Output Voltage
The output voltage of a variable frequency drive (VFD) should be selected to match the motor's rated voltage. In standard practice, VFDs are available in 220 V and 400 V series. For high-voltage motors such as 3 kV machines, use a 400 V class VFD together with input and output transformers: step down the motor supply from 3 kV to 400 V before the VFD, then step the VFD output back up to 3 kV for the motor.
Supply Voltage
An abnormal supply voltage is harmful to the VFD. Overvoltage can cause damage; for example, a line voltage rise from 380 V to 450 V may cause failure. If the supply voltage exceeds the range specified in the VFD manual, use a transformer or other means to correct the voltage to ensure safe operation.
Output Frequency
Maximum output frequency varies by model: common values include 50/60 Hz, 120 Hz, 240 Hz, or higher. VFDs that operate up to 50/60 Hz are typically used for speed control below rated speed and are common for higher-capacity general-purpose drives. VFDs with maximum frequencies above mains frequency are often small-capacity units. In regions above 50/60 Hz, since the output voltage may remain limited, the drive exhibits a constant-power characteristic. Note that available torque decreases in the high-speed region. For applications such as machine tools, speed can be adjusted within the constant-power range to suit workpiece diameter and material; running at higher speed under light load can increase throughput. Do not exceed the allowable maximum speed of the motor or the driven equipment.
Starting Torque and Low-Speed Torque
When starting a motor with a general-purpose VFD, the starting current is often smaller than when starting from the mains. Depending on the load's required starting torque, this reduced starting torque may be insufficient to start the load. Also, torque at low running speeds is typically less than rated torque. If the selected VFD and motor cannot provide the required starting or low-speed torque, select larger capacity equipment. For example, if an application requires 70% of the originally selected rated torque but the torque curve indicates only 50% is available, the VFD and motor capacity should be increased by at least 1.4 times (70/50) the originally selected capacity.
Cooling and Protection Structure
VFDs generate significant heat internally. To improve cooling efficiency, small-capacity drives are often open-frame, while larger units use forced-air cooling with fans. When a VFD is installed outdoors or in a harsh environment, install it on a dedicated panel or in a fully enclosed cabinet with appropriate heat-exchange or cooling arrangements.
Switching Between Mains and VFD
When switching a motor running on the mains to VFD operation, wait for the motor to come to a complete stop before switching to the VFD and restarting. Otherwise, large inrush currents and torque impacts are inevitable, which can trip protection devices or damage equipment. Some systems require transferring to VFD control without a full stop. For those applications, use a VFD model equipped with the appropriate transfer control option so the VFD synchronizes with the freely spinning motor after disconnecting the mains, then resumes power output.
VFD Capacity Selection Considerations
1. Selecting a VFD with power rating approximately equal to the motor power is generally best for efficient operation.
2. If the VFD and motor power rating steps do not match, choose a VFD power rating as close as possible to the motor power, slightly above the motor rating when feasible.
3. For motors that start and stop frequently, perform frequent braking, or operate under heavy-starting and frequent-duty conditions, select the next higher VFD power rating to ensure long-term, reliable operation.
4. If testing shows the motor has sufficient power margin, it may be possible to use a VFD with a rating below the motor's rated power. However, verify that instantaneous peak currents will not trigger overcurrent protection.
5. If VFD and motor power ratings differ, adjust any energy-saving program settings accordingly to achieve effective energy savings.
6. VFD rated capacity and parameters are specified for particular altitude and ambient temperature conditions, typically for altitudes below 1000 m and ambient temperatures at or below 40°C (some specifications use 25°C). If the operating environment exceeds these conditions, apply derating factors before finalizing the VFD model. Prolonged high ambient temperatures or installation inside poorly ventilated cabinets will shorten VFD life. Electronic components, especially electrolytic capacitors, suffer reduced life at elevated temperatures: for every 10°C rise above the rated temperature, service life roughly halves. Maintain as low an ambient temperature as practical, provide adequate ventilation and cooling, and consider selecting the next higher power rating to reduce operating temperature rise. At high altitudes the reduced air density decreases cooling effectiveness; above 1000 m, a typical derating is about 10% capacity loss per additional 100 m. Increase the VFD rating if necessary to prevent overheating.
7. If the application requires restarting after an instant power interruption, confirm the selected VFD supports this function. A VFD that restarts improperly when power is restored may cause overvoltage or overcurrent trips.
8. When using a sensor together with the VFD for speed control, ensure the sensor output type and signal level are compatible with the control input expected by the VFD.
