AC 3 phase induction motors

testybelchMechanics

Nov 14, 2013 (3 years and 10 months ago)

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AC 3 phase induction motors

The polyphase induction motor is self starting. Its speed falls to a small extent with load (
as

does a D.C. shunt wound motor) It has two main parts, the stator which is similar to a
stationary armature of an alternator, and the rotor, mounted on bearings within the stator.

Stator

This is provided with three sets of windings energised separately by
the individual phases of
a three phase supply, giving a rotating magnetic field, constant in magnitude and rotating
at the same angular frequency as that of the supply voltage.

Squirrel cage rotor


Rotor
-

There are no electrical or other connections made
to the rotor which is
built up of soft iron laminations fixed to the shaft and slotted to receive conductors.

The squirrel cage rotor has a single stout copper conductor bedded into slots,
these conductors being short circuited by heavy copper rings at bot
h ends. A similar
electrical type has windings on the rotor which are short circuited but are not in the form of
a squirrel cage.

Current flowing in the squirrel cage rotor conductors is an induced current and
cannot be controlled. When it is necessary to
vary the rotor current a three phase wound
rotor is used and the connections to the windings are brought out to slip rings across which
variable resistance's are connected and can reduce starting current, improve starting torque
and control speed.

Principa
l of operation

If a conductor set at right angles to a magnetic fields moves across the flux from left to
right, the direction of the induced voltage will be out of the paper, by lenz's law. If this
conductor is part of a complete circuit, them a current w
ill flow in the direction of this
voltage, and there will be a force on the conductor tending to urge it from right to left.


The same relative motion of field and conductor is obtained if the conductor is
stationary and the field moved form right to left
. If a current is switched on in the stator
setting up a rotating field then electromotive forces are set up in the rotor. The resulting
rotor currents give rise to a force in the conductors tending to move the rotor in the same
direction as the stator fie
ld motion. The speed of the rotor can never equal the speed of the
stator rotating field as there must always be relative motion between the conductors and
the rotating field. The greater the difference between the stator rotating field speed and the
rotor

speed the greater the relative speed of conductors and field, and the greater the force
on each conductor with more torque exerted on the whole.

Slip = Field speed
-

Rotor Speed/ Field speed

The greater the slip the greater the torque exerted. Light load
slip is about 2%,
full load slip is about 4 to 5%.


Torque and slip

Consider the frequency of the stator fields relative to each conductor. When the rotor is at
reat

this equals the alternation of the supply. If lightly loaded the slip is small, say only one
or two cycles per second.

Resistance of a squirrel cage rotor as a rule will be very small and its inductance
high. Its reactance will thus be large at the freque
ncy of the supply and much less when it is
running (induced reactance depends upon frequency).

XL = 2 p f L

XL = Inductive resistance

L = Inductance [ H ]

The power factor of the rotor will be low at starting and its torque small. P.F. of
0.2 to 0.4 on sta
rting.

If the resistance of the conductors was increased, the starting power factor is increased.


As the frequency difference between the rotating field and the rotating
conductors reduces so the Inductive resistance component reduces and so the power fa
ctor
increases improving efficiency and reducing current draw.


Wound rotor

If the resistance of the conductors was increased, the starting power factor is increased.
There are tow possible methods for attaining this. The first is to have a rotor squirrel cage
made of a suitable high resistance material say bronze rather than copp
er, a second method
is to use a wire wound rotor with the ends of the windings brought out via slip rings and
attached to high resistance's. However at working speeds more slip is needed for a given
torque. So what is gained in starting is lost in steady r
unning. When a large starting torque
is essential a Wound Rotor may be used, with external variable resistances which can be cut
out as the rotor speed increases. A less expensive solution is a dual squirrel cage rotor


Note:It is the resistance in the cl
osed circuit which determines the current in
the circuit induced by the rotating field


Comparisons of cage and slip ring rotors

Squirrel cage



Advantages



Cheaper and more robust



Slightly higher efficiency and power factor



Explosion proof, since abscence

of slip rings and brushes eliminates risk of sparking



Virtually constant speed machine



Disadvantages



High starting current ( 5 to 8 times F.L.)



