How to calculate unit of motor?

Replies

• 

  Ramani Aswath

  VA rating is three times average current (~40 amps) x average voltage (~420) between two phases. Divide by power factor.
  Approx.: 16800x3/(PF) VA
• 

  vijayppatel13

  power factor value??
  i have 0.999 pf in my main meter.
  so can i take 0.999 as a power factor?
• 

  Ramani Aswath

  You cannot. The motor can and probably will have a lower pf. Please check the original label.
• 

  vijayppatel13

  in my plant one motor is running on star delta starter it takes 80 amp average then i have provide drive in that motor, then it takes at 45 hz 75 amp(output of drive) and 62 amp(input of drive), so please tell me which ampere is require to take for unit calculation?
  input or output amp?
• 

  lal

  Input it would be. Motor and drive together should be considered. Drive would be dissipating a part of the energy it consumes as heat (losses) and transfer the rest to motor.
• 

  vijayppatel13

  then why input and output ampere has different? input amp is very low why?
• 

  lal

  When you calculate the power at input and output, you'll see the output is lesser than the input due to losses.
  
  Anyway, to answer the question, the motor driver is able to alter the voltage and frequency of power output to control the mechanical characteristics of motor - torque and speed. So the voltage and frequency of input power will be different from the same at output of drive.
• 

  vijayppatel13

  but sir.
  i have also put a energy meter in that panel.
  in energy meter it show output ampere instead of input.
  why this happen.?
• 

  lal

  Wait, it looks like I have perceived 'drive' the wrong way. Is drive simply a star-delta starter or a power electronic motor controller? And I assume we are talking about an induction motor.
• 

  vijayppatel13

  drive means inveter. frequency variable controller for motor.
• 

  lal

  Did you say the input current of drive from its spec? If so, it would not be more than the mentioned value unless overloaded!
• 

  vijayppatel13

  now i have provided inverter in 15kw motor
  inverter input current is15 and output current is 19 amp.
  now tell me how many power consumption of this motor?
• 

  lal

  Around 7kW if voltage is assumed to be 415V, 3phase, power factor to be 0.8 lagging and inverter efficiency around 80℅.
• 

  vijayppatel13

  how??
  please send formula for calculation........
• 

  lal

  Directly calculating motor power using the formula to deduce power cannot be used as converter output will be having a different voltage than the input. So the total input power to converter can be calculated and approximated to motor power assuming an 80% efficiency for conversion.
  
  Total input power to converter = 3^(1/2) x 415 x 15 x 0.8 (assuming 0.8 lagging p.f.)
  Total power = 8626W
  Assume 20% of total power is used by converter electricals and dissipated.
  Total motor power = 8626 x 80% = 6900VA
  Actual value may be more than that. So motor power can be assumed to be around 7kW.
• 

  vijayppatel13

  you have assume 0.8 lagging power factor..
  but in my main meter i have 0.999 power factor.
  so can i take 0.999 pf??
• 

  pankaj.bhatt376

  Yes you can assume 0.999 power factor