Induction Motor Report #2
Naresh Maraj- 66373
Comments Section…………………………………………………Final Grade………TABLE OF CONTENTS
Section Page no.
This report displays the information acquired from a thorough research undertaken on induction motors and the electromechanical principles associated with them. An induction motor is an electric motor in which the current in the rotor needed to generate torque is obtained by electromagnetic induction from the magnetic field of the stator winding. This motor works on the principle of Faraday’s Law and Lorrentz force law that will be explained in more details in the following sections. This is a very detailed and concise report that contains all of the relevant electromechanical information on induction motors.
The electromechanical principles utilised by induction motors, their operation and other important aspects were researched and a report was generated in order to obtain a better understanding of these electromechanical devices and portray it to the reader. Induction motors are the most widely used type of motors, especially as it regards to industrial applications.
In an induction /asynchronous motor the electric current in the rotor needed to produce torque is obtained by electromagnetic induction from the magnetic field of the stator windings (hence the name). The two main parts are the rotor (rotating component) and the stator (remains stationary in operation). Induction motors always run at a speed lower than synchronous speed (the speed of the rotating magnetic field in the stator).
Induction motors rely upon the induction of voltages and currents in it’s rotor circuit from the stator circuit. Machines with only amortisseur/ squirrel cage windings are called induction machines. The distinguishing feature of an induction motor is that no dc field current is required to operate machine. Induction machines can be used as generators or as motors but are rarely used as generators due to numerous disadvantages, for this reason induction machines are usually termed induction motors. There are two types of induction motors depending upon the type of input supply – (i) Single phase induction motor and (ii) Three phase induction motor. They can also be classified according to the type of rotor they contain (squirrel cage / slip ring or wound motor).
BODY OF REPORT
1.0 Principles of operation
In a DC motor, supply for both the stator and the rotor winding is needed. However, only the stator winding is fed with an AC supply in an induction motor. Alternating magnetic flux is generated around the stator winding due to AC supply. This alternating flux revolves with synchronous speed. The revolving flux is called a RMF (Rotating Magnetic Field).
The relative speed between stator RMF and rotor conductors cause an induced electromotive force (emf) in the rotor conductors, in accordance with Faraday’s law of electromagnetic induction. If the magnetic flux speed is the same as the rotor speed there will be no electromotive force and current because no flux lines will cut loop so there will be no force on rotor bar. The rotor will slow down, this will cause more flux lines to be cut again, and the force will cause the rotor to speed up. Therefore, the RMF speed should always be more than the rotor speed.
The rotor conductors are short circuited, and hence rotor current is produced due to induced emf hence the term induction motors. The induced current in the rotor will also produce alternating flux around it. This rotor flux lags behind the flux from the stator. The direction of induced rotor current, according to Lenz’s law is such that it will tend to oppose the cause of its production.
As the cause of production of rotor current is the relative velocity between rotating stator flux and the rotor, the rotor will try to catch up with the stator RMF. Thus the rotor rotates in the same direction as that of stator flux to minimize the relative velocity however the rotor never succeeds in catching up the synchronous speed. In accordance with Lorrentz force law there will be a current induced in bars of the rotor, hence force on the loop in such directions as to cause the loop to rotate generating torque. The rotor speed is proportional to the frequency of the AC power.
An Induction motor mainly consists of two parts called the Stator, which is the stationary part and the rotor, which rotates. Induction motors have the same physical stator as a synchronous machine, but the construction of the rotor is different. The two types of rotors that can be placed in the stator are the cage/ squirrel cage rotor (due to resemblance to hamster wheel) and wound rotor. A cage induction motor rotor comprises of a series of conducting bars laid into slots carved in the rotor face and shorted at either end by shorting rings. Insulated iron core lamina are packed inside the rotor to reduce eddy current losses.
Wound rotors have three-phase windings that mirror the stator windings. Each of the winding terminals is connected to separate slip rings on the rotor’s shaft. The slip rings on the wound rotor motor contain brushes that form an external, secondary circuit into which impedance (resistance) can be inserted. The stator is basically a three coil winding where the three coils are connected 120 degrees apart and a three phase AC power input is given to it (for 3 phase induction motors). The winding is passed through slots in the stator of the motor. Stators are constructed by stacking thin, highly permeable steel laminations inside a steel or cast iron frame. The 3 phases of the rotor windings are Y-connected. Cage induction motors are preferred over wound because they are more economical due to less maintenance (no wearing of brushes and slip rings).
