Almost all of the contactor motor starters were used on production equipment, primarily "Big Three" companies. To keep production lines running theydid repairs themselves in areas that they were able to service. They were also prompt to report any failures in the systems that they had purchased.
For those systems that utilized electric heaters and cycling temperature control the standard approach was a mechanical contactor for the enable/disable function and an SSR for the duty cycle control, with the overtemp sensors in series with the coil of the contactor.
The one single pole contactor that I saw failed was switching DC power to a 50 KW lighting load. That contactor did have magnetic blow-outs but it still welded shut on the "Make" operation. That was about 140 volts from a massive battery pack that my employer had sold them years earlier. That was in a vehicle crash test installation, and it was for one of quite a few high power lighting arrays.
The utility company did not want to provide service for 400 KW in that section of the building that would only be used a few minutes each day, so we sold them battery power units, which worked out quite well.

 

For those systems that utilized electric heaters and cycling temperature control the standard approach was a mechanical contactor for the enable/disable function and an SSR for the duty cycle control, with the overtemp sensors in series with the coil of the contactor.

Ok then it makes sense why contactor failure is rare in your experience. Resistive heating is the easiest duty a contactor will ever see, and it sounds like it probably wasn't even making/breaking current either; that was likely all done by the SSR. If the machine were a truck then your contactors were only doing the work of a ball hitch, where usually contactors do the work of a clutch disk. Sure they were technically carrying the load, but yeah not really.

 

Really, the majority of the applications were controlling motors, where no SSR was involved. Mechanical contactors as found in motor starter applications are much more tolerant of high voltage transient spikes than a solid stater control device. Even voltage spikes that arc across the open contacts will not destroy an adequate motor starter. (by adequate I mean selected per the manufacturers specifications.) So in a real world application the mechanical contactor motor starter is the way to go.

 

The days of VERY large cabinets of AC controlled relays/contactors has disappeared for some decades now, this in favour of the DC operated variety, together with the use of PLC/CNC etc.
Also 24VDC has become a standard for control voltage, due to preferred indicator voltage and DC operated devices.
Most of my customers operate large assembly line plants and reliability of each individual system is imperative, to reduce loss of production.
This: One of the reasons for the original request by GM for a production system improvement, hence the birth of the PLC!

 

Given that we have no additional information about the application besides the motor power rating, and given that any DC powered motor starter contactor would need an adequate DC power supply, And GIVEN that we have no hint about what sort of system this might be part of, I suggest that since this is not a CNC machine in a factory, that a mains voltage operated AC powered motor starter be used.
24 volts DC is fine for a system where those involved do not know to avoid grabbing live circuit elements. That is why many electrical cabinets have rather good locking means. The fact is that I HAVE had to work with DC powered control systems, that decision being made by folks who were and are in no way involved with keeping those DC powered systems operating. There have been frequent needs for repairs that would not exist in an ACpowered system.

 

GIVEN the TS came here, posted a single question and hasn't been back for 9 days to see the answers, I think this thread now serves as a play room for bored people to discuss the merits of various control voltages without compulsion to keep discussion limited to the original prompt.

 

@Stantor;
Exactly like someone throwing bloody raw meat in a shark tank and then watching a feeding frenzy.

 

I CAN take a hint, ST and S.T.
And still trying to share a bit of insight, based on experience.

With, occasionally, some opinion as well.

 

Don't take it personally, MB2. I am also guilty as charged, of committing the exact same mistake. Not once, but several times.

An improperly worded question, without schematics, with confusing descriptions, without clear requirements, and which the TS doesn't answer any of the questions posed to him/her, only creates a regenerative oscillation amongst old timers here.

I have been limiting the amount of replies that I actually post, for that reason. Not 100% there yet, but I'm working on it.

 

The unstated big advantage of using AC powered relays is that if somehow you get a coil connection reversed, everything still works. In a DC system with the diodes hidden inside the relays a reversed polarity error is very well hidden. Total job security for the local knows enough electric guy.

 

The unstated big advantage of using AC powered relays is that if somehow you get a coil connection reversed, everything still works.

Sounds a little like those that that I have come up against in N.A. in the reference post.
"We have always done it this way"
As I have related a few times here, e.g. the companies that have AC power magnetic devices such as solenoid valves, most of them have a current maintenance parts inventory of spare AC coils of every kind to replace on failure.
Some of these machines are specialized applications and are imported from Europe etc, , where it has been customary to commonly fit DC solenoids and contactors where necessary' The machines
that are designed to utilize DC only magnetics, have never failed due to coil failure with no inventory required.
I know this as I have serviced these machines over many decades. !!
As to the polarity error well hidden, I have never come across a DC coil operated device that did not show or state any included diode etc?