How to Size Overcurrent Protection for a Variable Frequency Drive
Variable frequency drives are used on electrical alternating current (AC) motors. These drives control the speed of AC motors in a very accurate fashion. In most all applications the frequency drives provide overcurrent protection for the motors. The main feed of electrical power to the frequency drive must still have some form of overcurrent protection to safely power the drive unit. Sizing the overcurrent protection, regardless of the specifications provided with the drive unit, may still require calculation. Does this Spark an idea?
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Things You'll Need
- Motor specifications
- Frequency drive power specifications
- National Electrical Code book
Instructions
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Obtain the electrical specifications from the motor's nameplate data tag. The data tag is generally placed on the motor, near the topside of the exterior frame. Record the operational voltage, the full load amperage, the horsepower and the power factor rating from the motor tag. As a particular example a motor may have the operational specifications of 480 volts at a 20-ampere full load capacity with a horsepower (hp) of 12 hp. The power factor rating is 1.0.
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Find the full power required to run this motor by calculating the full load wattage. A basic electrical formula for wattage is voltage times amperage (w = v X a). Multiply the 480 volts times the amperage of 20 amperes and the operation wattage is equal to 9600 watts. All frequency drives are rated in kilowatt (kW). One kilo is equal to 1,000. The motor will then use 9.6 kW of electrical power.
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Find the maximum power that the frequency drive can provide according to the specifications. In the above example, a 10 kW (10,000 watt) frequency drive will be needed to power the electric motor. It is always better to use a slightly larger drive unit than one that is too small to provide full power. Find the full load amperage of the frequency drive when it is providing full power to the motor at 480 volts. Divide the 10 kW by 480 volts to find that 20.83 amperes will be required.
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Use Article 430 Part C of the National Electrical Code book to determine the amount of overcurrent protection to be installed in this particular circuit. Since this size of motor is greater than 1 hp and will have a service factor of less than 1.15, a trip rating of 125 percent must be added to the full load value. Multiply 1.25 (125 percent) times 20.83 amperes. The frequency drive must have at least 26 amperes of fused protection on the power feed wires. This form of overcurrent protection can be a set of thermal overloads or a set of de-rated fuses. Consult the NEC handbook for various exceptions to overcurrent protection of electrical motors and frequency drives.
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Tips & Warnings
Follow all local electrical regulations whenever servicing or installing electrical equipment.
Always wait at least three to five minutes before touching any electrical parts on a frequency drive. Internal components must be fully discharged from the drive unit before handling.
