Solid Statements - SSRs vs EMRs

8/13/2019 Solid Statements - SSRs vs EMRs

1/5

Oct 2010

Solid-State Relays (SSRs) vsElectromechanical Relays(EMRs)

A study on worldwide relays marketfound the market size of solid staterelays to be between $200M and$600M, which represent 17% of totalmarket. However, this percentagecontinues to gradually increase everyyear.What are the reasons? Relays are not asignificant expense in the manufacturingprocess while down time is!

Consequently, higher-cost solid staterelays have an advantage over lower-cost electromechanical relays inindustrial automation applications due tohigher reliability and longer life, butthese are not the only advantages.

Longer life and higher reliability

During initial operation, both types ofrelay show similar levels of reliability.

Over time, however, the solid state relaywill gain the edge due to its mainadvantage: there are no moving parts.SSRs employ semiconductor switchingelements, such as thyristors, triacs,mosfet and transistors to switch the load

current. An EMR generateselectromagnetic force when inputvoltage is applied to the coil, theelectromagnetic force then moves thearmature that switches the contacts insynchronization. This simple principle ofoperation makes it possible tomanufacture EMRs at a reduced cost.The maximum electrical life of an EMRis the maximum permissible number ofswitch operations, at a specified contactload and under specified conditions.Electrical/mechanical life is generally

rated at 100,000 to max 500,000operations. SSR data sheets do notcarry an electrical life specification likeEMRs. Unlike the EMR, where life isdependent on actual switching load andnumber of cycles, SSR reliability ismainly determined by time-in-operation,rather the number of switching cycles.When SSRs are used within thepublished specifications, MTBF can be 2to 4 million hours, depending on themodel. A typical estimate for number of

operations is between 50 million and500 million under normal operatingconditions, but this is very muchapplication dependent.

Fast Switching

Solid state relays, depending upon themodel, can turn on between 20usec and10msec after the input signal is appliedand within 1/2 of an AC cycle after it is

removed (DC control versions).

EMRs, depending on type and ratings,can switch at a maximum of 10 to 20times a second, though the higher theswitching frequency of the application,

Crydom Inc.2320 Paseo de las Americas, Suite 201 San Diego, CA 92154

Tel.: +1 (877) 502 5500 - Fax: +1 (619) 210 1590 - E-

mail: [email protected]


2/5

the lower the MTBF of the EMR. Thismakes an EMR impractical in these

situations. When used in heatingapplications, the fast cycling of a SSRcan dramatically improve the life of theheater by reducing thermal stress.

Low Input Power Required

SSRs allow the switching of large loadswith very low input power. A low levellogic signal (TTL) can activate a relayswitching as much as 125 Amps. Therequired current would normally bearound 15mA. Crydom has a range ofspecial SSRs (e.g. HD/HA panel mountSSRs) that require only 2mA of inputcurrent. These SSRs have an internalcurrent regulator that always draws 2mAwithin the input voltage range.

Quiet Operation

Completely silent switching! This isbeneficial in medical applications,

environmental controls, lighting controlsor other areas where quiet operation isdesirable. The EMR, especially thecontactor, makes an acoustical noiseduring switching. This can create issuesand in a lot of cases disqualifies theEMR to be used in above applications.

No contact arcing and bounce

The bounce time of an EMR is theperiod from the first to the last closing oropening of a relay contact during thechangeover to the other switchingposition. Bouncing causes short-termcontact interruptions and are harmful tocontact life and are particularlyunwanted in applications where relays

are used for pulse counting, wherebounce can easily lead to false pulse

counting as contacts continue to makeand break the circuit. Contact bouncedoes not occur in semiconductor-basedSSRs as there are no contacts tobounce. The diagram below shows therelationship between switching time andcontact bounce time in EMRs andSSRs. In a random control SSR theclosing time is max 100microsec.

Shock and Vibration

Compared to SSRs, EMRs are more

susceptible to physical shock, vibrationand acceleration. Further, theorientation of the electromechanicalrelay relative to the shock or vibrationmust be considered in designs wherephysical movement is expected. Ideally,EMRs must be mounted so that anyshock or vibration is applied at rightangles to the operating direction of thearmature. When an EMRs coil is notenergized, the shock resistivity andnoise immunity are significantly affected

by the mounting direction. Determiningthe orientation of the armature in anEMR package can be a complicatingfactor in designing applications withEMRs. SSRs, by contrast, do not havemoving parts and are not as sensitive to


Tel.: +1 (877) 502 5500 - Fax: +1 (619) 210 1590 - E-



3/5

physical shock and vibration. TestingSSRs has shown functional shock

resistance greater than 10 times thanEMR.

