3. Motors
3.1 Introduction
The reliability and trouble free service of a motor depend on the superiority of its electrical and mechanical design, the accuracy and care with which the components are manufactured and assembled and on the stringency with which the motor is tested at the manufacturer’s works and at the site of installation. The operational duty requirements of motors vary from application to application.For common applications like fans, pumps, small flour mills etc. the motors may have standard characteristics. But for special applications like cranes, hoists, lifts, power press, machine tools,power station auxilliary etc. and for industries like coal mining, oil refineries, paper plants, sugar industry, cement industry, lift irrigation etc. specially designed motors with appropriate electrical and mechanical characteristics are available . The main mechanical features of such specially designed motors relate to the method of cooling (IC classification) and degree of ingress protection (IP classification). The methods of cooling and their designation are classified under I.S.6362-1971 and the degree of protection outlined in I.S. 4691-1968 . The method of cooling and their designations are given in table 3.1 below. The first characteristic numeral designates the mode of circulating the coolant and the second characteristic numeral designates the method of supplying the power necessary to circulate the coolant. Table 3.2 illustrates the capability of each degree of protection.The electrical characteristics of motors include voltage, frequency, insulation, temperature, class of duty ratings etc. The operation of nearly all driven machines can be classified into eight types of duties ranging from S1 to S8 as given in table 3.3.Testing of electric motors is a wide subject as there is large variation in the mechanical and electrical characteristics of motors depending on the type of industry, type of application etc. It is beyond the scope of this book to cover the pre-commission checks and tests of the wide variety of motors . What is attempted is to cover testing of motor installations of common applications.
3.2 Pre-commission checks
More than seventy percent of the motors employed in industries are induction motors. SCR controlled variable speed drives are also used where speed control is required. These high speed rotating dynamic machines call for testing of both the mechanical and electrical characteristics before commissioning the machine. The following pre-commission checks are recommended for industrial motors.
Mechanical Checks
· Check the alignment of the driving motor with the driven machine.
· Check the adequacy of foundation and size of foundation bolts and tightness of the motor to the foundation.
· Check the alround working clearance for safe operation and easy maintenance.
· Check whether the vibration is within permissible limits.
· Check the method of cooling and ensure that the requirement under IC classification is satisfied.
· Check the degree of protection against ingress of solid bodies/particles and liquids and ensure that the requirement under IP classification is satisfied.
· Check the bearings and air gap between stator and rotor.
Electrical checks
· Check whether facility to isolate the supply is provided within 3 metres. If push button switch is provided as isolation device, it should be provided with facility to lock.
· Check the size of cable for conformity with the approved drawing.
· Check the cable termination for
i. Proper crimping of cable conductors – check whether cable glands are of proper size and glanding is done properly.
ii. Proper seating of conductor sockets at motor terminals- check whether spring washers are used.
iii. Check the size of earthing conductor of glands. The size should be same as that used for the motor .
iv. Clamping of the cable at a distance of 45 cms from the termination.
v. Check whether cables are taken straight from the termination at the terminal box – there shall not be any tangential stress at the glands.
vi. Check whether double compression glanding is used in hazardous areas.
· Check whether duplicate earthing is provided for the body of the motor from diametrically opposite sides – check the size of the earthing conductors for adequacy.
· In hazardous areas, check whether earthing is done from inside of terminal boxes.
· Check the earth lead terminations and connections for
i. socketing of the earth lead ends (if round conductor)
ii. proper surface contact.
iii. removal of paint on the body of the equipment
iv. tinning of copper flats at terminations
v. effectiveness of bolting
· Check whether starting devices as per the approved scheme are provided for the motor.
· Check the cable connection from slip ring motors to rotor resistance cubicle.
· Check the enclosure of rotor resistance. The rotor resistance shall be inaccessible and enclosed for safety.
· Check the length of flexible connections to movable motors. The length shall be limited to 60 cms.
· Check whether flexible cables are enclosed in steel re-inforced PVC flexible conduits.
· Check the terminations for use of proper couplings and for adequate mechanical strength.
· Check the rating of capacitors for conformity with the approved scheme.
· Check the connections of motor capacitors. Capacitors shall either be connected from the motor terminal box or from the outgoing side of the motor starter. Suitable adapter boxes shall be provided for making the capacitor connection.
· Check whether the capacitor is provided with duplicate earthing.
· In the case of MV consumers, check whether ammeters of adequate rating is provided in the capacitor circuit.
· Check the utilisation category of contactors used in starters. The standardized utilisation categories of contactors for common applications of ac motors are given below:
AC-1 Non inductive or slightly inductive loads – resistance furnaces.
