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Solid State Motor Starters / Contactors 5
- Thread starter Bill99
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Not sure what your calling a solid state starter, is this like a reduced voltage starter or something? In general, solid state devices can not handle as high of voltages, currents seen by the starter on start-up but more importantly on disconnecting the motor load.
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Yes, there are reduced voltage starters (soft start) and then there are solid state on/off motor starters. See: as an example of on/off solid state starters.
We upgraded a customer to the on/off solid state starters since the electronic noise from mechanicals and soft starters was causing an ongoing problem.
If you size the solid state starter properly, and use a SS starter that has high voltage surge survivability; then they are effective.
You are correct that the voltage surge from disconnecting a motor load can be damaging. That is why we use the high surge models of starters.
We upgraded a customer to the on/off solid state starters since the electronic noise from mechanicals and soft starters was causing an ongoing problem.
If you size the solid state starter properly, and use a SS starter that has high voltage surge survivability; then they are effective.
You are correct that the voltage surge from disconnecting a motor load can be damaging. That is why we use the high surge models of starters.
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The big problem is really just cost. Any solid state power controller, soft start or not, will use SCRs as the switching devices. SCRs are not perfect conductors, so they reject about 1.5watts per running load amp per phase into the surrounding enclosure. If it is vented and/or on small loads this may not be a problem, but small electro-mechanical controls are far cheaper than solid state.
Then if you look at, for instance, a 75HP 460V motor pulling 100A FL, it will be rejecting 450W of heat continuously. That will require approx. 200cfm of air exchange to keep from overheating and that will introduce a lot of dust and contaminants, leading to eventual failure. If you seal up the box, i.e. NEMA 12, the starter will cook itself in short order. I once calculated the enclosure size necessary to dissipate the heat of a 75HP 460V solid state starter via skin effect, and it took so much surface area that the box became 90"H x 80"W x 16"D, and that was only if the surrounding air was 70F! So you have to use a Bypass Contactor to shunt power around the SCRs after they are gated fully on, and that adds cost.
There are plety of reasons why more people SHOULD use solid state, but this is the reason they don't; basically people are cheap when it comes to control devices. I have seen someone spend $250,000 for a machine, then try to start it with some piece of used junk across-the-line starter they bought at a surplus sale!
"Venditori de oleum-vipera non vigere excordis populi"
Then if you look at, for instance, a 75HP 460V motor pulling 100A FL, it will be rejecting 450W of heat continuously. That will require approx. 200cfm of air exchange to keep from overheating and that will introduce a lot of dust and contaminants, leading to eventual failure. If you seal up the box, i.e. NEMA 12, the starter will cook itself in short order. I once calculated the enclosure size necessary to dissipate the heat of a 75HP 460V solid state starter via skin effect, and it took so much surface area that the box became 90"H x 80"W x 16"D, and that was only if the surrounding air was 70F! So you have to use a Bypass Contactor to shunt power around the SCRs after they are gated fully on, and that adds cost.
There are plety of reasons why more people SHOULD use solid state, but this is the reason they don't; basically people are cheap when it comes to control devices. I have seen someone spend $250,000 for a machine, then try to start it with some piece of used junk across-the-line starter they bought at a surplus sale!
"Venditori de oleum-vipera non vigere excordis populi"
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Lakey,
No device on this planet is "100% fail safe"!
There are only 2 methods of switching an AC motor on and off, an electro-mechanical contactor or an SCR controller (lets leave out VFD transistors for this discussion since they are technically a DC switch and much more complicated to use). I would argue that SCRs are FAR MORE RELAIBLE than contactors.
Is a contactor fail safe? Let's review failure modes that could result in motor damage since all other failures (such as control circuit loss) are the same for both and technically "safe":
SCR controller: Shorted SCR? Keep in mind that 1 shorted SCR is not inherently unsafe to the motor, you need at least 2 shorted SCRs AND in separate phases in order to have a power path that could cause damage.
Contactor:
Welded contacts? If one contact welds, it holds the plunger in, allowing a complete path to all phases.
Swollen coil constricting an armature plunger and holding it in the closed position? Again, a runaway motor results.
A piece of foreign material falls into a contactor mechanism and jams it closed (such as drill tailings, a piece of control wire insulation residue from stipping the end, or the tail of a tie-wrap)? Same result as above.
Flash-over from ionized air during a high voltage spike? Although usually just a temporary spike to the motor, I mention this because it can cause winding damage, so air-gap contactors are still not "100%" fail safe.
I have seen all of these contactor failures far far more often than I have seen a solid state controller with 1 failed SCR, and I have seen even fewer multiple SCR failures. The same conditions that cause SCRs to short can also cause contacts to weld or flash over, and then you can elimiate all of the other mechanical issues mentioned above because SCR controls have not moving parts. And in every case of a failed SCR I have seen there was a simple backup plan of having shorted SCR detection that prevented subsequent operation and annunciated the failure for operators. Even if a contactor is used to bypass the SCRs, most shorted SCR detection circuits would pick up a stuck contactor as a similar condition and warn the operator. Add to that the feature found in most solid state controllers of being able to "shunt trip" an upstream circuit breaker in the unlikely event of multiple SCRs shorting, and you end up with a far more safe system. I have NEVER seen that kind feature on a "more reliable" contactor.
"Venditori de oleum-vipera non vigere excordis populi"
No device on this planet is "100% fail safe"!
There are only 2 methods of switching an AC motor on and off, an electro-mechanical contactor or an SCR controller (lets leave out VFD transistors for this discussion since they are technically a DC switch and much more complicated to use). I would argue that SCRs are FAR MORE RELAIBLE than contactors.
