Contactor generalizations
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This document discusses issues with contactors and relays in the field, including common customer complaints and causes of failures. It also covers the high costs associated with components and manufacturing processes. The desire for design changes to reduce costs while maintaining reliability is expressed. Standards and customer requirements that may limit design improvements are outlined. Finally, the document provides details on magnetic circuits, coil design considerations, and terms related to AC contactor coils.
Contactor generalizations
- 1. Contactor & Relay Issues ∙ Field Failures & Customer Complaints ∙ Cost and Improvement ∙ Agencies and Standards ∙ Magnetic Circuit ∙ Coil Design and Issues ∙ Contacts By Mike O’Dell 10/16/2011
- 2. Customer Complaints and Field Failures – in Order of Commonality ∙ Burned contacts ∙ Burned coils ∙ Open coils ∙ Stripped Screws/terminal threads or loose connection ∙ Noise ∙ Loose/misaligned springs
- 3. Field Failures Causes: Burned Contacts ∙ Mismatch between device current/voltage ratings and load ∙ Low voltage at the coil causing chattering and excessive arcing of contacts ∙ Bad riveting of contacts to carrier ∙ Environmental causes (temperature, humidity, minerals) Severity of Failure ∙ Arcing may propagate and create further damage and fire ∙ Costly to replace in field installed equipment ∙ Critical systems would require backup or redundant system
- 4. Field Failures Causes: Burned Coils ∙ Low voltage to the coil – below contactor pickup volts ∙ High voltage to the coil – exceeding coil rating ∙ Many nicked wire turns – insulation removed causing shorts ∙ Environmental causes Severity of Failure ∙ Burned coils release toxic gases and may potentially contribute to a fire ∙ Costly to replace in field installed equipment ∙ Critical systems would require backup or redundant system
- 5. Field Failures Causes: Open Coils ∙ Reliability issue with magnet wire termination – bad solder or crimp joint ∙ Voltage surge causing terminations to break ∙ Coil opens at another location such as outer winding due to previous damage ∙ Environmental causes Severity of Failure ∙ Failure to operate may be intermittent depending on nature of break ∙ Open coils are not normally a safety problem for the equipment ∙ Costly to replace in field installed equipment ∙ Critical systems would require backup or redundant system
- 6. Field Failures Causes: Stripped Screws – Customer Connections ∙ Threads not per specification ∙ Screws not started correctly ∙ Excessive tightening torque ∙ Environmental causes (corrosion) Severity of Failure ∙ Customer connection may not be secure. Potential fire hazard. ∙ Costly to replace in field installed equipment
- 7. High Cost Components and Processes in Contractors and Relays Silver Contacts ∙ Expensive silver alloys formulated and sized for required electrical life. ∙ Silver Oxide has low resistance ∙ Good mechanical strength and wear resistance ∙ Can be brazed or riveted to contact carrier ∙ Arc quenching parts/materials are added when needed
- 8. High Cost Components and Processes in Contractors and Relays Coils ∙ Mass of copper magnet wire – specific gages for different voltages. ∙ Stamped and formed terminals to meet customer connection requirements. Brass terminals typically tin-plated for soldering. ∙ Termination process costly – special tooling for strip/crimp or materials/labor for solder termination. ∙ Bobbin molds, material and time to mold add cost. ∙ Overmolds, housings, varnish, sealants
- 9. High Cost Components and Processes in Contractors and Relays Contactor / Relay Magnets ∙ Special alloy core-plated steel selected for specific application. Few steel mills produce this type of steel. Typically soft iron material that is magnetized when magnetic field created. ∙ Magnets used with AC coils are comprised of thin laminated sheets which limit the eddy current and heat. ∙ DC coil may use solid steel due to one way current – no eddy current
- 10. High Cost Components and Processes in Contractors and Relays Contactor / Relay Magnets • Stamping, assembly and finishing magnet (grinding) is costly. • Often have copper/aluminum shading rings for AC.
