PLC and motor brake ideas

sii

Senior Member
Location
Nebraska
I have been tasked with building a control system for a machine we have. On this machine are nine VFD's whose motors are connected to electric brakes, six of which are used to stop individual, massive inertial loads.

General shutdown procedure is rather than a three wire circuit I use a pushbutton connected to a plc input. When pushed, it initiates a stop command, ramping all motors to a stop, then deenergizing the brake, then firing a relay turning the machine off.

My concern here is that if power is lost to the machine, the brakes will immediately engage causing mechanical damage.

My idea is this but I'm looking for a better one: feed the plc and all of the brake circuits with a UPS. In addition to the normal stop input, wire the main control power to another input and if that input goes low, the stop circuit is initiated in the same manner as above. The UPS would have to be sized to power the plc and brakes through the shutdown process.

Any feedback on this would be appreciated. Thanks.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Usually on a brake motor, it's fairly difficult to damage the brake or the motor from powering it down; that is actually the EXPECTED operating mode. Ramping down with the VFD is the thing that is different. Personally, I wouildn't worry about it enough to want to spend a bunch of money on a UPS.
 

GoldDigger

Moderator
Staff member
The OP states a concern that deenergizing (applying) the brake will cause mechanical damage. What is the basis for this belief? Has an engineer looked at the situation? Is there a chance that suddenly applying the brake will cause a shear coupling to fail, removing the braking force completely? What will the safety consequences of that be?
 

sii

Senior Member
Location
Nebraska
Answers below:

The OP states a concern that deenergizing (applying) the brake will cause mechanical damage. What is the basis for this belief? The rotating mass is around 11 tons.

Has an engineer looked at the situation? Yes, the mechanical aspects of this machine will not change. No chance.

Is there a chance that suddenly applying the brake will cause a shear coupling to fail, removing the braking force completely? Yes, thus my question.

What will the safety consequences of that be? When exposed to the operators, the mass turns very, very slowly. I'm comfortable with that aspect. It is during the process that the rotating speed is much faster.
 

SceneryDriver

Senior Member
Location
New York, NY
Answers below:

The OP states a concern that deenergizing (applying) the brake will cause mechanical damage. What is the basis for this belief? The rotating mass is around 11 tons.

Has an engineer looked at the situation? Yes, the mechanical aspects of this machine will not change. No chance.

Is there a chance that suddenly applying the brake will cause a shear coupling to fail, removing the braking force completely? Yes, thus my question.

What will the safety consequences of that be? When exposed to the operators, the mass turns very, very slowly. I'm comfortable with that aspect. It is during the process that the rotating speed is much faster.
If your VFD's loose supply power, they will most likely fault and will not ramp your load to a stop. If dropping the brakes unexpectedly will cause a mechanical overload to the system, consider replacing the brakes with pneumatically operated units. With a proper accumulator tank, air supply will be independent of incoming power, and your UPS now only has to support the PLC and the needed pneumatic valve(s) for the brakes. Much less power needed than if you needed to operate the brake coils.


SceneryDriver
 
... My concern here is that if power is lost to the machine, the brakes will immediately engage causing mechanical damage.

My idea is this but I'm looking for a better one: feed the plc and all of the brake circuits with a UPS. In addition to the normal stop input, wire the main control power to another input and if that input goes low, the stop circuit is initiated in the same manner as above. The UPS would have to be sized to power the plc and brakes through the shutdown process. ...
Seems reasonable, if it's not a safety-critical process and it's OK to let them coast to a stop. If you have the budget for 66 tons of rotating mass, you certainly have the budget for a UPS.

You might use a phase-loss monitor. The main power doesn't have to be lost entirely; losing one phase could initiate the problem. And if it's not one of the two the phases that's feeding the main control power, the PLC won't get a "main power lost" signal.

I'm a little uncomfortable with a design that breaks something if the power fails. And with a design that places the brake on the other side of a shaft coupling.* I would be tempted to ask the mechanical engineers to revisit the design, possibly placing spring-loaded brakes on the big mass itself, so that nothing bad happens during a power failure or other reasonably-foreseeable failure-mode event. Actually, I think I'd ask them to revisit their entire Failure Modes & Effects analysis.

Just changing from electric to pneumatic brake actuation, by itself, won't improve anything. It sounds like the existing brake is simply too small to bring the big mass to a stop before the brake catches fire.


* if I'm reading between the lines correctly.
 

paulengr

Senior Member
Would there be a problem just letting the machine coast to a stop upon power failure, leaving braking out of the scenario?

