Relay Termod para sa Proteksyon sa Overload sa Motor: Mga Prinsipyo, Pagpili, ug Aplikasyon
Sa mga sistema sa kontrol sa motor, ang mga fuse gisagol sa pagprotekta sa short-circuit. Pero dili sila makaprotect kon ang sobrang init mao ang resulta sa mahimong matagamtam nga overloading, paborito nga forward-reverse operasyon, o undervoltage operasyon. Karon, ang mga relay termod gilarga gamiton para sa proteksyon sa overload sa motor. Ang relay termod usa ka device sa proteksyon nga nag-operate batas sa thermal effect sa electric current, ug iya usab usa ka klase sa current relay. Ito nag-operate sa paggihimo og init sa pag-flow sa current sa iyang heating element, nagresulta sa deformation sa bimetallic strip (gibuo sa duha ka metals nga may lain-lain nga expansion coefficients). Kon ang deformation nakaabot sa isang threshold, ito mag-actuate sa linkage mechanism, nagbubukas sa control circuit. Ini nagde-energize sa contactor ug nagdisconnect sa main circuit, sukad sa protektahan ang motor gikan sa overload.
Ang mga relay termod giclassify batas sa numero sa mga heating elements: two-pole ug three-pole types. Ang three-pole relays masudlan pa sa mga models nga may phase-loss protection ug wala. Common series include JR0, JR9, JR14, ug JR16. Ang time-current characteristic (ampere-second characteristic) sa mga relay termod typical nga nagpakita og inverse-time behavior nga match sa motor’s allowable overload curve: ang mas dako ang overload current, ang mas maong ang tripping time; conversely, ang mas gamay ang overload current, ang mas dugay ang tripping time. Sa proper selection, ang relay makapag-trip sa dili pa ang motor mabaton sa iyang thermal limit, sukad sa fully utilize ang motor’s overload capacity while preventing damage.
Konsekwensya sa ilang small size, simple structure, ug low cost, ang mga relay termod gilarga gamiton sa industrial applications para sa proteksyon sa motor.
I. Proteksyon sa Motors Pinaagi sa Relay Termod
Ang stator winding connection type sa motor nagdetermina sa overload ug phase-loss current characteristics, kung saan nagdictate sa appropriate type sa relay termod.
Star (Y) Connected Stator Windings
Sa star connection, ang line current equal sa phase current. Sa panahon sa motor overload, ang tanang three phase currents typical nga mogasto. Kon ang three-phase supply voltage balanced ug ang motor currents symmetrical, ang two-pole relay termod makaepektibo protektahan ang three-phase motor. Pero kon ang three-phase voltage severely unbalanced (e.g., 4% voltage imbalance can cause up to 25% current imbalance), o kon single-phase short circuit naay fault current dili pass through sa heating element, ang two-pole relay dili makaprotekta. Sa kasagaran, ang three-pole relay termod dapat gamiton.
Delta (Δ) Connected Stator Windings
Sa normal operation, ang line current (I) = 0.58 × phase current (Iφ), ug phase current Iφ = 0.58 × line current I. Kon usa ka supply phase nawala (e.g., usa ka fuse blow), sama sa Figure 1 (with phase B open), tungod sa equal winding impedance, Ic = Ia + Ib = 1.5Iφ, ug Ib = (2/3)Ic. Ini nagpakita nga ang line current dili accurate reflect sa phase current, sukad sa using line current for protection fails to detect true winding overload.
Kon phase loss occur under full load, Ia = 0.58Ie, Ib = 1.16Ie—ini overcurrent sufficient for a standard three-pole relay termod to trip. Pero kon 64% of rated load with phase loss, Ia = 0.37Ie, Ib = 0.75Ie. The overcurrent due to phase loss less than 20%, so a standard three-pole relay may not trip, yet one phase carries 58% more than its normal current, risking motor burnout. Therefore, for delta-connected motors, standard three-pole relay termod cannot provide effective protection; phase-loss protective relays must be used.
Kon usa ka stator winding breaks (e.g., loose connection between winding lead and terminal, such as open between A and B, as shown in Figure 2), then Ia = Ic = Iφ, and Ib = Iφ. Here, one line current equals phase current, just as in normal operation. In this case, a phase-loss protective relay can still provide protection, whereas phase-loss protection devices that rely on detecting supply-side phase loss will fail to operate.
II. Pagpili sa Relay Termod
Ang choosing and using ng mga relay termod usa ka well-known topic, pero ang motor burnout accidents due to improper selection and use still occur frequently. Therefore, beginners should note the following points in addition to following standard guidelines:
Understand the model, specifications, and characteristics of the motor to be protected.
Type Selection: In rural areas with frequent three-phase voltage imbalance, use standard three-pole thermal relays for star-connected motors, and phase-loss protective relays for delta-connected motors.
Current Rating Selection: Select the thermal relay’s rated current based on the motor’s rated current, then choose the heating element’s rated current. The adjustable range of the heating element’s setting current can be found in manufacturer tables. If the motor starting current is about 6 times the rated current and starting time is under 5 seconds, set the heating element’s current to equal the motor’s rated current. For motors with longer starting times, impact loads, or where shutdown is not allowed, set the current to 1.1–1.15 times the motor’s rated current.
Example: A motor has a rated current of 30.3 A, starting current 6 times rated, short starting time, and no impact load. Suitable models include JR0-40, JR0-60, or JR16-60. Using JR16-60: relay rated current is 60 A, three-pole type. Select a 32 A heating element, adjustable to around 30.3 A.
Connection Wire Selection: Using wires that are too thick or too thin affects heat dissipation and thus the thermal relay’s performance. Wire size should follow manufacturer instructions or electrical handbooks.
Motors with poor overload capacity or poor cooling: Set the thermal relay’s rated current to 60%–80% of the motor’s rated current.
Reset Mode: Thermal relays typically offer both manual and automatic reset modes, switchable via an adjustment screw. Manufacturers usually ship them in automatic reset mode. The choice depends on the control circuit. As a rule, even if the relay resets automatically, the protected motor should not restart automatically—otherwise, set the relay to manual reset to prevent repeated starts under fault conditions and equipment damage. For example, in manual start/stop circuits using push buttons, automatic reset is acceptable; in automatic start circuits, use manual reset.
III. Precautions During Use
To extend the service life of thermal relays and ensure optimal performance, observe the following:
Use connection wires at the relay terminals with cross-sections strictly according to specifications.
Thermal relays cannot provide short-circuit protection—fuses must be installed separately. They are unsuitable for motors with very long starting times, frequent operation, or intermittent duty cycles.
When installed with other devices, mount the thermal relay below them to avoid heat interference. Clean dust and dirt regularly.
After tripping, automatic reset occurs within 5 seconds; manual reset requires waiting 2 minutes before pressing the reset button.
After a short-circuit fault, check the heating element for damage and the bimetallic strip for deformation (never bend the bimetallic strip), but do not remove components.
When replacing a thermal relay, ensure the new one matches the original specifications.
Conclusion
Only by properly selecting, correctly wiring, and appropriately using thermal relays can effective overload protection for motors be achieved.