Limit AC Motor Inrush Current

Limit Inrush Current In AC Motor Air Conditioning Application

AC motors used in air conditioning equipment draw large inrush current at power-on. A recent customer of ours had a 115-VAC air conditioner installed in their travel trailer for park use. This 5,000-BTU unit required 16 amps for simultaneous compressor and blower motor operation. The trailer was also equipped with a load-shedding power distribution panel to protect its wiring harness from overload. If more than 20 amps was detected, the panel would immediately take the connected equipment offline. That’s why limiting inrush current of an air conditioner is very important.

AC Motor With Inrush Current Problem

The two AC motors in the air conditioner create a high inrush current at startup. Because of this initial high current demand, the trailer’s electrical fault protection feature would not allow the appliance to stay running. Our customer connected an ammeter to the circuit which showed that the air conditioner was drawing 30 amps at power-on.

This was 10 amps more than allowed by the power panel. They needed to find a way for limiting inrush current of the air conditioner to 20 amps. At this point the customer called us for assistance.

After performing some calculations, we recommended using two Ametherm MegaSurge® Inrush Current Limiters placed in series between the trailer’s power panel and power input to the air conditioner. With the MegaSurge® Inrush Current Limiters in place limiting inrush current, the customer’s air conditioner began to operate normally.

For additional information regarding inrush current visit Wikipedia.


Selecting the correct inrush current limiter for air conditioner

AC Motor Inrush Application Given

  • Limiting inrush current of an air conditioner was necessary for operation in the travel trailer
  • Maximum allowable inrush current for this application is 20A
  • Steady-state current is 16A as indicated by the air conditioner manufacturer
  • Inrush current for this type of AC motor is about 15x the steady-state current
  • Duration of the inrush for this type of circuit is typically 0.0167S or one 60Hz AC cycle
  • Peak VAC is 1.414 x VAC RMS

Inrush Calculations AC Motor

  • Inrush current based on our assumption = 15 x 16A = 240A
  • Cold resistance = peak voltage / maximum allowable inrush current = (115 VAC x 1.414) / 20.0A = 8.13Ω
  • Energy in joules = voltage x current x duration of the inrush = 115VAC x 240A x 0.0167S = 460.92J

Inrush Current Selection Criteria

  • Minimum cold resistance required: 9Ω
  • Energy dissipation required: 461J
  • Maximum steady state current: 16A at 115 VAC and 60Hz

AC Motor Inrush Solution

  • Ametherm part number: MS32 5R020 (x2)
  • Resistance: 5Ω (per device)
  • Steady-state current: 20A (per device)
  • Energy dissipation: 300J (per device)
  • Place both parts in series between the power panel and the air conditioner power lead


             You can download the MS32 5R040 datasheet here


To use our inrush current calculators visit our Calculators web page.


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