Motor Protection and co-ordination – an introduction

Electric motors are crucial components in virtually every industrial automation application or environment. Therefore, optimising their performance and reliability can play a major role in reducing costs and improving overall plant efficiency. Electric motors fail, and about half of them fail because of overheating through overload, phase failure, or insulation breakdown. Indeed, ABB says that a mere 10% rise in motor temperature can cut its life by as much as 50%. Other common causes of motor failure can be overloading through excess conveyor loading, dry pumps, or jammed driven equipment.

AC electric motors can be protected in several ways, the most common being electronic relay devices (mainly for fractional HP motors, but also suitable for larger motors), motor protection filters (which protect motors from the destructive affects of long cable runs between inverter and motor), and - perhaps most commonly - circuit breakers. Fuses, particularly high speed fuses, can also be used, and soft starts, although not motor protection devices in their own right, do help prevent damage to both motors and driven equipment with their ramp-up / ramp-down characteristics. In addition, intelligent control device for motor protection applications can be used with all major fieldbus systems, and also provide motor protection and control functions for stepper motors and DC servos.

But mainly, we are talking about AC motors here, for pump, conveyor and other drives. Take motor protection circuit breakers. These can be designed to meet global requirements for motor branch circuit protection. They will also provide a means of disconnection, branch-circuit short-circuit protection, and - additionally - overload protection. Such motor protection circuit breakers can often also be used in combination with contactors to provide compact and reliable two-component combination starter units. Typically, such devices will meet the required standards, including - for example - UL 489 and IEC 60947-2 standards as circuit breakers, as well as UL 508 and IEC 60947-4 for motor overload protection.

A typical modern motor protection relay will provide all basic motor protection features, including short circuit, load loss, load jam, and frequent starting protection. It will typically also offer imbalance current and phase-reversal protection, as well as provide optional voltage, I/O, and communications capabilities. Such relays will accurately track the heating effects of operating and cyclic overload currents. In many cases, reporting will be included, which may include load profiling, and motor start and trend data. This provides the user with an excellent feel for the way the protected motor and driven load is behaving.

Modern motor load controls prevent motor damage caused by overload, jamming or pump dry running. They may be surface mounted, in some cases, for ease of installation. True power and other parameters can be displayed on large LEDs. High and low power trips with adjustable on delays and trip timers are typical features, and the sensitivity of the low power trip used to detect dry running can, in many modern devices, be up to ten times that of conventional undercurrent relays. Analogue outputs and individual trip relays allow performance monitoring and pump / motor control.

Soft starts and co-ordination:

Many types of machinery and equipment benefit from being driven by soft starts. Such machinery includes pumps, compressors, crushers, fans and grinders, and indeed, any motor-driven machinery that would benefit from the ability to provide a 'soft' and smooth ramp up and ramp down to reduce wear and tear. For this reason, soft starts widely used throughout commerce and industry. However, they do need to be tested in accordance with certain criteria in terms of protection.

Manufacturers of soft starts may be required to carry out short circuit tests for their ranges. Part of international standard, IEC 60947-4-2, these will ensure that any short circuit in a control panel will not damage the soft start. Therefore, such tested soft starts, when used in conjunction with suitable short circuit protection devices (SCPDs) - such as high speed fuse links - can be offered, together with a general certificate of conformity to 60947-4-2, plus a copy of the short circuit test certificate. The end result is soft starts, which are ideal for panel builders and other OEMs as part of their own IEC 60439 system compliance.

The IEC 947 international standard now takes into account not only European requirements, but also those of the UL (1), NEMA (2) (USA) and JIS (3) (Japan) standards. This standard requires manufactures to meet specific requirements for the design and testing of soft starts. They cover a series of type test, including operating capability, temperature rise tests, EMC testing and short circuit testing.

Special circuit breaker equipment, or high-speed fuse links, have been used to great advantage in helping provide specialist motor soft start manufacturers with Type 2 co-ordination to the international standard IEC 60947-4-2 for their ranges of soft start devices. Co-ordination is crucial.

What are Types 1 & 2 co-ordination?

Type 1 co-ordination covers short-circuit conditions where the soft start cannot afterwards be operated again without repair or replacement of parts.

Type 2 co-ordination is different. In this case, after a short circuit, the equipment must not be dangerous, either to personnel or installations. Therefore, after any short-circuit, the soft start concerned should show neither deterioration, nor any altered settings. If contacts have become welded, they must be easy to separate. No replacement parts are allowed during the test, except for SCPDs, such as fuses, which must all be changed.

The actual definition of Type 2 co-ordination, according to BS EN 947-4-1 is:

'Effective limitation of the fault condition with no damage resulting to the components of the motor starter, other than light tack welding of the contactor main contacts which can easily be broken'.

However, three levels of co-ordination are covered by the standards:

• Type 1 co-ordination to BS EN 947-4-1.
• Type 2 co-ordination to BS EN 947-4-1.
• Total co-ordination to BS EN 947-6-2.

Type 2 co-ordination (IEC 947-4-1 and 947-4-2) applies to any type of motor starter and any type of short circuit protective device. Under IEC 947-4-2, soft start manufactures are required to state the soft start overload capability (such as, for example, class 10 at 6 x FLC for six seconds). The standard also means that product labelling must show the overload rating and number of starts per hour. Users should ensure that manufactures provide a certificate of conformity.

Damage must be avoided:

Soft start short circuit damage is usually a result of too much heat or too high a magnetic force, and both derive from the current.

• Excessive heat - This builds as a function of let through energy (I2t) during the clearing of the short circuit fault current by the SCPD. This may cause contacts to weld.
• Magnetic forces - These are related to the instantaneous peak let through current (Ip) passed by the SCPD. This is the current flowing in the circuit before the SCPD can open it. Such forces cause mechanical stress and may cause the contacts to fly apart, possibly causing case damage.

Such damage can be minimised in a number of ways before it occurs. Methods include: making soft starts larger and / or more resistant; reducing short circuit fault currents; and using better SCPDs.

Although far from being the only type of SCPD, for this type of application, high speed fuses are ideal, in many cases, for soft start short circuit protection because they are 'current limiting' and they must interrupt fault currents within the first half cycle of the current wave. They also have low peak let through currents and energies. Low peak let through currents provide real benefit for contactor contacts, helping to prevent welding, while low energy let-through helps provide overload relay (bi-metal) protection. In addition, a high back up protection for soft starts is provided by fuse links' short-circuit level to 200 kA. Moreover, high breaking capacity fuse switches often allow smaller motor frame sizes for given power ratings.

Co-ordination of SCPDs:

Co-ordination for protective equipment covers the contactor, thermal overload relay and SCPD, and the objective is to interrupt an overcurrent (which may be many times the rated motor current) or short-circuit current (10 times rated motor current) in enough time to avoid danger to personnel, or damage to equipment.

Type 2 co-ordination is achieved through careful selection of these components. As long as the performance characteristics are known, the components can be selected to provide Type 2 co-ordination and can then be tested as such under short circuit conditions. Soft start manufacturers must test with specific fuse sizes, types, and brands to verify that Type 2 co-ordination has been achieved. This is because IEC fuse standards do not specify maximum let-through currents and let-through energies.

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