AC-2 Slip ring motors: starting-switching off.
AC-3 Squirrel cage motors: Starting, switching off during running.
AC-4 Squirrel cage motors: Starting, plugging, inching.
· Check the current rating of the contactors and ensure that it is suitable for the motor for which it is used.
· Check whether the settings of over current relays in starters correspond to the motor loading.
· Check whether the protective arrangements provided are as per the approved scheme.
· Check the motor protection relays for the number of elements/ protections and their settings.
· Check whether relays are tested and set to suit the operating conditions.
· Check the rating of back up fuses for conformity with the approved scheme.
3.3 Pre commission tests
3.3.1 Measurement of insulation resistance
The insulation resistance between phases, between each phase and earth and between all phases together and earth are measured using insulation testers of appropriate voltage. (For detailed description of insulation testers and procedure for testing refer para 2.4.1.) The recommended voltage ratings of insulation testers is as given below:
MV motors – 1000 V tester
HV motors – 2.5 kV/5 kV tester
The measured value of insulation resistance should be more than 1 meg ohm for M.V motors and 20 meg ohm for HV motors. If satisfactory values are not obtained, the motor may be cleaned and dried and the insulation resistance measured again. The results may be recorded for future reference.
Polarisation Index
(Please refer para 2.4.2 also)
It is necessary to measure and record the polarisation index of HV motors. Measurement of polarisation index is a conclusive test for ascertaining the quality of insulation of HV motors. The value of insulation resistance after 1 minute and 10 minutes of starting the measurement are measured using HV insulation testers of appropriate voltage. The polarisation index (P.I. value) which is the ratio of the 10 minute value to the 1 minute value shall be more than 2.0.
3.3.2 High Voltage Test
High voltage test of motors are normally done at the manufacturer’s work site. This is done only once and not repeated at the site of installation. However in some special cases, HV test is conducted as a supplementary to the test conducted at the manufacturer’s work site. The manufacturer shall be consulted before such tests. The value of the test voltage shall be limited to 75-80 % of the value prescribed for the HV test. The insulation resistance may be measured before and after the HV test and recorded for future reference.
3.3.3 Measurement of tan delta for HV motors
(Please refer para 2.4.4 also)
The dielectric loss factor ‘tan delta’ is a measure of the quality of insulation of HV motors. As a standard practice, measurement of ‘tan delta’ is carried out for motors of 5 kV and above. A low dielectric loss factor is an indication of good insulation of the motor. Measurement of tan delta is done using ‘tan delta’ measuring equipment and the results are recorded for future reference. Future test results during periodical or break down maintenance may be compared with the initial values. The variation in the value of tan delta at different periods of interval shall be within reasonable limits.
3.3.4 No load test
The no load test is conducted to compare the actual values of current, power, power factor and losses with the designed values. The test results give an indication of the condition of the magnetic circuit
and also the mechanical losses. The motor is decoupled from the driven machine and run on no load. The direction of rotation may be checked and the no load speed measured using a tachometer. Measure the current and voltage of the three phases and compare with the factory test results. There shall not be notable variation between the measured values and the factory test results.The measured values of no load current, voltage, power and power factor may be documented for future reference.
3.3.5 Open circuit voltage ratio test
In the case of slip ring motors, the open circuit voltage ratio may be measured and recorded. The value is compared with the name plate details and recorded for future reference.
3.3.6 Earth continuity test
Earth continuity of the motor frame, cable glands, capacitor body and control gears shall be tested using an earth tester or an ohm meter for very low resistance measurement. Earth continuity of various non-current carrying metal parts with the main earth bus may be checked by touching the common lead of the tester/ohm meter on the body of the respective metal parts.
3.3.7 Measurement of shaft voltage and insulation resistance of bearing
This test is conducted only for very large capacity motors and generators. Due to non-uniformity in the air gaps and magnetic circuits, a very low voltage (of the order 0.5V to 2V) may be induced in the motor/generator shaft which will circulate a current (shaft current) through the bearings and external frame work. It is necessary to prevent this shaft current as it can cause damages to the costly bearings of large capacity motors. Shaft current is prevented by insulating the bearing support. The insulation resistance between the two sides of the bearing insulation is measured using 250V/500V insulation tester. The measured value of the insulation resistance shall be more than 1 meg ohm. The shaft voltage is measured between the shaft and the bottom of the insulated bearing with a low voltage voltmeter of range 0-5 V. The normal value of shaft voltage is below 1 volt. The measured values of shaft voltage and insulation resistance of the bearing are recorded for future reference.
