Three phase induction motors
Introduction
The three-phase induction machine is the most widely used among the rotating machines. An induction machine offers an almost constant speed within its normal operating range when operated off a constant-voltage, constant-frequency, and with the electric power source. The cost of purchasing these machines is very low, and they require little maintenance. However, speed control is difficult and very expensive. Therefore, the rotor current of the three phase induction motors is not induced by electrical connections, but rather the rotation of magnetic field.
Squirrel Cage Rotor construction
The rotor type of the motor is made up of a laminated core which is cylindrical in shape with conductors arranged in parallel. The parallel slots are used to house rotor conductors. Wires are not used as the rotor conductors, but rather heavy bars of copper, aluminium or alloys (Say, 2012). The following are some of the advantages of skewed rotor slot.
1. The locking tendency of the rotor is reduced, i.e. due to magnetic attraction and the tendency of the rotor teeth remains under stator teeth.
2. The relationship existing between the stator and rotor causes an increase in effective transformation ratio.
3. The increase of the length of the rotor conductor results in an increased rotor resistance.
The rotor bars are braced or welded electrically purposely to cause short-circuiting of end rings at both ends. We call the rotor squirrel cage because its construction is almost similar to the structure or house of a squirrel (Brittian, 2012). It is impossible to add an extra load to the armature circuit because the rotor bars are not temporarily short-circuited.
Source: (Toliyat & Kliman, 2004)
Three phase induction motor working principles
The stator core has slots which house the three separate single-phase windings. There will be the creation of the magnetic field when three currents positioned at 120 electrical apart pass through the windings. It is the most important part simply because there will be movements of the field in the stator core. The stator poles number and power source frequency determines the speed of the rotating magnetic field in the rotor (Toliyat & Kliman, 2004). The speed generated is known as synchronous speed and the following formula is used to determine it:
120 x Hertz frequency/ Poles number = Synchronous speed RPM
120xf / P= S
Where S = Synchronous speed
f = Applied frequency
p = Poles number
The rotor copper bars are cut, and voltages in the squirrel-cage winding bars are induced due to synchronous speed of the magnetic field. Currents in the bar rotor are set up by these induced voltages which create in the rotor core a field. Torque is developed as a result of the rotor field reacting with the stator field, hence causing the rotor to rotate (Chu, Chung, Thomas, Tuttle & Lee, 2003). The stator field synchronous speed is always faster than the speed of the rotor. Therefore, the rotor bars are cut from the stator field. There will be no cutting of the bar rotor if the rotor and the stator turn at the same speed causing no induced voltage.
Motors are designed in a manner that aluminium conductor impedance of the rotor is reduced and there is an increased efficiency in a high-speed range. With the high-speed impedance, the motor will have an increased torque in a low-speed range. The increase in the load will not reduce the speed of low-impedance motors in the right part of the maximum torque on the characteristic curve.
Regulation speed and slip percent
The speed regulation characteristic of the squirrel-cage induction motor is the best. You can measure the speed machine performance using percent slip. When measuring the speed machine performance, you will make use of the synchronized speed of rotating field of the stator as the benchmark. Due to the constant stator pole number and operating frequency, the synchronous speed will also be constant. At full load, we can get the number of revolutions per second due to slipping behind of the rotor when the stator field is rotating, and the rotor speed is reduced.
AC motor application
AC cooling fan
In everyday life, A.C. Motors play a very vital role in various areas such as air conditioning, water pumping, and so forth. The flexibility of the AC motor makes being widely used in various fields. AC cooling fans are used in electronic appliances by supplying a stream of air that is state in nature.
The computer and the fan work together in the same way that a thermostat interacts with an air conditioning unit so as to maintain the temperature. The AC fan will supply the machine with an increased air once the computer has notified it about the warmer environment.
Logic flow chart of AC cooling fan
Heat Sinks
The CPU chips reach very high temperatures when the machine is working and taking away the heat requires a lot of effort to pass air over the CPU. The heat sink is designed in a manner that it conducts heat. The bottom of the sink is flat to provide the large contact area with the CPU. The upper part has several fins with numerous air channels between them. The surface area of this section is increased by these fins, thus increasing cooling efficiency and also to increase the amount of heat that is taken away from the system. Convection is the basic thermodynamic principle used to eliminate heat from the system. The convection movement is created by AC Motor.
The AC motor takes in the electrical energy and converts it into mechanical energy. The mechanical energy causes the fan components to rotate, thus taking out air from the system.
The internal part of the processor directs the cooling device. One fan is located at the back and another one in front of the processor. The fan located at the back blows out the hot air while the front fan brings in cool air. There are many openings in the front of the CPU to allow the free movement of the air.
There is a great loss of energy associated with the rotor between the power input and output. Friction loss, eddy current, hysteresis loss and copper loss are some of the components of losses. The heat generated during motor operation indicates such energy loss; therefore, the fan plays a vital role in cooling down the system.
The rotor suspended inside the magnetic field of the motor conducts current when the magnetic field moves. The magnetic field is ever-changing inside the stator of the fan, and there will be the production of electric current in the rotor as the magnetic field rotates.
AC Motor Feedback
AC Motor resolver and AC Motor encoder are the two feedback controls used in the cooling fan. Both devices are used to sense direction, speed, and the output shaft position. They are greatly different, although they offer a similar solution in multiple applications.
AC Motor resolvers
It makes use of the transformer to cause the rotor to generate a voltage across an air gap. The device is very rugged and operates over a range of temperature since it lacks electronic components (Hoseason, 2009). The design of the device makes it resist shocks. Also, the device may be used in extreme conditions.
Optical encoder AC Motor
The air gap between a light source and photo detector allows passage of a beam of light which is interrupted by the rotating shutter as it crosses the region. Over time, the encoder wears out due to rotating shutter. This wear will affect the durability and dependably of the optical leader (Brown, 2012).
The following are some of the parameters AC motors can be adjusted for efficient control of its speed: Supply voltage, the number of stator poles, Constant V/F control of induction motor, and applied frequency.
Conclusion
I cannot imagine the world without electric motors. Motors are used in the smaller machine to the largest machine. Electric motors are in many diverse applications. There is quite a range of motors to choose from. The main purpose of motors in machines is to convert electrical energy into mechanical energy which is very useful in accomplishing many things in our daily life. Each motor has its unique characteristics, therefore, making each motor suitable for a specific application.
References
Brown, A. E. (2012). U.S. Patent No. 5,727,928. Washington, DC: U.S. Patent and Trademark Office.
Brittian, L. W. (2012). Three‐Phase Motors. Audel Electrical Trades Pocket Manual, 61-69.
Chu, H. W., Chung, E., Thomas, R. J., Tuttle, M. R., & Lee, S. M. (2003). U.S. Patent No. 6,643,128. Washington, DC: U.S. Patent and Trademark Office.
Hoseason, D. B. (2009). Squirrel-cage induction motors. Electrical Engineers, Journal of the Institution, 66(376), 410-425.
Say, M. G. (2012). The performance and design of alternating current machines: transformers, three-phase induction motors and synchronous machines. Sir Isaac Pitman & Sons.
Toliyat, H. A., & Kliman, G. B. (Eds.). (2004). Handbook of electric motors (Vol. 120). CRC press.
