Document wxQ7Z25NLnR3D8YDNZ9DxMbE

HOW THERMAL ELEMENTS WORK THERMAIAND MAGNETIC DEVICES -- B A-- -- B A--e 4 Blmetolilc principle is illustrated above. Two metals with unequal * coefficients of expansion, A ond 8, ere welded. When heat is applied the combination strip will bend, operating o circuit-opening mechanism inf & Heof to the bimetallic strip Is generally supplied by e coil connected In series with the load. Time required to operate decreases as lood increases. Strip returns to normal position ofter it has cooled 7b conrocf m#ctKtnifm 4 When bimetal strip bends, trip lotch moves up ond operates mechanism that opera contacts in lead line A Heaters used with bimetallic element can be replaced with one of different rat ing If load changes to confoct mocAon/im 5 Port of current to loodposset through bimetol element. Heot generated by overload current deflects strip C Hooter Is Inserted In manganese " alloy tube. When connected motor draws overcurrent, heot. de veloped within tube causes it to ex pand. A lever mechonlsm trips retoy O Cutowoy view of olloy-pool type 7 Heat developed within coil melts solder allowing ratchet to trip. *8 relay showing hooting grid end alloy pool. Ratchet wheel engoges .-Can reset relay ofter solder coolso spring-held tripping mechanism Protecting a circuit against;# overcurrent consists essentially Iq.Q. providing (i) a device to detect1!' floe of excess curreot and (2) a-L. aeaos of lnterruptlog the circuit. & Methods coooonly used In opening^ the circuit were discussed In tbe`i last fee pages under the heading, 4 "Circuit Breakers." Ve will com take-# up fundaoental principles in detect-!t ing excess current, as veil as# typical overcurrent detectors. |j Tberaal Devices. Plow of curreot through a resistor generates heat, $ the value of vhlcb varies as thew square of the current fleeing.' FCr&/ s aotor, the maxlmuo current that?* can be safely drove, and hence thej^ greatest ootpnt the unit cao de liver, is United by the teaperature rise vithin its winding. The thermaloverload device vas developed to peralt short-time overload, whlle^ still protecting against overload] that vouid injure the Insulation of,{ the notor* a vlndlng. The tberaal' relay consists; essentially of three parts: s lleoi-.f Producing means, a heot-resRonjiusi device, and a contact aecAdntsw.fl Oenerally .speahlog, theraal relays!' any be divided Into two classifica tions according to the type of; heat-responsive device: Hoe tattleend solder Root. Several forms are* illustrated to the panel to the** left. The heat-produolng aeaos Is; generally a heater coil. Thereof devices are connected in series; vlth .tbs load; hence the current; (loving, which produees heat wlthia'i the aotor*s viodings, vill generated a proportional oaount of beat within^ the theraal device. Referring to Pig. i, os see;] Illustrated the fundaaentsl prlocl*. pie in the operation of a binetaillc/ strip. Metals vith a different'] coefficient of expansion are bonded'! together. When heat la applied, escbfl atrip will expand at unequal rates, resulting In a bending action of the; whole. The beat Is supplied to the; bimetallic strip by e heater coil, which surrounds the elenent sod Is] connected in series with the loed. The blaetal theraal relay Is widely'; used today. The eolder-pooi type of therealt protector Is lilostrsted'iR Pig. ?] This relay consists of a containers filled with a specific amount of! solder. As with t*e bimetallic strip, a heater .surrounds tbcj cylinder holding the solder. Aj ratchet arm is connected to radial, vanes in the solder pool. The device la so calibrated that upofi] POWER July 1948* flow.of excess currant sufficient heat is generated within the coll to aelt the solder. The vanes are now`free to tarn, sod sines they are under eprlng teoslon the ratchet am will aovs. This aovssant operates s tripping aeebanisa that opens the circuit. Pig.6 illustrates the expansion- tube thermal relay. The element that responds to teaperature in this relay is a manganese-alloy tube. A heater' is Inserted io the tube supplying sufficient beat on overcurrent to oaase elongation of the tobe. This movement releases a latch aechanlai, which in turn operates the relay oontset. As the movement of the Hoy tube is very soall. a large Bultipllcetlon of lever ratios in the lotch nechanlam must be used. It is essential .in this type relay that all parts be held to close tolerance. The relay shown in Pig.IS ossa the beating offset oreated by a current Induced in a copper tube. As lo previous relays, the coil is connected in series with the load. A copper tube is inserted io the coll end through transformer action, curreot is Induced wlthlo the tube, generating beat. This heat in torn is transferred to a solder-ratchet relay. Since the heat generated depends upon transformer action, this relay la used io alternatingcurrent olrcults only. Thermal relays are moat commonly used to protect motors against overload. They are psrtlcolarly suited for this application because they oot only have time-lag characteristics, but, io addition, thermal relays ere available which act to sum up the heating effect of a series of minor overloads.' To illustrate this point, let os consider the action of the thermaloverload device connected to a aotor which Is subjected to periodic overloads. Each period of overload contributes to the temperatore rise wlthlo the aotor. Hence, it la easy to onderatand bow a aotor coold be damaged if subjected to a sufficient nuaber of these short-time over loads. Since the 'tberaal device operates on ths tsaperatore-rise principle, each period of aotor overload will act to raise the tem perature of the thermal relay. In thin instance, although each shorttime overload did not draw suffic ient curreot to operate the relay, the device will, nevertheless, oper ate before the resoltlng temperature of the aotor winding reaches a dangerous limit. Most thermal relays may be de signed to reset manually or autoDatlcally. Autoaatie reset may be an advantage if the'relay is some distance from the connected motor, end the motor le subjeeted to few overloads. It is good practice to use manually reset thermal relays to reduoe motor overloading by the aaebine operator. A combination starting switch and thermal-overload relay is shown in pig. li.Tbe device is well suited to operate and protect small dotors A theraal device incorporated In the witch consists of a small heater coll connected lo the motor circuit. The coll eurround8 e steel ratchet fastened to a fixed shaft. A solder-pool device operates the ratchet are. Pig. 12 shows overload relay with melting alloy element io place. Overcurrent of euffloient duration, allows ratchet of slsaent to turn opening circuit. Pressing button resets relay after cooling. T&eraontatle Protection.^Another type of thermal protective device designed to protect notors consists of a bimetallic strip, wblcb is near the motor winding. Should the stator winding overheat, the bi metallic element will expand and open the motor circuit. Note in this cane the heat supplied to trip the relay is the actual beat gener ated wlthlo the aotor wiodlog. The device aay.be designed to reset automatically when the temperature of the motor returns to normal, or it may be reset manually. In pro tecting small aotora the tripping mechanism Is connected directly io the motor circuit. Vbeo used with large motors, it operates to open the connected circuit breaker or aotor starter. Such relays are often used to operate an indleatlog device, warning the aaehine operator 1100 1 80 Norm<a> reft V N vmo! Zi 'U \ t X TYPICAL CHARACTERISTICS OF THERMAL ELEMEHTS 9 Tripping point of most (her ns a I devices chonges with tem perature of sur rounding olr. Ele ments with outomotlc compen sator are available to o to EO 90 40 60 Ambient temperature, deg. C f0WER ju)y 1948 1 fl^r,P ond reMt I "tlme Is shewn. Resetting may be automatic or monuol. Automotlc re set often used if motor to control distance Is large <4131 89