selection criteria for induction motors for vfd applications · selection criteria for induction...

Selection criteria for

Induction motors

for VFD applications

J. Srinivasan

js@primemover.co.in

VFD ensures energy saving, process control, lower starting

current and long service life for mechanical components

VFDS OFFER EXCELLENT ENERGY SAVING OPPORTUNITIES.

BUT THE MOTOR RELIABILITY HAS TO BE ENSURED

THROUGH PROPER SELECTION

Two learning experiences

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: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!!!!!

Speed in

RPM Power in kW

0 0

150 1

300 4

450 13

600 30

Fan Power increases in cube proportion to speed

600 30

750 59

900 103

1050 163

1200 243

1350 346

1500 475

The right selection is 475 kW rated at rated speed of 1492 rpm

Hoisting winch application

The motors would not lift!!!

• 132 kW, 6 pole- 6 nos • Identical everything, except

the motor brand

• Upon close comparison of

the data sheets, and after a

study at site, Pull out

• Brand A was successful

• Brand B did not hoist

• Identical components

study at site, Pull out

torque TPO of Brand B was

found to be 30% lower than

Brand A.

• Final solution was given

with higher size gearbox

Selection criteria for VFD driven motors

Efficiency

• VFD driven motors have

higher losses

• High efficiency motor keeps

it efficiency high

Reliability

• Torque characteristics

• Insulation stressa) Impulse voltage

b) Cable lengthit efficiency high

• Cumulative efficiency is

more important

b) Cable length

• Temperature rise

• Bearing current

• Noise level

• Vibration level

Additional joule losses under PWM

IEC:60034-17 / IS: 15880 – “Application guide- Cage

induction motors when fed from converters”

“IE” class-New efficiency standard

• North America with the core

regions US, Canada and

Mexico• Dimensions in inches

IEC 60034-30

Applicable the World over!

IEC: 60034-30 for efficiency & IEC: 60034-2-1 for testing of efficiencyIndian standard IS: 12615/2011 for efficiency & IS: 15999 for Testing of efficiency

MEPS for India is IE2- To become IE3 from January, 2014!!!

IE3 will have IE2 & IE2 will have IE1 efficiencies,

when operated through VFD

Output Output Output

kW 2 4 6 kW 2 4 6 kW 2 4 60.37 66.1 65.1 63.0 0.37 72.2 70.1 69.0 0.37 75.5 73.0 71.9

0.55 69.1 69.1 67.0 0.55 74.8 75.1 72.9 0.55 78.1 78.0 75.9

0.75 72.1 72.1 70.0 0.75 77.4 79.6 75.9 0.75 80.7 82.5 78.9

1.1 75.0 75.0 72.9 1.1 79.6 81.4 78.1 1.1 82.7 84.1 81.0

1.5 77.2 77.2 75.2 1.5 81.3 82.8 79.8 1.5 84.2 85.3 82.5

2.2 79.7 79.7 77.7 2.2 83.2 84.3 81.8 2.2 85.9 86.7 84.3

3.7 82.7 82.7 80.9 3.7 85.5 86.3 84.3 3.7 87.8 88.4 86.5

5.5 84.7 84.7 83.1 5.5 87.0 87.7 86.0 5.5 89.2 89.6 88.0

7.5 86.0 86.0 84.7 7.5 88.1 88.7 87.2 7.5 90.1 90.4 89.1

11 87.6 87.6 86.4 11 89.4 89.8 88.7 11 91.2 91.4 90.3

No. of Poles No. of Poles No. of Poles

IE1 IE2 IE3

1. The standard

stipulates that motors

must meet at least IE2

to be labeled Energy

Efficient.

15 88.7 88.7 87.7 15 90.3 90.6 89.7 15 91.9 92.1 91.2

18.5 89.3 89.3 88.6 18.5 90.9 91.2 90.4 18.5 92.4 92.6 91.7

22 89.9 89.9 89.2 22 91.3 91.6 90.9 22 92.7 93.0 92.2

30 90.7 90.7 90.2 30 92.0 92.3 91.7 30 93.3 93.6 92.9

37 91.2 91.2 90.8 37 92.5 92.7 92.2 37 93.7 93.9 93.3

45 91.7 91.7 91.4 45 92.9 93.1 92.7 45 94.0 94.2 93.7

55 92.1 92.1 91.9 55 93.2 93.5 93.1 55 94.3 94.6 94.1

75 92.7 92.7 92.6 75 93.8 94.0 93.7 75 94.7 95.0 94.6

90 93.0 93.0 92.9 90 94.1 94.2 94.0 90 95.0 95.2 94.9

110 93.3 93.3 93.3 110 94.3 94.5 94.3 110 95.2 95.4 95.1

125 93.5 93.5 93.5 125 94.5 94.7 94.5 125 95.3 95.6 95.3

132 93.5 93.5 93.5 132 94.6 94.7 94.6 132 95.4 95.6 95.4

160 93.8 93.8 93.8 160 94.8 94.9 94.8 160 95.6 95.8 95.6

200 94.0 94.0 94.0 200 95.0 95.1 95.0 200 95.8 96.0 95.8

250 94.0 94.0 94.0 250 95.0 95.1 95.0 250 95.8 96.0 95.8

315 94.0 94.0 94.0 315 95.0 95.1 95.0 315 95.8 96.0 95.8

355 94.0 94.0 94.0 355 95.0 95.1 95.0 355 95.8 96.0 95.8

375 94.0 94.0 94.0 375 95.0 95.1 95.0 375 95.8 96.0 95.8

2. IE1 tables are

given for

comparing the

efficiency of an IE2

motor when driven

by a VFD.

Cumulative efficiency

VFD speed variation

offers, nearly, 50%

more saving than the more saving than the

mechanical controls

Load torques & Power

Selecting for constant torque with VFDMixing, Mills, Cranes, Conveyors etc

Required

constant Torque

should be

available from

the motor, the motor,

throughout the

speed range.

Please always

specify the

Torque

Impulse voltage

Switching under PWM results in high dV/dt and the

rise time (tr) has direct influence on insulation life

Extra features for VFD duty Motors1. 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

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

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

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).

Insulated bearing

drives (VFDs).

Noise- IS: 12065 is the Indian standard

IEC 60034-17states:

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

5 dB to 15 dB for operation from U-converters. The actual values depend

upon the number of motor poles, the pulse pattern of the converter

Vibration- IS: 12075 is the Indian standard

Procedures recommended by IEC 60034-14 confirmed that

the vibration levels of induction motors increase when

these are fed by frequency converters

• VFD Duty

• NEMA / IEC

• Dual coat

Class F to F• Constant Torque

• Variable Torque

Torque derating for

Features for selection

High dV / dt

• Dual coat

• VPI/IGT

Torque derating for Constant Torque

or higher frame for Variable

Torque for F/B

PWM

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

60034-17- “guide for selection of

cage induction motors for VFD”

• Torque derating or higher frame

size for temperature limiting

selection should be addressedmotor fails

selection should be addressed

• Insulated bearing at NDE, if

required

• Winding & Bearing protection

accessories, if specified

• Encoder provision, if required

• Forced cooling arrangement