Menu
Search
Selection of Induction Motors for Industrial Applications (part 1)
Selection of Induction Motors for Industrial Applications (part 1) – photo credit: fwworldwide.com

Introduction to Induction Motors //

All types of industries are invariably required to install different types of electric motors as prime mover for driving process equipment participating in their respective production line up. The continuous process of technical development has resulted into availability of highly diversified types of electric motors.

Hence, an utmost care should be exercised in selection of most appropriate type of motor considering number of technical factors for each application, so that the motor would provide desired and optimum performance.

The characteristics of motors vary widely with the nature of their application and the type of duty they are expected to perform. For example, the applications like constant speed, constant torque, variable speed, continuous/intermittent duty, steep/sudden starts, frequent start/stops, etc. should be taken into consideration carefully when deciding for the type of a motor for that specific application.

Moreover, the motors are required to perform quite often under abnormal conditions during their total service life.

In view of above, an incorrect selection of motor always lands the industrial buyer into all sorts of problems, including premature failure of the motor, causing severe production curtailments.

Like one mentioned above, a number of other factors and design features like weather conditions, stringent system conditions, abnormal surroundings, hazardous area, duty cycle, motor efficiency, etc. should be considered while deciding the rating and subsequently drawing out the technical specifications of the motor.

Stator and Rotor Damages
Stator and Rotor Damages

Abnormal conditions and effects

The usual abnormal conditions encountered by the motors are given below.

1. Abnormal System Conditions

  1. Voltage
    1. Undervoltage
    2. Overvoltage
    3. Unbalance in 3-phase
    4. Single phasing
    5. Voltage surges
  2. Frequency
    1. Low frequency
    2. High frequency

2. Abnormal Operating conditions

  1. Locked rotor or stalled rotor
  2. Reswitching/Frequent start-stops
  3. Momentary interruption/Bus transfer
  4. Overloading
  5. Improper cable sizing

3. Environmental conditions

  1. High/low ambienttemperature
  2. High altitude
  3. High humidity
  4. Corrosive atmosphere
  5. Hazardous atmosphere/surroundings
  6. Exposure to steam/salt-laden air/oil vapour

4. Mechanical problems

  1. Seized bearings
  2. Incorrect alignment/foundation levelling
  3. Incorrect fixing of coupling
  4. High vibration mounting
  5. External shock due to load

5. Condition at location

  1. Poor ventilation
  2. Dirt accumulation
  3. Exposure to direct sunlight

Though, above mentioned abnormalities may prevail for short or long duration or may be transient in nature, major impact of the listed abnormal conditions is overheating of the motor along with one or several of the other effects as follows.

Change in the motor performance characteristics like drawl of more power and consequent deterioration in motor efficiency, etc.

Increase in mechanical stresses leading to:

  1. Shearing of shafts
  2. Damage to winding overhang
  3. Bearing failures
  4. Insulation failures

Increase in stator and rotor winding temperature leading to:

  1. Premature failure of stator or rotor insulation (For wound rotor motor)
  2. Increased fire hazard
  3. Breakage of rotor bar and/or end ring (For squirrel cage motors)

All the motors encounter few or several of these abnormalities during the course of their service lives. Consideration of listed abnormal conditions at design stage greatly helps to minimise the effects of abnormal conditions to maintain a consistent performance.


Design Considerations

Following are the most important design factors required to be considered when selecting a motor for any of the diversified industrial applications.

Output in kW/HP

There are two principle limitations for selecting the motor output:

1. Mechanical limitation

The breakdown torque, which is the maximum torque that the motor can produce when operating without stalling. This is a critical design factor in motor applications, particularly for the motors subjected to occasional extreme load conditions.

Another critical factor is the locked-rotor torque, which is the maximum torque that the motor can produce during startup from steady-state condition, a critical design feature for conveyor drives.

2. Electrical limitation due to insulation provided on the motor windings

The electrical load on the motor can be imposed till the winding insulation is able to withstand the prescribed temperature rise over an ambient for that particular class of insulation. Life of the motor greatly depends on the temperature rise of the windings.

Anticipated life-span of the motor can be achieved provided it is operated at its rated output without overloading and the prescribed preventive maintenance practices are religiously followed.

Speed of the Motor

Most of the motors are directly coupled with the driven equipment where in the speed of the motor and the driven equipment will be same.

In order to meet the speed of the driven equipment, the devices like gearbox, chains or belts are introduced between motor and driven equipment. In this case, it may be necessary to provide the rotor shaft suitable for its attachment with the speed decreasing or increasing device and hence the specification should include such specific requirement.