Is a contactor fail safe? Let's review failure modes that could result in motor damage since all other failures (such as control circuit loss) are the same for both and technically "safe":
SCR controller: Shorted SCR? Keep in mind that 1 shorted SCR is not inherently unsafe to the motor, you need at least 2 shorted SCRs AND in separate phases in order to have a power path that could cause damage.
Contactor:
Welded contacts? If one contact welds, it holds the plunger in, allowing a complete path to all phases.
Swollen coil constricting an armature plunger and holding it in the closed position? Again, a runaway motor results.
A piece of foreign material falls into a contactor mechanism and jams it closed (such as drill tailings, a piece of control wire insulation residue from stipping the end, or the tail of a tie-wrap)? Same result as above.
Flash-over from ionized air during a high voltage spike? Although usually just a temporary spike to the motor, I mention this because it can cause winding damage, so air-gap contactors are still not "100%" fail safe.
I have seen all of these contactor failures far far more often than I have seen a solid state controller with 1 failed SCR, and I have seen even fewer multiple SCR failures. The same conditions that cause SCRs to short can also cause contacts to weld or flash over, and then you can elimiate all of the other mechanical issues mentioned above because SCR controls have not moving parts. And in every case of a failed SCR I have seen there was a simple backup plan of having shorted SCR detection that prevented subsequent operation and annunciated the failure for operators. Even if a contactor is used to bypass the SCRs, most shorted SCR detection circuits would pick up a stuck contactor as a similar condition and warn the operator. Add to that the feature found in most solid state controllers of being able to "shunt trip" an upstream circuit breaker in the unlikely event of multiple SCRs shorting, and you end up with a far more safe system. I have NEVER seen that kind feature on a "more reliable" contactor.
"Venditori de oleum-vipera non vigere excordis populi"
Hello Bill99,
I would suppoprt everything jraef has said here.
The simple answer is that Solid State Controllers (be they solid state contactors or soft starters) are more expensive and than electro-mechanical alternatives. They also have a 'watts loss' associated with them that may drive the need for forced air ventillation or a means of bypass which also adds to the installed cost.
In case of failure cuases, as jraef has indicated a shorted SCR in one leg is far less of an issue than a welded contact in a contactor. However you should also be aware that SCR's are more vulnerable to voltage transients than electro-mechanical devices.
Those were the 'negatives' associated with Solid State Motor Control.
In my opinion there are far more 'positives' associated with Solid State Motor Control eg:
Solid State Contactor: Capable of high cyclic operation, no mechanical waer, no audible noise etc. This makes them extremely useful in certain inductrial applications and ideal for those domestic or commercial installation where audible noise could be a problem.
Soft Starters: Reduction and controlof both motor torque and motor current beyond that achieveable through the use of electro-mechanical reduced voltage starting methods. The level of control provided enhances the performance and reliability of the driven machine and acts to increase plant productivity. With the reduction in starting current there will be a reduction in voltage drop and/or disturbance to the supply network. This will impact positively on all users connected to the same supply line.
Hope the above is of use to you.
Regards,
GGOSS
I would suppoprt everything jraef has said here.
The simple answer is that Solid State Controllers (be they solid state contactors or soft starters) are more expensive and than electro-mechanical alternatives. They also have a 'watts loss' associated with them that may drive the need for forced air ventillation or a means of bypass which also adds to the installed cost.
In case of failure cuases, as jraef has indicated a shorted SCR in one leg is far less of an issue than a welded contact in a contactor. However you should also be aware that SCR's are more vulnerable to voltage transients than electro-mechanical devices.
Those were the 'negatives' associated with Solid State Motor Control.
In my opinion there are far more 'positives' associated with Solid State Motor Control eg:
Solid State Contactor: Capable of high cyclic operation, no mechanical waer, no audible noise etc. This makes them extremely useful in certain inductrial applications and ideal for those domestic or commercial installation where audible noise could be a problem.
Soft Starters: Reduction and controlof both motor torque and motor current beyond that achieveable through the use of electro-mechanical reduced voltage starting methods. The level of control provided enhances the performance and reliability of the driven machine and acts to increase plant productivity. With the reduction in starting current there will be a reduction in voltage drop and/or disturbance to the supply network. This will impact positively on all users connected to the same supply line.
Hope the above is of use to you.
Regards,
GGOSS
Ah, DC is a horse of a different color when it comes to solid state devices however. The leakage through an SCRs, although not usually significant on a AC system, can affect other components in a DC system. And in DC, the shorting of 1 SCR can mean conduction in the complete circuit! That is why you will often see an isolation contactor used in a DC circuit even if a solid state device is used.
"Venditori de oleum-vipera non vigere excordis populi"
"Venditori de oleum-vipera non vigere excordis populi"
To add to GGOSS's comments, we switched out many motor starters to solid state in order to reduce stray voltage problems, that hurt production. These were on 1/4 to 5 hp motors, primarily Baldor, on pumps and compressors, that were cycled on/off very frequently.
We all know that the arcs and sparks of electro mechanical relays cause some problems. As the frequency of motor starting increases, these problems can impact production or nearby equipment. If you google on "stray voltage" (in quotes), you will see many articles about monitoring and controlling of this.
University of Wisconsin:
Michigan State University:
describes stray voltage problem in an agricultural environment. We have found that replacing EMR motor starters with high-precision zero crossing SS motor starters improved the process.
University of Wisconsin:
Michigan State University:
describes stray voltage problem in an agricultural environment. We have found that replacing EMR motor starters with high-precision zero crossing SS motor starters improved the process.
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