Induction Motors
Fun Facts:
- The first electric Motor was designed by a scottish monk in 1740 his name is Andrew Gordon
- Andre Marie Ampere and Michael Faraday experimented with the principles of electromechanical motion in 1820s
- Moritz von Jacobi built the first usable electric DC motor on boat carrying 14 people across a river in 1834
- Tesla filed a patent for an induction motor in 1889
- in 1897 a 100hp induction motor had the same dimensions as a 7.5hp conventional motor
What do they look like?
DC Motor
Squirrel Cage Motor
Squirrel Cage Induction Motor:
Squireel cage induction motors are widely used in industries. Almost 70% of the industrial motors and
drives comes under these category. In Squirrel cage induction motors, rotor windings wound
in squirrel cage. These motors are very robust in construction and very cheap. These motors can
operate at any working conditions. Some of the advantages, disadvantages and applications of squirrel
cage induction motor compared with slip-ring induction motor are discussed below:
Advantages of Squirrel Cage Induction Motor:
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Squirrel Cage Induction motors are cheaper in cost compared to Slip Ring Induction motors.
Requires less maintenance and rugged construction. Because of the absence of slip rings,
brushes maintenance duration and cost associated with the wear and tear of brushes are
minimized
Squirrel Cage Induction Motors requires less conductor material than slip ring motor, hence
copper losses in squirrel cage motors are less results in higher efficiency compared to slip ring
induction motor
Squirrel cage motors are explosion proof due to the absence of brushes slip rings and brushes
which eliminates the risks of sparking.
Squirrel Cage motors are better cooled compared to slip ring induction motors
Squirrel cage motors operate at nearly constant speed, high over load capacity, and operates at
better power factor.
Squirrel Cage Induction Motor Disadvantages:
Some of disadvantages or demerits of squirrel cage induction motors are listed below:
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Main disadvantage of squirrel cage induction motor is that they have poor starting torque and
high starting currents. Starting torque will be in the order of 1.5 to 2 times the full load torque and
starting current is as high as 5 to 9 times the full load current. In slip ring induction motors,
higher starting torque can be attained by providing an external resistance in the rotor circuits
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during starting of the slip-ring induction motor. This arrangement in slip-ring induction motors
also reduces the high inrush currents during starting of induction motor.
Squirrel cage induction motors are more sensitive to the supply voltage fluctuations. When the
supply voltage is reduced, induction motor draws more current. During voltage
surges, increase in voltage saturates the magnetic components of the squirrel cage
induction motor.
Speed control is not possible in squirrel cage induction motor. This is one of the
major diadvantages of squirrel cage induction motors.
The total energy loss during starting of squirrel cage motor is more compared to slip ring motors.
This point is significant if the application involves frequent starting
Application of squirrel cage Induction Motor:
Squirrel Cage Induction Motors are widely used in Industrial applications than slip ring induction motors
due to cheaper in cost, rugged in construction, low maintenance. Squirrel Cage Induction Motors are
suitable for applications where the drive requires constant speed, low starting torque and no speed
control drives.
Wound Rotor Motor
Wound rotors are used in applications where high starting torque is required. External
resistances may be added to these rotors via slip rings shaft. These resistances serve to
increase the starting torque and ensure smooth starts.
However, these rotors are more expensive than induction motors. In the wound rotor, the rotor
windings are insulated to the ground. The slip rings and the brushes also require maintenance.
The starting current drawn by a wound rotor machine is lesser than that that of a squirrel cage
motor.
The wound rotor is designed to have the same number of poles as the stator winding of the
motor. The windings are designed to with stand high mechanical forces as these motors are
used for high-torque applications.
Wound Rotors are used for applications which require soft-starts and adjustable speeds
Squirrel cage rotors are the most common type of rotors found in induction motors. These
rotors are simple to construct, robust and relatively inexpensive.
They are particularly suited for low inertia loads. Their easy construction enables lower rotor
weight and lesser centirfugal force and windage losses.
Single-phase induction motors
A three phase motor may be run from a single phase power source. (Figure below) However, it will not selfstart. It may be hand started in either direction, coming up to speed in a few seconds. It will only develop 2/3
of the 3-φ power rating because one winding is not used.