Figure 1.7 Illustrating Construction of an Induction Motor
1.1 Equivalent Circuit
The equivalent Induction Motor Circuit allows performance characteristics evaluation for steady state conditions. The equivalent circuit can be used to visualize and calculate the flow of power through the motor. An induction motor is also called a singly excited machine (as opposed to a doubly excited synchronous machine), since power is only supplied to the stator circuit. An induction motor is based on the principle of the induction of voltages and currents. The voltage and current is induced in it’s rotor circuit from the stator circuit (transformer action).
Since the induction of voltages and currents in the rotor circuit of an induction motor is essentially a transformer operation, the equivalent circuit of an induction motor is very similar to the equivalent circuit of a transformer. Because an induction motor does not have an independent field circuit, it’s model will not contain an internal voltage source such as the internal generated voltage in asynchronous machine .We can derive the equivalent circuit of an induction motor from our knowledge of transformers and the variation of rotor frequency with speed.
Figure 1.8 Equivalent Circuit of a 3- Phase Induction Motor
In the figure above a) Represents the stator circuit model,
b) and c) represent the rotor models
d) Represents the complete equivalent circuit model
The equivalent circuit of the induction motor can be used to examine the losses and the efficiency of the machine. Efficiency for any device is usually ?=Power Out÷ Power In, taking into consideration the losses the equation ?= (PIN- PLOSS) ÷ PIN. Losses in the motor directly affect the cost of operating it and indirectly affects the motor rating. The efficiency is frequently determined by measuring the losses. Measurement standards are specified by certain societies such as ANSI, NEMA, and the Canadian Standards Association.
Induction motors are subjected to different types of losses that affect their efficiency. Mechanical losses include friction and windage. The mechanical losses are usually grouped with core motor losses and determined simultaneously.
Resistive / I2R losses are typically referred to as copper losses and are found in the stator and rotor windings of the induction machine. Usually the stator copper loss is about 33% and the rotor copper loss is about 15% of the total loss of the machine.
Rotational losses are typically about 14% of the total loss of the induction motor. Open-circuit or no-load core losses include the eddy current losses and hysteresis measured at no load. The sum of the mechanical and core losses is the no-load rotational loss. Core losses typically account for about 16% of the losses in the induction motor.
The stray loss includes anything not accounted for by the methods used to determine the preceding categories. Induction motor stray losses make up the remaining 22% of total losses.
1.3 Synchronous Speed
Synchronous speed is the rotational speed of the magnetic field in the stator winding of an induction motor. The synchronous speed is given by the formula Ns = (120f) ÷P where f = line voltage frequency in hertz, P = number of poles and Ns = synchronous speed in rpm (revolutions per minute). This formula shows that the rotor speed N bears a constant relationship with the field poles and the frequency of the generated voltage in the armature winding. A machine that runs at synchronous speed is a synchronous machine. Therefore, an AC machine in which the rotor moved at a speed and built a constant relationship between the frequency of the voltage in the armature winding and the number of poles is termed a Synchronous Machine.
Induction motors always operate slightly slower than synchronous speeds, In order for an induction motor to make torque there must be at least some difference between the stator field (synchronous) speed and the rotor speed, this difference is termed “slip.”
The voltage induced in a rotor bar of an induction motor depends on the speed of the rotor relative to the magnetic fields. Since the working of an induction motor depends on the rotor’s voltage and current, we are concerned with their relative speed. Two terms that are commonly used to define the relative motion of the rotor and the magnetic fields are slip speed, which is the difference between synchronous speed and rotor speed. I.e. N slip=N sync –N m where N slip = slip speed of the machine, N sync = speed of the magnetic fields and Nm = mechanical shaft speed of motor.
The other term used to describe the relative motion is slip, which is the relative speed expressed on a per-unit or a percentage basis. That is, slip is defined as S = (N slip ÷ N sync) × 100%, this equation can also be expressed in terms of angular velocity. Notice that if the rotor turns at synchronous speed, s = 0 while the rotor is stationary, s = 1. All normal motor speeds fall somewhere between those two limits. It is possible to express the mechanical speed of the rotor shaft in terms of synchronous speed and slip using the equations above.
1.6 Speed Control