Maximum switching capacity due tocontact erosion

EMR manufacturers specify their relaysin terms of maximum switching capacity.The maximum switching capacity(usually expressed in Volt-Amps orWatts) is provided in the relay datasheet (see chart). It substantially

derates with regard to maximum voltageor current capabilities. In addition, relayusers apply derating beyond therecommendations of the manufacturer inan effort to extend the contact life of therelay. Often this derating places theactual load that can be handled by anEMR within the operating range of anSSR. SSRs do not have contacts so nocontact erosionderating is required.

Other problems concerning contacts in aEMR regard the sensitivity to corrosion,

oxidation, or contaminates.

Phase angle control

This is another feature that permit theSSR to be used instead of EMR, thanks

mainly to the fast switching times.Phase angle control refers to a control

technique that provides a means ofvarying power to a load by altering thepoint in an AC half cycle where loadcurrent is permitted to flow through therelay. Each successive half cycle isvaried in the same manner. Morecurrent can be provided to the load byincreasing the portion of each half cyclethat the relay is in the on-state.Random turn-on relays are used inphase control circuits because the turnon point is determined by the timing of

the control to the relays input (max100usec delay). Turn off point is whenthe current goes through zero. Thiscontrol is obviously not possible with anEMR.

Limitations

On the other side the SSRs have a few

limitations that do not permit them tocover the complete market now coveredby the EMR.

- Cost issueThis is the main drawback against theEMR. The EMR consists, basically, of acoil and a output contact. In a SSR thereare various semiconductor componentssuch as thyristors, opto-isolators,protection devices, etc. In the past therehas been a rather large gap betweenthe price of EMR and the price of SSR.With continuous advancement inmanufacturing technology, this gap hasbeen reduced dramatically, making itinteresting to a wider number ofengineers.


Tel.: +1 (877) 502 5500 - Fax: +1 (619) 210 1590 - E-



4/5

- Thermal dissipationOne of the major considerations when

using a SSR is the generated heat. Atthe on state, solid state relays haveresidual voltage. This voltage,associated with the current that flowthrough the triacs, generates power(heat) that must be dissipated. Thismust be taken into consideration wheninstalling SSRs. The main effectivemethod of removing heat from the SSRpackage consists of an adequately sizedheat sink. In many cases it is sufficientto mount the SSR to the metal panel

housing. This is always dependant ofthe load current, quality of the SSR,panel material, spacing betweencomponents and ambient temperature.

- Leakage currentA solid state relay in the blocked statedoes not have an infinite impedanceacross its terminals. A low residualcurrent circulates in the load called theleakage current. This leakage currentmay be harmful in applications

controlling very low loads (smallsolenoid valves, etc) as this current maybe sufficient to maintain the loadsupplied. Considering that this value isnormally less then 1mA (10mA when asnubber circuit is included) this is rarelya problem for the application.

There are a few other limitations that areby-passed using various kinds ofprotection.

- (dV/dt) This is the voltagechange in relation to time that can ariseduring switching on, switching inductiveloads or mains interference. To avoidthis problem, an internal snubber circuit(RC network) is often used.

- Transient overvoltage. The ACmains voltage contains all kinds of

voltage spikes and transients. Thesetransients may result from other

components like motors, solenoids,switches, transformers, contactors, etc.If overvoltage protection is not provided,the thyristors used in SSRs mightexceed their breakdown voltage. Toprotect the SSR an internal (or external)varistor or transorb can be used.

Conclusions

To summarize, it is important to

understand the advantages andlimitations of the SSR. It then becomesclear as to why, in a technical context,the SSR is almost always preferred tothe EMR.

Advantages:- Long life and reliability- High switching frequency- No contact arcing and bounce- Maximum switching capacity- Vibration and shock resistance- No electromechanical noise

- Electromagnetic noise resistance- Phase angle control mode- High switching speed- Logic compatibility- Low input current

Limitations:- Higher cost- Thermal dissipation- Off-state leakage current


Tel.: +1 (877) 502 5500 - Fax: +1 (619) 210 1590 - E-



5/5

Documents

Solid Statements - SSRs vs EMRs