Electronic protective relays assure motor reliability
By FRED SHIRZADI
AN AC INDUCTION motor i s adependable device that provides long and trouble-free performance; h ow ever, it must be protected from numer ous types of malfunctions that cou ld be caused by outside sources. Such prob lems include thermal overload, single phasing, ground faults, u n derload, overvoltage, stall conditions, and incor rect phase sequence or reverse-p hase operation. Protection against su ch faults will prevent premature motor failures, eliminate expensive down time, reduce required mai nten ance, and generally boost produ ctivity of the process or driven machine.
Why do motors fail? Recently, the Electrical Research Associati on (ERA) in Leatherh ead, England, stud ied in stances of damage to more than 9000 motors in England, Finlan d and the United States. The study indicates the relative importance of the individu al causes of damage. This investigation revealed a typical fault rate of 2.5% per year and demonstrates major causes of failure, as shown in the accompanying table.
Approximately 25% of the motors exam ined were rated 50 hp and over . Repair costs for this group, however ,represent 80% of total repair costs. This emphasizes that larger -size mo tors need good protection, carefu l sys tem design, and proper maintenance of the motor and associated drive. To give an idea of the repair costs involved, it is estimated that in England alone annual costs run to about $40 million.
Note in the table that the thermal overloading (caused by contin uous overloading and also by stalling) is the major contributor to motor failures- 30 % of the total. This overloadi ng usu ally results in uniformly burn t-out windings. Of particular interest is the fact that these faults occurred in spite of th e use of normal prot ection schemes. Note also that an average of 14% of all defective motors failed because of single-phasing. In 19% of the failures, winding or bearing dam age was due to contamination, oil, humidity, etc. And 13% of the motor failures were caused by bearing dam age; 10% were due to aging of the terminals or windings; 5% were due to rotor problems.
What must modern motor protec tion d o? Many motor failures could be prevented by proper preventive action. That is to say, bearing failures are f requ ently caused by excessively tight pulleys, poor alignment, and similar mechanical faults; contamination fail ures could be prevented by better encapsulation and maintenance; and old-age failures could be prevented by timely replacement of worn-out compo nents. However, overload and single phasing, which represent 44 % of all failures in the study, must be taken care of by properly selected motor pro-· tection devices and systems.
A modern m otor protection system must provide advance warning of a fault to an operator (time permitting) or disconnect the load before damage occurs. In some cases, however , discon nection before fault occurrence may be impossible. Then the protection system must detect the fault condition as quickly as possible and minimize any damage during fault operation of the system .
In addition to providing maximum equipment protection, a well-d esigned system must m aximize the up-time of the overall system. In other words, ideal protection trips the motor only if there is an impending hazard and does not cause unw anted .(and often very expen sive) downtim e. Ideal protection can be furth er defined as such that results in optimum operation and con trol of the installation. A n example of this is a motor-start prevention system that blocks the starting attempt of a heavy-d uty motor until there is suffi cient therm al reserve in the motor to permit a start without overheating. A f r u itless (an d possibly damaging) starting attempt thus might be avoided.
Another example is a stalled-motor protection system which, with a serious fault in the drive, does not wait until the thermal protection system re sponds but disconnects immediately, thus avoiding an unnecessary motor temperature rise (and the associated long cooling period prior to a safe re start of the motor). This essentially means that, for optimum effectiveness, a motor-protection system must be ful ly integrated, combining all required protection functions in an integrated package working toward a common goal-to maintain proper motor opera tion effectively, not just merely to pre vent damage to a motor.
How,much heat can a motor take?
Since thermal overloading is among the leading causes of motor failures, it is well to examine just how hot a motor can get before damage occurs. As the ERA study showed, the most thermal critical parts of a motor are the stator windings. Depending on the class of insulation, the insulation material is selected for certain continuous operat ing temperatures and is a deciding fac tor in the life of a motor. In case of class B insulation, the average winding temperature is 120°C. Under fault con ditions, disconnection must take place as soon as the winding temperature reaches 165oC maximum. For short periods of time, however (as, for instance, during a heavy-duty start), higher temperatures can be permitted without any significant effect upon the motor life.
Motor specifications listed on the nameplate are always referred to at an ambient temperature of 40″C. Unless the manufacturer indicates otherwise, this means that the motor will deliver its full rated output at this tempera ture without overheating. At lower ambient temperatures the motor can handle greater loads; at higher am bient temperatures its loading must be reduced.
The operating temperature of the motor windings determines its insula tion and life. Assuming continuous operation of a motor at its rated tem perature of, say, 120″C and Class B insulation, a motor would have a life of 10 years and the aging process would be linear. If however, the same motor were subjected to 130″C operating tem peratures, its life would be cut in half, i.e., to 5 years. With the operating temperature of 140″C, its life would be cut in half again; i.e., down to 2.5 years, and so on. (See accompanying thermal curve.)
The above shows the importance of correctly setting the thermal protec tion system to keep the motor tempera ture as low as possible and thus get the most life out of the motor. Simple bimetal relays may permit a motor to run at 140 or 15o·c, greatly reducing its life.
Which thermal protection is best? Essentially, there are two basic thP.r mal-protection techniques -one relies on some means of measuring the wind ing temperature directly, and the other evaluates the motor winding tempera ture on the basis of current drawn by the motor.
On the surface, the direct winding temperature measurement method seems like the preferred way. Indeed, what could appear more reliable than a direct measurement? By placing a suit able temperature sensor within a wind ing (either solid-state or electrome chanical), one can obtain a continuous temperature reading. Using suitable circuitry, then, it is a simple matter to develop a protection system that would be fast, simple, and reliable.
