# Limit AC Motor Inrush Current

## Limit AC Motor Inrush Current In Air Conditioner Application

AC motors used in air conditioning equipment draw large inrush current at power-on. A recent customer of ours had a 115VAC air conditioner installed in their travel trailer for park use. This 15,000-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 can be 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 distribution panel. The customer needed to find a way for limiting inrush current of the air conditioner to 20 amps. At this point they called us for assistance.

After performing some calculations, we recommended an Ametherm MegaSurge® Inrush Current Limiter and instructed them to install it in series between the trailer’s power panel and power input to the air conditioner. With the MegaSurge® Inrush Current Limiter in place, the customer’s air conditioner began to operate normally.

## Selecting the correct inrush current limiter for air conditioner

### AC Motor Inrush Application Given

• Limiting inrush current to an air conditioner was necessary for normal operation in the travel trailer
• Maximum allowable inrush current for this application is 20A
• Steady-state current is 16A as specified by the air conditioner manufacturer
• Inrush current for this type of AC motor is typically 15x the steady-state current
• Peak voltage = 1.414 x VAC RMS
• Frequency f of U.S. line power is 60Hz

### Inrush Calculations AC Motor

• Inrush current I based on our assumption = 15 x 16.0A = 240.0A
• Cold resistance of inrush current limiter = peak voltage / maximum allowable inrush current = (115VAC x 1.414) / 20.0A = 8.13Ω
• Inductive impedance Xl = peak voltage / inrush current = (115 VAC x 1.414) / 240.0A = 0.677Ω
• Inductance L of motor in henrys = Xl / 2πf = 0.677Ω / 377.0 = 0.00179H
• Energy in joules = 0.5 x L x I2 = 0.5 x 0.00179H x 240.0A2 = 51.55J

### Inrush Current Limiter Selection Criteria

• Minimum cold resistance required: 9Ω
• Energy handling required: 52J
• Maximum steady state current: 16A at 115VAC

### AC Motor Inrush Current Solution

• Ametherm part number: MS35 10018 Inrush Current Limiter
• Install one inrush current limiter in series between the power panel output and air conditioner power lead