In case a variable speed drive is to be used for the speed variation, the motor should be compatible for this specific application. The standard motor may not provide desired performance when operated via variable speed drive.


Power Supply Voltage and Frequency Variations

Variations in the power supply parameters, i.e. voltage and frequency significantly affect overall performance of the motor. As provided in IS:325-1996, the permissible voltage variation is ±5 to ±10%, permissible frequency is 50Hz ± 3%, and permissible combined variation is ±6 %.

The effect of undervoltage is more serious than that of overvoltage.

The higher torque, resulting from overvoltage, can handle a little overload without undue heating of the winding, but only for a short duration. Continuous operation with undervoltage condition increases the current at the rate of about 20% for every 5% reduction in the supply voltage, increasing the rated copper loss.

This results into heating and prolonged temperature rise, and finally the burning of winding. During a motor start-up, the torque reduces by 10% for each 5% reduction in the supply voltage, causing more starting current and consequently more rapid heating of the
winding.

Large burned out induction motor
Large burned out induction motor

The motor offers reduced efficiency at either overvoltage or undervoltage. Power factor drops sharply with higher voltage and improves with lower voltage. Even when motor is lightly loaded, over-voltage cause rise in current and temperature thus reducing the life of motor. The variation in frequency by +5 % decreases the torque by about 10% and vice-versa at – 5% frequency, the torque increases by about 10%.

It is, therefore, of utmost importance to consider the combined effect of variation in voltage and frequency both when purchasing the motor.

Unbalance in the supply voltage results into a current unbalance of 6 to 10 times the percentage voltage unbalance. This in turn results into generation of negative sequence currents in the rotor causing its overheating and premature failure.

It is therefore vital to specify the permissible limits of variations in the power supply parameters for the motor in accordance with the requirement of the driven equipment. However, the permissible limits should never be more than provided in the applicable Indian Standard IS:325-1996 (Reaffirmed in 2002).

Will be continued very soon…

About Author //

author-pic

Ashok Parikh

Ashok Parikh - Working as Electrical Engineering Consultant located at Vadodara, India providing System Design & Engineering services to various industries, possessing 40 years of experience in diversified industries and consultancy.

30 Comments


  1. Prabhat Dashore
    May 16, 2016

    Dear Sir i want to run a 100 kg wheel from 1 single phase motor.
    kindly provide me specification for the same.


  2. Kizito
    Apr 29, 2016

    Dear Mr. Ashok,
    I am working on a small palm oil refinery project. In some step of production we will need induction motors with dual speed. These motors have been purchased but they have separated winding thus switching from one speed to another will need 2 different contactors. Is this kind of motor better than the one with changing poles (Dahlander)? Is the induced voltage in the winding not in action (for the separated winding motor) cause harm to the insulation? Can this induced voltage contributing to heat losses? Is there any voltage surge changing speed as this will be done without the motor stopping to a stand still? The motors are tiny and will be started directly (0.85/1.5kW, 3.1/3.5Amps & 700/1410rev per min under 380V). Thank you for teaching us everyday!

    Warm regards,

    Kizito


  3. Ghulam shabir
    Apr 07, 2016

    sir, for the industry there will be used 3 phase induction motor for many purpose these motor are connected with inverters.. so my question which type of harmonics or power quality produced on the motor.. and how we reduced these harmonics suggest me different techniques..

    GSM


    • Ashok R Parikh
      Apr 08, 2016

      Dear Mr. Ghulam Shabir,

      Whenever the motor is to be operated through inveter or variable frequency drive, specific care should be taken to build in technical specification the requirement of harmonics filter in the VFD itself so that the harmonics generated by VFD are suppressed and not passed on to either motor or electrical system. Usually, 3rd, 5th and so on level of harmonics are generated and magnitude of harmonics depends on the motor loading. The harmonics will cause more heating of and rapid insulation deterioration.

      I hope, this brief clarification will satisfy your comment.

      Ashok Parikh


  4. BASU JAIN
    Jul 16, 2015

    Sir, please tell me the way to test ” motor installed is optimum or under load or over load”?

    motors are already installed, so pls suggest me a best way, and work will not disturbed as well.


    • Ashok R Parikh
      Jul 16, 2015

      The testing of installed motors for underload or overload situations is not recommended at all. The overload test is the factory test performed only at the manufacturer’s works and can be witnessed by the purchaser. At site, the motor can be tested for its efficiency by performing no load losses test and load losses test. You can request your motor supplier to provide procedure for tests at site. I will try to provide some article for testing of motor when in service.

