Evgenii Konkin

Posted on May 11

Why Soft Starter Sizing Fails When Engineers Start With Motor HP

#engineering #electrical #motorcontrol #industrialautomation

A soft starter can be perfectly installed — and still be the wrong device.

Not because the motor is unusual.

Not because the voltage is wrong.

Not because someone wired the panel badly.

But because the starter was sized from motor horsepower instead of from actual starting duty.

That is where soft starter mistakes usually begin.

A 50 HP centrifugal pump and a 50 HP crusher can look similar on a one-line diagram.

They can even have similar full-load current.

But they do not impose the same thermal duty on the soft starter.

And that is why a starter that looks “large enough” on paper can still trip, overheat, or fail early in real service.

The Core Formula Behind Soft Starter Sizing

The calculator uses one main screening equation:

I_{rated,min} = FLA \times K_{app} \times K_{temp} \times K_{alt} \times K_{starts} \times K_{sf} \times K_{connection}

Where:

FLA = motor full-load current
Kapp = application duty factor
Ktemp = ambient temperature derating
Kalt = altitude derating
Kstarts = starts-per-hour derating
Ksf = service factor multiplier
Kconnection = connection factor

This is the real reason soft starter sizing is not just “pick something near motor current.”

The motor current is only the starting point.

The actual soft starter rating has to survive the application, the thermal environment, and the starting frequency.

Why Motor HP Is Not Enough

Many engineers still begin with motor power:

50 HP motor
460 V
standard industrial system
choose a starter frame that feels about right

But soft starters are not sized from HP alone.

They are sized from motor FLA plus derating.

If nameplate FLA is not available, it can be estimated from motor power. For a three-phase motor, the fallback relationship is:

FLA = \frac{HP \times 746}{V \times \sqrt{3} \times \eta \times PF}

But even that is only a fallback.

For real selection, nameplate FLA should win.

Because once the actual FLA is known, the more important question is no longer “What is the motor power?”

It becomes:

What duty will this starter really see?

The Part That Gets Missed in Real Projects

Soft starter problems usually come from one of these assumptions:

“It’s only a 75 HP motor, so this frame should be fine.”
“The current rating is above FLA, so we’re safe.”
“Starts per hour only affect motor heating.”
“Ambient conditions are not bad enough to matter.”
“Inside-delta always lets us pick a smaller unit.”

Every one of those shortcuts can produce an undersized selection.

And the worst part is that the system may still look reasonable during procurement.

The failure shows up later:

nuisance trips
thermal alarms
short SCR life
poor acceleration
repeated restart problems

At that point, the problem is blamed on commissioning.

But the real mistake happened during sizing.

One Example

Let’s take a case that looks ordinary at first:

Motor FLA = 96 A
Application = crusher
Ambient temperature = 50°C
Altitude = 2200 m
Starts per hour = 25
Service factor = 1.15
Connection = in-line

Now apply the duty factors.

Step 1 — Application duty

Because this is a crusher, the base duty is already heavy.

But frequent starts above 20 per hour force the model into severe duty.

So:

K_{app} = 1.50

Step 2 — Temperature derating

At 50°C, the calculator applies:

K_{temp} = 1.10

Step 3 — Altitude derating

At 2200 m:

K_{alt} = 1.12

Step 4 — Starts per hour

Because the motor starts more than 20 times per hour:

K_{starts} = 1.50

Step 5 — Service factor

For SF = 1.15:

K_{sf} = 1.05

Step 6 — Connection type

For standard in-line connection:

K_{connection} = 1.00

Now Multiply Everything

I_{rated,min} = 96 \times 1.50 \times 1.10 \times 1.12 \times 1.50 \times 1.05 \times 1.00

I_{rated,min} \approx 279 \text{ A}

So the required starter is not “something around 100 A.”

It is not even 170 A.

It must be rounded up to the next standard ladder value:

\text{Recommended size} = 290 \text{ A}

That is the kind of result that surprises people.

A motor with 96 A FLA can legitimately push you toward a 290 A soft starter when the duty is severe enough.

That is not an error.

That is the point of doing the derating correctly.

Why This Example Matters

A lot of engineers would look at a 96 A motor and assume that a 125 A or 170 A soft starter has enough margin.

But that logic ignores the actual thermal stress on the device.

In this case:

high starts per hour increase thermal cycling
high ambient temperature reduces cooling margin
altitude reduces heat rejection capability
crusher duty is already harder than a pump or fan

So the starter is not being selected for “steady motor current.”

It is being selected for survival under repeated real duty.

That is the difference between theoretical adequacy and field adequacy.

The Trap With Inside-Delta

There is one more detail worth mentioning.

Inside-delta connection can reduce the current seen by the soft starter, which is why the calculator uses:

K_{connection} = 0.58

for inside-delta applications.

That can save one or two frame sizes.

But many people misuse that reduction.

You only get that benefit if:

the soft starter actually supports inside-delta wiring
the motor terminal box has the required six accessible leads
the installation is designed for that topology

So inside-delta is not a shortcut.

It is a conditional design choice.

Treating it like a universal current discount is how equipment gets damaged.

Practical Takeaways

Soft starters should be sized from FLA, not from motor HP alone
Application duty can change the result dramatically even for the same motor current
Starts per hour, ambient temperature, and altitude are not secondary details — they can completely change the frame size
A starter rated above motor FLA can still be badly undersized
Inside-delta only works when both the product and motor wiring actually support it

Because again, the soft starter does not fail because the formula is complicated.

It fails because the selection logic was too simple.

Try It Yourself

If you want a fast way to check duty class, derating, and minimum required starter current before you finalize a selection, use the calculator here:

👉 Soft Starter Sizing

It lets you screen soft starter size from real motor FLA, application severity, thermal conditions, starts per hour, and connection type — before those assumptions turn into startup problems.

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