- 11. High Cost Components and Processes in Contractors and Relays Plastic ∙ Special insulative and track/arc resistive molding compound for contact boards and cross-arms. ∙ Molds and material are expensive and time to mold is costly.
- 12. High Cost Components and Processes in Contractors and Relays Contact Carrier Brass or copper, stamped, formed, plated, drilled and tapped Springs Labor cost to assemble
- 13. Desire Design Changes to Reduce Cost and Maintain or Increase Reliability ∙ Reduce or eliminate silver contacts – eliminate Cd ∙ Reduce coil size or eliminate coil completely ∙ Eliminate or reduce coil inrush current ∙ Reduce, reconfigure or eliminate the magnet/armature assembly ∙ Reduce housing size ∙ Less mechanical motion – fewer/smaller springs ∙ Survive non-standard voltages to coil ∙ Maintain or improve resistance to environmental issues
- 14. Contactor Cutaway LINE L1 L2 CONTROL CIRCUIT T1 T2 LOAD
- 15. Alternate Solutions Currently Available-list Advantages ∙ Solid State (MOSFET) and Semiconductor (Thyristor) Relays and Contactors – issues with heat, leakage and transients ∙ Latching relay – reduces coil size/no hum. Used often for lighting control.
- 16. Customer & Design Requirements Which May Limit Design Improvements ∙ Grandfathered materials/ratings in UL 508, IEC 60947 or other relevant standard such as ARI 780/790 ∙ Compatibility of field replacement with existing products ∙ Coil (control voltage) ratings ∙ Customer/end user mounting requirements ∙ Number of poles and contact arrangement required in applications. ∙ Auxiliary contact requirements ∙ Labeling ∙ RoHS and REACH ∙ Specific Engineering Test Requirements not covered in the Standards
- 17. Agency Standards UL 508 – Standard of Safety for Industrial Control Equipment ∙ Intended for control and accessory devices for starting, stopping, regulating, controlling or protecting electric motors. ∙ Requirements for construction, electrical clearances, insulation, grounding, marking, wiring. ∙ Overload, endurance, dielectric withstand, short circuit, over &under voltage and temperature are important tests. IEC 60947 (part 4 for contactors)-similar to UL508 with exception of IEC ratings ARI 780/790-97 – inactive standard intended for Definite Purpose contactors (used in air conditioning equipment). OEM’s request testing to the requirements of this standard – the electrical and mechanical life test minimums and temperature rise requirements are more stringent than UL 508. Other standards as applicable to specific product.
- 18. Magnetic Flux in Contactor Magnets A SMALL GAP IS LEFT TO BREAK THE MAGNETIC FIELD AND ALLOW ARMATURE TO DROP AWAY FREELY AFTER COIL IS DE-ENERGIZED ARMATURE SHADING COIL MAGNET
- 19. Magnetic Flux in Contactor Magnets
- 20. Magnetic Flux in Contactor Magnets
- 21. Magnetic Flux in Contactor Magnets
- 22. Magnetic Flux in Contactor Magnets
- 23. Magnetic Flux in Contactor Magnets
- 24. Magnetic Flux in Contactor Magnets
- 25. AC Contactor Coils - Terms ∙ Inrush current – current during the first few cycles of coil energization – before the armature closes onto the magnet. Inrush can be simulated by holding the armature in position. ∙ Sealed current – Current when armature is pulled in completely at coil rated voltage. ∙ Inrush current is much large than sealed current. Magnetic circuit when sealed increases impedance in coil circuit thereby reducing current. ∙ Pickup voltage – minimum control voltage which will cause the armature to start to move ∙ Seal in voltage – minimum control voltage required to cause the armature to seat against the pole faces of the magnet ∙ Drop out voltage – exists when the voltage is reduced to allow the contactor to open
- 26. AC Contactor Coils - Issues ∙ Low voltage – produces low currents and low magnetic pull. When the voltage is greater than the pick up voltage and less than the seal voltage the contactor may pick up but will not seal. As the coil is not designed to carry the greater current continuously, it will get hot and will either be damaged or burn out. The armature will chatter – creating noise and wearing magnetic pole faces. ∙ High voltage – Drawing higher than rated current will cause damage and possible failure. The excessive force of the armature closing will wear the pole faces prematurely. ∙ AC Hum – due to changing magnetic field, inducing mechanical vibration. Excessive noise can be caused by: broken shading coil, low voltage to coil, wrong coil, misalignment between magnet and armature.
