On our VFD page, we explain a few problems involved when starting up an electric motor:
Standard induction motors rotate at fixed speeds based on the voltage and frequency of the source of incoming power. The motor always runs at its peak load because the speed of the motor is fixed. However, because the motor rarely needs to run a peak load, energy is wasted, and you will end up with a more expensive motor than you expected.
Also, when starting an electric motor, the inrush of power leads takes the motor from idle to 100% power in seconds. That sudden inrush damages the motor leading to frequent repairs and perhaps replacement. In short, the lifespan of an expensive piece of equipment is shortened.
VFDs provide a solution to those problems and offer a great many other technical options to run a motor more efficiently than a motor does without control.
There is, however, an alternative to a VFD: Soft Starters.
It is relatively easy to understand the purpose behind a soft starter; it is used with an electric motor to reduce the load and torque temporarily during the motor’s startup process. Similar to a VFD, a soft starter reduces the surge of current during startup, which reduces the mechanical stress on the motor and shaft.
There is a great deal of benefits that soft starter offer. What follows is in no way a complete list, but it gives some of the more common benefits:
- Reduce Maintenance: the smooth startup process of an electric motor reduces the need for frequent maintenance, reducing downtime.
- Contactor Replacement: one issue from running a motor without using a soft starter is contactor replacement. Once a soft starter is introduced to the electric motor, the need to replace contactors falls as you see improvements in the operation of the motor. A soft starter also reduces the noise that comes with contactors when the motor is running.
- Reduced Torque Transients: sudden changes in either the direction or magnitude of the torque load leads to motor damage. That increased load reduces the lifespan of the motor. Significantly reducing the torque load during startup from applying a soft starter may limit any associated problems.
- Paying for Itself: Again similar to a VFD, switching to a soft starter reduces power consumption and pays for itself.
Soft Starter Applications
- Applications, where soft starters are typically used, are when:
- Situations where energy-efficiency of 99% or greater is needed.
- Large motors that need enormous inrush currents for conveyors, gears, or belt-driven systems.
- Piping surges where the direction of the fluid rapidly changes the direction that is eliminated by pumps.
Soft Starter Benefits
- Soft starters are more energy-efficient and cost-effective than VFDs above 10 HP
- Soft starters use silicon-controlled rectifiers, or thyristors, that reduce the heat generated by the motor during start-up.
- Where space is a concern, soft starters are usually smaller than VFDs.
- For applications where control of torque and steady, constant acceleration are not needed; or if the motor needs to limit current only during the startup inrush, then a soft starter is usually a more cost-effective solution over a VFD
- Emerson Control Techniques