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Thermal (Overload) Motor Relay Protection

Lovato motor protection relays

Winding failures in motor

The majority of winding failures in motor are either indirectlyor directlycaused by overloading(either prolonged o

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 a

approximate model if it is assumed that the motor is a homogeneous body, creating and dissipating heat at a rate
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proportional to temperature rise.

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

The temperature Tat any instant is given by:

where:

Tmax= final steady state temperature

= heating time constant

Temperature rise is proportional to the current squared:

where:

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

Therefore, it can be shown that, for any overload current I, thepermissible time tfor

this current to flow is:

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:

where:

I1= positive sequence current

I2= negative sequence current

and

at rated speed. A typical value of Kis 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 r, that defines the rate of cooling.

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

where:

= heating time constant

k= Ieq / Ith

A2= initial state of motor (cold or hot)Ith=thermal setting current

Equation 1takes 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.


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

time constant r.

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.


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

Resource:Network, Protection & Automation Guide Areva

About Author //


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Edvard Csanyi

Edvard- Electrical engineer, programmer and founder of EEP. Highly specialized for

design of LV high power busbar trunking (


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3.

tungpk

Nov 29, 2012

Hai, can I ask a question here?

I found a single line diagram, overload setting @ current/voltage module is 63A630A.

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

(reply)

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

(reply)

4. Stator Overheating Protection | EEP

Nov 19, 2012

[...] 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

[...]

(reply)
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5.

Introduction to Static Protection Relays | EEP

Sep 21, 2012

[...] 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, [...]

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6.

Functions Performed by Electrical Switchgear | EEP

Sep 14, 2012

[...] 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, [...]

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7.

Sizing and Protection of the Neutral Conductor (2) | EEP

Jul 31, 2012

[...] 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 [...]

(reply)

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