Why use start/stop wiht auxiliary contacts for motor starter?

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shespuzzling

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I think this is an easy one, but in a motor control circuit, what are the benefits of using a momentary stop switch, and momentary start switch in parallel with an auxiliary contact for a motor starter control circuit? Why not use just a simple single pole switch for on/off control?
 

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Any sort of power bump or brownout will drop the coil out and shut the motor down until someone manually restarts it.

Straight on/off control will attempt to ride through any sort of power bump which could result in a chattering/blown up starter and a damaged motor.
 
Cow said:
Any sort of power bump or brownout will drop the coil out and shut the motor down until someone manually restarts it.

Straight on/off control will attempt to ride through any sort of power bump which could result in a chattering/blown up starter and a damaged motor.
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with a single start stop station, it's not such a big deal.
the O/L will open the contactor, but if you have an involved
control situation, with many start and stop inputs, maintained
contacts no workee so well.

but... the cow speaketh the truth. momentary flutters will drop
out a held relay, and a manually closed switch will just sit there.
not always a good thing.
 
Sounds like a homework problem.
Please offer us your thoughts and we will be glad to critique them.
 
In addition to what has already been said, you can easily insert limit switches into the series of stop buttons to shut down when a certain condition is sensed, but would still need to manually restart it. Doing so with maintained switching scheme would automatically restart when the limit switch returned to normal condition.

Also can put limit switches or aux contacts from other components in series with that holding contact, open any of those switches and the contactor drops out. This may allow for conditions where the start bypasses the holding contact when pressed and your motor starts, but if other items in the series are not in normal operating condition it stops again when you release the start button.

I also run into "automatic restart" needs on some equipment. In particular irrigation systems - the POCO's may remotely shut down your system when total demand is high (they offer better rates if you subscribe to let them do this) to manage load on their system during peak demand periods. Many owners want that system to automatically restart when the POCO allows their system back on line. A simple timed contact parallel to the start button allows restarting, but since it times out and opens again if there is a malfunction in the system it shuts back down. If you held the start closed - most of those systems it bypasses some of the running limits but in this case no one is there to supervise and you don't want the start command to last more then a few seconds if there is a malfunction that a limit has otherwise opened the circuit for.

Multiple control stations - the conventional three way/four way switching circuit works, and uses same number of control conductors, but you don't really know if it is on or off state when there is some other limit switch putting it in an off condition. With momentary start/stop buttons, pressing any stop button will stop it regardless of other conditions, pressing any start button will start it if nothing is locking out the control circuit - like an unsatisfied limit switch or tripped motor overload.

There may be many more individual application examples of why this might be a better control scheme then a simple maintained contact as the primary operator interface.
 
kwired said:
There may be many more individual application examples of why this might be a better control scheme then a simple maintained contact as the primary operator interface.
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Sure you would have to know what the motor is being used for to make any sort of decision on how to control it.
 
OK...being nit-picky (is that a word?) The aux contact across the start button is labeled "B". In my vast experience, an "a" contact acts the same as the main contacts and a "b" contact acts opposite. Seems to add to the confusion factor. But...back to the OP question. You use a momentary push button so that if an overload trips or a power interruption drops out the starter, you must push the start button to restart the motor. If power returns or an O/L resets (some O/L relays can be setup to "auto reset") the motor would restart without any operator input if a switch was used. Could be a safety problem if other equipment must be a "start" configuration before starting the motor. Consider it a "lockout" type of circuit.
 
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This circuit has been in industrial use for probably 75-100 years but except for very simple applications, is rapidly fading away due to application of PLC controls. Prior to PLCs, this type of circuit allowed for safe interlocking of related devices using auxiliary contacts on the motor contractor or auxiliary relays. As a simple example, say conveyor #1 feeds conveyor #2. If #2 stops for any reason you would want #1 to stop automatically, which is easily accomplished by putting a contact from the #2 contractor in series with the contact shown as "B" ( that loop is usually called the "seal-in"). It would normally be an unsafe condition for #1 to automatically restart when #2 is restarted and you can see how such an arrangement could be especially hazardous in complex arrangement involving many pieces of interlocked equipment. The wiring of all that hard-wired interlocking with contacts was once the most difficult part of designing industrial systems. Today, PLCs perform all of such logic electronically, telling each piece of equipment when to run and stop with just a single contact.
 
It goes to functionality, is it okay for your motor after going into an overload condition to possibly restart? Most likely not! Good way to start a fire.
If the overload is set to auto reset either on purpose or accidently that's what's going to happen. ( And just because you'll set it up right, what about the next guy who replaces it. )
The only application I've seen that allows logic similar to that is emergency life support equipment where a high signal must be sent to turn off say a pump.
(Also as an after thought your circuit does not show an Estop... & I can't currently see your diagram so it would have to be a low enough voltage to be unplugged? or near enough to throw a disconnect without hunting for it, were the motor to have problems without an estop) Were you to decide have an Estop remember to use two NC contacts from the button in your circuit.
 
Bear in mind that the voltage in the circuit shown could be any standard 3 phase voltage up to 575 V. Although line voltage control circuits were common - and many are still in service today, around 50 years ago combination starter manufacturers started including a 120 v control power transformer for that part of the circuit - with grounded neutral.

With respect to e-stop - that is typically a push-pull button with a maintained NC contact installed at the source end of the circuit. With a circuit using the seal-in contact as shown, the motor won't automatically restart even if the e-stop is reset. For certain types of machinery modern international safety standards may require the use of a 'safety relay' that effectively locks out the equipment when an e-stop occurs.
 
Thanks all for your replies! Not a homework question :), just doing a little research and wanted to make sure I understood why it was set-up that way. So #1 is safety and protection of the motor, and #2 would be for ease of integrating additional controls, switches, etc. Makes sense. Thanks!
 
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