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By Luis Cordova

Why is inverter hotline coil resistance important?

Monitoring motor circuit resistance is the most crucial aspect for any manufacturer. Measuring resistance is critical for electric vehicle (EV) motors, brushless motors, and compressor motors for air conditioners (AC). An increase in resistance can significantly affect the balance of 3 phase motor circuits, cause heat build-up and lead to coil failures. Coil resistance is affected by physical properties such as the length of the wire, the temperature of the wire, wire material (copper, aluminum), and the type of wire. 

When a motor’s current flow becomes uneven, it may rapidly cause localized heating of problem connections, causing them to fail. Primarily failures occur by unbalanced resistance, which increases operating temperature encouraging thermal degradation. External factors such as energized heater circuits, fans, and pumps may cause rotors to function even when the motor is off. These external factors can cause increases in resistance, corrupt circuits, and wear out motors quickly.

What instrument can we use to test hotline coil resistance?

Resistance testing of motors used in EVs, brushless motors, or AC’s, gain better accuracy using Soken’s inverter hotline coil resistance meter DAC-HRI-3 series. These meters are suitable for temperature and resistance testing of energized coils in equipment driven with an inverter power supply. Generally, resistance determination uses either the temperature measuring method or the resistance method. The temperature method measures a partial temperature using a sensor but is affected by environmental temperatures. The resistance method measures the resistance and converts it using a temperature coefficient. The resistance method is more accurate as it can measure the authentic temperature rise of the motor. The Soken HRI-3 uses the resistance method because it is the superior method. The advantage of the Soken HRI-3 is its ability to measure resistance while the motor is running. It can measure the coil resistance directly and provide a precise resistance value. Meaning manufacturers are provided an accurate temperature and resistance measurement of a driven motor coil more efficiently.

How does the DAC-HRI-3 work? 

The DAC-HRI-3 uses a direct current (DC) blocking capacitor to prevent current flow into the alternating current (AC) power source. The DC current from the DC constant current of the meter is superimposed on the coil allowing for voltage drop measurement. A low pass filter (LPF) extracts the measurement from the voltage drop and calculates the resistance. Inverter noises are eliminated from the LPF using a CPU digital filter, providing accurate resistance results. Using the DC blocking component is the key to the HRI-3 measuring resistance while the motor is on. The HRI-3 combines automated features to measure resistance, graph their results, and provide data to any manufacturer. Soken recommends selecting an appropriate capacitor according to the rated current of the sample motor.

Currently, Hemmawit supplies all Soken products to many industries in Thailand. At Toshiba and Daikin, the HRI-3 provides accurate results to improve their products. These manufacturers know the quality of Japanese-made products and rely on Hemmawit to supply many types of Soken meters. For more than 20 years, Soken and Hemmawit have catered to the market in Thailand. Hemmawit manages the entire process of understanding your requirements, importing Soken products, training, after-sales service, and facilitating calibration of all Soken units.