However, the most serious objection to direct-measurement devices is the fact that they can only measure the winding temperature locally. The sen sor always measures the temperature at the point where it is located and is incapable of detecting temperature dif ferences in other parts of the motor. Also, it is incapable of detecting an overload condition that could result in a motor burnout. Some other common motor faults that such direct-measur ing sensors cannot guard against include rapidly occurring single-phas ing, a short-circuit, and ground leak age. And it is only under certain condi tions that a false restart attempt can be prevented.
In addition to these operational shortcomings of direct temperature sensing, there are several practical objections as well. For instance, the cost of fitting thermistors and the associated tripping device can be high. Also, a request for in-winding sensors tends to increase motor delivery time.
Another consideration is the ability to perform a functional check. Trip circuits, especially those associated with thermistors, are arranged so that protection actuates a trip whenever the lines to the thermistors are inter rupted. Checking the response temper ature and the quality of the thermal coupling at the winding, however, is virtually impossible . Yet, a thermistor installed in a motor windi ng is often subjected to severe mec h a nical stresses, and removal .of a damaged thermistor from motor windings is not a straightforward matter.
A motor’s operati ng conditions are q uite accurately described by the amount of current it draws. With cor rect interpretation, the input current can serve as a mea suring stick for determining wh en to sh ut off a motor. Indeed, the current drawn by a motor at any given time uniquely defines the amount of electrical power that the motor is con sum ing. This, of course, directly relates to the temperature rise in the motor wi ndings. In addition, the electric current is quite easy to mea sure in comparison with several other physical variables that could also be used to monitor motor performance.
Therefore, it appears that a protec tion system that judges the motor con dition strictly on the basis of motor current and is entirely external to the motor provides optimu m protection as well as functional flexibility .
Det ermining the shutdown criteria.During motor startup and under high 0verloads, the major power losses occur in the windings (stator and rotor), where the thermal capacities deter mine the permissibl e load and the per missible starting time. By adjusting the tripping time of the OL protective relay, its characteristics can be tailored to the individual motor. A suitable starting point for perfor ming this adjustment is the permissible stalling time of the cold motor in conjunction with the corresponding current.
Typical time/current characteristics used to adjust tripping time in a mod ern electronic motor protection t::Jit are shown by curves in the accompa ny ing sketch. The caption shows how these curves are used.
This procedure will also result in adequate protection even under the following applications and conditions:
1. Heavy starting current (fans, centrif uges, elevators, etc.)
2. Explosion-proof motors
3. Submerged motors
4. Hermetically sealed refrigeration compressors.
The adjustment of the tripping time permits the thermal capacity of the motor to be used in the most cost effective manner while reducing the possibility of nuisance trips. If a plant with motors running at their thermal limits is shut down, the motors must be given sufficient time to cool off before restarting them. An attempt to restart a hot motor will fail, since the protec tive system will trip and disconnect the motor.
Other motor protection functions. A modern electronic motor protection system can sense and display numerous types of motor faults by monitoring the motor current. Fault displays make troubleshooting quick and easy by pin pointing the nature of each fault.
To begin with, consider a fault asso ciated with phase loss (popularly known as “single-phasing”). Even though the danger to large motors in this case is well known, the possibility of its occurrence is quite real-a blown fuse on one phase win result in single phasing. This condition, however, can be easily detected with an electronic motor protection system independent of motor loading by simply monitoring each phase. Thus, a motor can be shut down as soon as single-phasing is spot ted.
Next there is ground-fault detection.
Most insulation faults in motors result in a leakage to the grounded parts of a motor. As a rule, it is practically impossible to prevent the occurrence of these f aults. However, it is possible to detect them quickly and limit the dam age to a minimum.
In the insulated-neutral system (with high -impedance grounding) or the resonant-grounded system (with ground-fau lt n eutralizers), only rela tively small leakage currents occur, wh ich usually allow the motor to remain in operation for a short time. While central neutral-point monitoring reports the occurren ce of a ground fault in general, the motor branch cir cuit involved can be indicated separate ly, permitting required repairs.
In th e grounded -neutral system, ground-fault currents can build up rap idly to short-circuit magnitude, in which case the motor must be shut down quickly to limit the damage. This is achieved by an electronic protection system.
With the large, heavy-duty motors used in industrial plants, occasional mechanical jamming or stalling leads to extremely heavy currents. Under these conditions, it is best to disconnect the motor as soon as possible. This eliminates unnecessary mechanical and thermal loads on the motor and dam age to the associated power-transmis sion train.
To prevent nuisance trips, however, the system must differentiate between a current surge caused by a startup and a surge due to mechanical jamming or stalling while in operation. This is accomplished by a logic circuit that enables protection only after the motor has been started and is running. This protection usually disconnects the mo tor within a fraction of a second after jamming occurs.
Another type of fault is an underload condition, when the motor current falls below some minimum value. This might very well indicate a problem in the overall system-a broken gear in the power train (effectively removing the mechanical loading from the mo tor), or clogged filters on fans. In the event of such a fault, the underload protection responds after a suitable time lag. The trip current or signal for the underload protection can be ad justed over a relatively wide range to accommodate a variety of motors and conditions.
Finally, there is always the possibili ty of a motor running in the reverse direction ( with th e wrong phase sequence). This can result in signifi cant damage to the motor and possibly to the associated power-transmission train. Furthermore, improper phase sequencing may also cause accidents, injuring plant operators.
As in single-phasing, this condition can be sensed independent of motor loading; the protective system simply monitors the phase sequence and responds if it is wrong.