TRAINING REPORT
INDUCTION MOTORS AND THEIR USES
MADE BY : RISHAV JAIN
CONTENTS S.No. Topic PAGE NO. 1 2 3 4 5 6 7 8 9
ACKNOWLEDGEMENT COMPANY OVERVIEW BASICS OF INDUCTION MOTORS CONSTRUCTION OF THREE PHASE MOTORS CHOSING THE CORRECT TYPE OF MOTOR 12 WINDING USES OF MOTORS TESTING CONCLUSION
2 3 5 11 18 20 25 31
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ACKNOWLEDGEMENT On completing my training at ABB Motors, Faridabad, I would like to thank Mr. Khusro Khan for providing help in all situations required and for working hard to give me the best opportunities to learn all that I could in this short period of time. I also feel obliged to express my gratitude to Mr. Anjan Chatterjee, HR Co-ordinator, for granting me the position as a trainee in this company. Mr. Khan and other members of the production department were very helpful in explaining, in detail, every important aspect of the manufacturing process. They also provided me with helpful tips for concluding the training with this report.
Mr. Khusro Khan
JITENDER
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COMPANY OVERVIEW ABB motors (India) is one of the six manufacturing units of ABB India. The factory is located at Faridabad about 40km from Delhi in the state of Haryana in north central part of India. The technical backup is received from ABB Motors in European countries. • ABB is pioneer to introduce high efficiency (eff 2) motors in India in the year 1991 • ABB was again pioneer to introduce EFF1 motors for frame 315 & 355 in the year 1998 • It is the only motor factory in India to be accredited for IMS (Integrated Management System) combination of ISO 9001 – 2000, ISO 14001 – 1996 & OHSAS 180001 – 1999. • We are the only company in India to offer complete range of EFF1 (High Efficiency motors) in frame 100 to 355. Product Range • • • • • • • • • • •
Standard three-phase TEFC (HX & HX+)motors, IS:325 Standard three-phase SPDP motors, IS:325 High Efficiency EFF1 Motors ( Frame 100 to 280) M2000 motors, (Eff1 Frame 315 to 400) Ring frame motors, IS:2972 Part III Auxiliary motors for a.c. electric locomotives Auxiliary motors for three-phase electric locomotives Non-sparking motors type ‘n’, IS:9628 Increased safety motors type ‘e’, IS:6381 Roller table motors Flame Proof Motors (Frame 112 / 132 are in production, Frame 100 under development, Frame 160 under Certification) • Crane duty Motors • Brake Motors (externally mounted) 4
• • • •
Induction Generators Windmill Generators Motors for frequency converter drive Custom built motors
Manufacturing Range • • • • • •
• • •
Type Output Voltage Frequency Duty Ambient Altitude Insulation Frame
Three-phase squirrel-cage induction motors 0.18 ... 500 kW; 0.25 ... 675 hp 415 V; 220 ... 660 V 50 Hz; 25 ... 60 Hz S1 ... S8 acc. to IS:12824 45oC; 20 ... 70oC up to 1 000 m Class ‘F’/Class ‘H’ (VPI on request) HX71- HX280 & M2BA315 - 400
Special Features • High Efficiency – ABB motors are engineered for high efficiency for minimum life cycle costs. • Low noise level – The motors are designed to ensure low noise level. • Accessories – Space Heaters, thermistors, RTD and BTD’s on request. • Terminal Box – Terminal Box suitable for both copper and aluminum cables. Terminal box size sufficient for easy termination and maintenance of cable. • All motors are provided with deep groove ball bearings with C3 clearance for high load capacity. • Special care taken at the time of order for considering right bearing at the time of order (e.g. Roller Bearings for belt Application or Insulated bearing for VFD Application). • Dual mounting holes – Motors are provided with 6 nos. mounting holes instead of 4 nos. as standard practice in the industry. This facilitates easy replacement and easy inventory management. 5
BASICS OF INDUCTION MOTORS AC (Alternating current) and DC (Direct current) motors While there are only three general types of DC motors, there are many different AC motor types. This is because each type is confined to a narrow band of operating characteristics. These characteristics include torque, speed, and electrical service (single-phase or polyphase). These operating characteristics are used to determine a given motor’s suitability for a given application. What makes an AC motor different from a DC motor? In a DC motor, electrical power is conducted directly to the armature through brushes and a commutator. An AC motor does not need a commutator to reverse the polarity of the current , as AC changes polarity “naturally.” Also, where the DC motor works by changing the polarity of the current running through the armature (the rotating part of the motor), the AC motor works by changing the polarity of the current running through the stator (the stationary part of the motor).
