Selection criteria for Induction motors for VFD applications. J. Srinivasan

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1 Selection criteria for Induction motors for VFD applications J. Srinivasan

2 VFD ensures energy saving, process control, lower starting current and long service life for mechanical components Two learning experiences VFD S OFFER EXCELLENT ENERGY SAVING OPPORTUNITIES. BUT THE MOTOR RELIABILITY HAS TO BE ENSURED THROUGH PROPER SELECTION

3 400 kw, 1485 rpm motor failed on overload Application: FD Fan in 135 TPH Boiler x 2 nos, Fan BkW-358 kw-vfd 1. As per the process requirement, at 60% of Boiler MCR condition, (One Fan Operating condition) Fan shaft power-335 kw at 1330 RPM. 2. Two Fan Operating Condition, Fan shaft Power-358 kw at 1485 RPM. Problem faced: Winding burnout due to overload in 2 nos VFD failed once-all 3 on different occasions Why? Selection of 400 kw for this application was wrong!!!!!

4 Fan Power increases in cube proportion to speed Speed in RPM Power in kw The right selection is 475 kw rated at rated speed of 1492 rpm

5 Hoisting winch application The motors would not lift!!! 132 kw, 6 pole-6 nos Identical everything, except the motor brand Brand A was successful Brand B did not hoist Identical components Upon close comparison of the data sheets, and after a study at site, Pull out torque T PO of Brand B was found to be 30% lower than Brand A. Final solution was given with higher size gearbox

6 Selection criteria for VFD driven motors Efficiency VFD driven motors have higher losses High efficiency motor keeps it efficiency high Cumulative efficiency is more important Reliability Torque characteristics Insulation stress a) Impulse voltage b) Cable length Temperature rise Bearing current Noise level Vibration level

7 Additional joule losses under PWM IEC: / IS: Application guide-cage induction motors when fed from converters

8 IE class-new efficiency standard North America with the core regions US, Canada and Mexico Dimensions in inches IEC Applicable the World over! IEC: for efficiency & IEC: for testing of efficiency Indian standard IS: 12615/2011 for efficiency & IS: for Testing of efficiency MEPS for India is IE2- To become IE3 from January, 2014!!!

9 IE3 will have IE2 & IE2 will have IE1 efficiencies, when operated through VFD IE1 IE2 IE3 Output No. of Poles Output No. of Poles Output No. of Poles kw kw kw The standard stipulates 72.1 that 72.1motors must 77.2 meet at 77.2 least 75.2 IE to be 82.7 labeled 82.7Energy Efficient IE1tables are given 91.2 for comparing the efficiency of 93.3 an IE motor when 93.5 driven by a VFD

10 Cumulative efficiency VFD speed variation offers, nearly, 50% more saving than the mechanical controls

11 Load torques & Power

12 Selecting for constant torque with VFD Mixing, Mills, Cranes, Conveyors etc Required constant Torque should be available from the motor, throughout the speed range. Please always specify the Torque

13 Impulse voltage Switching under PWM results in high dv/dtand the rise time (tr) has direct influence on insulation life Extra features for VFD duty Motors 1. Magnet wire with increased dielectric strength- Dual coat wire 2. Additional cycles of varnish dip or Vacuum-pressure impregnated 3. heavy-duty lacing or taping of end-turns

14 Cable for VFD loads The length of the cable connecting the VFD to the motor, the rise time of the VFD output voltage pulse and the size of the motor are the main variables affecting the winding insulation For retrofit applications, VFDs having a rise time of at least 0.5ms should be chosen and the switching frequency for VFDs should be selected below 5 khz. The cable length should be restricted below 30 meters

15 Temperature rise of winding Voltage harmonic distortion causes additional joule heating in the lamination steel and operations at low speeds causes ventilation reduction. Hence Torque derating or oversizing of self cooled motor should be done

16 Insulated bearing When a stray current in an electric motor uses a bearing as its path to ground, bearing damage can occur. The most common causes of stray electric currents are: asymmetry in the motor s magnetic circuit, unshielded power cables, and fast switching pulse width modulated (PWM) frequency converters used in modern variable frequency drives (VFDs).

17 Noise- IS: is the Indian standard IEC states: A-weighted noise level increase in the range of 1 db to 6 db occurs for operation with I-converters up to rated frequency compared with operation on a sinusoidal voltage at rated frequency, and in the range of 5dBto15dBforoperationfromU-converters.Theactualvaluesdepend uponthenumberofmotorpoles,thepulsepatternoftheconverter

18 Vibration- IS: is the Indian standard Procedures recommended by IEC confirmed that the vibration levels of induction motors increase when these are fed by frequency converters

19 Features for selection VFD Duty NEMA / IEC Class F to F High dv / dt PWM Dual coat VPI/IGT Constant Torque Variable Torque Torque derating for Constant Torque or higher frame for Variable Torque for F/B

20 Existing motor & New purchase Existing motor Use with double shielded cables Limit the cable length below 10 meters Include a line reactor 3% or 5% Replace with VFD-duty when motor fails VFD-duty motor Comply with IS: 15880/2009, IEC guide for selection of cage induction motors for VFD Torque derating or higher frame size for temperature limiting selection should be addressed Insulated bearing at NDE, if required Winding & Bearing protection accessories, if specified Encoder provision, if required Forced cooling arrangement

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