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24 — Calculators

Setting a motor overload relay

An overload relay in the delta branch does not see the line current but the winding current — smaller by a factor of root three. Set the nameplate value there and the motor is protected forty percent too high. It is one of the most common mistakes in star-delta circuits.

Input

In the delta branch the relay only sees the winding current, not the line current.

For comparison with the calculated target. Initially equal to the motor current.

The time in which the relay trips at 7.2 times the set value. Given in the data sheet; class 10 means up to 10 s, class 20 up to 20 s.

To check whether the relay survives the start.

Result

Value to set
—A
Assessment
—
Estimated tripping time at starting currentAn approximation from the trip class. The characteristic in the data sheet governs.
—s
Deviation of the setting from the target
—%

A guide, not a design to standard. We give no warranty for the correctness of the results, the applicable standards and case-by-case verification govern.

Worked example

The default case the calculator starts with, worked through once. Change the values above and it recalculates immediately.

Inputs

Rated motor current
55 A
Location of the overload relay
In the line feeder (direct on line)
Value set on the relay
55 A
Tripping time of the trip class
10 s
Starting current
385 A
Duration of the start
4 s

Result

Value to set
55 A
Assessment
The setting matches the calculated target.
Estimated tripping time at starting current
10.6 s
Deviation of the setting from the target
0 %

Formula

  • Direct on line: I_e = I_N
  • In the delta branch: I_e = I_N / √3
  • Approximate tripping time: t ≈ t_class · (7.2 / (I / I_e))²

Assumptions and standards

  • The setting follows the rated motor current for the actual duty type. Intermittent duty or different cooling call for different values.
  • The tripping time is an approximation from the thermal model and only serves to estimate whether the start gets through. The characteristic in the device data sheet governs.
  • An overload relay protects against overload, not against short circuit. Short-circuit protection is a separate device and a separate design.

Frequently asked

Why root three?

Because in delta connection the line current divides between three windings, and each winding carries only the line current divided by root three. With the relay in that branch, it measures this smaller current. At 55 A motor current that is around 32 A.

What if the relay trips during starting?

Then the trip class does not match the starting time. A higher class allows more time — but the motor has to withstand it thermally as well. With heavy starts, checking that motor and relay match is not a formality.

Is a motor protection circuit breaker enough on its own?

It combines overload and short-circuit release in one device and suffices for many applications. Where short-circuit currents at the point of installation are high, its breaking capacity has to match; otherwise a fuse upstream is needed.

Calculating is the easy part.

A formula gives you a number. Designing a plant also demands installation method, grouping, discrimination, the standards in force and a look at the installed base. That is what we take on.

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