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TUEL: Phase Advancer and method for speed control and power factor
correction of three phase induction motor
FIELD OF INVENTION
The invention relates to the control of speed and power factor of three phase
induction motor in the field of A.C electrical machines of electrical engineering.
BACK GROUND OF THE INVENTION AND PRIOR ART
3 Phase induction motors are of two types e.g. squirrel cage type and phase
wound type. Both types of motors are extensively used in the Industry for a host
of conventional applications ranging from pumps, grinders, blowers, conveyers,
hauling machines, rolling mills, machine tool drives, drives for chemical
processes, utilities etc. to some recent applications in traction. Squirrel cage
motors are used as industrial workhorse on applications where change in speed,
torque and p.f. is not desirable i.e. power flow to the drive is ensured at almost
constant speed and p.f. Once this motor starts with the desired starting torque, it
is expected to provide mechanical power at almost constant running torque
thereby keeping the speed constant. But many a times due to process
requirements in chemicals industry and in some machine tooling and traction
duties, change in speed, power factor and torque is required to be changed at
frequent intervals and some times even for small intervals. For such duties cage
motors can not be used. For such duties and applications only three phase
wound rotor motors are suitable. Whereas change in speed and torque of wound
rotor motor is done to suit a particular process requirement, improvement in
power factor of the motor is also sought in order to reduce the input current to
the motor, thereby relieving the connecting cables from electrical stress which

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ultimately results in saving of electrical energy and consequently reduction of
electrical bill to the consumer.
The following methods are commonly used for change in speed of Induction
motors.
(a) Change from Stator side: Under this the following methods are commonly
available.
(i) Change of Stator Poles: This method is suitable only for squirrel
cage induction motors. Change in stator poles are done by the
following methods,
(aa) Method of Consequent poles
(bb) Poles amplitude modulation
(ii) Chang of Frequency
(iii) Line Voltage Control
(b) change from Rotor side
(i) Rotor Resistance Control
(ii) E.M.F injection.
Method of Consequent Poles:
In this method change in the number of poles is effected by making change in
the stator winding connections with the help of suitable switching arrangement.
The number of stator poles can be changed in the ratio of 2:1 by simple change
in the stator coil connection. This method is not at all suitable for phase wound
motor as reconnection of the rotor winding/coils becomes difficult to give same
number of poles as that of the stator. If two independent sets of stator windings

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are employed, each arranged for pole changing we may get four synchronous
speeds. The schematic arrangement is described below.
Figure 2.2.1(a) shows two coils ai - ai and a2 - a2 belonging to phase a of the
stator winding. Assuming that each such coils are short pitched and are designed
to carry equal current, both in clockwise and anticlockwise direction, each coil
forms a north pole and consequent south poles are created in the intervening
space between the coils. It means that we have created 4 poles out of two coils
i.e. a low speed connection (giving a synchronous speed of 1500 r.p.m.) is
achieved. If now current in one of the two coils are reversed by means of a
controller as shown in Figure 2.2.1(b), the number of poles becomes half of the
previous case i.e. a2 poles construction is achieved which gives 3000 r.p.m. as
synchronous speed i.e. a high speed domain. We can have series/parallel
connection of connection of phase group of individual phases as shown in Figure
2.2.1 (c) & 2.2.1 (d), i.e. the phases can be now connected in star/delta to give
two speed operation resulting in constant - torque, constant H.P. and variable -
torque characteristics.
Pole Amplitude Modulation:
This method of pole changing does not necessarily gives 2:1 speed change but
some other speeds in between. One such method is described below.
Figure 2.2.2 (a) shows the flux distribution of one phase of a 3 phase winding
which are actually sine waves but for simplicity they are shown as rectangular
waves.
We can express the flux density B0 at an angle 0 measured mechanically around
the perimeter of the core as

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B0=ASin (P/2)θ (2.2.2.1)
Where A is the amplitude and P is the number of poles; here A = CSinke & P- 8
assumed, where k is a integer and C = a constant. Then the field is given by
B0 = CSinkθ Sin (P/2)θ
= C/2[Cos (P/2 -k )θ - Cos (P/2 + k)θ] (2.2.2.2)
From equation 2.2.2.2. it is clear that the resultant B0 is combination of 6 pole
field and 8 pole field. To remove one of the poles not desired, the windings of
the 3 phases can be spaced electrically by 2 πr/3, where r is given by any of the
following equations,
( m/r) = l/3(l+2k/p) or (m/r) = 1/3(1- 2k/p) (2.2.2.3)
If P = 8 and k = 1, then from equation 2.2.2.2 the number of poles after
modulation is 6 or 10. Now if we want lesser speed i.e. if speed corresponding
to 10 poles then if m = 1 and r =4, from equation 2.2.2.3 the lower pole number
6 vanishes. It means that with a 8 pole field we get a modulated field of 10 pole.
This is achieved by having each phase winding in two equal halves and then by
reversing the second half of winding with respect to the first half. Another
method to get the desired modulation is to omit a fixed section form each half
and reversing the half of remaining winding with respect to the first half.
Complete process of pole amplitude modulation has been given in the Figure
2.2.2 (i) to 2.2. (v).

