contactor generalizations

By Mike O’Dell

10/16/2011

∙ Field Failures & Customer Complaints

∙ Cost and Improvement

∙ Agencies and Standards

∙ Magnetic Circuit

∙ Coil Design and Issues

∙ Contacts

Contactor & Relay Issues

∙ Burned contacts

∙ Burned coils

∙ Open coils

∙ Stripped Screws/terminal threads or loose connection

∙ Noise

∙ Loose/misaligned springs

Customer Complaints and Field Failures – in Order of Commonality

∙ 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)

Burned Contacts

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

Field Failures Causes:

∙ 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

Burned Coils

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


∙ 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

Open Coils

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


∙ Critical systems would require backup or redundant system


∙ Threads not per specification

∙ Screws not started correctly

∙ Excessive tightening torque

∙ Environmental causes (corrosion)

Stripped Screws – Customer Connections

Severity of Failure

∙ Customer connection may not be secure. Potential fire hazard.



∙ 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

Silver Contacts

High Cost Components and Processes in Contractors and Relays

∙ 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

Coils


∙ 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


Contactor / Relay Magnets

• Stamping, assembly and finishing magnet (grinding) is costly.

• Often have copper/aluminum shading rings for AC.


Contactor / Relay Magnets

∙ 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.


Plastic

Brass or copper, stamped, formed, plated, drilled and tapped

Springs

Labor cost to assemble


Contact Carrier

∙ 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

Desire Design Changes to Reduce Cost and Maintain or Increase Reliability

L1 L2

T1 T2

LINE

LOAD

CONTROL

CIRCUIT

Contactor Cutaway

∙ 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.

Alternate Solutions Currently Available-list Advantages

∙ 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

Customer & Design Requirements Which May Limit Design Improvements

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.

Agency Standards

ARMATURE

MAGNET

A SMALL GAP IS LEFT TO BREAK THE MAGNETIC

FIELD AND ALLOW ARMATURE TO DROP AWAY

FREELY AFTER COIL IS DE-ENERGIZED

SHADING COIL

Magnetic Flux in Contactor Magnets

Magnetic Flux in Contactor Magnets

∙ 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

AC Contactor Coils - Terms

∙ 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.

AC Contactor Coils - Issues

∙ 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

Magnet Wire

Coil Design – Magnetic Wire

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.

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.

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

DC Coil Design

∙ “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

Design Methods – DC Coils

∙ 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.

Controlling Arcing

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

AC 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

DC Contacts