MCC Start/Stop

[mbrooke]

Ok, forgive me but I'm about to ask the dumbest question I've ever asked on this forum. Whats the point of the start/stop button concept on MCCs and motor controllers? Why not simply start based on a call for rotation or an on/off disconnect concept?

 

[petersonra]

Ok, forgive me but I'm about to ask the dumbest question I've ever asked on this forum. Whats the point of the start/stop button concept on MCCs and motor controllers? Why not simply start based on a call for rotation or an on/off disconnect concept?

It is as much a matter of personal preference as anything else in some cases. A lot of times there is a hand off auto switch we're in the auto position some automatic means exist such as a PLC contact or a pressure switch is used to start and stop the motor. In the hand position or maybe a start-and-stop set of push button to start and stop the motor. Some people don't like the idea all the motors starting by itself again when it is been started by hand. Other times in the hand position the motor just runs all the time.

 

[mbrooke]

It is as much a matter of personal preference as anything else in some cases. A lot of times there is a hand off auto switch we're in the auto position some automatic means exist such as a PLC contact or a pressure switch is used to start and stop the motor. In the hand position or maybe a start-and-stop set of push button to start and stop the motor. Some people don't like the idea all the motors starting by itself again when it is been started by hand. Other times in the hand position the motor just runs all the time.

For conveyers I could understand. Pumps and air handlers I've never understood except from a fire alarm standpoint.

By any chance, does start/stop have anything to do with inrush after a power outage?

 

[ptonsparky]

Don’t forget sticks, A few years ago we updated the controls on a small industrial due to a change from the way it was fueled. NG to wood. We asked how it was to operate. Half the time it was ok. Except when it didn’t. That is when the HOA, and short pieces of wood came into play. The wood was to hold the starters in when Hand would not. Turned out no one working there knew the proper sequence. We knew by spring startup. Occasionally Hand could be used but we knew when because it was logged. Sticks were no longer in use.

 

[retirede]

For conveyers I could understand. Pumps and air handlers I've never understood except from a fire alarm standpoint.

By any chance, does start/stop have anything to do with inrush after a power outage?

I think it’s more safety than inrush. A lot of things could be dangerous if they start unexpectedly.
When a customer of our equipment requested “automatic restart after power interruption”, would would post warning signs all over it.

 

[mbrooke]

I think it’s more safety than inrush. A lot of things could be dangerous if they start unexpectedly.
When a customer of our equipment requested “automatic restart after power interruption”, would would post warning signs all over it.

Makes sense, though you can't easilly size an MCC OCPD at 250%.

 

[bwat]

Makes sense, though you can't easilly size an MCC OCPD at 250%.

Why not? And what does that have to do with starting unexpectedly?

 

[paulengr]

This is the two wire vs three wire concept. Think about an operator: if it has an on/off switch and it stops because the overload or an interlock is off, it could suddenly restart at any time unexpectedly where a start button requires an intentional act. Also stop is not accidental and everybody can easily figure out how to shut it down.

NFPA 79 makes it mandatory:

The stupid one is the breaker/disconnect standards call for green off lights and red on. 79 is the opposite although MCCs go either way.

 

[mbrooke]

Why not? And what does that have to do with starting unexpectedly?

Code doesn't let up upsize the OCPD for the feeder from the MDB to the MCC. Just from the MCC to the motors.

Lets say you have an MCC controlling 20+ motors. The inrush for all of them coming on at once would be rather large.

 

[mbrooke]

This is the two wire vs three wire concept. Think about an operator: if it has an on/off switch and it stops because the overload or an interlock is off, it could suddenly restart at any time unexpectedly where a start button requires an intentional act. Also stop is not accidental and everybody can easily figure out how to shut it down.

NFPA 79 makes it mandatory:

The stupid one is the breaker/disconnect standards call for green off lights and red on. 79 is the opposite although MCCs go either way.

Any idea exactly what 79 calls for?

 

[petersonra]

This is the two wire vs three wire concept. Think about an operator: if it has an on/off switch and it stops because the overload or an interlock is off, it could suddenly restart at any time unexpectedly where a start button requires an intentional act. Also stop is not accidental and everybody can easily figure out how to shut it down.

Lots of motors start and stop by themselves and it is quite safe. Sump pumps, air compressors, etc.

Now if it is part of a machine, which would be where NFPA79 would apply, I am inclined to agree that a manual operation should be terminated on power failure.

 

[bwat]

Code doesn't let up upsize the OCPD for the feeder from the MDB to the MCC. Just from the MCC to the motors.

Lets say you have an MCC controlling 20+ motors. The inrush for all of them coming on at once would be rather large.

First, this is different than what your post #6 says. That was about MCC OCPD.

Second, you can make the feeder wire and OCPD to the MCC larger if you’d like. There’s nothing stopping that.

