Soft Starters

What it does

An across-the-line starter applies full voltage instantly. The motor draws locked-rotor current, typically six to eight times its full-load rating, for the second or two it takes to accelerate, and the pump accelerates the water column with it. A soft starter puts back-to-back thyristors in each phase and phases them on progressively, so the voltage ramps from an initial value to full over a set time. Torque follows the square of voltage, so the motor accelerates gently and the current peak is reduced to whatever the current limit setting allows, commonly 250 to 350 percent of full-load current.

Once the motor is at speed, most soft starters close an internal or external bypass contactor and the thyristors carry no current until the next start. On a stop command, the starter can ramp the voltage down over a set time, a soft stop, which lets the pump decelerate gradually and the check valve close slowly instead of slamming shut on a reversing column.

Where a soft starter fits

The soft starter sits between the two extremes. It fixes the starting problems for a fraction of the cost, heat, and complexity of a drive. It does nothing for a process that needs speed control, and a station that installs soft starters to save energy has been sold the wrong device.

Settings that matter on a pump

Initial voltage or torque

The starting point of the ramp, typically 30 to 50 percent. Too low and the motor sits without turning while it heats; too high and the start is nearly across the line.

Ramp time

Time from initial voltage to full, typically 5 to 20 seconds on a pump. Longer is gentler and heats the motor longer; the ramp must end with the motor at speed.

Current limit

The ceiling on starting current, commonly 300 percent of full-load amps. The starter holds voltage at whatever keeps current under the limit until the motor accelerates. On a weak supply this is the setting that stops the lights dimming.

Kick start

A brief pulse of higher voltage at the beginning to break a pump free of grit or a stiff seal, then the ramp. Useful on pumps that sit; unnecessary otherwise.

Soft stop time

The voltage ramp-down on stop, typically 5 to 30 seconds. Long enough that the check valve closes gently; short enough that the pump does not run below the speed that moves water for long, which matters with a large static head where the pump stops moving water early in the ramp.

Stall and thermal protection

The starter monitors current and time during the ramp and trips if the motor does not accelerate, and models motor thermal state across starts. Set from the motor data, not left at defaults.

Phase loss and imbalance

Built into most starters; confirm they are enabled and set.

Bypass

Thyristors dissipate on the order of a watt per amp per phase while conducting. A 100 amp starter without bypass makes a few hundred watts continuously, needs a heatsink and enclosure cooling, and has three semiconductors in the motor circuit around the clock. With a bypass contactor closed at full speed, the thyristors carry current only during the ramp, the heat drops to almost nothing in run, and a thyristor failure does not take the pump down until the next start. Most modern soft starters include the bypass internally. Specify it, and confirm the bypass is rated for the motor full-load current and the contactor duty.

Integration with the panel

• Control: a run input and a stop input, with a run and a fault output back to the controller. Many starters also offer a network interface for current and diagnostics.
• Protection: the starter provides motor overload, phase loss, and stall protection. Short-circuit protection upstream is chosen to match the starter listing and the panel SCCR; semiconductor fuses are required for some ratings.
• HOA: the same as any starter. The soft starter is the starter; the HOA and the controller call it.
• Run confirmation: from the starter run output or the bypass contactor auxiliary, plus current where available. A starter in the ramp is not yet at speed; use the at-speed or bypass-closed signal for the proof where the logic cares.
• Parameters recorded on the drawings and in the commissioning record, as with a drive.