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Yes usually for safety reasons brakes are energized to release.

Most VFDs respond to power loss by going into coast to stop. However several of them have a special power loss mode where they utilize the stored energy in the load as a power source and decelerate to stop when input power is lost. I can't remember if it can be regenerative thus keeping the brakes and PLC alive too or not. Most PLCs have enough power in their power supplies to last about 100 ms. Beyond that you need a little more.


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Tony S

Senior Member
In a similar situation I had one hell of a fight convincing management that cutting power on an emergency stop wasn’t always a good idea. A 50 ton centrifuge in freefall isn’t in a hurry to stop.
I lined a couple of managers up, gave each a stopwatch and proceeded to have fun. In freefall, 100 seconds to stop, controlled deceleration, 8 seconds.
At the end of the day, anyone caught up in the machine wouldn’t stand a chance.

I found out later that before I joined the company they had tried fitting a brake to the motor shaft. All it did was wreck the drive train.
 

Besoeker3

Senior Member
Location
UK
Just changing from electric to pneumatic brake actuation, by itself, won't improve anything. It sounds like the existing brake is simply too small to bring the big mass to a stop before the brake catches fire.
That would be my take based on the information provided.
 

paulengr

Senior Member
Yes usually for safety reasons brakes are energized to release.

Most VFDs respond to power loss by going into coast to stop. However several of them have a special power loss mode where they utilize the stored energy in the load as a power source and decelerate to stop when input power is lost. I can't remember if it can be regenerative thus keeping the brakes and PLC alive too or not. Most PLCs have enough power in their power supplies to last about 100 ms. Beyond that you need a little more.


Sent from my SM-T350 using Tapatalk
In this case the brake actuator is not the main inertial mass so this approach won’t be much different.
 

RobertKLR

Member
Location
Texas
I'm late to the party on this one but at a Weyerhaeuser plant in Texas I connected a water pump to use as a brake that just pumped water out of a bucket back into the same bucket. The idea was to use the water to absorb the bulk of the braking force of a large roller on a corrugator. By playing with the restriction on the pump I was able to find a good rate of flow that would brake the roller quickly. I had to use a 15 gallon water tank because repeated stops heated the original 5 gallons pretty quick. It worked well after a lot of tinkering but the system was never used. Later we did use a similar pneumatic system (small compressor pump) for a much smaller load.
 

Besoeker3

Senior Member
Location
UK
Usually on a brake motor, it's fairly difficult to damage the brake or the motor from powering it down; that is actually the EXPECTED operating mode. Ramping down with the VFD is the thing that is different. Personally, I wouildn't worry about it enough to want to spend a bunch of money on a UPS.
That would be my take also.
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
I think in this situation using an eddy current brake to dissipate most of the kinetic energy, and then using friction braking to bring it to a stop would be the best solution. But it sounds like they don't want to change or add anything mechanical?
 

sii

Senior Member
Location
Nebraska
Sorry, I haven't looked at this thread for awhile, been busy and not terribly fond of the new site.

Anyway, it is correct to say that I will not be allowed to change any mechanical aspects.

I've done some testing of the machine and a couple seconds of deceleration, or even coasting before engaging the brakes will be sufficient to protect the mechanical elements. I don't need them to stop, just slow down. Integrating a UPS to ride through just a few seconds of power loss is simple.
 

Fulthrotl

~~Please excuse the mess. Sig under construction~~
Sorry, I haven't looked at this thread for awhile, been busy and not terribly fond of the new site.

Anyway, it is correct to say that I will not be allowed to change any mechanical aspects.

I've done some testing of the machine and a couple seconds of deceleration, or even coasting before engaging the brakes will be sufficient to protect the mechanical elements. I don't need them to stop, just slow down. Integrating a UPS to ride through just a few seconds of power loss is simple.
if the concern is one of safety, is there guarding with interlocks on the moving parts now?
if you lose power now, you drop the brakes when they de energize, and it's a red button stop, correct?

in the event of a power failure, if the ups kept the plc powered up, and the safety door interlocks
energized until it coasted to a stop, would that solve the problem? after all, with no power, it isn't gonna
restart until it gets power again anyway.

i'd put a timer on a plc that if you lost power, it held the safety doors closed until however long it took to coast to
a stop, and i'd also put a timer in to prevent it from re starting if power came back on before the stop sequence was done.

and i'd put a timer on the brakes to keep them hot for 5 seconds after you lose power.
 
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