3-φmotor runs from 1-φ power, but does not start.
The single coil of a single phase induction motor does not produce a rotating magnetic field, but a pulsating
field reaching maximum intensity at 0o and 180o electrical. (Figure below)
Single phase stator produces a nonrotating, pulsating magnetic field.
Another view is that the single coil excited by a single phase current produces two counter rotating
magnetic field phasors, coinciding twice per revolution at 0o (Figure above-a) and 180o (figure e). When the
phasors rotate to 90o and -90o they cancel in figure b. At 45o and -45o (figure c) they are partially additive
along the +x axis and cancel along the y axis. An analogous situation exists in figure d. The sum of these
two phasors is a phasor stationary in space, but alternating polarity in time. Thus, no starting torque is
developed.
However, if the rotor is rotated forward at a bit less than the synchronous speed, It will develop maximum
torque at 10% slip with respect to the forward rotating phasor. Less torque will be developed above or
below 10% slip. The rotor will see 200% - 10% slip with respect to the counter rotating magnetic field
phasor. Little torque (see torque vs slip curve) other than a double freqency ripple is developed from the
counter rotating phasor. Thus, the single phase coil will develop torque, once the rotor is started. If the rotor
is started in the reverse direction, it will develop a similar large torque as it nears the speed of the backward
rotating phasor.
Single phase induction motors have a copper or aluminum squirrel cage embedded in a cylinder of steel
laminations, typical of poly-phase induction motors.
Permanent-split capacitor motor
One way to solve the single phase problem is to build a 2-phase motor, deriving 2-phase power from single
phase. This requires a motor with two windings spaced apart 90o electrical, fed with two phases of current
displaced 90o in time. This is called a permanent-split capacitor motor in Figure below.
Permanent-split capacitor induction motor.
This type of motor suffers increased current magnitude and backward time shift as the motor comes up to
speed, with torque pulsations at full speed. The solution is to keep the capacitor (impedance) small to
minimize losses. The losses are less than for a shaded pole motor. This motor configuration works well up
to 1/4 horsepower (200watt), though, usually applied to smaller motors. The direction of the motor is easily
reversed by switching the capacitor in series with the other winding. This type of motor can be adapted for
use as a servo motor, described elsewhere is this chapter.
Single phase induction motor with embedded stator coils.
Single phase induction motors may have coils embedded into the stator as shown in Figure above for larger
size motors. Though, the smaller sizes use less complex to build concentrated windings with salient poles.
Capacitor-start induction motor
In Figure below a larger capacitor may be used to start a single phase induction motor via the auxiliary
winding if it is switched out by a centrifugal switch once the motor is up to speed. Moreover, the auxiliary
winding may be many more turns of heavier wire than used in a resistance split-phase motor to mitigate
excessive temperature rise. The result is that more starting torque is available for heavy loads like air
conditioning compressors. This motor configuration works so well that it is available in multi-horsepower
(multi-kilowatt) sizes.
Capacitor-start induction motor.
Capacitor-run motor induction motor
A variation of the capacitor-start motor (Figure below) is to start the motor with a relatively large capacitor
for high starting torque, but leave a smaller value capacitor in place after starting to improve running
characteristics while not drawing excessive current. The additional complexity of the capacitor-run motor is
justified for larger size motors.
Capacitor-run motor induction motor.
A motor starting capacitor may be a double-anode non-polar electrolytic capacitor which could be two + to +
(or - to -) series connected polarized electrolytic capacitors. Such AC rated electrolytic capacitors have such
high losses that they can only be used for intermittent duty (1 second on, 60 seconds off) like motor starting.
A capacitor for motor running must not be of electrolytic construction, but a lower loss polymer type.
Resistance split-phase motor induction motor
If an auxiliary winding of much fewer turns of smaller wire is placed at 90o electrical to the main winding, it
can start a single phase induction motor. (Figure below) With lower inductance and higher resistance, the
current will experience less phase shift than the main winding. About 30o of phase difference may be
obtained. This coil produces a moderate starting torque, which is disconnected by a centrifugal switch at 3/4
of synchronous speed. This simple (no capacitor) arrangement serves well for motors up to 1/3 horsepower
(250 watts) driving easily started loads.