      Other tests performed at site are measurement of insulation resistance and dc resistance measurement of windings to know about their health. These tests are performed under regular preventive maintenance program.

      The motor is required to be stopped for the tests. In case standby is available, the drive in service may be stopped and standby may be started. The testing of all the motors may be carried out in turns.


  5. aarif
    Dec 30, 2014

    dear sir ,
    i am working as electrical design engineer in building construction field and have no idea about motor selection for fire fighting or plumbing to top of the building .
    can you guide me with some example for the same.


    • Ashok Parikh
      Dec 31, 2014

      Dear Mr. Aarif,

      You can select the standard squirrel cage induction motor of suitable capacity based on hp of fire fighting pump. You have to select either horizontal foot mounted or vertical flange mounted motor as required for pump.

      Regards,
      Ashok R Parikh


      • aarif
        Jan 06, 2015

        dear sir,
        Thanks for the response . actually i have to pump the water from -1 m from ground to +40 m head at the speed 2400 LPM with maintaining a pressure of 3.2kgs/sq .cms at top most . now how i can calculate the capacity of motor in hp


  6. Gopal Agrawal
    Oct 26, 2014

    Sorry Sir,
    previous motor was 15HP capacity…………

    Thank you in anticipation.

    Gopal agrawal


  7. Gopal Agrawal
    Oct 26, 2014

    Dear sir, can you explain the following situation?
    We had installed a 3PH, 415V, 1440 RPM, Induction motor (with capacitor for PF correction) on a thread rolling machine with flywheel.
    The Star-Delta manual starter had a starting current of 25 Amps and it took about 2-3 minutes for the motor to pickup suffecient speed and decreased current to about 17 Amps before Delta connection. On Delta switching the current surges to 85 Amps then rapidly decreasing to constant value of about 4 Amps without load.
    This motor on full load had current of about 18-20 Amps with heating problems.

    We replaced the above motor with a 3PH, 415V, 1440 RPM, 25 HP, induction motor.
    This motor on Star Connection had a starting current of 100 Amps which rapidly decreases to about 2 amps and on Delta switching had no load current of 14 Amps.
    This motor worked satisfactorily on star connection on load with full load current of 5 Amps and Delta Full Load current of 16 Amps.

    How the motor is working in star connection with just 5 Amps full load current?

    Please comment on power requirement and consumption.

    Regards,
    Gopal Agrawal


    • Ashok Parikh
      Oct 28, 2014

      Dear Mr. Gopal Agrawal,

      You have not mentioned the rating of first motor and duty cycle for thread rolling machine, i.e. whether continuous, intermittent, cyclic, etc.

      It is essential to set time for changeover from star to delta. The time should be set such that the changeover takes place at the moment the motor achieves its full speed in star. If the time is set in this manner, there will be hardly any current surge at time of changeover.

      As 25 hp motor is now working satisfactory, it seems that previous motor rating might not have been selected after proper technical evaluation.

      Power consumption will solely depend on load taken on the motor.

      Regards,
      Ashok R. Parikh


  8. Roy
    Oct 10, 2014

    What is the relation between stator current and induction motor speed (Assume the motor load is constant and type of motor is squirrel cage induction motor)

    Thanks and Regards,

    Roy


    • Ashok Parikh
      Oct 11, 2014

      Dear Mr. Roy,

      Kindly note that motor load or current is proportional to speed. If motor speed is reduced, it will not be able to maintain full load. The torque is also reduced. You may see motor characteristics curves for speed vs. torque, speed vs. load, efficiency, etc. which are supplied by the motor manufacturers for all the motors to understand above phenomena.

      Regards,
      Ashok R. Parikh


  9. Muhammad Fahim
    Aug 29, 2014

    What are the effects of humidity on three phase induction motors?


    • Ashok Parikh
      Aug 30, 2014

      Dear Mr. Fahim,

      The motor winding insulation is greatly affected by humidity, as moisture condenses over insulation and dielectric strength of insulating materials gradually starts deteriorating. The winding insulation of motors to be installed in coastal areas/humid surroundings is given the specific treatment if covered in the specification.

      In order to prevent moisture condensation, usually the motors rated 11kW and above are provided anti-condensation heaters. These heaters are automatically switched on when the the motor is taken out of service and vice-versa.


  10. Kamin Dave
    Aug 24, 2014

    Can we close induction machine on out of phase closing with alternate source when main source failed? If it yes, then how and if it is no then why?


