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Drives & Motion·6 min read·

VFD or soft starter: what each one is actually for

One controls speed, the other only controls starting. The cost, panel space and process questions that decide it.

Short answer

Use a soft starter when the only problem is inrush current or mechanical shock at start, and the motor runs at full speed once it is going. Use a VFD when the process genuinely benefits from varying speed, which on a centrifugal pump or fan usually pays for the drive in energy alone. A soft starter is cheaper, smaller, produces no harmonics and cannot vary speed at all.

The drive loop, and the line that closes itSetpointfrom the PLCDriveV/f, vectorMotor3 phaseLoadENCCLOSED LOOP ONLYWithout the dashed line the drive is guessing at speed from current and voltage. That is the whole difference between sensorless vector and closed loop.

A soft starter and a VFD look like adjacent products and answer different questions. Choosing between them starts with what problem is actually being solved.

What a soft starter does

It ramps the voltage to the motor over a few seconds at start, limiting inrush current and the mechanical shock of a direct on line start.

Once the motor is at speed, an internal bypass contactor closes and the motor runs directly across the line. The starter is out of circuit and doing nothing until the next stop.

That last point carries most of the consequences. No harmonics in normal running, no losses, no cooling requirement worth discussing, and no ability whatsoever to change speed.

What a VFD does

It synthesises a variable frequency, variable voltage supply, and it is in circuit all the time.

That gives speed control, controlled acceleration and deceleration, torque control, and on centrifugal loads a large energy saving. It also brings harmonics, switching losses, a cooling requirement, more panel space, and a longer commissioning.

The cube law, which decides most cases

For a centrifugal pump or fan, flow is proportional to speed, pressure to speed squared, and power to speed cubed.

Running at eighty percent speed uses about half the power. At fifty percent speed, about an eighth.

Where a system currently runs a fixed speed pump and throttles the flow with a valve, that valve is converting the difference into heat and noise. A drive removes it, and on a pump of any size the payback is usually short enough that the energy argument alone settles the question.

The corollary is important. On a constant torque load, a conveyor or a positive displacement pump, the cube law does not apply and the energy argument mostly disappears. There, the case for a drive has to be made on process control.

When a soft starter is the right answer

  • The load runs at one speed and always will.
  • The problem is inrush current, either because the supply is weak or because the distributor limits it.
  • The problem is mechanical: a belt drive that slips, a coupling that shocks, a pump that hammers a column of water.
  • Panel space is tight and harmonics are a concern.

When neither is right

Worth stating. A small motor on a stiff supply driving a low inertia load, started a few times a day, is fine direct on line. It is cheaper, has fewer failure modes and needs no commissioning.

Fitting a drive to every motor because drives are modern is a real pattern and it adds cost, failure modes and harmonics for nothing.

The one that catches people

A soft starter cannot slow a motor down and it cannot stop it faster. Soft stop ramps the voltage down, which on a loaded pump takes the pressure away gently and is genuinely useful for water hammer, but it is not braking.

If the requirement is to decelerate a load in a controlled time, that is a drive with a braking resistor, and no soft starter will do it.

Common questions

Can a soft starter save energy?
Essentially no. Once the motor is up to speed a soft starter bypasses itself and the motor runs directly across the line. Energy saving comes from running slower, which only a VFD can do. Claims otherwise usually relate to a small optimisation at light load that rarely repays the complexity.
How much energy does a VFD save on a pump?
On a centrifugal pump or fan, power varies roughly with the cube of speed, so running at 80 percent speed uses about half the power. Where flow is currently throttled by a valve, replacing the throttle with speed control often pays back the drive within a year or two.
Do VFDs cause harmonics?
Yes. A standard six pulse drive draws non sinusoidal current and injects harmonics back into the supply. On a small installation nobody notices. On a site with many drives it can matter, and the answers are line reactors, DC chokes, twelve pulse drives or active front ends, in ascending order of cost.

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