induction motor construction about rotors

1. BABA SAHEB BHIMRAO AMBEDKAR
UNIVERSITY
presented by,
Nisha Bharti
Electrical
engineering
6th semester
Roll No-118028

2. Contents
Why we need deep bar And double cage rotor
Deep-bar cage rotors
Double-cage rotors
Torque-slip characteristics
Emf –injection in rotor circuit
Slip ring induction motor
Applications

3. Introduction
 Induction motor consists of two parts
namely:1-stator 2-rotor
 Stator is a stationary part and rotor is a
rotating part of motor.
 Induction motor has two types of rotors :
a) squirrel cage b) phase wound or
wound rotor
 Induction motor having wound rotor is
known as slip ring induction motor

4. Why we need deep bar & double
cage rotor
 A higher resistance gives higher torque and
lower starting line current at higher power
factor .
 But for normal operation when running a
low rotor resistances is required so that slip
is low and efficiency is high.
 Therefore for good starting performances
,the rotor resistances should be high and
under normal operation speeds ,the rotor
resistance is low.
 The starting torque can be increased by
using bar material of higher resistivity.

5. Deep bar cage rotor

6. Deep bar cage rotor
 Earlier cage rotors have lower starting torque . but
on increasing resistivity of bar materials torque can
be increased.
 Deep bar cage rotor with deep and narrow bars is
given.
 Bar may be assumed to be made of number of
narrow layers connected in parallel . as shown in
diagram A&B.
 In diagram topmost layer element A is linked with
minimum linkage flux therefore minimum leakage
flux .therefore its leakage inductance is maximum.

7.  At starting condition, the rotor –frequency is equal
to the supply frequency.
 The bottom layer offers more impedance to flow of
current than the top layer element A . Therefore
maximum current flow through the top layer than
the bottom layer.
 Because of the unequal current distribution the
effective rotor resistances increases and leakage
reactance decreases.
 With high rotor resistances at starting conditions ,
starting torque is relatively higher and starting
current is relatively low.

8.  UNDER NORMAL CONDITION- in this
condition the slip and rotor frequency are very
small. The reactances of all the bar are small
compared to their resistances .
 The impedance of the bar are nearly equal, so
current flows through all the parts of the bar
equally.
 The resulting large cross section area makes
rotor resistances quite small, resulting in good
efficiency at low slips.

9. Double cage rotor

10. Double cage induction motors
 In induction motor with two rotor windings or cage is
used for obtaining high torque and low starting
current.
 The stator of double cage rotor induction motor is
similar to that of ordinary induction motor .
 In double cage rotor there are two layers of bars.
each layer is short circuited by end ring .
 The outer cage bar have a smaller cross section
area than the inner bars and made of higher
resistivity like bronze, aluminum , brass
 The inner cage is made of low resistance copper.
 There is a slit between top and bottom slots .

11.  AT STARTING CONDITION- the voltage
induced in the rotor is same as the supply
frequency.
 Hence the leakage reactance is of the inner
cage winding is more than that of the outer
cage Winding.
 Therefore the most of the starting current is
flowing in the outer cage winding which offers
low resistance to the flow of current.
 The high resistance outer cage winding ,
therefore develops a high starting torque.
 Rotor speed increases ,the frequency of the
rotor Emf decreases

12.  AT NORMAL CONDITION – the leakage
reactances of both the winding becomes negligibly
small.
 The rotor current division between the two cages
is governed mainly by their resistances . Since
the resistances of outer cage is 5 to 6 times more
than the inner cage .
 most of the rotor current flows through the inner
cage . Hence under normal operating speed ,
torque is developed mainly by the low resistance
inner cage .

13. Torque – slip characteristics of
double cage induction motor

15. Applications
 Used in irrigational and industrial purposes.
 Used in lathe machines and drilling
Machines.

16. Conclusion
 For low torque requirements an ordinary
cage motor is generally used .
 For higher torque requirements a deep
bar cage motors is used .
 A double cage rotor is used for higher
torques
 For larger size motors with very large
starting torques and exceptionally long
starting periods slip rings construction is
used.

17. Emf injection in rotor
 Injecting slip frequency Emf into rotor
side - When the speed control of three
phase induction motor is done by adding
resistance in rotor circuit, some part of power
called, the slip power is lost as I2R losses.
 Therefore the efficiency of three phase
induction motor is reduced by this method of
speed control. This slip power loss can be
recovered and supplied back in order to
improve the overall efficiency of three phase
induction motor and this scheme of
recovering the power is called slip power
recovery scheme and this is done by
connecting an external source of Emf of slip
frequency to the rotor circuit.

18.  The injected Emf can either oppose the rotor
induced Emf or aids the rotor induced Emf. If it
oppose the rotor induced Emf, the total rotor
resistance increases and hence speed
decreases and if the injected Emf aids the main
rotor Emf the total decreases and hence speed
increases.
 Therefore by injecting induced Emf in rotor
circuit the speed can be easily controlled. The
main advantage of this type of speed control of
three phase induction motor is that wide range
of speed control is possible whether its above
normal or below normal speed.

19. Wound or slip ring motor
 The wound rotor consists of a slotted
armature . Insulated conductors are put
into the slots .
 The rotors winding are connected to form
star winding .the open end of star circuit
are brought outside the rotor and
connected to the three insulated slip rings
.slip rings are mounted on the shaft with
brushes resting on them.
 Slip rings and brushes are provided to
connecting external resistors in the rotor
circuit.

20. References
 Ashfaq husain ,electric machines”
 Power electrical.com