Document MGb0BwZz9ewLmNZM0Q35Y35Ex

PROTECTIVE DEVICES EQUIPPED WITH THERMAL OR: magnetic elements to detect overcurrent flow 4 4 Switch toed for storting froctionol- 1 O Reloy having on alloy-pool element. 4 0 Relay may be odjusted to reset ou- I * hp motor. Thermo! element is shown Top terminals in series with line " tomatically or monuofly at shown . * 1 A- Mognali< overload reloy with doth- 1 C Heat to operofe results from current 1C Magnetic trip provided with time lag I *v pot for time log on short overloads * induced In center copper tube by coil * V for surges. Fast trip on short circuit comdined thermal magnetic that the dotor is reaching a dangerous teaperature. Ambient Temperatore. lost theraal devices are affected by chaoses la the eurroaodlog teaperature. This ie Illustrated in Plg.9. It le thus good praotioe to have both notor and protective device subjected to the same ambleot teaperature. there this Is not practicable, the varia tion in surrounding teaperaturs should bs taken into conslderatloa lo selecting the proper tbernal relsj. To Illustrate, If the motor le outdoors and the. thermal relay loelde, both *111 generally be at the sane teaperature lo *ara weather, to vlnter, the relay *111 be subjected to a higher teaperature then the motor. This is not serious since ve *111 obtain adequate dotor protection, even though *e do llait the notor output. If the eotor Is located at a point vhere the aablent teaperature la higher tbeo that surrounding the protective theraal relay, It *111 be necessary to select s heeter coll elth lover rating to get adequate protection. A theraal device Is available to coopensate for variations In the aablent temperature about the relay, hen conditions are such that the relay aablent teaperature changes, a eoapensatiog eleaent built Into this type of relay recalibrates tha tripping eechanlsn. As a result, the theraal eleaent is oot affected by local teaperature conditions, but trips only on notor current. Maintenance. Theraal relays ordinarily require little mnlntenr anee other than occasional inspec tion. Relay tenainal connections should be checked for tightness slnco any heat generated at these points *111 affect the relay's noraal operation. One of the aost frequent causes of false tripping or failure to trip on overload Is poor selection of the theraal heater or.coll. Manufacturer's recoaaendatlona should be followed In selecting theraal relays. The tripping point of an over load protective device say be checked by passim a low-voltage current through it. Por theraal relays used for aotcr protection, the.point of tripping any be checked vitbaut removing the relay froa ItB noraal oouotlog. Throogh the use of a step-down transformer, In which the secondary voltage is but s fe* volts, high current can be obtolned to check theraal relays. Wheo so used, tho secondary of the test transformer Is connected directly to tho theraal device; the value of current any be varied by selecting the proper taps on the secondory of the trnosforaer. A clamp-on a&soeter simplifies current measurement. A comparison should be node between the tripping character istic curve supplied with the relay and tho result obtained through thle test. Magnetic Devices. The earlleet overload device, other than the fuse ae the staple magnetic relay. It consists ofa wire coll connected in series with the elrcult being protected. A metal plunger located within the coll la mechanically connected to some type of circuit Interrupter. When a predetermined value of current flows through tit*; coll, the resulting magnetic action raises the plunger, which in tun opens the circuit. This relay will operate Instan taneously when Its calibrated valst of current passes through the coll.': It Is thus not suited for motor protection for tso reasons: (i) If the relay is calibrated to operate st a point slightly above the noraal full-load rating of the eon-, neoted motor, the relay will trip uoder the current surge on start-; lng. (2) If the relay Is calibrated to carry the heavy starting current; `It will not offer overload protec4 tlon to the motor onee It Is running Time delay was originally lntnr duced In this typo relay by the addition of leather bellows and calibrated air valve. On overload the plunger acted to force trapped air within the leather bellows out through the calibrated air valve.; The length of time delay la coo- trolled by adjustment of tbs valve openlog. The leather bellows Intro-' duced a maintenance problem in that It le necessary to keep the hello** soft with oil. If allowed to dri out they will crack. 5 Time delay is also Introduced by the addition of a dnebpot as above In Pig.14. The attachment prevent* the relay froa operating cn harales* surges and ebort-tlae overloaded When overload curreot flows, the plunger is pulled upward by magnetic, action. Time delay Is Introduced hfl regulating the alee of opeclol through which the doshpot oil must! pass, lo addition, the viscosity of] the oil used will affect the period of time lag. A variation la the] 90 (414) POWER July I940J daahpot arrangement Is shown In Pig. 16. Thermo-Magnetic Devices. The circuit breaker shown in Pig. 19 combines the magnetto and thermal principles, giving accurato inverse- . time overload protection. As In the eurve of Pig. 3D, the resulting graph is a combination of both thermal and. magnetic characteris tic curves. Magnetic action protects the connected motor on starting and quickly disconnects it on large load peaks. Continuous overload or a series of load peaks, which would In effect add up to a dangerous overload on the motor, are cootrolled by the thermal eleaent. Circuit breakers, which use a magnetic elemeot to detect the flow of raise current, shown in Pig. 16. operate Instantaneously on short circuits, and provide time delay for short period overloads. Owing lo the absence of any heating Process, there Is little heat loss ltbin the relay. This Is an Im portant consideration where circuit breakers have to be grouped to- of this type are not effected by changes la ambient teaperature. Doth thermal aod magnetic deree* ore usually rated at 123* of ^l`Aoad eurrent rating of the connected motor, por an alternatlogcorrent equirrel-cage motor, this point would correspond to the moil- h,*^D,abls temperature, that *ouid be produced lo tbs motor. *4 dlrct*currcnt motors the sajmua safe commutating ability, vltion * olndlng temperature, th .*. conaldered lo determining eaiimtto current. POWIK July 1948 1 ft Retarding action of bimetallic, TM which operote* ogoirut pull of mognetle element, prevent* breaker from operating on momentary surges Both elements are contained In the circuit breaker shown below 4 7 Combining feature* of both * A element*. Thermal operate* on overlood* while mognotlc trip hondles short-circuit foultt action atone' trips Croats instantly Conioet trip tstooso <4151 91