UNIT 3. INDUCTION MOTORS. OBJECTIVE. The aim of this chapter is to gather knowledge about the following topics of. Induction motors. 1. Construction, types . PDF | Introduction: The three phase induction motors are simple in construction, rugged, low cost and easy to maintain. They run at a constant. Can the induction motor run at the synchronous speed, why? – If rotor runs at the synchronous speed, then it will appear stationary to the rotating magnetic field.
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Working principle of three phase induction motor. Equivalent Circuit. Slip. Different Speed Control Methods. Stator. Induction Motor Equivalent Circuit. 5. Torque and Power Characteristics. 6. Operation Beyond the Range 0 < s < 1. 7. Determination of Motor Constants. 8. AN AC Induction Motor Fundamentals created naturally in the stator because of the nature of Author: Rakesh Parekh the supply. DC motors depend either on.
The stator windings are connected directly to the two. Internally they are connected in such a way, that on applying AC supply, a rotating magnetic Virtually all electrical motors use magnetic field rotation field is created. This is because the squirrel cage where: This total assembly resembles the look of a squirrel cage, The magnetic field produced in the rotor because of the which gives the rotor its name.
The rotor slots are not induced voltage is alternating in nature. Instead, they are given a To reduce the relative speed, with respect to the stator, skew for two main reasons. The rotor runs slower than the speed of the stator field.
This speed is called The second reason is to help reduce the locking ten- the Base Speed Nb. The rotor teeth tend to remain locked under the stator teeth due to direct magnetic attraction The difference between NS and Nb is called the slip. The between the two. This happens when the number of slip varies with the load. An increase in load will cause stator teeth are equal to the number of rotor teeth.
A decrease in load will cause the rotor to speed up or decrease slip. The rotor is mounted on the shaft using bearings on The slip is expressed as a percentage and can be each end; one end of the shaft is normally kept longer determined with the following formula: The generated torque forces the where: They are: When the supply voltage is applied, current in the main winding lags the supply voltage due to the There are probably more single-phase AC induction main winding impedance.
It is logical that the least expensive, low- ing on the starting mechanism impedance. Interaction est maintenance type motor should be used most between magnetic fields generated by the main wind- often. The single-phase AC induction motor best fits ing and the starting mechanism generates a resultant this description. The motor As the name suggests, this type of motor has only one starts rotating in the direction of the resultant magnetic stator winding main winding and operates with a field.
From The single-phase induction motor is not self-starting. Due all single-phase motors are generally used for to induction, the rotor is energized. This will cause the phase AC induction motors are further classified as rotor to vibrate, but not to rotate. Hence, the single- described in the following sections.
It has two windings: The main winding, which is of a heavier wire, keeps the Input motor running the rest of the time. This makes the start winding an auxiliary Start Winding winding once the motor reaches the running speed. The motor draws high starting current, ing capacitor. Refer to Good applications for split-phase motors include small Figure 9 for torque-speed curve. Avoid using this type of motor in any applications lers.
Winding Refer to Figure 9 for torque-speed curve. A modified version of the capacitor start motor is the Start Winding resistance start motor. In this motor type, the starting capacitor is replaced by a resistor. The resistance start Permanent split-capacitor motors have a wide variety motor is used in applications where the starting torque of applications depending on the design. These include requirement is less than that provided by the capacitor fans, blowers with low starting torque needs and inter- start motor.
Apart from the cost, this motor does not offer mittent cycling uses, such as adjusting mechanisms, any major advantage over the capacitor start motor. Starting is by means of a design that auxiliary winding like the capacitor start motor for high rings a continuous copper loop around a small portion starting torque. Like a PSC motor, it also has a run type of each of the motor poles.
This to lag behind the field in the unshaded area. The allows high overload torque. Mechanically, the shaded-pole motor construction Start Cap Centrifugal Switch allows high-volume production. This motor is costly due to start and run Supply Line capacitors and centrifugal switch.
Unshaded Portion of Pole It is able to handle applications too demanding for any other kind of single-phase motor. These include wood- working machinery, air compressors, high-pressure The shaded-pole motor has many positive features but water pumps, vacuum pumps and other high torque it also has several disadvantages. The low initial cost suits the shaded-pole motors to low horsepower or light duty applications.
Perhaps their larg- est use is in multi-speed fans for household use. But the low torque, low efficiency and less sturdy mechanical features make shaded-pole motors impractical for most industrial or commercial use, where higher cycle rates or continuous duty are the norm. Figure 9 shows the torque-speed curves of various kinds of single-phase AC induction motors.