Resistance split-phase motor induction motor.
This motor has more starting torque than a shaded pole motor (next section), but not as much as a two
phase motor built from the same parts. The current density in the auxiliary winding is so high during starting
that the consequent rapid temperature rise precludes frequent restarting or slow starting loads.
Nola power factor corrrector
Frank Nola of NASA proposed a power factor corrector for improving the efficiency of AC induction motors
in the mid 1970's. It is based on the premise that induction motors are inefficient at less than full load. This
inefficiency correlates with a low power factor. The less than unity power factor is due to magnetizing
current required by the stator. This fixed current is a larger proportion of total motor current as motor load is
decreased. At light load, the full magnetizing current is not required. It could be reduced by decreasing the
applied voltage, improving the power factor and efficiency. The power factor corrector senses power factor,
and decreases motor voltage, thus restoring a higher power factor and decreasing losses.
Since single-phase motors are about 2 to 4 times as inefficient as three-phase motors, there is potential
energy savings for 1-φ motors. There is no savings for a fully loaded motor since all the stator magnetizing
current is required. The voltage cannot be reduced. But there is potential savings from a less than fully
loaded motor. A nominal 117 VAC motor is designed to work at as high as 127 VAC, as low as 104 VAC.
That means that it is not fully loaded when operated at greater than 104 VAC, for example, a 117 VAC
refrigerator. It is safe for the power factor controller to lower the line voltage to 104-110 VAC. The higher the
initial line voltage, the greater the potential savings. Of course, if the power company delivers closer to 110
VAC, the motor will operate more efficiently without any add-on device.
Any substantially idle, 25% FLC or less, single phase induction motor is a candidate for a PFC. Though, it
needs to operate a large number of hours per year. And the more time it idles, as in a lumber saw, punch
press, or conveyor, the greater the possibility of paying for the controller in a few years operation. It should
be easier to pay for it by a factor of three as compared to the more efficient 3-φ-motor. The cost of a PFC
cannot be recovered for a motor operating only a few hours per day. [7]
Summary: Single-phase induction motors
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Single-phase induction motors are not self-starting without an auxiliary stator winding driven by an
out of phase current of near 90o. Once started the auxiliary winding is optional.
The auxiliary winding of a permanent-split capacitor motor has a capacitor in series with it during
starting and running.
A capacitor-start induction motoronly has a capacitor in series with the auxiliary winding during
starting.
A capacitor-run motor typically has a large non-polarized electrolytic capacitor in series with the
auxiliary winding for starting, then a smaller non-electrolytic capacitor during running.
The auxiliary winding of a resistance split-phase motor develops a phase difference versus the main
winding during starting by virtue of the difference in resistance.
More poles means less RPM but more Torque at the same power!
Electrical motor efficiency is the ratio between the shaft output power - and the electrical input power.
Electrical Motor Efficiency when Shaft Output is measured in Watt
If power output is measured in Watt (W), efficiency can be expressed as:
ηm = Pout / Pin
(1)
where
ηm = motor efficiency
Pout = shaft power out (Watt, W)
Pin = electric power in to the motor (Watt, W)
Electrical Motor Efficiency when Shaft Output is measured in Horsepower
If power output is measured in horsepower (hp), efficiency can be expressed as:
ηm = Pout 746 / Pin
(2)
where
Pout = shaft power out (horsepower, hp)
Pin = electric power in to the motor (Watt, W)
Primary and Secondary Resistance Losses
The electrical power lost in the primary rotor and secondary stator winding resistance are also called copper
losses. The copper loss varies with the load in proportion to the current squared - and can be expressed as
Pcl = R I2
(3)
where
Pcl = stator winding - copper loss (W)
R = resistance (Ω)
I = current (Amp)
Iron Losses
These losses are the result of magnetic energy dissipated when when the motors magnetic field is applied to the
stator core.
