PRACTICAL -7
AIM:
STUDY OF PAM MOTORS.
OBJECTIVE;
AFTER STUDYING THIS EXPERIMENT, IT WILL BE POSSIBLE TO;
(1) Identify the Areas in Which the
Pam Motor Is Better Replacement As Compare To Normal Induction Motors.
(2) Identify The Advantage Of Pam
Motors.
Significance; .Motor efficiency is generally highest
when motors operate within about 50% of their rated load. However, for much
application the load placed on a motor varies over time. In some applications
like fans and pumps, the load varies with the cube of the air or water
velocity So, when the velocity is
reduced by 20% , the load on the motor is reduced by 50% .For applications where
the load varies with time , some means of speed control is usually employed.
Traditionally speed is controlled with fluid couplings, eddy current settings, throttles.
But these controls are inefficient. Because speed is varied by introducing
friction. Thereby energy is wasted. Multi speed motors generally contain two or
more than two sets of windings. This allows the motor to Operate at two or more
than two different speeds. Here fine tuning I left to traditional speed
controls. Multi speed motors provide a fairly low cost approach to motor speed
control. These motors typically Cost from 30-80% more than a similar sized
single speed motor. Savings vary depending on the Application but savings of
8-10% are typical. Multi speed motors are not appropriate for all motor applications.
Therefore, PAM motor is developed for covering the wide spectrum of application
with economy.
THEORY:
The
Dahlander connection is one of the approaches to connect the single
winding to obtain two speeds. In recent years. a new method of "pole
Amplitude Modulation" developed by G.H. Raweliffe and his team .The advantage of this approach is that
it requires only single winding and can yield close ratio as well as wide
-ratio speed.
The
PAM motor is similar in construction to a conventional squirrel cage motor
except for the stator winding coil connections. Because they use a single winding,
they have the same frame size as single speed motors for the same horse power. So
it is easy to install as a retrofit. The winding coil connections are usually
same as that of a standard induction motor for one of the speed The other speed
is obtained by reversing the currents in one half of the coils in each phase
with respect to the other half in a proper sequence. For this change over operation,
comparatively simple control gear is sufficient and requires only 6 leads to be
brought out from the stator winding. The basic
Scheme
of connection is shown in the figure.
In
PAM motor windings, the coils in each phase are sub-divided into two sections
in a specific manner depending upon the speed ratio. PAM motors have been built
in USA and UK up to ratings of 10,000 H.P.
SPACE HARMONICS OF PAM WINDING:
In
a conventional motor winding, the m.m.f wave form is symmetrical and the major
harmonics can easily be controlled by the proper pitching while in PAM motors winding,
the m.m.f gives rise to non-identical pole pairs. The irregularity in the wave
form introduces additional space harmonics such as sub-harmonics. Medium
harmonics and higher order harmonics. It is not possible to completely
eliminate the space harmonics, but they can be minimized to an acceptable limit
by appropriate design methods.
Advantages of PAM motors: The following are the advantages of PAM
motors.
1. PAM motor is a rugged motor
identical in construction to the squirrel cage motor. So it is well suited for
applications like power stations, mines, chemical plants etc. where environment
is hostile.
2. It is simple to maintain. So
normally trained person can maintain it which is not the case in electronic
drives.
3. Compared to multi winding motors,
they are more compact and their first cost is less.
4. In starting high inertia loads like
fans for power stations, use of PAM motor can lead to energy saving during
starting. When started on low speed winding, the starting current is lower
leading to lower voltage dips.
5. Normal 2-speed motors give 2:1
speed ratio. PAM motors can give close speed ratio in the range of 1.5:1 which
is best suited for energy conservation.
PAM Motor application areas: The majority of PAM motor applications are
falling torque for energy conservation. The possible energy conservation
application areas are
(a) Thermal power station
(b) Water/ sewerage pumping installations
(c) Fan for ventilation installations (buildings,
mines, etc)
In
the case of thermal power station, it is known that auxiliaries consume about
12% of the power generated. IN INDIA. Single speed squirrel cage motors are
used for auxiliaries. The load on power station is not constant and the requirement
of cooling air, cooling water etc. varies with load and also with season. Hence
PAM motors are cost effective choice for this type of application. The major
drives in a power station where PAM motors can be applied are
> Induced draft (ID)
fan
> Force draft fan
> Primary air fan
> Coal and ash handling
plan
> Cooling tower fans
etc.
Pumping
stations (city water sewage pumping) are designed anticipating future increase
of demand. Ovision of a second speed on
the pump can lead to energy saving. There are also application in which the
suction side water level s vary considerably. Mine and building ventilation
needs change with climate. A two speed fan can save energy by adjusting the
flow to seasonal demand.
List of figure:
2.17,2.18,