(have to consider busbar ratings and existence of MCC main of course)

 

[Jraef]

“3 wire control” is where you have a Start-Stop control scheme, whether via push buttons or a PLC, held in (“sealed”) by an aux contact of a contactor. The concept is all about “low voltage dropout”, where the contactor will drop out at 70% of the control voltage and not pick up again unless you have 80%, values that are in NEMA design specs and most MIL specs. This prevents unexpected restarting after a power loss; in other words it’s all about safety.

But there is nothing mandating 3 wire control or even how it is accomplished. I once built an entire MCC for some barge mounted pumps that were going to be manually controlled but needed low voltage dropout, using Molded Case Circuit Breakers that were UL listed as Manual Motor Starters had adjustable thermal trips, by adding an Under Voltage Release coil to them. The breaker handle was the On-Off control, the UVR would drop them out under the same conditions as a coil.

There are some OSHA safety requirements that can be met by using 3 wire control, but they can also be met in other ways, this is just the least expensive.

On-Off without a seal-in contact is called “2 wire control” and is fine for loads that you WANT to come back on after a power loss without intervention, like sump pumps and maybe some types of fans. But all aspects of safe operation must be considered.

This all has nothing whatsoever to do with starting current or inrush.

 

[mbrooke]

First, this is different than what your post #6 says. That was about MCC OCPD.

Second, you can make the feeder wire and OCPD to the MCC larger if you’d like. There’s nothing stopping that.

(have to consider busbar ratings and existence of MCC main of course)

By MCC OCPD I was think the feed to the MCC itself... similarly a motor OCPD would be the feed from the MCC to the motor. Thats just how I think it in words. My appologies for confusion.

 

[mbrooke]

“3 wire control” is where you have a Start-Stop control scheme, whether via push buttons or a PLC, held in (“sealed”) by an aux contact of a contactor. The concept is all about “low voltage dropout”, where the contactor will drop out at 70% of the control voltage and not pick up again unless you have 80%, values that are in NEMA design specs and most MIL specs. This prevents unexpected restarting after a power loss; in other words it’s all about safety.

But there is nothing mandating 3 wire control or even how it is accomplished. I once built an entire MCC for some barge mounted pumps that were going to be manually controlled but needed low voltage dropout, using Molded Case Circuit Breakers that were UL listed as Manual Motor Starters had adjustable thermal trips, by adding an Under Voltage Release coil to them. The breaker handle was the On-Off control, the UVR would drop them out under the same conditions as a coil.

There are some OSHA safety requirements that can be met by using 3 wire control, but they can also be met in other ways, this is just the least expensive.

On-Off without a seal-in contact is called “2 wire control” and is fine for loads that you WANT to come back on after a power loss without intervention, like sump pumps and maybe some types of fans. But all aspects of safe operation must be considered.

This all has nothing whatsoever to do with starting current or inrush.

What does your experience typically show for supply, return and exhaust fans? HVAC pumps and water pumps?

 

[kwired]

Code doesn't let up upsize the OCPD for the feeder from the MDB to the MCC. Just from the MCC to the motors.

Lets say you have an MCC controlling 20+ motors. The inrush for all of them coming on at once would be rather large.

Nothing stops you from having all 20 motors automatically controlled, I think it is just standard configuration to use start-stop if not otherwise specified.

If auto starting after a power failure is going to be problem then you may need some logic in the controls to start certain loads at different times. Some of which may already going to happen because of interlocking something that must prove something else is up to speed, temperature, pressure, etc. before it will be allowed to start.

 

[don_resqcapt19]

If auto starting after a power failure is going to be problem then you may need some logic in the controls to start certain loads at different times. Some of which may already going to happen because of interlocking something that must prove something else is up to speed, temperature, pressure, etc. before it will be allowed to start.

Before PLCs and other logic controls were common, sometimes a motor driven cam switches were used for the auto restart to avoid starting all of the motors at the same time. This was on an industrial waste treatment plant with a number of blower and aerator motors. I think there were 6 or 8 NO cam switches set to close in sequence about 10 seconds apart, wired in parallel with the start buttons.

 

[petersonra]

What does your experience typically show for supply, return and exhaust fans? HVAC pumps and water pumps?

IMO, this kind of stuff needs to start back up automatically after a power failure.

 

[mbrooke]

Before PLCs and other logic controls were common, sometimes a motor driven cam switches were used for the auto restart to avoid starting all of the motors at the same time. This was on an industrial waste treatment plant with a number of blower and aerator motors. I think there were 6 or 8 NO cam switches set to close in sequence about 10 seconds apart, wired in parallel with the start buttons.

I take it all starting at the same time would blow the OCPD feeding the MCC?

 

[mbrooke]

IMO, this kind of stuff needs to start back up automatically after a power failure.

IMO, I'd say you're correct!