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Disadvantages of Pole Changing:
(i) Synchronous speeds obtained by this method is in steps i.e. 300r.p.m.
(for 2 poles), 1500 r.p.m.(4 poles), 1000 r.p.m. (6 poles), 750 r.p.m (8
poles) and so on. No. intermediate speeds such as 2900 or 1600 or
1550 or 1100 or 1050 or 950 or 800 or 760 r.p.m.s can be obtained by
this method.
(ii) This method can not be employed for phase wound motors as
reconnection of the rotor winding/coils becomes difficult to give same
number of poles as that of the stator. In other words this method is
suitable for squirrel cage motors only.
(iii) Very cumbersome for industrial use.
(iv) Motor has to be stopped to make suitable pole changes desired.
Frequency control or (V/f) Control:
This method is commonly called V.F.D method. A change of input frequency
changes the synchronous speed and provides a direct method of speed
control. The air gap flux per pole is given by
ø = (l/4.44KwlTphl)(V/f) (2.2.4.1)
Where Kwl = winding factor of stator per phase,
Tphl= turns per phase or stator,
V = Applied voltage per phase,
f = frequency of supply
From equation (2.2.4.1) it is evident that if f is decreased air gap flux ø will
increase. If f is decreased over a wide range air gap flux ø will also increase

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simultaneously and it may saturate the field poles. To avoid saturation and to
keep the magnetization current within limits, the applied voltage must be
reduced in direct proportion to the frequency, i.e. when f is varied, V must
also be varied such that the ratio V/f remains constant. Now a days variable
(V, f) supply from constant (V,f) source is arranged by the converter -
inverter arrangement as shown schematically in Figure 2.2.4(a).
Disadvantages of (V/f) Control:
(i) This method is basically suitable for obtaining speeds at or below rated
or base speed,
(ii) For less than rated speed N0 r.p.m. (Corresponding to base/rated
frequency
f0, (V/f) is kept constant and hence motor provides a constant torque
only.
(iii) The starting torque at reduced frequency is not reduced in the same
proportion because rotor power factor improves with reduction in
frequency. Since power is product of torque and speed, operation at
reduced frequency i.e. at reduced speed results in lesser output and
hence loss in efficiency.
(iv) For speeds higher than rated or synchronous speed f is required to be
increased and so V needed to keep (V/f) constant is more than the
rated value which can be provided by the inverter and also becomes

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detrimental to motor's insulation grading of the stator winding. Hence
the method remains fit only for sub synchronous speed of the motor.
(v) A very costly speed control device. The cost escalates exponentially
with increase in motor output H.P. size,
(vi) In case the V.F.D kit develops any snag, inhouse repair is not possible.
Line Voltage Control:
The torque developed by an induction motor is proportional to square of
voltage. A very large voltage change is required for a small speed change.
Disadvantages of Line Voltage Control:
(i) A very large voltage change is required for a small change in speed,
(ii) Suitable for small motors of a few H.P. It has a limited used for fan
type loads whose
torque change is proportional to the square of speed,
(iii) Not feasible for large industrial motors.
(iv) Obviously the voltage should be reduced below the rated value only,
(v) Motor gets overheated on repeated speed change using this method.
Rotor Resistance Control:
This type speed and torque control is only possible for slip ring induction
motors. As rotor resistance is increased (which can be done by adding
resistance externally to the rotor) the motor slip increases and hence speed
decreases for a given load torque. The speed-torque characteristics as
obtained by this method is given in Figure 2.2.8.

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Steps of starting resistances r1, r2, r3 rn are given by
r1 = R1 (1-α)
r2= αr1, r3=α2r1 ,rn=αn-1r1
where r1, r2, r3 , rn are various step resistances, R1 =(r1 + r2 + r3 + ....
+rn + rm)Ω rm = motor resistance in Ω, α = (sm) 1/n, sm = slip at full load,
total resistance steps = n
Disadvantages of Rotor Resistance Control:
(i) Increase in input power to the.motor.
(ii) Power is wasted as heat in the external resistance connected to the
rotor,
(iii) Operating motor efficiency decreases,
(iv) The speed is not suitable for large range of speed changes,
(v) Not feasible for large capacity motor,
(vi) Speed above synchronous range not achievable.
Injecting EMF in the Rotor:
This method is only suitable for slip ring type induction motors. In this
method a slip frequency emf is injected into the rotor circuit for speed
control. A 3 phase voltage Vi from external source at slip frequency is
injected into the rotor circuit for speed control.
Disadvantages:
(i) An auxiliary machine is required for voltage injection.

