2. The motor selection process is not as simple as you might think.
While selecting a motor, you will need to consider
• Speed,
• Power Ratings,
• load torques,
• losses,
• Efficiency etc.
We will discuss all these factors in detail. Motor selection can be
easy and at the same time very difficult.
Motor Selection-
3. The selection of a motor is a surprisingly complex process. And
to that end we must ask ourselves what do we expect from a
motor? the answer is simple; We are looking for a motor to
drive a load.
In this answer we have not indicated any requirement about
speed, nor have we said anything about starting condition, nor
have we said anything about speed control. In fact, we are
simply content that the device is made to rotate.
4. If we consider many of the applications which we come across in everyday
life, we find that we do not require anything special from a motor. For
instance, within reason does the speed of a food mixer or the speed of
vacuum cleaner really matter .
For the food mixer, 300r/min is probably just as effective as 270 r/min.
for electric drill it is normally starting of load so there are no problems
getting it started. For the vacuum cleaner there is no need to adjust the
speed from 900r/min to 899 r/min.
5. Coming from a domestic introduction to the application of electric motors
it does therefore come as somewhat of a surprise that engineers are
concerned as to whether a factory lathe should operate at 300 r/min or
400 r/min or whether a motor will be able to start a passenger lift when
carrying 10 people. Or whether a roller in a strip mill will ensure that the
strip expansion is taken up.
Given the demands imposed by the applications, what are our difficulties in
selecting a motor. There are three problems to be addressed as follows:
6. •In most instances there are two or more types of motor
which are suitable for our needs, so we must make a choice
rather than have it made for us.
•In some instances, it is not simply a matter of matching the
motor to the load when running; there is also the matter of
starting the load. Thus, the starting torque able to accelerate
the load and the acceleration time need consideration,
•There is imposed on motor selection environmental
factors. For instance, can the motor operate under water or
in a hot surrounding or in a dusty atmosphere?
The 3 Factors to Consider are
7. Speed:
Most electric Motors operate at quite high speeds, say between 500 and 3000 rpm, Often it is
easier to change speed by means of mechanical gears.
Most motors operate within small speed range e.g. an induction motor might operate between
1430 and 1470 rpm according to load. At the extreme condition, a comparable synchronous
motor would run at 1500 rpm regardless of the load. On the other hand, commutator motor
with series winding can accept wide speed variation as instanced by railway trains which seldom
operate at constant speed such is the nature of trains.
In several specialist applications, precise speed control is essential. In the past, this requirement
gave rise to a wide range of specially designed motors which were extremely complicated in
their design they were a tribute to the ingenuity of engineers in time gone by. However,
improvements in power electronics systems have consigned such motors to the scrap heap and
we will look at methods of controlling the speed of d.c shunt motors and cage rotor induction
motors.
8. Power Rating & Duty Cycle
Basically, a motor is expected to operate continuously with a rated power output.
The rating is dependent on the ability of the motors to dissipate waste heat, the
heat which comes from the loss in the windings, the eddy current losses in the
rotor and stator cores, windage and friction loss. The losses cause the windings
to become warmer and if the insulation gets too warm it will break down. The
rating depends on limiting the load such that it will not overheat the winding
insulation.
The motors instanced are those which provide simple drives. Let us consider
what would happen if a motor were cold and it were loaded to its rated value. It
would not immediately heat to the maximum permitted temperature rather there
would be a considerable period until that temperature was reached. The time
would take hours.
10. Initially, we could at the time of being cold have increased the load. This would
cause the motor to heat more quickly, this is acceptable so long as we do not
continue with the overload beyond a certain time or where the maximum operating
temperature is reached. This approach leads to short term ratings which assume
higher loading followed by periods of switch off to allow cooling. Lift motors
operate in similar load cycles.
It is possible to use a 10KW motor to deliver 20KW for a very short period. Or for
that matter to deliver say 11KW for 1 hr. motors for such cyclical duties are often
available in standard forms for duty periods such as 10, 30 or 60 min. where there
are shorter duty periods, motors are generally specifically designed for there
intended applications.
There is quite a range of controlled-speed motors, but the two most common are
the conventional dc motor and the variable frequency cage rotor induction motor.
