Motor Control Circuit Question

[Charlie_in_TX]

We have all seen the standard circuit. Starting from the X1 side of the transformer connected to the fuse connected to 'START'/'STOP' circuit connected to the contactor coil connected to the overload protection finally returning to the X2 grounded side of the transformer. The 'START' button has a aux contact in parallel to latch the start signal. A safety benefit is that when power is removed from the contactor coil for any reason, upon the power returning to normal, the start sequence must be re-initiated.

Standard stuff. I had it explained to me as controls on the hot side, load in the middle and safety permissives on the right grounded side. The permissives are always normally closed.

But why? Is it in the NEC someplace? Is it some other code?

I am working on an industrial automation system. I work for the controls contractor. The electrical contractor is building the MCC. They are attempting to change the above standard.

Design issues (but are they code violations?)
1. 'HAND''OFF''AUTO' rotary switch. In this system 'HAND' should be labeled 'ON'. It applies power to the contactor coil until it is turned 'OFF'. The problem? If there is a motor shutdown (because overload heaters, emergency shutdown device (ESD), etc.), when the the condition is corrected, the motor will start immediately without a new 'START' command.
2. Reverse logic in the ESD. Let me explain. They are using the NO contact in the ESD to power a relay. The ESD is at the motor, the relay is 150ft away in the MCC. A NC contact in the relay is used in the HOT side of the control circuit. Problems. A. The permissive is in the HOT side of the load not the grounded, 'neutral' side. B. The permissive is not 'fail safe'. It is possible for the motor to run without the cable to the ESD connected.

 

[Smart $]

...Is it in the NEC someplace? Is it some other code?

...

Essentially not covered in NEC.

Suggest you look to NFPA 79:

http://www.nfpa.org/aboutthecodes/AboutTheCodes.asp?DocNum=79&cookie_test=1

...and perhaps NEMA standards.

 

[kwired]

Essentially not covered in NEC.

Suggest you look to NFPA 79:

http://www.nfpa.org/aboutthecodes/AboutTheCodes.asp?DocNum=79&cookie_test=1

...and perhaps NEMA standards.

I have never liked the idea of control devices of any kind in the grounded conductor. A ground fault in the right place will bypass these controls without anyone ever knowing.

 

[Charlie_in_TX]

As usual I left out important info. These system will be on a ship being used in the oil field. Therefor, the DNV, ABS, etc will be the certifying/approving bodies.

 

[Charlie_in_TX]

There is info near what I am looking for in NFPA79
"[FONT=&quot]H.2 Measures to Minimize Risk in the Event of[/FONT] [FONT=&quot]Failure.[/FONT]
[FONT=&quot]Use of proven circuit techniques and components[/FONT]
[FONT=&quot]measures to minimize risk in the event of failure include[/FONT]
[FONT=&quot]the use of proven circuit techniques and components.[/FONT]
[FONT=&quot]These measures include, but are not limited to, the[/FONT]
[FONT=&quot]following:[/FONT]
[FONT=&quot]?[/FONT]
[FONT=&quot](3) Stopping by de-energizing[/FONT]
[FONT=&quot]?[/FONT]
[FONT=&quot](6) Circuit design to reduce the possibility of[/FONT]
[FONT=&quot]failures causing undesirable operations"[/FONT]

 

[GeorgeB]

I have never liked the idea of control devices of any kind in the grounded conductor. A ground fault in the right place will bypass these controls without anyone ever knowing.

IME, at least 90% of factory wired motor starters (loose and in MCCs) have the overload relay contacts on the grounded side ... and the NO aux contact prewired for 3-wire (start/stop) control. As an electronics guy at the time I began working in a panel shop, I wanted to put in my designs

start ... stop ... overload contact(s) ... coil

The electrician building the panel said he could do that, but that it would add time and confuse experienced electricians who expected it otherwise, specifically

stop ... start ... coil ... overload contact(s)

When I first programed microprocessors and PLCs, I wanted to follow the "real" convention ... but it used an extra word of memory over having the start first ... and then memory mattered much more than today. In very few cases did we bring the overload contacts in as an input ...

lda start
or aux
and stop
out coil

vs

lda stop
push
lda start (with later units, the push above was automatic here)
or aux
and stack
out coil

 

[Barndog]

We have all seen the standard circuit. Starting from the X1 side of the transformer connected to the fuse connected to 'START'/'STOP' circuit connected to the contactor coil connected to the overload protection finally returning to the X2 grounded side of the transformer. The 'START' button has a aux contact in parallel to latch the start signal. A safety benefit is that when power is removed from the contactor coil for any reason, upon the power returning to normal, the start sequence must be re-initiated.