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The many types of AC motor may be split into two main groups: single-phase and poly-phase. Single Phase: A single-phase power system has one coil in the generator. Therefore, one alternating voltage is generated. The voltage curve of a single-phase AC generator is shown in Figure
Single-phase motors are generally motors with KW ratings of one or below. (These are generally called fractional KW motors.) They are generally used to operate mechanical devices and machines requiring a relatively small amount of power. Types of single-phase motors include: shaded-pole, capacitor, split-phase, repulsion, series (AC or universal) and synchronous. However, the single-phase motor is generally not used because it is inefficient, expensive to operate, and is not self-starting. Three-Phase Three-phase or poly-phase motors run on three-phase power. A three-phase power system has three coils in the generator. Therefore, three separate and distinct voltages will be generated. The voltage curve is shown in Figure
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Types of three-phase motors include: induction (squirrelcage or wound), rotor types, commutator, and synchronous. The Squirrel Cage Induction motor Induction refers to electrically charging a conductor by putting it near a charged body. Induction Principle The principle of the induction motor was first discovered by Arago in 1824. He observed that if a non-magnetic metal disk and a compass are pivoted with their axes parallel, so that one (or both) of the compass poles are located near the edge of the disk, spinning the disk will cause the compass needle to rotate. The direction of the induced rotation in the compass is always the same as that imparted to the disk. Applying the Induction Principle to AC Motor AC motor works by changing the polarity of the current running through the stator (the stationary part of the motor). The stator plays the role of the metallic disk described above. A rotating magnetic field is established in the stator. The conductor, called the Rotor, “follows” the rotating magnetic field by beginning to rotate, just like the compass needle described above. The induction motor uses a rotor of a special design. It resembles a cage used for exercising squirrels. This is why it is called a squirrel cage rotor. The rotor consists of circular end rings joined together with metal bars. Note that the metal bars are placed directly opposite each other and provide a complete circuit within the rotor, regardless of the rotor's position. Rotors normally have several bars, but only a few are shown here for clarity.
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FIGURE: THE ROTOR OF A SQUIRREL CAGE INDUCTION MOTOR Squirrel cage motors are usually chosen over other types of motors because of their simplicity, ruggedness and reliability. Because of these features, squirrel-cage motors have practically become the accepted standard for AC, allpurpose, constant speed motor applications. Without a doubt, the squirrel-cage motor is the workhorse of the industry. The Squirrel Cage Induction Motor has certain advantages over the DC motor. • There are only two points of mechanical wear on the squirrel cage motor: the two bearings. • Because it has no commutator, there are no brushes to wear. This keeps maintenance minimal. • No sparks are generated to create a possible fire hazard. Three- Phase Motor An induction motor depends upon an electrically rotating magnetic field, not a mechanically rotating one. (A mechanically rotating field would work, but an electrically rotating magnetic field has significant advantages.) How is an electrically rotating field obtained? It all starts with the phase displacement of a three-phase power system.
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Three-phase power can be thought of as three different single-phase power supplies. They are called A, B, and C. In the three-phase motor, each phase of the power supply is provided with its own set of poles, located directly across from each other on the stator, and offset equally from each of the other two phases’ poles.