Low starting torque

Wound rotor with slip rings



Advantages



High starting torquw



Lower starting current



Speed ca
n be varied if required

Disadvantages



cost



Danger from sparking

For small squirrel cage motors direct on line starting with starter current of
about 5 x full load . With larger squirrel cage where the torque increases as speed of load
increases ( fans, bow

thrusters, etc.) reduced voltage starting may be obtained with star
delta or auto transformer starters.

In modern marine practices the wound rotor with slip rings is seldom found. To
obtain high starting torque with starting currents of about 3.5 x full l
oad the induction motor
rotor is provided with two cages.

a.

AN outer cage in shallow slots with high resistance ( bronze)

b.

An inner cage in deeper slots of low resistance (copper)

On starting the inner cage is very reactive with low torque and little current
(See graph above).The outer cage has high torque with most of the rotor current when
starting. During running most of the current is in the inner cage of low resistance as at
small slip the inductive reactance is low.

These two cage induction motors may be

started direct on line and are widely
used in marine practice.



Skewed conductors (windings)

Magnetic hum



Two possible sources of magnetic hum , commonly heard in transformers, are



Attraction and repulsion alternately of laminations



Magnetic
striction i.e. when the poles in a bar are aligned the bar has a tendency to expand.

Synchronous motors



Advantages



The ease with which the power factor can be controlled. An overexcited synchronous
motor with a leading power factor can be operated in paral
lel with induction motors
having a lagging power factor to improve the overall power factor of the supply system



The speed is constant and independent of the load. This characteristic is mainly of
use when the motor is required to drive another alternator
to generate a supply at a
frequency, as in frequency changers

Uses
-
A.C. electric propulsion schemes but generally not for auxiliary purposes.



Disadvantages



Cost per h.p. is greater than induction motors



D.C. supply is necessary for the rotor excitation. Th
is is usually provided by a small
D.C. generator carried on an extension of the shaft.



Some arrangement must be provided for starting and synchronising the motor. Two
possible methods are by pony motor, or by incorporating a wound rotor induction
windings
which may be opened when up to speed and a D.C. voltage applied

Graph of induction motors showing effect of increasing the ratio of
resistance to inductance.


Full load occurs at around 40 % torque. It can be seen that varying the
resistance will change the degree of slip and hence speed. For R = 2X the motor is not self
starting as it never reaches full starting torque. It will also be expensive to run due to t
he
high heat losses through the resistance's.


At reduced load the power factor is much reduced. Because of this it is very
inefficient to place an oversized motor on a load, or to have several motors only partly
loaded.

The effects of frequency and volta
ge change on an
induction motor.

Effects of voltage change

At constant voltage if frequency is increased from 50Hz to 60Hz there is an increased
Inductive resistance XL. As stator flux is reduced this effects the starting torque increasing
starting current

demand. Higher speed increases power output. If a centrifugal pump or fan
the power increase is proportional to the speed cubed ( [60 / 50]3 = 1.728) giving a 73%
increase in power demand.

At constant voltage if the frequency is decreased from 60Hz to 50H
z the stator
flux is increased but the speed is reduced by a 83%. Unless the load is reduced the machine
will run hotter than normal. Starters and contactors could be adversely affected. A 440v
60Hz system supplied from a 415v 50Hz shore supply runs at 83%

speed, slightly hotter
but should run without damage.

Effects of frequency change

At constant frequency if voltage is reduced this has little effect on speed (less than 5%) but
increased current for same power. Torque is proportional to the square of the
voltage
therefore there is a corresponding and greater drop in available starting torque, this leads to
longer run up times and the possibility of stalling.

As induction motors very really run at full load, a large voltage reduction would be required
to ca
use a damaging current.

At constant frequency if voltage is increased gives a stronger stator flux
depending on slot design and original flux density this could increase stator iron losses
sufficiently to cause overheating.

Squirrel cage


wound rotor