Standard stuff. I had it explained to me as controls on the hot side, load in the middle and safety permissives on the right grounded side. The permissives are always normally closed.

But why? Is it in the NEC someplace? Is it some other code?

I am working on an industrial automation system. I work for the controls contractor. The electrical contractor is building the MCC. They are attempting to change the above standard.

Design issues (but are they code violations?)
1. 'HAND''OFF''AUTO' rotary switch. In this system 'HAND' should be labeled 'ON'. It applies power to the contactor coil until it is turned 'OFF'. The problem? If there is a motor shutdown (because overload heaters, emergency shutdown device (ESD), etc.), when the the condition is corrected, the motor will start immediately without a new 'START' command.
2. Reverse logic in the ESD. Let me explain. They are using the NO contact in the ESD to power a relay. The ESD is at the motor, the relay is 150ft away in the MCC. A NC contact in the relay is used in the HOT side of the control circuit. Problems. A. The permissive is in the HOT side of the load not the grounded, 'neutral' side. B. The permissive is not 'fail safe'. It is possible for the motor to run without the cable to the ESD connected.

The hand off auto switches we use here the hand is momentary so if you are not at the starter box and go to reset the overload it will not start back up.

 

[Electrician4life]

start ... stop ... overload contact(s) ... coil

While I see the advantage of putting the OL's before the coil, what is the advantage of placing the start ahead of the stop?

 

[kwired]

IME, at least 90% of factory wired motor starters (loose and in MCCs) have the overload relay contacts on the grounded side ... and the NO aux contact prewired for 3-wire (start/stop) control. As an electronics guy at the time I began working in a panel shop, I wanted to put in my designs

start ... stop ... overload contact(s) ... coil

The electrician building the panel said he could do that, but that it would add time and confuse experienced electricians who expected it otherwise, specifically

stop ... start ... coil ... overload contact(s)

When I first programed microprocessors and PLCs, I wanted to follow the "real" convention ... but it used an extra word of memory over having the start first ... and then memory mattered much more than today. In very few cases did we bring the overload contacts in as an input ...

lda start
or aux
and stop
out coil

vs

lda stop
push
lda start (with later units, the push above was automatic here)
or aux
and stack
out coil

NEMA starters usually are pre wired in the fashion you mention. If I am using IEC starters and you typically need to wire everything with them I usually will not put the overload in the grounded line. The overload will usually be the last control device before the coil in that case.

The hand off auto switches we use here the hand is momentary so if you are not at the starter box and go to reset the overload it will not start back up.

You could also wire a hand off auto with a start button that is used to start it in the hand position, and you would need to turn the HOA to off to stop it. Pump panels come prewired this way.

While I see the advantage of putting the OL's before the coil, what is the advantage of placing the start ahead of the stop?

I don't like it when it is done that way. If both buttons are pressed at same time the controlled device is energized, If start button would stick you can not use stop button to stop the control. Placing stop first is more fail safe.

 

[GeorgeB]

While I see the advantage of putting the OL's before the coil, what is the advantage of placing the start ahead of the stop?

AT ONE TIME, it was difficult (and used additional memory) with AB PLCs to do a parallel branch that didn't start on the left (supply, logically) rail. With Boolean programming, it required an extra word.

IMO, there is no advantage in doing it before the "stop" logic beyond that.

Introductory motor control training I've seen often does "start" "stop" "coil" for examples of both starters and "latch" circuits. I wish they didn't.

 

[pfalcon]

You won't find code that specifically states how to design a circuit - motor or otherwise. What you will find are spot notices that say things like "After a fault reset the machine must not restart without operator action" or something similar.

OSHA will hold you to using standard practices that are proven to work. OSHA will hold you to using listed products that work.

Where do you get those practices? In bits and pieces scattered throughout codes and standards. In the end most boil down to some simple rules:

* Always fail safe
Therefore if something requires energy to move then safeties cut the energy. This is why MOST safety devices are normally open but held closed devices. When they fail they pop open and kill the circuit.
* Never restart
If a fault condition occurs the equipment must never restart simply because the fault was reset. If it does then you risk people or equipment in the path getting ambushed. Idiots may hold a reset in and burn a motor.

There are a few more but the two above apply to the OP. They're kinda like using the bathroom; everyone in the industry KNOWS you must do these things. OSHA WILL ding you for failure to do these things. But nobody typically explains them.