FIG: THREE PAIRS OF FIELD COILS ON THE STATOR, SET 120 DEG APART The three currents start at different times. Phase B starts 120° later than phase A and phase C starts 120° later than phase B. This is shown on the sine wave graph in Figure, which indicates the way the magnetic field will point at various times in the cycle.
FIG: Magnetic Field Rotation Providing Torque To Turn The Motor Introducing these different phase currents into three field coils 120° apart on the stator produces a rotating magnetic field, and the magnetic poles are in constant rotation. The
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magnetic poles chase each other, simultaneously inducing electric currents in the rotor (generally, bars of copper imbedded in a laminated iron core). The induced currents set up their own magnetic fields, in opposition to the magnetic field that caused the currents. The resulting attractions and repulsions provide the torque to turn the motor, and keep it turning. Walking through one revolution of the motor First, the A poles of the stator are magnetized by phase A. Then, the B poles are magnetized by phase B. The rotor turns, due to the induced current. Then, the C poles are magnetized by phase C. The rotor turns, due to the induced current. The rotor has completed one-half turn at this point.
FIG: Rotating Magnetic Field Turns the Motor Now, the A poles of the stator are magnetized again, but the current flow is in the opposite direction. This causes the magnetic field to continue to rotate, and the rotor follows. Then, the B poles are magnetized by phase B. The rotor turns, due to the induced current. Then, the C poles are magnetized by phase C. The rotor turns, due to the induced current.
FIG: Rotating Magnetic Field Turns the Motor
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The rotor has completed one full revolution at this point, and the process repeats itself.
CONSTRUCTION OF THREE PHASE MOTORS The three-phase motor is probably the simplest and most rugged of all electric motors. To get a perspective on how important the three-phase motor is, all you need to know is that this motor is used in nine out of ten industrial applications. All three-phase motors are constructed with a number of individually wound electrical coils. Regardless of how many individual coils there are in a three-phase motor, the individual coils will always be wired together (series or parallel) to produce three distinct windings, which are called phases. Each phase will always contain one-third of the total number of individual coils. As we mentioned, these phases are referred to as phase A, phase B and phase C. Three-phase motors vary from fractional KW size to several thousand KW. These motors have a fairly constant speed characteristic but a wide variety of torque characteristics. They are made for practically every standard voltage and frequency.
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CHOSING THE CORRECT TYPE OF MOTOR In order to select the correct motor type for a given application, it is necessary to understand the load requirements first. To understand these requirements, you need to be familiar with the concepts of force, work, torque, power and KW, and how they relate to speed. Force, Work and Torque Work is done when a force overcomes a resistance. Work = Distance x Force In the case of an electric motor, force is not exerted in a line, but in a circle, about a cylindrical shaft. This turning force is called torque. Torque = Radial Distance x Force Power and KW Power takes into consideration how fast work is accomplished. It is the rate of doing work. Power = Work/Time The reason for this difference is the amount of work that can be delivered in a given amount of time. Obviously, a larger motor should be able to deliver more work in a given time than one that is considerably smaller. It is this difference that determines the power rating of the motor.
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Motors are rated in KW (HP). One KW is equal to 746 watts.
