Motors & Drives Information

Typical Question

I have several motors that were manufactured for three-phase 380VAC, 50Hz. I need to run them on a three-phase 380VAC, 60Hz feed. What will be the effect of the configuration? Will the motors overheat? Will increased frequency affect the motors’ lifespan?

Discussion Group Answers

• If the motor winding will take it—usually it will if new, but not if it is old—you need to increase the applied voltage to maintain constant volts per hertz. Since 60 Hz is 120% of 50 Hz, the 60 Hz nominal voltage of the motor would be 456 volts; that is close enough that 480 volts 60 Hz should not hurt it.

• I have numerous motors of all kinds that were once run on 60 Hz, now running on 50 Hz and all the transformers as well. I’m trying to come up with a method and objective tests to quantify how the motors are performing at 50 Hz, and if the transformers are experiencing core saturation at 5 Hz. Have any of you seen a wholesale change of power from 60 Hz to 50 Hz? What would be key things to check in trying to isolate the critical few motors or other loads that should be changed to 50HZ rated equipment?

• Basically, transformers are a lot like motors, except that transformer has no rotating secondary. Transformers have a rated current and rated voltage. If the applied voltage at a given frequency remains constant at rated voltage and frequency, performance should remain the same. If the current stays within the rated value, no significant increase in heating should occur.

• From a pure technical viewpoint, the flux per pole will be reduced based on the equation:
  
  Flux/p = 100 * E / (4.44 * Hz * N * Kp * Kd)

  E = RMS Volts between lines (380 for your case)
  Hz = Frequency in Hertz (50 or 60)
  Kp = Winding pitch factor (constant for each motor)
  Kd = Winding distribution factor (constant for each motor)
  N = Winding turns in series per phase (constant for each motor)
  At original 50 HZ, Flux 50 = 100*380/(4.44*50*N*Kp*Kd)
  At 60 HZ, Flux 60 = 100*380 / ( 4.44*60*N *Kp*Kd)
  

  And the relation of magnetic flux developed in the motor is:
  
  Flux 50/Flux 60 = 50/60 = 0.833 (Only 83.3 % of the 50 HZ flux).
  
  This weakened flux, proportionally produces only 83.3% torque for the same rotor current.
  
  T = B*L*i2

  T = Torque
  B = Air gap Flux density
  i2 = Rotor current

  The speed will be increased following the synchronous flux speed.
  
  RPMs = 120 * Hz / p, where p = number of motor poles (constant for each motor)
  
  The speed increases in the ratio 60/50 = 1.2 ( 120% of original speed at 50 Hz)
  
  The mechanical power developed at the shaft is HP = T * RPM / 5250 (This is the same because the speed increased proportional to the torque reduction).
  
  In conclusion, “The motor will develop the same output power but with lower torque capacity”. This implies problems accelerating heavy inertia or high torque loads.

# posted by Medical Information : 2:59 AM