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(ii) Used as Schrage Motor, series and shunt exciters where method of
speed and torque control were cumbersome,
(iii) Loss in efficiency.
OBJECTS OF THE INVENTION
- The object of the invention is to develop a novel phase advancer for
smooth starting of phase wound induction motor both at no load and load
condition
- Another object of the invention is to improve motor power factor up to
even 1 as well as leading power saving considerable electrical energy
power.
- Also the object of the invention is to increase motor efficiency and
improve running torque.
- Yet another object of the invention is speed-torque change over a large
range as desired.
- Still other object of the invention is to provide fairly large starting torque
to the induction motor with low starting current, thereby eliminating the
need of starting resistances.
- Moreover, the object of this invention is to develop a phase advancer
ensuring low power loss, noise level and maintenance cost of the set up.

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DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The Proposed novel phase advancer typically has a two body construction i.e. a
stator and a rotor as shown in Fig 3.2(a) and 3.2(b). The stator and rotor has
even number of poles. The stator has both a conductive or inductive circuit to
achieve suitable field amplification both positive or negative, which is regulated
to have the desired speed, p.f. and torque of the motor. The design of stator and
rotor has been done to suit the H.P. ratings of the induction motor. The phase
advancer rotor is electrically connected to the rotor circuit of 3 phase induction
motor to vectorically add or subtract the output e.m.f. of the phase advancer to
or from the rotor voltage of the induction motor so that desired speed, p.f. and
torque is achieved with the minimum expense of power to do so. During start of
the induction motor the rotor of the phase advancer provides suitable rotor
e.m.f. which does not necessitate the use of rotor starting resistances.
Stator:
This has multiple number of slots in iron core to house the designed windings
in multiple phases. Selection of core material and number of slots per phase is
determined to suit the amplification / Compression of air gap flux. The windings
are typically used phase windings for providing desired amplification /
Compression of total flux both positive or negative, in the air gap. It has both a
conductive or inductive circuit to achieve this. Total number of poles are selected
for overall amplification / Compression. A heat sensitive transducer has been
placed on the stator to prevent the machine from overheating.

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Rotor:
This is an armature type construction having a number of poles. The core
material and number of slots determine the rotor flux. The windings are so
designed to carry the full load rotor current and the starting current of the motor
and also the starting overcurrents. This is electrically connected to the rotor
circuit of the motor.
Complete electrical connection diagram for the phase advancer is as shown in
the Fig 3.5(a).
Performance:
For starting phase wound induction motor with the help of the proposed novel
phase advancer no external starting resistances are required. The motor is given
a reduced voltage supply from an auto transformer and the phase advancer is
kept stationary. Rotor flux is assisted by stator amplification winding and an
e.m.f. is fed to the rotor circuit which is vectorically added to the rotor flux to
ultimately develop a starting torque of sufficient value (phasor diagram
described at Appendix "A"). Thus saving of energy is achieved during starting
by avoiding loss of energy in the rotor starting resistances. For normal running of
the motor to drive a load, the phase advancer is kept disconnected from the
rotor circuit by a soft two way switch. When increase in driving torque at
reduced speed is desired from the motor the phase advancer is reinserted into
the rotor circuit. Parameters of stator winding are changed externally and the net
air gap flux of the rotor gets reduced which results in drop in speed. The phase
advancer may be driven by an external drive machine e.g. a.d.c. shunt motor to

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change power factor of motor and increase in torque. The phase advancer has
been tested on a 7.5. h.p., 415 volt, 3 phase, 50Hz wound rotor induction motor.
However wound rotor induction motors of any horsepower output can be
operated with the proposed novel phase advancer with suitable change in the
design only. The test results are given in the Table 1 and Table 2.
Torque, Power factor and speed of a 3 phase induction motor:
(a) The Torque equation of an unaided 3 phase induction
motor is given by

(b)When the phase advancer is out of circuit, the motor
works as an ordinary induction motor, the torque, slip and
rotor current is as load to be met by the motor.
(c) When aided by the novel phase advancer,
Please refer Fig 1 and 2.

