Thermal replica protection (ANSI 49) — thermal model for motor and transformer

The [guide on motor protection — overload class 10/20/30](/guides/nen-3140/motorbeveiliging-overbelastingsklasse-10-20-30) covers how a thermal overload relay protects a motor via a time-current curve with a fixed class designation. This article covers the underlying, more refined technique applied to both motors and transformers: the thermal replica model, denoted by ANSI device number 49 (machine or transformer thermal relay).

Why a fixed current threshold is not enough

A simple overcurrent protection only looks at the actual current at this moment. The real temperature of a winding, however, depends not only on the current right now, but also on how warm the winding already was before this moment: a motor that has just come through a heavy start, or a transformer that has already been running close to its rated load for some time, has much less thermal margin left than the same machine that has just stood idle for a long period — even though the actual current is identical in both cases.

The thermal replica model

A relay with function 49 solves this by continuously maintaining not only the actual current, but a computational model of the winding temperature — hence the term "thermal replica": the relay electronically mimics the thermal behaviour of the protected machine.

The temperature rise is calculated according to an I²t relationship with a thermal time constant: the higher the current above the rated value, the faster the modelled temperature rises; the longer the time constant of the machine (larger mass, more cooling capacity), the slower that rise.
The model has a memory function: when the current drops, the modelled temperature gradually cools down according to the same time constant, instead of instantly jumping to zero — exactly as a real winding does not cool instantly the moment the load disappears.
Where available, the model is corrected with an actually measured temperature (for example an RTD in the winding or in the top-oil of a transformer), so the relay does not rely only on a calculated model but checks it against the actual, measured temperature.

The difference from overload classes 10/20/30

The overload classes 10/20/30 of a simpler motor protection relay in effect describe a fixed time-current curve tuned to an average thermal behaviour for that class, without an explicit, continuously updated thermal memory of the history. A full thermal replica relay (49) goes a step further: it continuously tracks the modelled temperature, including cooling between successive load peaks, and can therefore make a more accurate distinction between a machine that is already "hot" going into a new peak and a machine starting "cold" — a distinction that a pure time-current curve without memory does not make.

Note: for a transformer, alongside the winding temperature the top-oil temperature is often modelled or measured separately (see the guide on the OTI/WTI oil/winding temperature indicator) — the winding thermal replica (49) and the physical OTI/WTI measurement are two different, complementary methods for monitoring the same underlying risk (insulation ageing due to excessive winding temperature).

Practical relevance

When configuring a thermal replica relay for a motor or transformer, it is important to enter the correct thermal time constant (heating and cooling time constants can differ) and the correct overload level based on the manufacturer's data for the machine, and — where available — to link an actual temperature measurement to the model, so the relay does not rely purely on a generic assumption but follows the actual thermal behaviour of the specific machine.

Common mistakes

Configuring only a fixed current threshold or overload class without using the memory function of a thermal replica relay, so repeated short overloads with too little cooling time in between are not correctly recognised.
Using an identical time constant for heating and cooling, while many machines in reality cool down more slowly than they heat up (less effective cooling at standstill than during operation) — a too optimistic cooling time constant can lead to a premature, unjustified re-energisation.
Not updating the thermal model after a motor or transformer replacement with different thermal characteristics, so the protection no longer matches the actually connected machine.
Confusing the thermal replica protection with the physical OTI/WTI temperature measurement of a transformer — both are valuable, but protect via a different principle (modelled versus directly measured).

Motor protection — overload class 10/20/30
Oil/winding temperature indicator (OTI/WTI) for a power transformer
Transformer differential protection (87T) — percentage differential
[Current transformer protection class (5P/10P) and knee-point voltage](/guides/practical/stroomtransformator-beveiligingsklasse-5p10p-knikpuntspanning)