Stray Losses
Stray losses are the losses that remains after primary copper and secondary losses, iron losses and mechanical
losses. The largest contribution to the stray losses is harmonic energies generated when the motor operates
under load. These energies are dissipated as currents in the copper windings, harmonic flux components in the
iron parts, leakage in the laminate core.
Mechanical Losses
Mechanical losses includes friction in the motor bearings and the fan for air cooling.
NEMA Design B Electrical Motors
Electrical motors constructed according NEMA Design B must meet the efficiencies below:
1)
Power
(hp)
Minimum Nominal Efficiency1)
1-4
78.8
5-9
84.0
10 - 19
85.5
20 - 49
88.5
50 - 99
90.2
100 - 124
91.7
> 125
92.4
NEMA Design B, Single Speed 1200, 1800, 3600 RPM. Open Drip Proof (ODP) or Totally Enclosed Fan
Cooled (TEFC) motors 1 hp and larger that operate more than 500 hours per year.
The power factor of an AC electric power system is defined as the ratio of the active (true or real) power to
the apparent power
where
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Active (Real or True) Power is measured in watts (W) and is the power drawn by the electrical resistance
of a system doing useful work.
Apparent Power is measured in volt-amperes (VA) and is the voltage on an AC system multiplied by all
the current that flows in it. It is the vector sum of the active and the reactive power.
Reactive Power is measured in volt-amperes reactive (VAR). Reactive Power is power stored in and
discharged by inductive motors, transformers and solenoids
Reactive power is required for the magnetization of a motor but doesn't perform any action. The reactive power
required by inductive loads increases the amounts of apparent power - measured in kilovolt amps (kVA) - in the
distribution system. Increasing of the reactive and apparent power will cause the power factor - PF - to decrease.
Power Factor
It is common to define the Power Factor - PF - as the cosine of the phase angle between voltage and current - or
the "cosφ".
PF = cos φ
where
PF = power factor
φ = phase angle between voltage and current
The power factor defined by IEEE and IEC is the ratio between the applied active (true) power - and
the apparent power, and can in general be expressed as:
PF = P / S
(1)
where
PF = power factor
P = active (true or real) power (Watts)
S = apparent power (VA, volts amps)
A low power factor is the result of inductive loads such as transformers and electric motors. Unlike resistive loads
creating heat by consuming kilowatts, inductive loads require a current flow to create magnetic fields to produce
the desired work.
Power factor is an important measurement in electrical AC systems because
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an overall power factor less than 1 indicates that the electricity supplier need to provide more generating
capacity than actually required
the current waveform distortion that contributes to reduced power factor is caused by voltage waveform
distortion and overheating in the neutral cables of three-phase systems
International standards such as IEC 61000-3-2 have been established to control current waveform distortion by
introducing limits for the amplitude of current harmonics.
Example - Power Factor
A industrial plant draws 200 A at 400 V and the supply transformer and backup UPS is rated 200 A × 400 V = 80
kVA.
If the power factor - PF - of the loads is only 0.7 - only
80 kVA × 0.7
= 56 kW
of real power is consumed by the system. If the power factor is close to 1 (purely resistive circuit) the supply
system with transformers, cables, switchgear and UPS could be made considerably smaller.
Any power factor less than 1 means that the circuit's wiring has to carry more current than what would be
necessary with zero reactance in the circuit to deliver the same amount of (true) power to the resistive load.
A low power factor is expensive and inefficient and some utility companies may charge additional fees when the
power factor is less than 0.95. A lowpower factor will reduce the electrical system's distribution capacity by
increasing the current flow and causing voltage drops.
"Leading" or "Lagging" Power Factors
Power factors are usually stated as "leading" or "lagging" to show the sign of the phase angle.
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With a purely resistive load current and voltage changes polarity in step and the power factor will be 1.
Electrical energy flows in a single direction across the network in each cycle.
Inductive loads - transformers, motors and wound coils - consumes reactive power with current waveform
lagging the voltage.
Capacitive loads - capacitor banks or buried cables - generates reactive power with current phase leading
the voltage.
