Induction Motors
- Gary Foster
An induction motor or asynchronous motor is an AC electric motor in which the electric current in the rotor needed to produce torque is obtained by electromagnetic induction from the magnetic field of the stator winding. An induction motor can therefore be made without electrical connections to the rotor. An induction motor's rotor can be either wound type or squirrel-cage type. -Wikipedia-
DKIST uses induction motors in many applications.
Mount movement
Coude movement
Enclosure movement
Facilities
Hoists, cranes, lifts
Most of these induction motors are 3 phase, 4 pole, 480VAC, 3 phase, 60 Hz.
Most of our induction motors are controlled by variable speed drives. - which is addressed in a separate topic.
Several applications use multiple motors and drives operating in tandem to move large loads. The mechanical load is shared by multiple motors and drives.
Fun Fact: Three phase electrical power and induction motors were invented in 1887 by an incredibly smart Serbian inventor named Nikola Tesla.
I care that they don’t have any of their own ..
Nikola Tesla
The Great Inventor
Induction motors are simple.
Copper wire is wound three interleaved windings on the stator. These windings do not rotate..
The rotor has several conducting bars or windings fixed around a rotating shaft.
Motors with wire windings on the rotor are called 'wound rotor' … because the rotor conductors are wound with wire … duh.
Motors with bars on the rotor are call 'squirrel cage rotor' … because the rotor conductors look like a squirrel cage … if you use your imagination. …
Induction motors only require 3 or 4 wires for electrical power connection.
The 3 phase electrical supply excites coils in the fixed (stator) windings. FMI: 3 Phase Current Vectors
Three phase power circulates through the coils. Current passing through the coils produces a magnetic field.
The coils are wound with three phases equally spaced around the circumference of the stator. The 'Resultant Flux Vector is the addition of the three phases.
Each phase produces a magnetic field that goes up and down as the AC voltage changes. The magnetic fields produce a magnetic flux along the direction of the voltage. A force in a specific direction is also called a ‘Vector’.
The combination of three magnetic vectors produces a (resultant) rotating magnetic force. FMI: 3 Phase Space Vectors
The rotating shaft of the motor has conductors arranged in a circle that react to the magnetic field. Most of the induction motors that we use have iron bars as the conductors. The circular collection of iron bars is commonly called a Squirrel Cage.
A current is induced in each bar as the magnetic field rotates past it. The current flowing through the bar creates it own magnetic field. The magnetic field in the bar is opposite in polarity to the rotating magnetic field. Magnetic opposites attract: so the magnetized bar is attracted to the rotating magnetic field.
Motor direction of rotation
The three power phases can be connected to the motor in any order. The relationship between phases determines the direction of the stator magnetic vector. The rotor follows the stator magnetic field.
In the chart above, as rotation occurs from left to right.: Phase 2 follows Phase 1. Phase 3 follows Phase 2.
Imagine if the phases in the chart above travelled right to left - instead of left to right. Then Phase 2 follows Phase 3. Phase 1 follows Phase 2. The stator magnetic field (and the motor rotor) will rotate in the opposite direction.
To reverse the direction of rotation: reverse any two phase connection from the electrical supply to the stator windings.
Speed and torque
The above chars display a 2 pole stator - which means 1 North - South pole for each phase.
Induction motor rotor speed is almost equal to the speed of rotation of the stator magnetic field (synchronous speed). Synchronous speed can be found by this formula :
In real terms: for a 2-pole motor (as shown above), and a 60Hz electrical supply the stator magnetic field makes 60 revolutions per second. This results in synchronous speed of 3600 RPM.
But, we can add more poles around the circumference of the rotor. If we double the number of coils, then we have 4 N - S pairs of magnetic field producers. The magnetic field rotates only 30 times per second and the resulting synchronous speed is 1800 Hz. A 6 pole motor will run at 1200 RPM. an 8 pole motor will run at 900 RPM.
Why would we want to do that?
In a 4 pole stator: distance between each N - S pole pair is half the distance for the 2 pole stator. The magnetic fields for each phase are closer together. The fields will be closer to each passing segment of the rotor for a longer time and will attract the rotor with a higher force. The motor will be able to deliver a higher torque than the motor with 2 pole stator.