Putting it all together, Torque, KW, and speed are all interrelated when turning a load. KW is proportional to torque and speed. The following formula ties them all together: T=974*KW/rated speed This means that if either speed or torque remains constant while the other increases, KW increases. Conversely, if either torque or speed decreases while the other remains constant, KW will decrease. Below is a chart that shows the relationship of KW, torque and speed. Speed Increases Torque Constant
KW Increases
Speed Decreases Torque Constant
KW Decreases
Speed Constant Torque Increases
KW Increases
Speed Constant Torque Decreases
KW Decreases
Speed Increases Torque Decreases
KW Increases
Speed Decreases Torque Increases
KW Decreases
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Speed Control for an AC Motor Because each motor type has its own characteristics of KW, torque and speed, different motor types are more suited for different applications. The basic characteristics of each AC motor type are determined by the design of the motor and the supply voltage used. The induction motor is basically a constant speed device. The speed at which an induction stator field rotates is called its Synchronous Speed. This is because it is synchronized to the frequency of the AC power at all times. The speed of the rotating field is always independent of load changes on the motor, provided the line frequency is constant. Synchronous speed is determined by the number of poles that the motor has, and the frequency being supplied to it. The equation for determining the synchronous speed of a motor is: N = 120f/P Where: N = the synchronous speed of the motor in revolutions per minute (RPM) f = the frequency supplied to the motor in Hertz (Hz) P = the number of poles the motor has
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Motors designed for 50 Hertz use (standard in the INDIA) have synchronous speeds as follows: Poles 2 4 6 8 10
RPM 3000 1500 1000 750 600
Induction motors do not run at synchronous speed; they run at Full Load Speed, which is the rotational speed of the rotor. Full load speed is always slower. The percent reduction in speed is called Percent Slip. The slip is required to develop rotational torque. The higher the torque, the greater the slip. The motor speed, under normal load conditions, is rarely more than 10% below synchronous speed. If the motor is not driving a load, it will accelerate to nearly synchronous speed. As the load increases, the percent slip increases. For example, a motor with a 2.8% slip and 1800 rpm synchronous speed would have a slip of 50 rpm, and a full load speed of 1750 rpm (1800 - 50 = 1750 rpm). It is this full load speed that will be found on the motor's nameplate. From the formula, it is evident that the supply frequency and number of poles are the only variables that determine the speed of the motor . Varying the voltage is not a good way to change the speed of the motor. In fact, if the voltage is changed by more than
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10%, the motor may be damaged. This is because the starting torque varies as the square of the applied voltage. Because the frequency or number of poles must be changed to change the speed of an AC motor, two methods of speed control are available. These are: • Changing the frequency applied to the motor Changing the frequency requires a device called an Adjustable Frequency Drive to be inserted upstream from the motor. This device converts the incoming 50 Hz into any desired frequency , allowing the motor to run at virtually any speed. For example, by adjusting the frequency to 30 Hz, the motor can be made to run only half as fast. • Using a multi-speed motor Multi-speed AC motors are designed with windings that may be reconnected to form different numbers of poles. They are operated at a constant frequency. Two-speed motors usually have one winding that may be connected to provide two speeds, one of which is half the other. Motors with more than two speeds usually include many windings. These can be connected many ways to provide different speeds. Starting the Motor A Starter is a device that is used to start a motor from a stop. The across-the-line starter is by far the most common. This type of starter places the motor directly across the full voltage of the supply lines, hence the name: “across-theline.” When an induction motor is placed across-the-line, it will accelerate to full speed in a matter of seconds.
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Reversing the Motor To reverse a motor all we need to do is reverse the order in which the Line Power is fed to the motor. This wiring change is accomplished by “swapping” two of the phases of power. In short, A motor wired with phases ABC to run forward, would have its phases wired CBA to run in reverse Braking the Motor Two common methods used for breaking a motor are DC Injection Braking and Dynamic Braking. i) DC Injection Braking: DC injection braking is a method of braking in which direct current (DC) is applied to the stationary windings of an AC motor after the AC voltage is removed. This is an efficient and effective method of braking most AC motors. DC injection braking provides a quick and smooth braking action on all types of loads, including high-speed and high-inertia loads. ii) Dynamic Braking: Dynamic braking is another method for braking a motor. It is achieved by reconnecting a running motor to act as a generator immediately after it is turned off, rapidly stopping the motor. The generator action converts the mechanical energy of rotation to electrical energy that can be dissipated as heat in a resistor.
