How to Size Overcurrent Protection for Frequency Inverter (2)
Continued from the article - How to Size Overcurrent Protection for Frequency Inverter (1), we dig deeply to how-to.
2. Find the full power required to run this ac 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 20 amperes and the operation wattage is equal to 9600 w. All frequency inverters (variable speed drives, VSDs, variable frequency drives, VFDs) are rated in kilowatt (kW). One kilo is equal to 1,000. The ac motor will use 9.6 kW of electrical power.
3. Find the maximum power that variable frequency drive (VFD) can provide according to the specifications.
In the example above, a 10 kW (10,000 watt) frequency inverter will be needed to power the electric ac motor. The important point here is that 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 frequency inverters (variable speed drives, VSDs, variable frequency drives, VFDs) 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.
4. Use Article 430 Part C of the National Electrical Code (NEC) 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% must be added to the full load value. Multiply 1.25 (125%) times 20.83 amperes. The frequency inverter must have at least 26 A 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 ac motors and frequency inverters.