Without going into all the math, here is the equation for torque produced by an induction motor
Full-load Torque, at the motor shaft, can be found by:
In imperial units
T = 5252 Php / nr
where:
T = full load torque (lb ft)
Php = rated horsepower
nr = rated rotational speed (rev/min, rpm)
In metric units
T = 9550 PkW / nr
where:
T = rated torque (Nm)
PkW = rated power (kW)
nr = rated rotational speed (rpm)
TLDR:
1HP, 2 pole motor -> 1.4 ft-lb ~ 1.9 N-m
1HP, 4 pole motor -> 2.9 ft-lb ~ 3.9 N-m
1HP, 6 pole motor -> 4.3 ft-lb ~ 5.9 N-m
1HP, 8 pole motor -> 5.8 ft-lb ~ 7.9 N-m
a 5 HP, 4 pole induction motor can produce ~ 14 ft-lb of torque
Induction Motor Operating Characteristic
Induction motor speed and torque
Induction motors produce highest torque when the rotating magnetic field produced by the stator is near the speed of the rotating shaft. The magnetic field produced by the stator is near each rotor bar for a longer time and can transfer (induce) more energy into the rotor to magnetize it and produce a current.,
Torque produced at zero rotor speed is called Starting Torque. Starting Torque is ~50% of Full Torque. This varies depending on the design of the rotor bars. Mechanical load must be less than motor starting torque to begin rotation.
Torque during acceleration can vary based on motor design.
The motor will accelerate the load to the speed corresponding to pullout torque (maximum torque), then continue to accelerate to near synchronous speed. Then rotor speed will decrease until rotor speed is at the point on the curve where motor torque equals load torque. As load torque changes, speed will change along the torque curve until motor torque equals load torque.
Once within the normal operating range, the motor can deliver 0 - 100% (pullout) torque with only a slight change in speed.
The difference between magnetic field speed and rotor speed is called slip (see below).
The change in speed is called droop (see below).
If load torque increases to the pullout torque of the motor then speed and torque will suddenly decrease. Which is probably why it is called pullout torque ...
Slip
The rotor will never reach the speed of stator flux. If it did, there would be no relative speed between the stator field and rotor conductors, no induced rotor currents and, therefore, no torque to drive the rotor. Rotor speed is always less than Stator field speed. This difference in speed depends upon load on the motor.
The difference between the synchronous speed of the rotating stator field and the actual rotor speed is called 'slip'. Slip is usually expressed as a percentage of synchronous speed:
Slip = s = (Stator Speed – Rotor Speed) / Stator Speed × 100 %
Slip speed = (Stator Speed – Rotor Speed)
In an induction motor, the change in slip from no-load to full-load is~ 0.1% (no load) to 3% (full load).
When connected to a fixed frequency supply speed is more or less constant.
Droop
In the normal operating range, power is transferred by the relative motion of the stator magnetic field across the rotor conductors. This relative motion is the slip between stator field and rotor.
When operated from a constant electrical supply, then as the mechanical load increases more power is required at the rotor. Without added power, the rotor slows. As the rotors slows, each rotor conductor moves faster through the magnetic field (which is rotating a synchronous speed) and spends longer time within the magnetic field. More energy is transferred from the magnetic field to the rotor. Rotor speed settles at a slightly lower speed while delivering the higher torque.
This feature of induction motor operation only applies when the drive voltage is constant.
Generator or Motor
If an external mechanical source rotates the induction motor shaft while the magnetic field is rotating, the motor will generate and develop a voltage that will drive current into the stator. This can occur during braking or holding a load against gravity.
If the motor is operated in the generating region, it will deliver energy back into the source. This can cause overvoltage or overcurrent and must be managed by the power circuit.Motor Cooling
Electrical motors generate heat as a result of the electrical and mechanical losses during operation. Losses are higher during starting, dynamic braking. and increased loading.
The most common cooling means is Totally Enclosed Fan Cooled
enclosed case, with cooling fins on the motor casing
axial, rotor driven fan mounted on the end of the motor opposite the rotor
this works well for motors operated near synchronous speed, installed where the internal windings wlll not be exposed to contaminants.
Other common cooling options:
Motor Electrical Circuits
All Induction Motors require:
disconnecting means, with Lock Out / Tag Out to permit safe service
short circuit / overcurrent protection
motor overload protection
Some induction motors may require:
overspeed protection
overtemperature protection
brakes to stop the mechanical load
…
Induction Motor Trade-offs
More poles = more copper and more complex stator. This makes the motors bigger and heavier.
Four pole, squirrel cage motors are the most common type and are manufactured in much higher quantity than any other configuration. So, four pole induction motors cost less than other pole configurations..
For more information
Induction Motors training folder
Animations of Electrical Machines
Rotation Principle of induction motor
General interest: Electrical Academia