SELECTING THE RIGHT INVERTER AND / OR FREQUENCY CONVERTER FOR YOUR APPLICATION

SMPS APPLICATIONS: SOME LOADS (SUCH AS SWITCH-MODE POWER SUPPLIES, OR SMPS) ARE EXTREMELY REACTIVE, WITH A POOR POWER FACTOR AT THEIR INPUT DUE TO VERY HIGH PEAK-CURRENT AND HIGH HARMONICS CONTENT AT THE INPUT. CONSEQUENTLY, SELECTING A HIGH-FREQUENCY PWM TOPOLOGY TYPE INVERTER / FREQUENCY CONVERTER (ANY MODEL NUMBER BEGINNING WITH THE "GL" PREFIX)) WITH A CAPACITY (VA) EQUAL TO OR ONLY SLIGHTLY HIGHER THAN THE POWER RATING OF THE LOAD WILL RESULT IN AN OVERLOADED INVERTER / FREQUENCY CONVERTER WITH A DISTORTED WAVESHAPE AT THE OUTPUT. THEREFORE, IN ORDER TO ENSURE MAXIMUM RELIABILITY FROM THE INVERTER / FREQUENCY CONVERTER, PLEASE NOTE THE FOLLOWING: IF YOUR PARTICULAR APPLICATION CALLS FOR THE USE OF A HIGHLY REACTIVE LOAD (SUCH AS AN SMPS) IT IS STRONGLY RECOMMENDED THAT YOU SELECT AN INVERTER / FREQUENCY CONVERTER WITH A POWER OUTPUT RATING OF AT LEAST THREE TIMES THE LOAD RATING (IN WATTS). FOR EXAMPLE, IF A SYSTEM IS ENERGIZED BY AN SMPS RATED AT 300 WATTS INPUT CONSUMPTION, THE PROPER SIZE INVERTER AND / OR FREQUENCY CONVERTER TO DRIVE THIS POWER SUPPLY IS 1 KVA.

MOTOR APPLICATIONS: IF YOUR PARTICULAR APPLICATION CALLS FOR THE USE OF A COMPRESSOR / PUMP OR OTHER MOTOR, YOU MUST FIND OUT WHAT THE SURGE CURRENT REQUIREMENT IS IN ORDER FOR THE MOTOR TO START TO ROTATE. THIS IS OFTEN CHARACTERIZED AS THE LOCKED ROTOR CURRENT OF THE MOTOR. IT IS STRONGLY RECOMMENDED THAT YOU SELECT AN INVERTER / FREQUENCY CONVERTER WITH A POWER OUTPUT VA RATING AT LEAST EQUAL TO THE PRODUCT OF THE LOCKED ROTOR CURRENT MULTIPLIED BY OUTPUT VOLTAGE. THIS WILL GIVE THE LOAD RATING (IN VA). ANOTHER CONSIDERATION WHEN STARTING A MOTOR IS THE POSSIBILITY OF REACTIVE ENERGY BEING FED BACK TO THE INVERTER / FREQUENCY CONVERTER AC OUTPUT. WHEN SELECTING AN INVERTER / FREQUENCY CONVERTER, A MINIMUM LOAD MAY BE REQUIRED IN ORDER TO ABSORB THE REACTIVE ENERGY. IT IS SUGGESTED THAT, WHENEVER POSSIBLE THE LOW-FREQUENCY TRANSFORMER ISOLATED GS SERIES OF INVERTERS AND FREQUENCY CONVERTERS BE USED FOR APPLICATIONS INVOLVING MOTORS, BECAUSE THE AC OUTPUT CIRCUITRY OF THESE MODELS IS MORE TOLERANT OF THIS REACTIVE ENERGY ISSUE AND NO MINIMUM LOAD IS REQUIRED FOR THE MOTOR APPLICATION.

FURTHER INFORMATION AND APPLICATION SUPPORT ON THIS TOPIC CAN FOUND IN THE ARTICLE ENTITLED "ONLINE UPSs SAVE 'MISSION-CRITICAL' APPLICATIONS," PUBLISHED IN THE SEPTEMBER 1, 1998 EDITION OF ELECTRONIC DESIGN MAGAZINE (COPIES OF THIS AND OTHER RELEVANT ARTICLES ARE AVAILABLE FROM THE FACTORY UPON REQUEST).