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
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
20. 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
21. 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.
22. 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
23. 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.
24. 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.
25. 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
26. 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
27. 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.
28. 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
29. 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