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WINDING A very brief explanation of the winding process has been provided below according to the different stages the motor goes under this process. 1. Coil making – Copper wires are arranged in order to form coils according to the specifications demanded by the customer. These include number of wires used, thickness of the wire and number of turns of these wires. 2. Mounting / Coil insertion – These copper coils are then manually inserted into the depressions of the rotor accordingly to distinguish the three different phases. Insulation paper is also inserted between each coil to provide prevention from short circuits. 3. Connection – The three phases are then produced upwards to the control panel where they are fitted with the help of PVC insulation and screws. Shaping and pressing of the coils is also done in this stage of the process. 4. In-process testing – A few tests are also performed before the motor is sent in for impregnation. These include: a) DCR test – Used to test the resistance of the copper coils inserted into the rotor. b) Surge test – This checks if there is any fault in the insulation between the coils. c) HV test – This is used to check if the motor breaks down if high voltage is applied to its ends.
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5. Impregnation – There are two basic types of
impregnations. a) Flood impregnation - Once fitted with its (unimpregnated) coils and provided with all necessary winding overhang supports and end winding connections, the entire stator assembly is flooded with resin and then cured at elevated temperature. b) Vacuum Pressure impregnation - This method is employed to obtain a nearly 100 % filling of the voids in the slot guaranteeing the excellent moisture and mechanical strength. 6. Baking – This is the final stage of winding under which the motor is placed in an oven and is heated at a temperature of 145 to 155 degrees C for 6 hours. This is done to increase insulation by hardening the liquid produced from the previous stage.
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USES OF MOTORS A few examples showing the different uses and purposes of electrical motors have been provided below. 1.
Roller Table Motors Introduction In Rolling Mills, the stock is fed to the rolling stands and removed from them by roller tables, the rollers of which are driven either in groups by a common motor or individually, each roller having its own motor. Roller table motors are special purpose motors, with special torque speed characteristics to meet severe conditions of frequent starts and reversal. In addition, the high ambient temperature, humidity and risk of fine dust particles from the process infiltrating the motor are constant threat to system reliability. Classification They can be generally classified into two groups: i) Constant speed roller table motors: These motors are switched directly on the line and designed for frequent start, stops and reversal. The acceleration factor B is a characteristic figure that describes the behavior of the motor when frequently switched. Its value is determined as the product of total inertia (expresed in GD2) to be accelerated and the maximum no. of starts per hour of the motor without exceeding the permissible temperature limit.
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ii) Variable speed roller table motors: These motors are fed through PWM frequency converter. A PWM converter has many advantages e.g. minimum reactive power consumption, less converter loss, multi-drive applications, etc. The greatest advantage of using frequency controlled ac roller table is a possibility to avoid a gear box by optimal selection of the pole number and the nominal frequency. Special Features Mechanical features: • Stator frames and end-shields are of cast iron grade FG- 260 or higher. • Degree of protection IP55 or IP56 • Mounting B3 or B5 or B35 • Cooling IC 0041 (surface cooled) • Circular Cooling ribs (for better heat transfer) • 63 series single row deep groove ball bearings, roller bearing on DE and ZZ bearing on request • Epoxy Paint • Plug and socket arrangement (on request) Electrical Features • High starting torque design( constant speed) • Low starting current (constant speed) • High stall time (constant speed) • High pull out torque (variable speed) • Nominal Frequency selection according to required speed range (variable speed) • Special care for PWM frequency converter supply • Star connection • Insulation class F or H • Voltage range 380 to 660 V 22
2.
Textile Motors – Introduction These motors, also known as ring frame motors are used for driving spinning frames in textile industries. Rating of these motors are as per IS:2972 part III and are available in both single speed and double speed. The starting torque and the speed-torque characteristics of the spinning frame motor is typical since it should be neither too high nor too low. If it is too high, the acceleration will be rapid and there will be snatch and break-up of the yarn. If it is too low, the tension of the yarn is insufficient and as such the yarn gets entangled and cause breakage. The torque values of the motors have therefore to be very carefully specified by suitably matching with that of the load. Classification Spinning frame motors are generally available in 4 pole and 6 pole single speed application and 6/4 and 8/6 pole two speed applications. Special Performance Requirement Following Values of various torque characteristics for spinning frame motors have been stipulated taking into consideration their performance requirements when started at rated voltage. • •
Starting torque: 150 to 200% of full load torque Pull Out torque: 200 to 275 % of full load torque
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3.
