Selection Criteria for Motors

👉Selection Criteria for Motors in Special Purpose Machine (SPM) :

 Choosing the right motor for Special Purpose Machines (SPM) is crucial for optimal performance and efficiency. The selection process involves several key criteria to ensure the motor meets the specific requirements of the application: 

👉 Power Requirements: Determine the power (horsepower or kilowatts) needed to drive the machine effectively. The motor should be capable of handling the load and providing the required torque. 

 👉Speed Requirements: Identify the operating speed (RPM) required for the SPM. Motors can be selected based on the required constant speed or adjustable speed settings (via variable frequency drives). 

 👉Torque Demand: Evaluate the torque characteristics of the machine. Motors should be selected based on their ability to provide both starting torque and continuous torque under operational conditions. 

 👉Voltage and Frequency: The motor must be compatible with the available electrical supply in terms of voltage and frequency (e.g., 230V, 400V, 50Hz, or 60Hz). 

 👉Duty Cycle: Consider the motor's duty cycle, which refers to how long the motor will operate under load. Choose between continuous, intermittent, or short-time duty motors depending on the machine’s operational time. 

 👉Efficiency: Opt for energy-efficient motors to minimize operational costs and ensure long-term reliability, particularly for continuous operations. 

 👉Size and Mounting Type: Ensure that the motor fits within the available space in the machine and can be easily mounted according to the design. 

 👉Environment and Safety: Consider the operating environment, including temperature, humidity, and potential exposure to hazardous materials, to choose a motor with the appropriate IP rating and protection class. 

👉 Control and Automation Compatibility: Ensure the motor can integrate with control systems, such as PLCs, and is compatible with automation needs. Selecting the right motor is essential for machine performance, reliability, and cost-effectiveness. Careful consideration of these factors ensures efficient operation and long-term success in SPM applications.

👉Available Power Supply (AC or DC )  : Motor is selected as per available power source of AC or DC. If AC course available . Select Induction motor. DC source available select DC Motor as per load requirement.

👉 Starting Characteristics : 

      A. DOL 

      B . Star-Delta 

     C . VFD

👉 Operating Conditions ( Indoor/Outdoor ) : Selected motor operator in indoor or outdoor. If motor operated Outdoor  need to consider Canopy & for V1 mounting arrangement Top plate cowl to be consider.

👉 Acceleration Characteristics : Motor acceleration time depends upon the load Gd2 value. If load Gd2 value is more than motor Gd2 Value . Then starting time is more.

👉 Overload Capacity : Minimum overload capacity of motor is 160% of rated load for 15 Sec.

👉 Ambient Temperatures : Motor suitable for required ambient temperatures.

👉Thermal Safety : Need to check requirements of PTC ,RTD , BTD & Space Hearer

👉 Application : Motor select to considering application. ( Variable Toque or Constat Torque application )

Hazardous Area Motors ( FLP & Non Sparking Motors )

 

Hazardous Area Motors

As name it shows that the motor used in hazardous environments like coal mines, chemical & gas plant. Therefore creating a need for a very specific design suitable to such condition. The hazardous areas is define as those where explosive atmosphere is present or is expected to present or it is expected to be present in quantities which merit requirements of special precautions. The construction, installation and use of equipment is designed specifically to suit the hazardous environment. IS 5572 standard classifies hazardous areas in three zone depending upon the frequency and duration for which dangerous concentrations are likely to present. The classification of areas where flammable gas or vapor risks may arise in order to permit the proper selection of electrical apparatus for used. Following are the classification and selection of equipment’s as per IS 5572:

Zone	Classification of area	Selection of equipment
Zone 0	Explosive atmosphere is present continuously or for longer periods or frequency	Intrinsically safe or pressurized electrical equipment to be used.
Zone 1	Explosive atmosphere is present under normal operation conditions.	Electrical equipment must be used Flame proof enclosure Ex (d) conforming to IS/IEC 60079-1.
Zone 2	Explosive atmosphere is present only under abnormal operation condition and for short period only.	Equipment with type of protection Ex (e) in accordance with IS/IEC 60079-1 may be used without any special enclosure .Equipment with IS/IEC 60079-15 are also permitted for use.

The area classification shall also take into consideration of gas group and temperature classification, depending upon the properties of material handle. For example: Zone 1 IIA T3.

Where,

 Hazardous Area Zone = Zone 1

 Gas Group = II A

Temperature Class = T3

    Ø Flame Proof Motor : Type EX ( d )  

  

These motor are used in Coal Mines , Petro Chemicals & Chemicals industries , Fertilizer , Solvent Extraction Plant, Paints & Varnish industry , LPG Bottling Plant , Agro Chemicals ,Drugs & Pharmaceuticals etc. Following precaution to be taken while selection of FLP Motors:

1. Temperature Class: The classification of temperature class T1 to T6. This is already discuss in previous blog . The maximum surface temperature under worse operating condition must not exceed the ignition temperature of gas. The maximum surface temperature is refer to that surface which is coming in contact with the explosive gas. In case of Flame Proof  Ex ( d ) motors , this refer to external surface temperature whereas  in case of Non-Sparking EX ( nA ) motors , this refer to the internal temperature.  

