Technical Articles
Motors are the backbone of the industry. Their use is also increasing in commercial establishments. Hence, Protection of motor is extremely important so as to keep these processes functioning safely and continuously.
The main purpose of motor protection system is to prevent excessive temperature built up in the windings because of over-current and short-circuit current. Following are the reasons for over-current.
IEC 60947-4-1:2023 require that the thermal overload relay and SCPD are co-ordinated to ensure that they operate satisfactorily under all load and fault conditions. Following aspects need to be considered to achieve proper co-ordination.
Co-ordination means matching the characteristics of SCPD and down stream equipment to ensure that the let-through energy and peak cut-off current do not rise above the levels that the circuit can withstand.
Improper co-ordination can lead to
As per the standard two types of co-ordination are permissible, Type "1 " and "2".
Type "1" co-ordination requires that under short-circuit conditions, the contactor or the starter shall cause no danger to persons or installation. The motor feeder may not be suitable for further service without repair and replacement of parts (Not remaining suitable is NOT a requirement and hence you may find separating in a different sentence could avoid possibility of misconception)
Type "2" co-ordination requires that under short-circuit conditions, the contactor or the starter shall cause no danger to persons or installation and shall be suitable for further use. However contact welding is recognized. Also the time-current characteristics of the over load protection device should not change. This in other words means safety, low down time and continued protection.
Recommended combination needs to be proven through short-circuit tests at
Test at Prospective current "r" is done to verify the performance under fault conditions practically possible at the motor feeder end. These faults are normally associated with the motor and the associated feeder. Prospective current "r" is specified according to the rated operational current ('e, AC-3) of the feeder. If the motor feeder is not specified according to utilization category AC-3, the prospective current "r" shall correspond to the highest rated operational current for any utilization category claimed by the manufacturer. The values are mentioned below.
The values are mentioned below.
| Rated operational current le (A) | Prospective current "r" (kA) |
|---|---|
| le <= 12 | 1 |
| 12 < le <= 50 | 3 |
| 50 < le <= 100 | 5 |
| 100 < le <= 250 | 10 |
| 250 < le <= 500 | 18 |
| 500 < le <= 800 | 30 |
| 500 < le <= 1300 | 42 |
| 1300 < le | Subject to agreement between manufacturer and user. |
Test at Conditional short-circuit current Iq is carried out to verify the performance under system level faults. lq is declared by the manufacturer. This is the maximum fault current that the feeder can withstand. Generally the declared value of Iq is 50 kA.
S-D-F, which incorporates H.R.C fuses, is the most efficient and popular in the industry. S-D-F, like conventional fuse-switch units, is capable of switching and protecting electrical circuits. In addition these are also suitable for isolating down stream equipment. MCCB was primarily used for protection of distribution circuits. However, with the development of current limiting MCCBs, it has become possible to employ MCCBs in motor feeders also. With the availability of various accessories, MCCB as SCPD offers several advantages.
MMS can be used in two ways. It can be used directly for switching of a motor. This is very cost effective. However downside is limited electrical life of MMS compared to that of a contactor. Moreover, a separate undervoltage protection is required. Alternately, MMS can be used along with a contactor. Since, MMS combines thermal as well as short circuit protection, it will trip and interrupt even small overloads (which otherwise could be interrupted by a contactor).
For Star-Delta motor feeders, the motor winding is connected in star. When it reaches a certain speed the motor winding connection is changed to delta. In case of Open transition from star to delta, there is some time difference between opening of the star contactor and closing of the delta contactor. During this period there is no voltage across the motor terminal and the motor will momentarily act as a generator. When the delta contactor closes, full line voltage appears across the motor terminal. If the motor emf and the line voltage add up, the transient current peaks may reach up to 18In. Also the motor will experience a jerk, which in some cases may be critical.
In case of open transition star-delta starting (most common practice), it's an established fact that the transient current peaks during change-over from star to delta are in the order of 18 times the line current (In). As the maximum magnetic threshold of a MMS is 14In and as it is a current peak sensing device, such conditions will definitely lead to nuisance tripping of MMSs during change-over from star to delta mode. Both the above facts i.e. 18 times transient peak and nuisance tripping of MMS have been verified through inhouse tests as well.
Hence, to avoid nuisance tripping, it is technically correct to increase the MMS rating for star/delta starting so that the ratio of instantaneous release setting to the motor full load current is at least 18. However, this will lead to loss in thermal overload protection offered by the MMS (as the MMS rating will be higher than the full load current of the motor). This aspect can be addressed by providing an additional thermal overload relay in the phase circuit.
In case of close transition, the change over from star to delta will take place through three resistors. These resistors do not allow full line voltage to appear across the motor terminal and also there will be no break in the supply to the motor. Hence, there will be no jerk to the motor and transient current peaks will also get eliminated.
Effective motor protection should protect motor and the associated feeder against any overcurrent including short circuit current. More and more users demand Type '2' coordination because it helps to ensure a safe working environment. In view of down times and maintenance costs, though Type '2' co-ordination has higher initial costs, in the long term will prove economical. Manufacturer having all the products in its product portfolio is better place to recommend the combinations for proper Type '2' co-ordination.
Sandeep Pawar,
General Manager - Product ManagementElectrical Engineer with over 20 years of experience in Product Management and Application Engineering for Protection Relays, Power Quality Solutions, and Intelligent Motor Control Centres (MCCs). Possesses strong expertise in LV and MV power system protection schemes, motor protection, monitoring, and control applications. Well-versed in both conventional and intelligent motor management systems, with extensive knowledge of protection, control, and condition monitoring solutions.
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