- 27. Coil Design – Magnetic Wire Magnet Wire ∙ Copper & Aluminum wire ∙ Round, Rectangular or Square X-Section ∙ Ratings from 105C to 220C ∙ Many insulation materials ranging from Polyvinyl (105) to Aromatic Polyamide (220) ∙ Insulation thickness can vary ∙ Wire gages from 4/0 to 46 AWG
- 28. Coil Design Windings ∙ Precision wound – turns are laid side by side & wire traverses from one end of the bobbin to the other and back again during winding. Reduces voltage gradient with the coils and prevent accidental shorts. Winding thickness consistent ∙ Random wound – Wire traverses back and forth across the bobbin but consecutive turns are not always adjacent. Winding thickness varies. Packs more copper in smaller space due to tighter nesting. Less costly because of higher winding speed.
- 29. Coil Design ∙ Coils can be encapsulated, over molded, varnished ∙ AC coils must withstand inrush current until the contactor closes ∙ Volts/turn useful parameter to help select wire gage and number of turns based on limits of bobbin design ∙ I²R heating, surface area and wire insulation affect wire size selection ∙ Various coil voltages are used in the same coil/contactor family ∙ UL listed insulation systems often required for contactors in N.A.
- 30. DC Coil Design ∙ Very common today at low voltage <12 volts. Design become costly at higher voltages. ∙ Depends on magnet/armature design – ideally a DC coil will have a solid magnet/armature (not always the case due to product extensions). ∙ DC coil needs high resistance due to lack of inductance in magnet circuit – thinner magnet wire than equivalent AC coil. ∙ Higher numbers of amp-turns are needed than in AC coils due to lower current. ∙ Diodes often used to reduce DC spike during de-energization of coil
- 31. Design Methods – DC Coils ∙ “True” DC coil ∙ One large (tall) single winding to absorb/dissipate heat. ∙ Two winding DC coil ∙ One “pickup” winding to absorb inrush current. ∙ One “hold” winding capable of lower power after contactor closes. ∙ Late break auxiliary contact removes pickup winding from circuit during contact closure. ∙ Electronic DC coil ∙ Primarily 24VDC product
- 32. Controlling Arcing ∙ Arcing phenomena varies with electrode/contact material and contamination ∙ To breakdown a large air gap a minimum of 320V is needed ∙ Very small gaps will generate an arc with an intense electric field ∙ Minimum voltage to sustain an arc in air with small gaps is around 12 volts for most contact materials (less for Gold) Above the minimum arc voltage: ∙ Properly designed and operating device some arcing when contacts come together and more arcing at contact separation ∙ DC arc can be sustained at gaps roughly proportional to voltage – 10 to 20 V/cm.
- 33. AC Contacts AC arc suppression ∙ Arcing occurs at greater than 12VAC and is greatest when opening contacts ∙ AC may have several sets of contacts to make/break all legs ∙ Self extinguishing due to current crossing zero. ∙ Anode/Cathode side is random – movable and stationary contacts erode at equal rates ∙ Higher currents and voltage require additional means to quench the arc after the first half cycle
- 34. DC Contacts DC arc suppression ∙ DC requires only one set of contacts per device ∙ Rapid opening of contacts with enough air gap is necessary to break arc ∙ Arc splitters commonly used with low voltage contactors ∙ Rapid closing may cause contact bounce and accelerated erosion ∙ Current flows in one direction and one contact will be anode and the other will be cathode