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Inductive and capacitive loads stores energy in magnetic or electric fields in the devices during parts of the AC
cycles. The energy is returned back to the power source during the rest of the cycles.
Power Factor for a Three-Phase Motor
The total power required by an inductive device as a motor or similar consists of
Active (true or real) power (measured in kilowatts, kW)
Reactive power - the nonworking power caused by the magnetizing current, required to operate the device
(measured in kilovars, kVAR)
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The power factor for a three-phase electric motor can be expressed as:
PF = P / [(3)1/2 U I]
(2)
where
PF = power factor
P = power applied (W, watts)
U = voltage (V)
I = current (A, amps)
Typical Motor Power Factors
Power Factor
Power (hp)
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Speed (rpm)
1/2 load
3/4 load
full load
0-5
1800
0.72
0.82
0.84
5 - 20
1800
0.74
0.84
0.86
20 - 100
1800
0.79
0.86
0.89
100 - 300
1800
0.81
0.88
0.91
1 hp = 745.7 W
Main Objectives
The main objectives while starting an induction motor are:
1. To handle high-starting current
2. To achieve high-starting torque.
As we know, rotor resistance determines starting torque. Usually, this rotor resistance is small, giving small
starting torque, but good running conditions. So, the squirrel-cage motor can run only with low-starting loads.
If the rotor resistance is increased by some means, then the slip and speed at which maximum torque occurs can
be shifted. For that purpose, external resistance can be introduced in the rotor circuit, which is done inthe case of
slip ring or wound rotor type motors.
When power is applied to a stationary rotor, excessive current will start flowing.
This happens due to the fact that there is a transformer action between the stator winding and the rotor winding,
and the rotor conductors are short-circuited. This causes heavy current flow through the rotor. If, for reducing this
heavy starting current, starting voltage applied is reduced then it affects the starting torque as well.
Methods of Starting the Motor
To get everything out, the following method of starting is generally used:
1. DOL starting
2. Auto transformer starting
3.
Star–delta starting.
Losses Calculation
The following are the losses in an induction motor:
1. Core loss in the stator and the rotor
2. Stator and rotor copper losses
3. Friction and windage loss.
Core loss is due to the main and leakage fluxes. As the voltage is assumed constant, the core loss can also be
approximated as a constant. DC can measure the stator resistance. The hysteresis and eddy current loss in the
conductors increase the resistance, and the effective resistance is taken at 1.2 times the DC resistance.
The rotor copper loss is calculated by subtracting the stator copper loss from the total measured loss or the rotor
I2R loss. The friction and windage loss may be assumed constant, irrespective of the load.
Efficiency = Rotor output/stator input
Output = Input – Losses
Example With Calculations
Consider a three-phase 440 V, 50 Hz, six-pole induction motor. The motor takes 50 kW at 960 rpm for a certain
load. Assume stator losses of 1 kW and friction and windage loss of 1.5 kW.
To determine the percentage slip, rotor copper loss, rotor output, and efficiency of the motor, perform the
following function:
Percentage slip
The synchronous speed of the motor = (50 ×120) / 6 = 6000 / 6 = 1000 rpm
Slip = (Synchronous speed – Actual speed) = 1000 – 960 = 40 rpm
Percentage slip = [(40 / 1000) × 100] = 4% = 0.04
Rotor copper loss
Rotor input = 50 1 = 49 kW
Rotor copper loss = Rotor input × Slip = 49 × 0.04 = 1.96 kW
Rotor output
Rotor output = Rotor input – Rotor copper loss – Friction and Windage loss
= 49 – 1.96 + 1.5
= 49 – 3.46
= 45.54 kW
Motor efficiency
Motor efficiency = Rotor output/Motor input
= 45.54 / 50 = 0.9108
= 91.08%
Full load motor efficiency varies from about 85 % to 97 %, related motor losses being broken down roughly as follows:
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Friction and windage, 5 % – 15 %
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Iron or core losses, 15 % – 25 %
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Stator losses, 25 % – 40 %
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Rotor losses, 15 % – 25 %
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Stray load losses, 10 % – 20 %.