 

2. Classification of Hazardous Gases:  Hazardous gases are classified as per IS/IEC 60079-1 and are associated with flame proof enclosures. Hazardous gases, their group specification and ignition temperature have been specified in IS/IEC 60079-20.

 

3. Enclosure & Cooling: These motor are designed that the frame temperature will remain below the ignition temperature of gas-air mixture involved. The frame, end shield, terminal boxes and bearing covers of all motor made of grey cast iron. All cast iron parts forming flame proof enclosures are subjected to hydraulic pressure test after final machining as per IS/IEC 60079-1. Type of cooling as per IS/IEC 60034-6.

 

4. Degree of Protection: Degree of protection as per IS/IEC 60034-5. All flanged motors are additionally provided with oil tight shaft protection on driving end side. Drain plug is not permissible in FLP motors.

 

5. Paint: All motors are given special treatment of primer and paint to internal as well as external surface. All external surface are enclosed with epoxy polyamide base acid/alkali resistant paint of dark admiralty Grey Shade ( No. 632 as per IS : 5 ) or as per customer requirement.

 

6. Name Plate:  Stainless steel name plate is provided on each motor . Special data such as efficiency , starting current , starting torque, gas group,temperature class & statutory approval references are also provided with usual name plate details.

 

7. Cable Entries:  Motor for mining application ( i.e. coal mines & oil mines ) provided with compound filling sealing box. Cable entries suitable for flame proof glands ( for application in hazardous area Gas   Group II A and II B only ) can be provided with flame proof glands. A cable sealing box is mandatory for all motors for use in coal mines and oil mines.

 

8. Statutory Approvals & Licenses: Motors used in hazardous area need statutory approvals from various statutory authorities depending upon there area of jurisdiction before marking. Statutory / licensing accord their approval /License base on the test report issued by their recognized test laboratories. Following are the Statutory Authority:

§  PESO Approved Laboratory : For testing & certification

§  Director General of Mines Safety ( DGMS ) : For approving Coal Mines

§  Petroleum & Explosives Safety Organization ( PESO ) : for approving all areas where explosive liquids / gases are stored & transported

§  Bureau of Indian Standards ( BIS ) : for Licensing 

Ø Non Sparking Motor : Type Ex ( nA )

Non sparking motor provides protection against auto ignition of surrounding gases which may be released under abnormal operating condition. These motor are designed such that the limiting temperature of all parts in continuous operation does not exceed 200 °C  i.e. Temperature class T3 , as per IS/IEC 60079-15. Motor enclosure is made of Aluminum upto 80 frame motor & Cast Iron form frame size 90. Other feature i.e. degree of protections , Paint & name plate is similar to FPL motor as discuss in above. 

 

Basic Formula & Performance Calculations of 3-Phase Induction Motor

 

 Basic Formula & Performance Calculations of 3-Phase Induction Motor 

 1.  Torque : 

  Torque in Kgm  =   ( kW x 974 ) / Full Load RPM

  Torque in Nm   =   Torque in Kgm x 9.81

  Starting Torque =   [ (Rated Voltage /Reduce Voltage )^2 ]  x Reduce Voltage

  Starting Torque % of Full Load Torque = ( Starting Torque /Rated Torque ) x100 

2.  Speed : 

Synchronous Speed (Ns)   =  ( 120 x Frequency ) / No. of Pole 

% Slip = [( Synchronous Speed - Full Load Speed  ) / Synchronous Speed] * 100

Speed at 50 Hz =  [ Speed ( Actual RPM ) x 50 ] / Frequency 

3.  Power : 

     Power in kW =  (√3 x Line Voltage x Line Current x p.f.) / 1000 

    Power Conversion : 

     A.  HP to kW  =  HP X 0.746

     B.  kW to HP  =  kW / 0.746  

4.  Current :    

   Full Load Current in Amp. = ( kW x 1000 ) / (√3 x Line Voltage x efficiency x p.f.)

     

 5.  Motor Starting Time :  

      Motor Starting Time   ∝  Motor Rated Speed  X ( Load GD²  +  Motor GD² )

                                                                         Accelerating Torque

    

    * Starting time is directly proportional to load  GD²

    * So higher the load GD²  (in case of fans), higher is the starting time

  

  6. Performance Calculations of 3 Phase Induction Motor 

      Where , 

        I0    :  No Load Current in Amp.