Crane Duty Motors – Introduction ABB Crane Duty Squirrel Cage Induction Motors incorporating State-of-the-art technology, are specially designed to service the cranes and hoists. These motors have unique torque-speed characteristics, which results in rapid acceleration causing minimum heat losses in rotor circuit. ABB crane duty motors are robust in construction and extremely sound in electrical and mechanical design. These motors can also be used for similar applications in intermittent duty. These motors have high starting torque with low starting current and are suitable for frequent starts, stops and reversing operation. Further, rapid acceleration is achieved by high pull out torque/ rotor inertia ratio. Special Features The ABB Crane Duty Motors stand apart for many reasons: • • • • • •
High starting torque Low starting current Frequent Starts/Stops and Reversals High Pull out torque Low Rotor Inertia Extra thermal endurance
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4.
Dual Speed Motors – Introduction A double speed motor permits operation of the loads at speeds corresponding to two different pole numbers. These motors are preferred in applications such as fan, pump, blower, where adjustable speed operation leads to substantial energy saving. These motors also find applications in machine tools where it is required to achieve different torque and speed combinations from operation point of view. Classification These are classified according to the different speed ratios. The desired speed ratio from a double speed motors could be an integer or a fraction. i) Integral Speed Ratio Motors: 4/2 and 8/4 pole combinations give integral speed ratio. Integral speed ratio can be obtained by two types of winding configuration. Separate Winding: The stator contains two separate windings placed one above another, each corresponding to the two pole numbers. Only three leads are brought out from each windings. Dahalander Winding: The stator is wound for one speed and provision is given for reconnecting the same winding for the second speed. All the six ends of the winding are brought out to the terminals and a connection diagram is provided to facilitate change in the speed of the motors. ii) Fractional Speed Ratio Motors: Pole combination 8/6 and 6/4 give fractional speed ratio. These motors are wound with two separate windings, as described in integral speed ratio above.
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TESTING Testing can be categorized into 2 types, Routine and Type tests. Routine tests are performed on every motor after it is assembled since these are the requirements according to ‘IS:325’ standards. Type tests are performed usually to one of the motors of a series of similar motors or by a request of the customer. These are needed if the motor will run in special conditions. The various routine tests are briefly explained below: 1. Visual inspection: During the visual inspection the following points are checked: • The rating plate values. • Fans, fan motors, main terminal box, terminal blocks and other auxiliary devices are of correct type. • Paint, if for client inspection, is of correct type. 2. High-voltage test: High-voltage test is carried out for windings, temperature detectors and space heaters to ensure that there are no weak points in insulation. The measuring principle for windings is shown in fig. 1.
U
kV
V W
A
Detector, heater
B
Fig. 1. High voltage test arrangements for the stator phase U.
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By standard motors, and when the star point is available, the windings can be tested separately as in figure 1A. In other cases all the phases are tested together as shown in figure 1B. The test voltages used are at least equal to values given in IS 4029. Measuring voltage of phase windings is 2000 V (2 x UN + 1000 = 2000 V for all sizes ≤ 690 V) for two minutes. For auxiliaries test voltage is usually 1500 V and testing time is 15 seconds. 3. Insulation resistance measurement: The purpose of this measurement is to check that the insulation level of windings and auxiliary devices of the motor is high enough to ensure safe operation of the motor. The insulation resistance of windings is measured as shown in fig. 2. The measuring voltage is 500 VDC. For type tested motors this measurement is done after the temperature rise test. For routine tested motors the test is done at the ambient temperature. Also space heaters and temperature detectors are tested with 500 VDC.
