Starting motors requires more energy than maintaining motor speeds.
With AC motors, starting often requires about 6 times the current used while the motor is operating at full speed. Along with the magnitude of the current, the staring power draw begins as almost fully reactive (VArs) and transitions to mostly real (Watts).
The most common way to start motors is across-the-line, drawing the transient power required for the start from the AC power system. If the motors are large enough, relative to the AC power system, the power flow will cause power system voltage to sag, extending, or even preventing the motor from starting. When this occurs, with an AC motor, additional motor starting equipment is required.
- A Dynamic VAr system can be used. It operates directly on the power system voltage. By injecting VArs, the motor bus voltage can be controlled directly and dynamically. The motor can be started at the torque level required/desired.
- A Reduced Voltage Soft Starter (RVSS) limits the amount of current available to the motor through phase-angle control. Limiting the current limits the starting acceleration as well, so the motor is started gradually, with a lesser impact on the power system.
- A Variable (adjustable) Frequency Drive (VFD) converts the 60 Hz AC power to DC, then back to AC at a variable frequency. The motor is initially "turned over" at low frequency, and the frequency gradually increased as the motor speeds up. Once started, the VFD is bypassed.
Generalities aren't perfect, but are right more often than wrong. Here are some:
- A Dynamic VAr system is a bus-level solution and can start multiple motors without modification. It's also harmonic-free. Motor acceleration depends upon the targeted starting voltage.
- An RVSS is a proven solution where reduced acceleration (reduced torque) starts are acceptable. It is a harmonic generator.
- A VFD allows a motor is remain on its speed/torque curve. Its very often the most expensive solution, and it generates harmonics.
- A combined Dynamic VAr system/RVSS is a common solution allowing both power bus voltage stability and reduced acceleration starting.