The VFD
Definition: a converter that transforms fixed-voltage, fixed-frequency AC into variable-voltage, variable-frequency AC is called a VFD (variable frequency drive).
Effect: it reduces the impact load when a motor starts, controls motor speed, stretches the starting time, smooths the current — achieving soft starting — and can also improve the efficiency of the grid and the motor. In practice, VFDs are mainly used for energy saving: through regulation they change output voltage, current, and frequency. Motors that need speed control generally use VFDs.
Drawbacks:
- High cost — much more expensive than microcomputer-based protection.
- Modern VFDs almost all use PWM control; this pulse-modulated operation generates higher harmonic currents on the supply side and distorts the voltage waveform. Once the power system is polluted by harmonics, mild effects reduce operating efficiency; severe effects can damage equipment and even endanger the safe operation of the power system.
- VFDs trip relatively often on overload. Motors themselves tolerate overload well, and with motor parameters set correctly in the VFD, motor overload rarely appears; the VFD itself, with poorer overload capability, raises overload alarms more easily.
The Soft Starter
Definition and effect: connected in series between the supply and the controlled motor, it regulates AC voltage by controlling its internal thyristor conduction angle via microcomputer, so the motor’s input voltage rises gradually from zero along a preset function until full voltage is applied at the end of starting — that is soft starting. During the process, starting torque and speed rise progressively until the thyristors conduct fully and the motor works on its rated-voltage characteristic, achieving a smooth start with reduced starting current and avoiding overcurrent trips. Starting ends when the motor reaches rated speed, which then receives rated voltage for normal running.
Drawbacks:
- It cannot change the supply frequency, so it cannot start a motor from zero voltage and zero frequency — no zero-impact starting.
- It cannot vary speed.
- After starting the motor, the soft starter exits the system and loses its protective role.
Summary of Differences
- A VFD is used where speed control is needed; its output changes not only voltage but also frequency.
- A soft starter is essentially a voltage regulator that protects the motor: for reduced-voltage starting it changes only voltage, not frequency.
- A VFD has all the functions of a soft starter, but its structure is more complex and its price much higher.
- VFDs control electrical equipment by adjusting frequency — they can vary speed and start, like industrial variable-frequency pumps. Soft starting basically works by changing the effective series behavior in the circuit so current rises gradually — used to start high-power motors while reducing the grid disturbance of direct-on-line starting.
