor trip. Alternative methods of detecting overload current employ directly or indirectly heated hi-metal strips. Excessive current in any of the supply cables will cause deflection of the bi-metal strip through temperature rise. Thickness of the strips is used to delay tripping when a motor starts. (A thick bi-metal strip takes longer to heat.) Short-circuit protection is also a requirement for motors of over 0.5 kW. Fuses of the cartridge/high rupture capacity (HRC) design are employed to provide the necessary rapid interruption of high fault current. Because short-circuit current may be high enough to damage normal motor contacts, the fuses may be arranged to break first in the event of short circuit. The secondary function of fuses is to provide back-up for the other protective devices.
C. Fuses of the cartridge/high rupture capacity (HRC) design are employed to provide the necessary rapid interruption of high fault current. Because short-circuit current may be high enough to damage normal motor contacts, the fuses may be arranged to break first in the event of short circuit. The secondary function of fuses is to provide back-up for the other protective devices.
D. Induction motor development of torque:
The torque is produced by the interaction between the magnetic field of the stator and the magnetic field of the rotor.
The shaft torque produced is given by T x Φ.I where Φ is the flux produced by the series connected stator winding and I is the armature (and supply) current in the rotor. As Φ is produced by the same current the torque is essentially $\displaystyle \small \mathrm{T\ \alpha\ I^2}$ which makes this single phase a.c. motor more powerful than the induction types.
E. Condition to be satisfied for achieving maximum running torque in an induction motor:
Maximum Torque at any time:
from the torque equation we can see $\displaystyle \small \mathrm{ \frac{SR_2}{{R_2^2+(SX_2)^2}}}$ should be maximum.
or $\displaystyle \small \mathrm{ \frac{R_2}{{(\frac{R_2}{\sqrt{s}}-\sqrt{s}X_2)^2+2R_2X_2}}}$ should be maximum.
or $\displaystyle \small \mathrm{\frac{R_2}{\sqrt{s}}-\sqrt{s}X_2 =0}$
or $\displaystyle \small \mathrm{R_2 = sX_2}$
Thus the Torque will be maximum when, $\displaystyle \small \mathrm{R_2 = sX_2}$ which is $\displaystyle \small \mathrm{T_{max}= \frac{KE_2^2}{2X_2} }$
From the above expression, the following conclusions can be drawn
i. Maximum torque is independent of rotor circuit resistance.
ii. Maximum torque varies inversely as standstill reactance of the rotor. Therefore to have maximum torque, standstill reactance (i.e. inductance) of the rotor should be kept as small as possible.
iii. The slip at which the maximum torque occurs depends upon the resistance of the rotor.
Starting torque (S=1):
$\displaystyle \small \mathrm{T\ =K_1 \times \frac{E_2^2R_2}{{R_2^2+(X_2)^2}}}$
$\displaystyle \small \mathrm{T_{st}\ =K_2 \times \frac{R_2}{{R_2^2+(X_2)^2}}}$
where, $\displaystyle \small \mathrm{K_2= K_1 \times E_2^2 }$
Maximum Starting Torque:
$\displaystyle \small \mathrm{\frac{dT_{st}}{dR_2}\ =K_2 \times\left ( \frac{1}{{R_2^2+(X_2)^2}}-\frac{R_2(2R_2)}{{(R_2^2+X_2^2)^2}} \right )=0 }$
$\displaystyle \small \mathrm{R_2=X_2}$
Thus it is clear the maximum starting torque will be when, $\displaystyle \small \mathrm{R_2=X_2}$
Starting torque of a squirrel-cage motor.
The squirrel cage rotor resistance is fixed and small as compared to its reactance which is very large especially at start (because at standstill the frequency of rotor current is equal to that of supply frequency). Hence, the starting current $\displaystyle \small \mathrm{I_2 }$ of the rotor, though very large in magnitude, lags by a very large angle behind $\displaystyle \small \mathrm{E_2 }$ ; consequently the starting torque per ampere is very poor. It is roughly 1.5 times the full-load torque although the starting current is 5 to 7 times the full-load current. Thus such motors are not suitable for applications where these have to be started against heavy loads. Starting torque of a slip ring motor.
In a slip ring motor the torque is increased by improving its power factor by adding external resistance in the rotor circuit from the star. connected rheostat ; as the motor gains speed the rheostat resistance is gradually cut out. This additional resistance, however, increases the rotor impedance and so reduces the rotor current. At first, the effect of improved power factor predominates the current-decreasing effect of impedance, hence starting torque is increased. But after a certain point, the effect of increased impedance predominates the effect of improved power factor and so the torque starts decreasing.
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