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(i) Total rotor induced emf (phasor) value as
corrected by the phase advancer is given by

Where, Eresuitant = Corrected rotor emf per phase
ø1 = initial power factor angle of rotor emf
δ = angle of injected emf
Reference Phasor E2

where ø2 -ø1 is the change in power factor.
Hence motor power p.f. changes or gets improved.
(d) When stator magnetic amplifying circuit is switched on, a
combination of external impedances of Z value when
added, give their own flux output. These fluxes interact
with the rotor flux and links phase advancer armature to
give required injected emf in the rotor as | Eiected | <δ
where 6 is measured from sE2
(e) If the stator amplifying circuit present the stator flux ψ1
and rotor flux
ψ2 gives
---- ---- --

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(f) Therefore the net rotor emf is then

(h) At constant torque but variable output speed and power
factor both change.
Table 1:
(a) It is observed from readings at ser Nos. 1 & 2 that on light
load the induction motor drawing a current of 2.2 Amp at
a very low power factor 0.359 with an input of 640 Waltt.
With the phase advancer pressed into service the input
current was reduced to 1.6 Amp (i.e. a reduction of almost
27%) while the power input to the motor remained the
same. Percentage drop in speed achieved was 3.42%
from no load speed of 1460 r.p.m and no load p.f.
improved from 0.359 to 0.5. Obviously the output of the
induction motor has increased.

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(b) On fairly good load the induction motor drawing a current
of 3.6 Amp at p.f. of 0.834 with an input power of 2200
watt without the phase advancer. When the novel Phase
advancer was used the speed was reduced by 8.22% the
input current reduced drastically to 2.7 Amp (i.e. reduced
by 25%), p.f. from 0.834 to unity and further to input
current of 2.1 Amp (i.e. a reduction of 41.6%) to a power
factor 0.912 with an input power remaining almost same,
when speed was reduced by 14.38%.
Table 2: It is observed from reading at ser Nos. 3 & 5 that the power input to
the motor was reduced by 17% when field amplification was used, with no
change in the power factor the speed was successfully increased from 1270
rpm to 1420 i.e. 11.81% which indicated that motor torque has changed
substantially.
Table 3: A comparative study of the performance of a SRIM under three
conditions imposed viz. condition 1, with normal running i.e. no external
resistance added to its rotor circuit, condition 2, with external resistance
added to its rotor circuit and condition 3, induction motor aided with the
Novel Phase Advancer is given at Table 3. It is observed that the
performance of the SRIM when aided with the Novel Phase Advancer is
greatly enhanced.

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WE CLAIM
1. A novel phase advancer and a method to control speed torque and power
factor of three phase induction motor comprising a stator (1), rotor (2),
core (3), slots (4), windings (5) and a thermistor (6), altogether
characterized to provide desired amplification / compression of air gap
flux.
2. The stator (1) of the novel phase advancer as claimed in claim (1) having
multiple number of slots in the core to house the designed windings in
multiple phases.
3. The core (3) of the novel phase advancer stator as claimed in claims (1)
and (2) with the characteristics to provide desired amplification /
compression of flux.
4. The slots (4) per phase in the stator of the novel phase advancer as
claimed in claims (1), (2), and (3) to house the designed stator windings.
5. The windings (5) of the novel phase advancer stator as claimed in claims
(1), (2), (3) and (4) to carry the load currents providing desired
amplification / compression of total flux both positive or negative, in the
air gap, having both a conductive or inductive circuit.
6. The poles of the phase advancer stator as claimed in claims (1), (2), (3),
(4) and (5), selected for over all amplification / compression.

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7. The thermistor (6) in the novel phase advancer stator as claimed in claims
(1), (2), (3), (4) and (5) to sense the heat generated and trip stator and
rotor circuits.
8. The rotor (2) of the novel phase advancer as claimed in claim (1) having
armature type construction and number of poles.
9. The core (7) of the rotor of the novel phase advancer s claimed in claims
(1) and (8) to determine the rotor flux.
10.The slots (8) per phase of the novel phase advancer rotor core as claimed
in claims (1), (8) and (9) to determine the rotor flux.
11.The windings (9) of the rotor of the novel phase advancer as claimed in
claims (1), (8), (9) and (10) to carry the full load rotor current, starting
current of the motor and the starting over currents.
12.The method of starting, speed control and power factor corrections of
phase wound induction motor by the novel phase advancer through a soft
two way switch (10).
Dated this 5th Day of DECEMBER 2007.

This method is primarily designed for speed and torque control of a slip ring
induction motor. The method employs an auxiliary machine for amplification of
rotor emf of the slip ring induction motor. The machine is connected to the rotor
electrically at slip frequency to amplify the rotor voltage so that starting of the
motor, speed, power factor and running torque control etc. even from remote
are done singly by this machine. This machine has been named as a Novel Phase
advancer.