While the stator is industrial and commercial applications. They are the same as that of the squirrel cage motor, it has a set classified either as squirrel cage or wound-rotor of windings on the rotor which are not short-circuited, motors. These are helpful in adding external resistors and contactors.
These motors are self-starting and use no capacitor, start winding, centrifugal switch or other starting The slip necessary to generate the maximum torque device. In the slip-ring motor, the effective rotor They produce medium to high degrees of starting resistance is increased by adding external resistance torque. The power capabilities and efficiency in these through the slip rings. Thus, it is possible to get higher motors range from medium to high compared to their slip and hence, the pull-out torque at a lower speed.
Popular applications include grinders, lathes, drill presses, pumps, A particularly high resistance can result in the pull-out compressors, conveyors, also printing equipment, farm torque occurring at almost zero speed, providing a very equipment, electronic cooling and other mechanical high pull-out torque at a low starting current. As the duty applications. Once the motor reaches the base Squirrel Cage Motor speed, external resistors are removed from the rotor. Here, the rotor is of the squirrel cage standard induction motor.
The power This motor type is ideal for very high inertia loads, ratings range from one-third to several hundred horse- where it is required to generate the pull-out torque at power in the three-phase motors. Motors of this type, almost zero speed and accelerate to full speed in the rated one horsepower or larger, cost less and can start minimum time with minimum current draw.
In most applications, the equation would be: This type of motor during the transient operations. During acceleration, the motor should sup- compressors, conveyer belts, hoists and elevators.
In drives with large inertia, such as electric trains, the motor torque must exceed the load torque by a large amount in order to get adequate acceleration. In drives requiring fast transient response, the motor torque should be maintained at the highest value and the motor load system should be designed with the low- est possible inertia.
Therefore, it assists the motor developed torque T and maintains the drive motion by extracting energy from the stored kinetic energy. To summarize, in order to get steady state rotation of the motor, the torque developed by the motor T should always be equal to the torque requirement of the load Tl. The torque-speed curve of the typical three-phase induction motor is shown in Figure The Locked typically 0.
As the motor accelerates, both the torque and the current will tend to alter with rotor speed if the voltage is maintained constant. The actual full-load slip induction motors can vary considerably between of a particular motor is dependant on the motor design. The magne- fixed voltage will drop a little to the minimum torque, tizing current is independent of the load but is depen- known as the pull-up torque, as the motor accelerates dant on the design of the stator and the stator voltage.
The working current of the motor is directly on the terminal voltage and the rotor design. AN The tendency for the large machines and high-speed In most drives, the electrical time constant of the motor machines is to exhibit a low magnetizing current, while is negligible as compared to its mechanical time con- for the low-speed machines and small machines the stant.
Therefore, during transient operation, the motor tendency is to exhibit a high magnetizing current. A low magnetizing current indicates a low iron loss, As an example, Figure 12 shows torque-speed curves while a high magnetizing current indicates an increase of the motor with two different loads. The system can in iron loss and a resultant reduction in the operating be termed as stable, when the operation will be efficiency. In the first case, at a new speed, the motor high-speed motors.
The operating PF and efficiencies torque T is greater than the load torque Tl. Conse- are generally quoted on the motor data sheets. For example, Constant operation to X. Hence, at point X, the system is stable.
Torque, Variable Speed Load screw compressors, In the second case, a decrease in the speed causes conveyors, feeders , Variable Torque, Variable Speed the load torque Tl to become greater than the motor Load fan, pump , Constant Power Load traction torque T , the drive decelerates and the operating drives , Constant Power, Constant Torque Load coiler point moves away from Y. Thus, at point Y, the system is developed motor torque is equal to the load torque unstable.
The motor will operate in a steady state at This shows that, while in the first case, the motor a fixed speed. The response of the motor to any selection for driving the given load is the right one; in disturbance gives us an idea about the stability of the the second case, the selected motor is not the right motor load system. This concept helps us in quickly choice and requires changing for driving the given load.
The typical existing loads with their torque-speed curves are described in the following sections. In contrast, the power is linearly proportional to the speed. Equipment, such as Torque screw compressors, conveyors and feeders, have this type of characteristic. In this type of Power load, as speed increases, the torque is constant with the power linearly increasing. When the torque starts to Speed decrease, the power then remains constant.
The torque is the square of the speed, while the power is the cube of the speed. This is the typical torque-speed Torque characteristic of a fan or a pump.
Typical applications include Speed extruders and screw pumps. The torque is inversely proportional to the speed, which theoretically means infinite torque at zero speed and zero torque at infinite speed. In practice, there is always a finite value to the breakaway torque required.
This type of load is characteristic of the trac- tion drives, which require high torque at low speeds for Torque the initial acceleration and then a much reduced torque when at running speed.
