Soft starter and variable speed drive

The difference is the soft starter is primarily a mechanical soft start. You reduce the voltage and limit the current to ease the mechanical stresses associated with motor starting. You do have the potential to reduce the current associated with starting, but this benefit is highly dependent on many variable and can change dependent on the particular voltage condition at the time of starting. Medium Voltage Motors do not have the generous amounts of starting torque many people take for granted with 460v motors. Most Medium Voltage Motors (2300v and up) have between 70 -85% starting torque (this various from supplier to supplier and is also a function of the rotor construction - die cast vs copper bar).

When you use a reduce voltage starter, think of it as a reduced torque starter. They do not call them reduced current starters for a reason. Your starting torque will drop by the square of the voltage so a 80% RVS will result in 64% of the torque available (.80 x .80 = .64) Because you have a medium voltage motor that may only have 75% of its full load torque during acceleration, you now only have 64% of the available 75% during start. 64% of 75% is 48%. If your speed torque demand exceeds the available 48% pull up torque, your motor will stall.

On a centrifugal load, this is usually not a problem as long as you can tolerate 350% inrush, but there are some serious asterisks with that statement. Are you trying to load the pump during starting? You have the issue of line notching that occurs when the SCR's fire which causes additional voltage demand (the scr's firing is basically a short between phases), the size of your transformer relative to the linear load - voltage drop during starting and flicker restriction requirements by the utility.

Constant torque loads are even more problematic if you have to start the equipment loaded or if you begin loading the equipment before the motor is up to full speed.

The soft start works mind you, it just takes some due diligence.

The variable speed drive is a completely different technology. PWM drives can actually vary the frequency and the voltage with a fixed v/hz ratio. By reducing the frequency, but maintaining the ratio to voltage, you can smoothly accelerate the motor and generate full starting torque in the process while NEVER EXCEEDING THE FULL LOAD AMPS OF THE MOTOR. The gives you unlimited starts with your motor because the motor never sees more than it's Full Load Amps it would see running at full load under normal conditions.

Think of the variable speed drive like a dimmer switch on your ceiling fan. You can ease the switch up and slowly make the motor go faster and faster (by the way...that's a totally different technology (using DC power and I just made the comparison to provide a loose visual aid).

Once the motor is up to speed:
The Reduced Voltage Starter will have a bypass that closes after the motor is up to full speed and now your motor is running at full speed, producing full torque and the SCR's are no longer in circuit.

The variable speed drive can still trim load based on the speed reference from the user. (many ways to do this) and provides additional benefits such as process control, huge huge huge electrical savings and excellent motor protection.