VFD Selection: Getting It Right the First Time
Selecting the wrong VFD wastes money and can damage your motor. Oversizing wastes budget; undersizing causes overheating and tripping. This guide walks you through the exact steps SISL's engineers use when sizing VFDs for factories across Bangladesh.
Step 1: Determine Motor Power Rating
Check your motor nameplate for the rated power in kW (or HP). This is the starting point for VFD selection.
- 1 HP = 0.746 kW (for conversion)
- Always use the motor nameplate current, not just kW, for final VFD selection
- If the motor nameplate is not readable, measure the running current with a clamp meter under full load
Rule: The VFD rated output current must be equal to or greater than the motor full load current (FLC).
Step 2: Identify Your Voltage Class
Bangladesh uses 50Hz power supply. Match the VFD input voltage to your power supply:
- Single-phase 220V: For small motors up to 2.2kW. Use FR-D720S series.
- Three-phase 200V: Less common in Bangladesh. Use FR-D720 / FR-E820 / FR-A820 series.
- Three-phase 380-400V: Standard industrial supply in Bangladesh. Use FR-D740 / FR-E840 / FR-F840 / FR-A840 series.
Important: You can use a single-phase input VFD to drive a three-phase motor, but the motor power is limited to 2.2kW maximum.
Step 3: Identify Your Load Type
The load type determines which VFD series and sizing approach to use:
- Variable torque loads (pumps, fans, blowers): Torque decreases with speed. VFD can often be sized at motor rating. Best series: FR-F800, FR-E800.
- Constant torque loads (conveyors, mixers, extruders): Torque stays constant regardless of speed. VFD must be sized at or above motor rating. Best series: FR-E800, FR-A800.
- High starting torque loads (cranes, crushers, ball mills): Need 150-250% torque at startup. Must use FR-A800 series with advanced vector control.
Step 4: Consider the Operating Environment
- Temperature: Standard VFDs operate up to 40-50 degree Celsius. In hot environments (boiler rooms, furnace areas), derate the VFD by 1-3% per degree above 40 degree Celsius, or choose a larger frame size.
- Dust and humidity: For dusty or humid environments (cement plants, textile mills), consider IP54/IP55 rated enclosures or install the VFD in a separate control panel with cooling.
- Altitude: Above 1000m, derate VFD output current. Not typically an issue in Bangladesh (mostly sea-level).
- Motor cable length: Long cables (over 50m) cause voltage reflection that can damage motor insulation. For cables over 100m, use output reactors or dV/dt filters. FR-F800 supports up to 500m cable length.
Step 5: Select Required Features
Match features to your application needs:
- Safety (STO): Required for EU machinery directive compliance, crane applications, and anywhere emergency stop must disable motor torque. Choose FR-E800 or FR-A800.
- PID control: For closed-loop pressure/flow/temperature control. All Mitsubishi VFD series have built-in PID. FR-F800 has the most advanced adaptive PID.
- Communication: For PLC integration, choose VFDs with Ethernet, CC-Link, or Modbus support. FR-E800 and FR-A800 have built-in Ethernet.
- Multi-motor: For controlling multiple motors from one VFD (e.g., multi-pump systems). FR-A800 supports direct multi-motor control.
Step 6: Verify and Order
Before ordering, verify:
- VFD output current is greater than or equal to motor FLC
- Voltage class matches your supply
- Required features (STO, Ethernet, PID) are included
- Environmental conditions are within VFD specifications
- Motor cable length is within limits
Use our VFD Finder to get automated recommendations, or Energy Calculator to estimate savings. For expert assistance, contact SISL at WhatsApp +8801329721616. Our engineers will verify your selection and provide the best price in Bangladesh.