11. Motor Load Torques:
Load torques can be considered under two categories:
Constant torque
Fan or pump-type torque
The constant torque load can be instanced by the passenger lift. If we consider the
arrangement. Let us assume loads causing a force of 5000 N in the lifting cable. If the
lift were to rise at a speed of 1 m/s then the power required is 5000W. this requires
that the motor produces 2πn1T , where n is the motor rotational speed and T the
torque required. If the speed were to be doubled to 2 m/s. then the power would be
10000W= 2πn2T. However, the rotational speed will double to provide twice the
rising speed for the lift hence we find that the torque remains the same that is it is
constant.
12. This does not address the torque requirements during the acceleration and
deceleration of the lift. To start the lift rising extra torque will be required the
greater the desired acceleration, the greater the torque needed. Also, we have the
problem that the torque must be produced even at standstill. Thus, when we
consider a so-called constant torque load, we are not considering the acceleration
and deceleration period. The torque requirements for lift operation are shown in
graph below;
13. Fans and pumps total load is only due to the drive against air (in the
case of a fan) or against a fluid in the case of a pump. Usually, the drive
against air is proportional to the speed. Pumps have similar
characteristics, but the proportionality of the drive requires a higher
power for the speed usually something approaching.
Both characteristics have the advantage that the motor hardly requires
any torque at starting. In practice there is a static friction to overcome,
so a practical load characteristics for a fan and that for an induction
motor driving it shows difference between the motor and load torque
shows there is a good advantage causing rapid acceleration to the
operating condition as shown in graph.
14.
15. The motor & its environment:
When choosing a motor there is one factor which is independent of the
circuitry of the motor the factor is the environment in which it is designed to
operate. Towards this end there are four ranges of environmental activity
which are considered:
1.Ingress of materials
2.Ingress of water
3.Cooling arrangement
4.Cooling power
16. Not all motors operate in a clean atmosphere although most operate in an
atmosphere which contains nothing more than a little dust. There are
international electro technical commission standards which provide for a
range of levels of protection, starting with the need to keep out solids about
the size of a screw to a tennis ball. In such a motor clearly, we are not
concerned about the dust entering the motor, but we are concerned with
keeping hands out. Progressively the range allow for smaller and smaller
bodies until we wish to keep out even dust. Apart from the build-up of dust
on moving parts there is the hazard of igniting the dust.
17. Water could, affect insulation but if we expect the motor normally to operate in a dry
place e.g driving a cassette player there is no need to protect it from water. However,
some motors might experience dripping water or the occasional jet of water and in
extreme situation be immersed in water. These situations therefore give rise to
different casing designs for the motors, to afford them correct ingress protection.
Motors are generally cooled by a fan mounted on the end of the rotor causing air to
pass between the rotor and stator. However, if we have sealed out dust then we
probably could not pass air (unless it were filtered) through the motor and the heat
would simply have to be released through the surface of the casing. Motors immersed
in water may very well be in colder situations and surface cooling thereby made
easier.
18. Finally, the fan need not be mounted on the rotor shaft but could be a separate unit
with its own motor. Therefore, we need to consider the way the cooling circuit is
powered.
There are therefore several ranges to be considered and any of the motors which we
have described can experience any of the environmental factors listed above.
19. Machine efficiency:
We observed the losses which can arise in an electrical machine. Ideally,
we would hope that the power into a machine would equal the power out.
When considering a machine in a dynamic state e.g. accelerating the
power need not be equal because the magnetic and mechanical systems
will be changing their stored energies and therefore absorbing some of
the power. It is necessary to be able to predict these changes if we wish to
predict the response to a control system demand such as a step change.
20. If we take a closed steel ring which has been
completely demagnetized and measure the flux
density with increasing values of the magnetic field
strength, the relationship between the two quantities
is represented by curve OAC. If the value of H is then
reduced it is found that the flux density follows curve
CD and that when H has been reduced to zero, the flux
density remaining in the steel is OD and is referred to
as the remanent flux density.
