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2. ELECTRIC MOTORS 2. ELECTRIC MOTORS
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Bureau of Energy Efficiency
Syllabus
Electric motors: Types, Losses in induction motors, Motor efficiency, Factors affecting
motor performance, Rewinding and motor replacement issues, Energy saving opportunities
with energy efficient motors.
2.1
Introduction
Motors convert electrical energy into mechanical energy by the interaction between the mag-
netic fields set up in the stator and rotor windings. Industrial electric motors can be broadly clas-
sified as induction motors, direct current motors or synchronous motors. All motor types have
the same four operating components: stator (stationary windings), rotor (rotating windings),
bearings, and frame (enclosure).
2.2
Motor Types
Induction Motors
Induction motors are the most commonly used prime mover for
various equipments in industrial applications. In induction
motors, the induced magnetic field of the stator winding induces
a current in the rotor. This induced rotor current produces a sec-
ond magnetic field, which tries to oppose the stator magnetic
field, and this causes the rotor to rotate.
The 3-phase squirrel cage motor is the workhorse of industry;
it is rugged and reliable, and is by far the most common motor
type used in industry. These motors drive pumps, blowers and
fans, compressors, conveyers and production lines. The 3-phase
induction motor has three windings each connected to a separate phase of the power supply.
Direct-Current Motors
Direct-Current motors, as the name implies, use direct-unidirectional, current. Direct current
motors are used in special applications- where high torque starting or where smooth accelera-
tion over a broad speed range is required.
Synchronous Motors
AC power is fed to the stator of the synchronous motor. The rotor is fed by DC from a separate
source. The rotor magnetic field locks onto the stator rotating magnetic field and rotates at the same
speed. The speed of the rotor is a function of the supply frequency and the number of magnetic poles
in the stator. While induction motors rotate with a slip, i.e., rpm is less than the synchronous speed,
the synchronous motor rotate with no slip, i.e., the RPM is same as the synchronous speed governed
by supply frequency and number of poles. The slip energy is provided by the D.C. excitation power 2. Electric Motors
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2.3
Motor Characteristics
Motor Speed
The speed of a motor is the number of revolutions in a given time frame, typically revolutions
per minute (RPM). The speed of an AC motor depends on the frequency of the input power and
the number of poles for which the motor is wound. The synchronous speed in RPM is given by
the following equation, where the frequency is in hertz or cycles per second:
Indian motors have synchronous speeds like 3000 / 1500 / 1000 / 750 / 600 / 500 / 375 RPM
corresponding to no. of poles being 2, 4, 6, 8, 10, 12, 16 (always even) and given the mains
frequency of 50 cycles / sec.
The actual speed, with which the motor operates, will be less than the synchronous speed.
The difference between synchronous and full load speed is called slip and is measured in per-
cent. It is calculated using this equation:
As per relation stated above, the speed of an AC motor is determined by the number of
motor poles and by the input frequency. It can also be seen that theoretically speed of an AC
motor can be varied infinitely by changing the frequency. Manufacturer's guidelines should be
referred for practical limits to speed variation. With the addition of a Variable Frequency Drive
(VFD), the speed of the motor can be decreased as well as increased.
Power Factor
The power factor of the motor is given as:
As the load on the motor comes down, the magnitude of the active current reduces.
However, there is no corresponding reduction in the magnetizing current, which is propor-
tional to supply voltage with the result that the motor power factor reduces, with a reduction in
applied load. Induction motors, especially those operating below their rated capacity, are the
main reason for low power factor in electric systems.
2.4
Motor Efficiency
Two important attributes relating to efficiency of electricity use by A.C. Induction motors are
efficiency (
), defined as the ratio of the mechanical energy delivered at the rotating shaft to
the electrical energy input at its terminals, and power factor (PF). Motors, like other inductive
loads, are characterized by power factors less than one. As a result, the total current draw need-
ed to deliver the same real power is higher than for a load characterized by a higher PF. An
important effect of operating with a PF less than one is that resistance losses in wiring upstream
of the motor will be higher, since these are proportional to the square of the current. Thus, both
a high value for
and a PF close to unity are desired for efficient overall operation in a plant.
Squirrel cage motors are normally more efficient than slip-ring motors, and higher-speed
motors are normally more efficient than lower-speed motors. Efficiency is also a function of
120
× Frequency
Synchronous Speed (RPM) =
No. of Poles
kW
Power Factor = Cos
= kVA
Synchronous Speed Full Load Rated Speed
Slip (%) =
× 100
Synchronous Speed motor temperature. Totally-enclosed, fan-cooled (TEFC) motors are more efficient than screen-
protected, drip-proof (SPDP) motors. Also, as with most equipment, motor efficiency increas-
es with the rated capacity.
The efficiency of a motor is determined by intrinsic losses that can be reduced only by
changes in motor design. Intrinsic losses are of two types: fixed losses - independent of motor
load, and variable losses - dependent on load.
Fixed losses consist of magnetic core losses and friction and windage losses. Magnetic core
losses (sometimes called iron losses) consist of eddy current and hysteresis losses in the stator.
They vary with the core material and geometry and with input voltage.
Friction and windage losses are caused by friction in the bearings of the motor and aerody-
namic losses associated with the ventilation fan and other rotating parts.
Variable losses consist of resistance losses in the stator and in the rotor and miscellaneous
stray losses. Resistance to current flow in the stator and rotor result in heat generation that is
proportional to the resistance of the material and the square of the current (I
2
R). Stray losses
arise from a variety of sources and are difficult to either measure directly or to calculate, but are
generally proportional to the square of the rotor current.
Part-load performance characteristics of a motor also depend on its design. Both
and PF
fall to very low levels at low loads. The Figures 2.1 shows the effect of load on power factor
and efficiency. It can be seen that power factor drops sharply at part loads. The Figure 2.2 shows
the effect of speed on power factor.
Field Tests for Determining Efficiency
No Load Test: The motor is run at rated voltage and frequency without any shaft load. Input
power, current, frequency and voltage are noted. The no load P.F. is quite low and hence low
PF wattmeters are required. From the input power, stator I
2
R losses under no load are subtract-
ed to give the sum of Friction and Windage (F&W) and core losses. To separate core and F &
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Figure 2.1 % Load vs. Power factor, Efficiency
Figure 2.2 Speed vs. Power factor W losses, test is repeated at variable voltages. It is useful to plot no-load input kW versus
Voltage; the intercept is Friction & Windage kW loss component.
F&W and core losses = No load power (watts) - (No load current)
2
× Stator resistance
Stator and Rotor I
2
R Losses: The stator winding resistance is directly measured by a bridge
or volt amp method. The resistance must be corrected to the operating temperature. For mod-
ern motors, the operating temperature is likely to be in the range of 100°C to 120°C and nec-
essary correction should be made. Correction to 75°C may be inaccurate. The correction fac-
tor is given as follows :
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The rotor resistance can be determined from locked rotor test at reduced frequency, but rotor
I
2
R losses are measured from measurement of rotor slip.
Rotor I
2
R losses = Slip
× (Stator Input Stator I
2
R Losses Core Loss)
Accurate measurement of slip is possible by stroboscope or non-contact type tachometer.
Slip also must be corrected to operating temperature.
Stray Load Losses: These lo