U1
MΩ
U2
V1
V2
W1
W2
Fig. 2. Insulation resistance measurement of stator windings. 4. Resistance measurement at the ambient temperature:
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Resistances of windings are measured at ambient temperature • to check that the connections of the windings are correct •
to find out eventual unbalance between phases
• to measure the accurate ‘cold’ resistance value so that the temperature rise can be determined after a temperature rise test (type test) The resistance values of the standard terminals are measured between terminals U1-U2, V1-V2, W1-W2 regardless of the Y/D -connection of the motor. Resistances are measured by ohmmeter with 4-wire method. Resistances of possible temperature detectors and space heaters are measured from their terminal blocks. All measured values are compared to the calculated values. 5. Terminal markings and direction of rotation: According to standard IS: 4728 the motors rotate clockwise (facing to the drive-end of a motor) and the phases of supply are connected to terminal box in order L1, L2, L3 -> U, V, W. Should the motor rotate counterclockwise chance the place of two leads. 6. No load point at 50 Hz frequency: The motor is driven at no load at 50 Hz frequency and rated voltage (for standard 50 Hz ratings) with a free shaft extension. The stator current, voltage and input power are measured, recorded and compared to the designed values. 7. Locked rotor point at 50 Hz frequency: The rotor of the motor is locked mechanically. The 50 Hz voltage selected gives approximately the rated current. The stator current, voltage and input power are
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measured, recorded and compared to the designed values.
Some of the type tests asked for by the customers are also mentioned below: 1. No load test: Motor is to be run at no load at rated voltage and frequency with a free shaft extension. The stator current, voltage and input power are measured, recorded for different readings of voltages to calculate the friction and windage and iron losses of the motor at rated conditions. 2. Locked rotor test: The rotor of the motor is locked. The voltage is being adjusted in order to get the rated current, 1.5 times the rated current and double of the rated current. Temperature rise test: The temperature rise test is carried out to determine the temperature rise of the winding of the motor. For EEx e, Ex nA and Ex N motors also the temperature rise of the motor is determined. An asynchronous machine as a motor is tested by feeding it from the supply that gives the rated voltage and current. It is also possible to test a machine as an asynchronous generator with the ATEML testing systems. Performance of the temperature rise test
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The temperatures of the motor are recorded during the test. The motor is to run until all temperatures have become stable, change of temperature less than 1°C per hour as per IS:12802. After the motor has been stopped the temperature rise of stator winding is measured by the resistance method. After the motor has been stopped, the resistance values are read for a suitable time (approximately 2 minutes). The following table shows at which time latest the resistance value for calculating the temperature rise should be measured. Rated power Time after electrical disconnection 0-50 30 s 50-200 kW 90 s 200-5000 kW 120 s The temperature rise for copper is determined as Θ =
R2 ( 235 + t 1 ) − t a − 235 R1
Where R2 = hot resistance value R1 = cold resistance value t1 = temperature for the cold resistance (°C) ta = temperature of the cooling medium (°C) 235 = temperature coefficient of resistance for copper Acceptable temperature rises of the windings according to the standard IS325/IEC 34-2 are Θ = 80 K for insulation class B 105 K for insulation class F 4. Efficiency determination: The efficiency is calculated from the total losses, which are determined by summation of the loss components in accordance with IS 4029/ IEC 34-2.
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5. Overload test: Overload test is performed to check if the torque of the motor is high enough to handle overloads for a short period. Test is performed at 160% of rated torque for 15 seconds (IS 4029/ IEC34-1).
6. Over-speed test: By ABB Motors the over speed test is normally carried out at 120% of the synchronous speed at no load for two (2) minutes. 7. Vibration level test: The vibration level test is made in accordance with IS 12075/ IEC 34-14, under no load with motor in a state of free suspension. Results can be expressed either as vibration velocity (mm/s) or acceleration (mm/s²). 8. Sound level test: By ABB Motors the A-weighted sound pressure level is measured on a motor with free shaft extension. The measurement is done at different points around the machine and limits specified as per IS 12065. 9. Partial loads: Partial loads are measured at 50% and 75% of the rated load and the efficiencies are determined for these loads. The other loading points can be determined as per the requirement.
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