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HomeFrequency ConverterDetermining frequency converter size

Determining frequency converter size

Frequency converters are becoming almost standard part of aquatics equipment room packages. Most frequency converters are fairly simple to install and operate however, they are quite complex with respect to their sophisticated hardware and software implementations. Frequency converter functionality and operation can be greatly improved by understanding basic frequency converter theory, terminology and interfacing options.

The frequency converter must be sized for at least 2 times the full-load amps of the motor to allow for the high inrush of current during motor starting. For example, a typical 20 hp premium efficient motor is rated for 49 amps at full-load and 220 volts. A minimum of 98 amps are required for sizing the frequency converter for start up conditions. Most frequency converters limit the current output to 150% of full-load amps because torque and current are not proportional above 150% torque. A typical 20hp frequency converter is rated for 1.5 times full-load amps or 74 amps and is not sufficient for the application. A typical 30 hp frequency converter is rated for 108 amps at 220 volts. The minimum frequency converter size would be 30hp. A 40 hp frequency converter would be recommended.

Alternate methods for determining frequency converter size are used by manufacturers. Some manufacturers choose to oversize the motor instead of the method described above. A successful method for determining frequency converter horsepower is to multiply the demand horsepower calculated at the minimum capacity by the turndown ratio and again by 2.5 to allow for start up conditions. This is a conservative method for determining frequency converter horsepower and the equipment must be sized for full-motor torque.

The frequency converter must be programmed for full torque boost with the least amount of time delay at start up. The time delay limits the inrush of current and creates a soft start. Soft starting a motor under load is not recommended. The frequency converter soft-start feature must be disabled.

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