    • Ashok Parikh
      Aug 26, 2014

      Dear Mr. Kamin,

      It is not advisable to close any induction machine on out of phase closing with alternate source in the event of failure of main source, as it will cause current unbalance in 3-phase windings due to irregular voltage. Moreover, when main supply fails, the motor starts costing down (gradually to standstill condition). If supply from alternate source is imposed before total standstill situation, the voltage induced in stator due to residual magnetism of rotor and supply voltage from alternate source are likely to superimpose and cause heavy voltage rise, which may ultimately lead to stator winding failure. Specifically for large rated motor, this care should be taken and that is why usually the under voltage trip is ensured for large rated motors.
      Even the circuits breakers are usually tested for out of phase switching.


  11. zubair
    Aug 10, 2014

    Hi. I would like to have a Pdf version of all 3 selection of asynchronous motors material.


  12. sakthi
    Jul 04, 2014

    why induction motors were widely used in industries? pls list out its characteristics and advantages also applications


    • Ashok Parikh
      Jul 08, 2014

      Dear Sakthi,

      Induction motors are widely used in the industries for motion control system applications. Not only in industries, but even in most of the home appliances, the induction motors are used. The induction motors are simple and of rugged design, low maintenance, low cost and most suitable means for such applications. With Indian and International Standards almost similar for the induction motors, squirrel cage or wound rotor, the replacement is extremely easy and fast reducing downtime of outage, which is vital for sustaining production levels. Another advantage of induction motors is their direct connection to the power supply. The motors are available in all the ratings from smallest to the largest to exactly suit the driven equipment requirements. Now the concept of energy conservation is also applied due to availability energy efficient motors in different efficiency levels as well.

      I request you to go through any of the books for Electrical Machines, in which you will find very good and detailed discussion for the electrical and mechanical characteristics.

      I hope, you get your answer in brief.


  13. Souraj Ali
    Mar 28, 2014

    sir what is the important starting inductor motor in the industrial application?


  14. Sachin Holambe
    Mar 08, 2014

    sir, for the industry there is need of wireless monitoring of the 3 phase induction motor parameters (current,voltage, speed, temperature ….etc ).
    If ,Yes how the scheme is useful for the industry.


    • Ashok Parikh
      Mar 09, 2014

      Dear Sachin,

      You will appreciate that usually, large nos.of different type/rating of motors would be installed in the industries for varied applications. Monitoring of parameters adopting wireless method for marge nos. of motors would be mammoth task involving additional heavy cost. The method could be deployed for extremely critical motors based on GSM technology. However as per prevailing trend, monitoring of even critical motors is carried out via DSC/SCADA systems.


  15. Manish Gandhi
    Oct 13, 2013

    Sir, I have to do a Cost Benefit Analysis of Using a VFD on blower of 300 kw. …how to do it???


    • Ashok Parikh
      Oct 14, 2013

      Dear Manish,

      First of all, 300 kW motor compatible for operating via VFD will be required to be procured.
      You may get the costs as follows.
      i. Costs of normal motor and VFD compatible energy efficient motor.
      ii. Costs of normal star delta starter and VFD with back-up module.
      iii. Cost of input power per unit
      You have to draw out pattern of load variations in blower and derive anticipated energy drawl assuming normal motor operation and energy drawl when operated via VFD. Find out difference in drawl of energy (kWh) and arrive at saving of energy cost by operating via VFD.
      Find out the capital investment cost difference and compare with saving due to lesser energy cost. This will provide you with fairly good simple payback period for additional capital investment to be made for motor and VFD.
      I hope, this will help you.

  16. […] terms used frequently in intermittent duty drives and hoisting are defined as follows:1. DutyOperation of the motor at the declared load(s) including starting, electric braking, no load and rest and de-energised […]

  17. […] = 60; The split-phase (SP), or more accurately, the resistance-start, split-phase, induction run motor,is recommended for medium-duty applications.It can run at constant speed even under varying load […]

  18. […] Motors for Industrial Applications (part 2) – photo by TCD Systems Continued from first part: Selection of Induction Motors for Industrial Applications (part 1) Design Considerations (cont.)Motor EfficiencyThe new IEC 60034-30 motor efficiency standard could […]


  19. jeewangarg
    May 03, 2013

    Sir why we take 3 times the starting current of any motor.

Leave a Comment

Tell us what you're thinking... we care about your opinion!

Time limit is exhausted. Please reload CAPTCHA.

Get PDF