The breakdown torque is the highest of all ous operating and constructional parameters of a the NEMA types. It can handle heavy overloads motor.
The two most widely used parameters are the for a short duration. The locked rotor torque is good enough to NEMA sets standards for a wide range of electrical start many loads encountered in the industrial products, including motors.
NEMA is primarily associ- applications. The motor effi- ated with motors used in North America. The standards ciency and full-load PF are comparatively high, developed represent the general industry practices and contributing to the popularity of the design.
The are supported by manufacturers of electrical equip- typical applications include pumps, fans and ment. These standards can be found in the NEMA machine tools. Standard Publication No. These motors are IEC intended for operation near full speed without IEC is a European-based organization that publishes great overloads. The starting current is low. The starting current and counterpart of the NEMA. The IEC standards are full-load speed are low. The high slip values associated with motors used in many countries.
The motors with changing loads and subsequent sharp which meet or exceed these standards are referred to changes in the motor speed, such as in as IEC motors. Their hoists, oil-well pumping, wire-drawing machines, typical torque-speed curves are shown in Figure The speed regulation is poor, making the design suitable only for punch presses, cranes, elevators and oil well pumps.
Design D motor. Where NEMA usually efficiency, high starting currents and lower full-load specifies continuous, intermittent or special duty running currents. The IEC Design N motors are specify temperature rise insulation class , frame size similar to NEMA Design B motors, the most common physical dimension of the motor , enclosure type, motors for industrial applications. TABLE 1: Duty Cycle Type Description 1 S1 Continuous running Operation at constant load of sufficient duration to reach the thermal equilibrium.
For this type of duty, the starting current does not significantly affect the temperature rise. There is no rest period.
There is braking no rest period. The period of duty is too short to reach the thermal equilibrium. This duty includes frequent overloads that may exceed the full loads. Amps Rated full-load supply current. Rated motor output. M Rated full-load speed of the motor.
Hertz Rated supply frequency. Duty Motor load condition, whether it is continuos load, short time, periodic, etc. Date Date of manufacturing.
Class Insulation Insulation class used for the motor construction. This specifies max. Service Factor Factor by which the motor can be overloaded beyond the full load. NEMA Nom. Motor operating efficiency at full load. Efficiency PH Specifies number of stator phases of the motor. Pole Specifies number of poles of the motor. Specifies the motor safety standard. Also, mechanical brakes require regular Apart from the nonlinear characteristics of the induction maintenance.
Let us look at them one by one. In these Earlier motors tended to be over designed to drive a types of loads, the torque is proportional to the square specific load over its entire range. This resulted in a of the speed and the power is proportional to the cube highly inefficient driving system, as a significant part of of speed. Variable speed, depending upon the load the input power was not doing any useful work.
Most of requirement, provides significant energy saving. In many flow control frequency and the number of poles. Although this is an effective means of When an induction motor starts, it will draw very high control, it wastes energy because of the high losses inrush current due to the absence of the back EMF at and reduces the life of the motor valve due to start. This results in higher power loss in the transmis- generated heat.
The high When the supply line is delivering the power at a PF inrush current may cause the voltage to dip in the less than unity, the motor draws current rich in harmon- supply line, which may affect the performance of other ics. This results in higher rotor loss affecting the motor utility equipment connected on the same supply line.
The torque generated by the motor will be pulsating in nature due to harmonics. At high speed, the pulsat- When the motor is operated at a minimum load i. But at low speed, the pulsat- the magnetizing current and is almost purely inductive. This As a result, the PF is very low, typically as low as 0. When the load is increased, the working current begins to rise. Rotor voltage is induced in the rotor windings rather than being physically connected by wires.
Induction Motor Rotor: The rotor is the rotating part of the electromagnetic circuit. The most common type of rotor is the squirrel cage rotor. The rotor comprises of a cylindrical laminated core with axially placed parallel slots for carrying the conductors. Each slot carries a copper, aluminum, or alloy bar. The rotor of three-phase induction motors frequently is likewise implied as an anchor. The purpose behind this name is the anchor shape of the rotors used within quite early electrical devices.
Induction motor has the same physical stator as a synchronous machine with an alternate rotor development. Induction motor might be worked as either motors or generator. On the other hand, they are fundamentally used as induction motors. When the motor is connected to a single-phase power supply, the main winding carries an alternating current. It is logical that the least expensive, most reduced upkeep sort engine ought to be utilized most regularly.
These are of different types based on their way of starting since these are of not self starting. Those are split phase, shaded pole and capacitor motors. Again capacitor motors are capacitor start, capacitor run and permanent capacitor motors. Permanent capacitor motor is shown below.