Thermal (Overload) Motor Relay Protection
Thermal (Overload) Motor Relay Protection (on photo: RTX relay with MNX contactor; credit:

Winding failures in motor //

The majority of winding failures in motor are either indirectly or directly caused by overloading (either prolonged or cyclic), operation on unbalanced supply voltage, or single phasing, which all lead through excessive heating to the deterioration of the winding insulation until an electrical fault occurs.

The generally accepted rule is that insulation life is halved for each 10° C rise in temperature above the rated value, modified by the length of time spent at the higher temperature. As an electrical machine has a relatively large heat storage capacity, it follows that infrequent overloads of short duration may not adversely affect the machine.

However, sustained overloads of only a few percent may result in premature ageing and insulation failure. Furthermore, the thermal withstand capability of the motor is affected by heating in the winding prior to a fault.

It is therefore important that the relay characteristic takes account of the extremes of zero and full-load pre-fault current known respectively as the ‘Cold‘ and ‘Hot‘ conditions.

The variety of motor designs, diverse applications, variety of possible abnormal operating conditions and resulting modes of failure result in a complex thermal relationship.

A generic mathematical model that is accurate is therefore impossible to create. However, it is possible to develop an approximate model if it is assumed that the motor is a homogeneous body, creating and dissipating heat at a rate proportional to temperature rise.

This is the principle behind the ‘thermal replica’ model of a motor used for overload protection.

The temperature T at any instant is given by:

Formula temperature T

Tmax = final steady state temperature
τ = heating time constant

Temperature rise is proportional to the current squared:

Formula temperature rise

I= current which, if flowing continuously, produces temperature Tmax in the motor

Therefore, it can be shown that, for any overload current I, the permissible time t for this current to flow is:

Formula permissible time t

In general, the supply to which a motor is connected may contain both positive and negative sequence components, and both components of current give rise to heating in the motor.

Therefore, the thermal replica should take into account both of these components, a typical equation for the equivalent current being:

Formula equivalent current

I1 = positive sequence current
I2 = negative sequence current


Formula K

at rated speed. A typical value of K is 3.

Finally, the thermal replica model needs to take into account the fact that the motor will tend to cool down during periods of light load, and the initial state of the motor. The motor will have a cooling time constant τr, that defines the rate of cooling.

Hence, the final thermal model can be expressed as followin Equation 1:

Formula final thermal model


τ = heating time constant
k = Ieq / Ith
A2 = initial state of motor (cold or hot)
Ith =thermal setting current

Equation 1 takes into account the ‘cold’ and ‘hot’ characteristics defined in IEC 60255, part 8.

Some relays may use a dual slope characteristic for the heating time constant, and hence two values of the heating time constant are required. Switching between the two values takes place at a pre-defined motor current. This may be used to obtain better tripping performance during starting on motors that use a star-delta starter. During starting, the motor windings carry full line current, while in the ‘run’ condition, they carry only 57% of the current seen by the relay.

Similarly, when the motor is disconnected from the supply, the heating time constant τ is set equal to the cooling time constant τr.

Since the relay should ideally be matched to the protected motor and be capable of close sustained overload protection, a wide range of relay adjustment is desirable together with good accuracy and low thermal overshoot.

Typical relay setting curves are shown in Figure 1.

Figure 1: Thermal overload characteristic curves
Figure 1: Thermal overload characteristic curves; Cold curves. Initial thermal state 0%

Resource: Network, Protection & Automation Guide – Areva

About Author //


Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on


  1. Gurubalan
    Jul 20, 2016


    In our factory man cooler fans running at 24×7 so that bearing & winding issue happening at frequent,to avoid the winding failure ,can I put the relay in fan like mixy….

  2. […] the motor will continue to run hot. For this reason, it makes sense to begin your troubleshooting by checking for motor overload. Because 30% of motor failures are caused by overloading, it is important to understand how to […]

  3. Avinash Shavili
    Oct 21, 2015

    Eep website very useful to study and know about total electrical subject.

  4. […] starting of the machine after the Motor cooled down to the safe temperature level. Locked Rotor ProtectionThe ideal relay should provide protection against both starting and running locked rotor condition. […]

  5. vetri
    Sep 22, 2013

    can you please tell me once the motor getting heat motor IR value what will happen?

  6. tungpk
    Nov 29, 2012

    Hai, can I ask a question here?

    I found a single line diagram, overload setting @ current/voltage module is 63A…630A.
    Breaker is 250AF/250AT
    Load is 15.75kW
    My calculation: 15.75*1000/sqrt(3)/440V/0.8=25.8A

    My question is
    1) is the overload setting too high? should 25.8A within the overload setting? Can I say minimum value of overload setting cannot lower than 25.8A?
    2) is the MCCB rating too high?current only 25.8A, but the breaker is 250AF?normally, we refer to which value, AF or AT (Because I have seen some stated 160AF/100AT)?why there is two AF and AT for MCCB?

    Thank you

    • Edvard
      Nov 30, 2012

      AT stands for Ampere Trip (circuit breaker size)
      AF stands for Ampere Frame (frame size of the circuit breaker)

      Normally AF should be greater or equal to AT.

      Thermal overload protection Ir can be adjusted in amps from 0.4 to 1 times the rating of the trip unit. This actually depends on the manufacturer.

      Your example shows that circuit breaker is over dimensioned. CB should be at least 100A breaker with Ir set to 0,4.

      Kind regards

  7. […] later, it is the practice for motors rated less than about 1500 hp to provide either replica-type thermal-overload relays or long-time inverse-time-overcurrent relays or direct-acting tripping devices to disconnect a […]

  8. […] is based on the use of analogue electronic devices instead of coils and magnets to create the relay characteristic. Early versions used discrete devices such as transistors and diodes in conjunction with resistors, […]

  9. […] are three main functions, namely:1. Circuit protection takes three main fault types into account:Overloads,Short-circuits, (Both of which adversely affect the lifetime of cables and loads)Insulation faults, […]

  10. […] and Protection of the Neutral Conductor (1) Protection of the neutral conductorProtection against overloadIf the neutral conductor is correctly sized (including harmonics), no specific protection of the […]

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