        Ifl    :  Load Current in Amp.

        W0  :  No load Power in Watts.

        Wfl  :  Load Power in Watts . 

         Ns   :  Synchronous Speed . 

         N     :  Full load Speed.  

       Ravg  : Average resistance 

      Resistance in STAR Connected motor    =  Ravg / 2

      Resistance in DELTA Connected motor  =  Ravg  x 1.5

 

      Resistance Per Phase ( Rph )  =  (RII*((235+95)/(235+T1)))

     Where , 

       RII  =  Resistance in STAR / DELTA

       T1   =  Initial Ambient Temp.

     No Load Loss Calculation : 

     No Load Cu Loss ( Vcuo )  =  I0*I0*Rph 

     Windage & Friction Loss  (Vr)  =  1 % of kW ( Assumption  )

     Constant Loss [ Iron Loss ] ( Vfe )  =  (Wo-Vcuo-Vr) 

    

     Load Loss Calculation :

     Stator Cu loss  ( Vcu1 )   =  (Ifl*Ifl*Rph)

     RPM @ 50 Hz   =  [ (RPM*50) / Frequency) ]

     Slip ( S )   =  (Ns-N)/Ns 

     Rotor Cu Loss ( Vcu2 )   =  (Wfl-Vcu1-Vfe)*Slip 

     Stray Loss  ( Vs )  =  (0.5/100)*Wfl  ( Assumption  )

     Total Loss  =  Vr+Vfe+Vcu1+Vcu2 +Vs

    Performance : 

      Output  =  (Input-losses)

     % Efficiency  =  (Output / Input) x 100 

      Power Factor  =  (Input) / (1.732*V*Ifl) 

      * The method of determining efficiency of machine or motor depends on number of    assumption. Now days IS: 15999- IEC -60034-2 std . use for performance calculation of motor. This will explain in future blog. 

    

    

Motor Starting Time

 

Motor Starting Time

It is the time required to accelerate the motor and the load from standstill to rated speed.

                                       Motor rated speed x ( Load GD ² + Motor GD ² )

Starting time ∝  ...........................................................................................................

                                           Accelerating torque 

1)    Starting time is directly proportional to load GD²

§  So higher the load GD² (in case of fans), higher is the starting time

2)    It is inversely proportional to accelerating torque

§  Accelerating torque is (average motor torque – load torque)

§  Higher load torque reduces accelerating torque & thus increases the starting time

§  Higher average torque, i.e. higher starting torque and pull out torque increases the accelerating torque.

3)    Starting time is directly proportional to motor rated speed

§  Higher the rated speed (in case of 2P motors), higher is the starting time

 

·        Thermal withstand time of motor

It is the time the motor can withstand without any damage, when it draws the starting current while accelerating with full load, from standstill to motor rated speed. This is declared separately for motor running under hot condition and cold condition.

It depends on:

1)    Method of starting – DOL / Star-Delta / VFD

2)    Insulation class & limiting temperature rise

3)    Motor starting current in amperes

·        Number of consecutive starts

This indicates the frequency of safe starts. During start, the motor is subjected to heavy inrush of current in stator winding which causes rapid temperature rise in the windings. The life of motor insulation reduces when the temperature of the windings rises high. The life of a motor largely depends on the life of winding insulation. If there are too many starts with insufficient cool down time, the temperature rise causes problems. At start, there is also very high energy dissipation in the rotor causing the rotor bar temperature to rise dramatically. If the bars get too hot, the rotor will be damaged. If the motor is hot, the amount of energy that can be safely dissipated is reduced, therefore the starts must also be reduced. More than one consecutive starts are frequently required at the time of trials and commissioning of motor.

Frequency of safe and permissible consecutive starts depends on :

1)    Method of starting

2)    Time per start

3)    Thermal withstand time of motor

4)    Insulation class & limit of temperature rise

5)    If starting is with load or without load

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Space Heater for Motor

 

Space Heater for Motor

As the name itself indicate that it is used for heating purpose of motor or panel. In  large electric motor or panels, if the motor is out of service then condensation form due to fluctuating temperature ,even in seal enclosure. This may result in electric components corrosion and insulation decrease. Such motor or panel used after long time then there is chance of damage in motor or panel due to insulation failure. The space heater keeps the internal temperature of the motor above the ambient dew point while the motor is OFF. The heater helps to prevent condensation from forming inside the motor which can be damaging to motor windings, bearings and electrical connections. Wire type space heater is used in motor or panel. This space heater is wound circular type on the winding, which is outside to the stator core. Below figure shows the assembly of space heater. It is very important to switch off the space heater whenever the motor is running or panel is operating condition.

A. Assembly of Space Heater on Motor winding

 

B. Actual View of Space Heater