If H is increased in the reverse direction the flux density decreases until at some value OE, the flux
has been reduced to zero. The magnetic field strength OE required to wipe out the residual
magnetism is termed the coercive force. Further increases H causes the flux density to grow in the
reverse direction as represented by curve EF. If the reversed magnetic field strength OL is adjusted
to the same value as the maximum value Ok in the initial direction the final flux density LF is the
same as KC.
Hysteresis:
21. If the hysteresis loops for a given steel ring are determined for different
maximum values of the magnetic field strength they are found to lie within one
another. The apexes A, C, D, and E of the respective loop lie on the B/H curve
determined with increasing value of H. it will be seen that the value of the
remanent flux density depend upon the value of the peak magnetization thus
corresponding to a maximum magnetization that is approaching saturation, the
remanent flux density is OY. The value of the remanent flux density obtained
when the maximum reaches the saturation value of the material is termed the
remanence of the material. Thus for the material having the hysteresis loops. The
remanence is approximately OY.
The value of the coercive force varies from OP for loop A to OQ for loop E; and
the value of the coercive force when the maximum magnetization reaches the
saturation value of the material is termed the coercivity.
22. Losses
Now that we are fully aware of the implication of hysteresis, we can
proceed to consider the losses which occur in motors and
generators.
AC machines are more complicated because of reactance in the
circuits so we will simply be considering only dc machines. Even
here the components experience flux reversal and therefore the a.c
effects such as hysteresis and eddy current losses are experienced
but on a limited scale. The losses in d.c machines can be classified
under the headings below
23. Armature losses:
I2R loss in armature winding. The resistance of an armature can be measured by the
voltmeter-ammeter method. If the resistance measurement is made at room
temperature, the resistance at normal working temperature should be calculated.
Whence the resistance R1 at room temperature should be calculated. Thus, if the
resistance at normal working temperature of say 15 C and rises to 50 C temperature
rise of the winding after the machine has been operating on full load for 3 or 4 hr,
then; Resistance at 65C=R1 x 1+(0.00426 x 65)/ 1+ (0.00426 x 15 ) = 1.2R1
24. Core loss:
Core loss in the armature core due to hysteresis and eddy currents. Hysteresis
losses are dependent on the quality of the steel. It is approximately proportional
to the square of the flux.
The eddy-current loss is due to circulating currents set up in the steel
laminations. Had the core been of solid steel for a two-pole machine then if the
armature were rotated emf would be generated in the core in the same way as
they are generated in conductors placed in armature.
The rotation being assumed clockwise when the armature is viewed from the
right hand side of the machine. Owing to the very low resistance of the core
these eddy currents would be considerable and would cause a large loss of power
in, and excessive heating of the armature.
25. Commutator losses:
Loss due to the contact resistance between the brushes and the segments. This loss
is dependent upon the quality of the brushes.
For carbon brushes the p.d between a brush and the commutator over a wide
range of current is usually about 1 V per positive set of the of brushes and 1 V per
negative set, so that the total contact resistance loss in watts is approximately
2×total armature current.
Loss due to the friction between the brushes and the commutator. This loss
depends upon the total brush pressure the coefficient of friction ad the peripheral
speed of the commutator.
26. Excitation losses:
Loss in the shunt circuit equal to the product of the shunt current and
terminal voltage. In shunt generators this loss increases a little between no
load and full load since the shunt current must be increased to maintain
the terminal voltage constant; but in shunt and compound motors it
remains approximately constant.
Losses in series compole and compensating winding. These losses are
proportional to square of the armature current.
27. Bearing friction and windage losses:
The bearing friction loss is roughly proportional to the speed, but
the windage loss namely the power absorbed in setting up
circulating currents of air is proportional to the cube of the speed.
The windage loss is very small unless the machine is fitted with
cooling fan.
28. The efficiency of a DC motor:
If Ra = total resistance of armature circuit
I = input current
Is = shunt current
Ia = armature current
The total loss in the armature circuit= I2
aRa
If V= terminal voltage, loss in shunt circuit= IsV. This
includes the loss in the shunt regulating resistor.
If C= some of core, friction and windage losses,
Total losses= I2
aRa + IsV + C
Input power= IV
Output power = input power - losses
Output power= IV – I2
aRa – IsV– C
Efficiency = Output power/input power x 100
