DKIST uses induction motors in many applications.
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.
Three phase electrical power and induction motors were invented in 1887 by an incredibly smart Serbian inventor named Nikola Tesla.
Induction motors are simple.
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.
Speed and torque
The above graphics display a 2 pole stator - which means 1 North - South pole for each phase.
Induction motor rotor speed is almost equal to 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.
Why would you 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, 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
n imperial units the Full-load Torque can be found by:
T = 5252 Php / nr
where
T = full load torque (lb ft)
Php = rated horsepower
nr = rated rotational speed (rev/min, rpm)
In metric units the rated torque can be found by:
T = 9550 PkW / nr
where
T = rated torque (Nm)
PkW = rated power (kW)
nr = rated rotational speed (rpm)
TLDR:
a 5 HP, 4 pole induction motor can produce ~ 14 ft-lb of torque
or, 1HP -> 2.9 ft-lb ~ 3.9 N-m
Induction Motor Operating Characteristic
Torque capability of an induction motor
Torque during acceleration can vary based on motor design.
Once within the normal operating range, the motor can deliver 0 - 100% (pullout) torque with only a slight change in speed. The change in speed is called slip (see below).
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.
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.
For more information
Induction Motors training folder
Animations of Electrical Machines
Rotation Principle of induction motor
General interest: Electrical Academia