VFD or star-delta starter: choosing how to start an induction motor

Choosing a motor starter is really a question about the load: how much torque it needs at standstill, whether it ever needs to run at anything other than full speed, and how much disturbance the supply can take when the motor starts. Answer those three and the right starter usually picks itself.

The motor is connected straight to the supply. It is the simplest and cheapest method and gives full starting torque, but the inrush current is typically six to eight times the rated current. DOL is common on small motors where the supply can absorb that surge without other equipment noticing.

The motor starts with its windings connected in star, which reduces the starting current and starting torque to roughly one third of their DOL values, then switches to delta for running. It needs a motor with all six winding ends brought out and a load that can start with reduced torque, such as a fan or a centrifugal pump starting against a closed valve.

The weak point is the changeover. There is a brief open transition, and if the motor has not reached close to full speed the current and torque spike at the switch.

A soft starter uses thyristors to ramp the voltage up over a set time, giving a smooth start and stop with lower inrush than DOL. It is a good choice when the aim is simply to protect the mechanics and the supply at start-up, and the motor will always run at full speed once it is going.

A VFD controls both voltage and frequency, so the motor can produce full torque from standstill at close to its rated current, and run at whatever speed the process needs. Starting is gentle on the supply and the mechanics, and acceleration and deceleration are fully controllable.

The largest benefit is usually energy. On centrifugal pumps and fans, power falls roughly with the cube of speed, so running at a reduced speed instead of throttling with a valve or damper can save a substantial share of the motor's energy use.

A drive is not a drop-in replacement for a contactor. It needs to be engineered into the system properly.

Use DOL for small motors on a stiff supply. Use star-delta or a soft starter where the load starts light, always runs at full speed and you mainly need to limit the starting surge. Choose a VFD when the process benefits from speed control, when the load needs high torque from standstill, or when the energy saving on a pump or fan will pay for the drive.

Every installation is different, so treat this as a starting point for the conversation rather than the answer to it.