Motor Protection- Abb

Heikki Autio – Breakers & Switches

Motor Protection Type 2 Co-ordination © ABB | Slide 1

Program

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Main electrical parameters of motors

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Basics of fuse links

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Main solutions for switching and protection of motors

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Main standard prescriptions for coordination methods

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SOC Selected Optimized Coordination

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Summary

Main electrical parameters of motors Motor data: Rated parameters

P = Rated motor power [kW] V = Supply voltage [V] cos  = Power factor  = Efficiency Ie = Rated current [A]

P Ie    cos   3  V

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Main electrical parameters of motors Motor data: Starting parameters

In accordance to the IEC 60947 - 4 - 1 standard the starting current is defined as: Ia = 7.2 x Ie

With reference to the characteristics of the most common motors is given a value for the typical maximum initial starting current defined as: Ip = 12 x Ie

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Main electrical parameters of motors

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With reference to the starting typical current waveform of the motor (in red) the previous parameters (Ie – Ia – Ip) are drawing as below: Ie = Rated nominal current t [s]

Ia = Rated starting current (7.2 x In) Ip = Maximum peak inrush current (12 x In)

Ie

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I [A] Ia=7.2xIe

Ip=12xIe

Switching and protection of motors In order to realize the motor coordination, these kinds of devices are usually used:

Fuse (short-circuit protection)

Contactor (operation cycles-switching)

External thermal/electronic release (overload protection)

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Fuse link 

Fuse link (known as fuse) is protection device that protects electrical circuit against overcurrent and short-circuit current

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Fuse link provides superior current limiting properties 

Optimal solution for motor protection

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Fuse links structure 

Fuse link breaks circuit under excessive current by melting the fuse element inside the fuse body

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Arch that is formed between blade contacts is cooled down by silica sand inside the fuse body

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All energy create by short circuit current is kept inside the fuse body 

No emission of gases and no need for arching space

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Contactor 

It is the device intended to carry out the switch on/switch off operations of the motor under normal conditions and also to disconnect the motor from the supply network in case of over currents detected by the overload relay which commands the tripping.

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Besides, the contactor shall be chosen so as to be suitable to carry the rated current of the motor with reference to the category AC-3.

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The contactor carrying out motor operations allows to realize a number of operations much higher than those which the switch could perform.

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Overload relay 

It is the device intended to realize the protection against motor overload and it has usually the function of commanding the opening of the contactor for those over currents lower than the operating currents of the fuse link

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Motor starting current With reference to the time current curve of the fuse link and the thermal protection must be possible to realize a solution with the characteristics given in the picture: the starting current of the motor is always under the tripping curve of the protection devices in order to permit the starter of the motor Time current characteristic of the fuse link

t [s]

Overload protection

Motor starting current

Ie

I [A]

Ia=7.2xIe Ip=12xIe

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Main solution for switching and protection of motors DOL (direct-on-line) starting is perhaps the most traditional system and consists in connecting the motor directly to the supply network, thus carrying out starting at full voltage. This requires a big inrush current and develops a high starting torque with very reduced acceleration times. It generates violent accelerations which stress the transmission components such as belts and joints. Star-delta () starting is the best known system and maybe the most common starting system at reduced voltage; it is used to start the motor reducing the mechanical stresses and limiting the current values during starting; on the other hand, it makes available a reduced inrush torque. The motor starts with the winding in the Y connection, the current absorbed by the motor is reduced by 3 times in comparison with its rated current. When 80% of the speed is reached, switching is carried out by changing over to the delta () phase. In this phase, the motor absorbs a current reducing by 1.73 times the rated current, completes starting and continues in the ON position with full load conditions.

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Main solution for switching and protection of motors The most common used circuits of starters are: Starter supply with full voltage starter (Direct online) Starter initially supply with a partial voltage and current value starter (Star-Delta)

Direct on Line Starter DOL diagram

Switch Fuse

Contactor

Overload Relay

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Star-Delta Starter Y/ diagram

Switch Fuse

Line Contactor

Overload Relay

 Contactor

Y Contactor

Cootrdination types The Standard reference for motor starters is IEC 60947-4-1. This Standard gives also the definitions for the classification of motor starters.

Starter type Type 1 = Damages to the contactor and to the overload relay are acceptable. The starters may be inoperative after each operation. Therefore the starter shall be inspected and the contactor and/or the overload relay. There shall not be damages for the users. Type 2 = No damages to the overload relay or other parts have occurred; except that welding of contactor or starter contacts is permitted, if they are easily separated without significant deformation. Only fuse needs to be changed Comparison

Type 1 initial economic advantage - expensive maintenance - stops on the plant due to faults.

Type 2 initial high investment - service continuity - easy maintenance. © ABB | Slide 14

Short circuit test modality Duty cicle:

O - CO at “Iq” current O - CO at “r” current O : operation of the fuse link under short circuit condition CO:closing by contactor under short circuit condition and operation of the fuse link © ABB | Slide 15

Short circuit test modality

Iq : Full short circuit current (Maximum short circuit current admitted by the combination) r: critical short circuit current (Depending from contactor size) (1 - 3 - 5 - 10 kA) © ABB ABB SACE

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Solutions with other short circuit protecting device Solution with soft starters

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As a control component of the motor, this device bases its functioning on power semiconductors, also called tyristors, which allow to control and slowly increase the voltage applied to the motor, making it start slowly and limiting the starting currents.

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Once the ramp time has elapsed, the tyristors are usually by-passed with a contactor, and the line is directly connected to the motor.

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This means that tyristors do not remain constantly in function, thus reducing any possible problem due to temperature-rise.

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Solutions with other short circuit protecting device

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Selected Optimized Coordination (SOC) http://applications.it.abb.com/SOC/Page/Selection.aspx http://bit.do/motorprotection

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How to use Selected Optimized Coordination user interface (SOC)

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With a few mouse click it is possible to search for and select coordination table(s) most appropriate for the current purpose

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Following filters are available 

Protection Device

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Rated Voltage

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Short-Circuit current (Iq current)

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Coordination Type

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Overload Relay

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Motor Rated Power

How to use Selected Optimized Coordination user interface (SOC) 

You can select more than one filter at the time

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How to use Selected Optimized Coordination user interface (SOC)

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How to use Selected Optimized Coordination user interface (SOC) 

Each result links to the whole coordination table

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How to use Selected Optimized Coordination user interface (SOC)

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How to use Selected Optimized Coordination user interface (SOC) Table example

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Summary of Fusible Type 2 Motor Coordination

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Ready made Type 2 coordination tables that are based on actual test results 

Fast and easy selection motor control devices

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No need to do expensive tests

Very high limiting capability of fuse link 

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The cheapest and the most optimized selection

After fault only fuse link replacement is needed 

Keeps same protection level as before fault. No derating needed

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No risk of re-energization

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