3.3.8 Oscilloscopic Tests
In the case of HV motors, it is necessary to record the transient inrush current at the instant of starting and the subsequent steady state starting current. In the case of large capacity HV. motors (1500 kW and above) it is also necessary to record the “Snap shot” voltage dip at the instant of starting. This is done using storage type oscilloscopes. For measuring the transient inrush current of HV motors, the secondary from the protection core of the CT (not from the metering core as the metering core may saturate when the current exceeds five times the full load current) is connected to a CIV resistor (current-into-voltage resistor). The voltage developed across the resistor is proportional to the current passed and hence the voltage can be recorded on the oscilloscope, calibrated to read the corresponding current. During transit and usage it is possible that one or more resistors of the CIV network may either get disconnected or open circuited altering the value of the CIV resistor grid. Hence before commencement of the oscilloscope recording, the value of the CIV resistor must be measured accurately with a micro ohm meter or a milli ohm meter. The oscillograms taken during precommission tests are documented for future reference and analysis.
3.3.9 Measurement of capacitor current
Where individual capacitors are installed for power factor compensation, the current drawn by the capacitors in each phase shall be measured and checked with the current rating given in the name plate of the capacitor. The value shall be within reasonable limits of ± 10%. The measured value shall be recorded for future reference. The reading of the ammeter provided in the capacitor circuit shall be checked to ensure accuracy.
3.3.10 Partial Discharge measurement
It is preferable to measure the quantity of partial discharge from windings of large capacity HV motors. Partial Discharge is measured using a partial discharge measuring probe. The measured values are recorded for future reference. The quantity of partial discharge in subsequent years gives an idea of the ageing of the insulation. In the case of substantially increased partial discharge, corrective action shall be taken by replacing coils susceptible to failures. The assessment on the condition of the insulation is made by comparing the quantity of partial discharge with the test results of previous years.
3.3.11 Testing and Setting of Relays
Smaller motors are provided with over current relays only. Generally, thermal over current relays are used. These are tested and set to correspond to the actual loading condition. Larger MV motors and HV motors are provided with composite motor protection relays of many elements. A motor is said to be satisfactorily protected if the relay has atleast five elements. The operation of all these elements are tested using a secondary injection relay test kit and the relay is set to suit the operating conditions of the motor. The motor protection relay is tested and set in consultation with the manufacturer of the particular relay. The details of the thermal characteristic of the motor may be obtained from the motor manufacturer.
3.4 Commissioning of large capacity motors
After completing the pre-commission tests, repeat the checks once again and ensure the following before starting the motor for trial run.
1. The surrounding space is clear and free from unwanted objects.
2. The rotor is free to rotate.
3. Adequate resistance is inserted in the rotor circuit in the case of slip ring motors.
4. Brushes are properly bedded to the curvature of the slip rings with correct pressure and that the brush holders are sufficiently tight on the supporting rods.
5. The space heater supply is switched off.
6. Oil has been let into the bearing in sufficient quantity (in the case of forced or combined lubricated bearing)
7. The driven equipment is ready.
Now switch on to start the motor. The trial run must be on No Load. During the trial run the temperature of bearings shall be observed every 10-15 minutes till constant temperature is attained, but in no case for less than 8-10 hours. If the temperature rises above the prescribed limits, the motor hall be stopped and the corresponding bearing checked.
If the motor runs satisfactorily on No Load for sufficiently long time, the motor may be put on Load. Immediately after the motor is put on Load, the vibrations at bearings, frame etc. shall be measured using a Vibrometer. The vibration shall be within specified limits. The readings of all the indicators on the control board shall be noted.
Motors are assigned a maximum permissible number of start which is related to the heating of the stator and rotor windings at the end of each cycle of operation. Hence too frequent trial starts can cause undesirable over heating of the motor windings and consequent damage to the insulation of the stator and rotor windings. The starting cycle shall be repeated only after complete cooling of the motor to the original condition unless the motor is designed for frequent starting. For any motor, there is a limit for the number of starts per hour which depends on the moment of inertia and other facors of the motor and dirven load. A fair idea of the state of the motor is given by the reading of various thermometers mounted on the motor. The mean temperature rise of the winding and also the maximum temperature rise of the winding and the stator core shall not exceed the specified limits.
The state of insulation of the stator winding shall be assessed by regular measurement of the insulation resistance in hot state of the winding and on comparison of the dielectric absorption coefficient K which is usually taken as the ratio of insulation resistance at 60 seconds to the value at 15 seconds.
Written by John on September 22nd, 2008 with
1 comment.
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#1. August 10th, 2011, at 5:25 AM.
Thanks for the post, I’ll keep checking back for more stuff, bookmarked!