'Soft-Stop' Mechanical Brake for Induction Motor??? 2

[elfman]

Known:
Class 1, Division 1, Group C & D environment
5 HP Induction Motor with a impeller type blade attached.
Local (at motor)Disconnect.
Blade can be accessed before motor comes to a stop.
In a non-hazardous environment we typically supply a solenoid to latch the access till motor spins down. Haven't had any luck finding a commercially available solenoid that has certs to operate in hazardous environment.

Question, is there a way to mechanically 'soft-stop' the motor/blade when power is cut via the disconnect?

Any comments, ideas or discussion welcome.

Thanks in advance.

"To be or not to be, that is the question" - William Shakespeare via Hamlet

 

[jraef]

There are any number of mechanical brake options available, but "soft" is a relative term. Most spring loaded mechanical brakes work by simply killing power to a solenoid coil that is holding the spring mechanism open, so when power is cut, it fully engages at its maximum set tension. But the amount of tension on the brake pads itself is adjustable in many versions, so that may be the only method of soft stopping available in that type of brake; let the pads slip a lot under light tension. That then means more wear and more frequent replacement, but may be the simplest. I know that Dings makes hazardous location rated brakes, but you would need to investigate whether the tension is adjustable. One more word of caution on this concept, ex-proof brakes have to have a thermal over ride that prevents its own temperature from becoming incendiary, so if this has to stop quite frequently, you may get in trouble there too.




"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[BrianPetersen]

Or you can leave the motor alone, and instead of using a solenoid-locking interlock switch, you could use an air cylinder to operate a shot pin to keep the lid shut. Put the valve outside the classified area. If you do this, your interlock strategy should prove not only that the lid is shut but also that the shot-pin is engaged before allowing motor start-up. There are some fiber-optic-based safety rated interlocking systems out there which will allow this to be done while still allowing "control reliable" circuit/system performance to be maintained and then only the fiber-optic cable has to go into the classified area; the electrical panel with the safety relay for the fiber-optic system can be outside the classified area.

 

[iceworm]

Not answering your question, but have you considered a VFD? I have use them quite successfully as a brake - years back, I'm sure there is much better stuff on the market now. Locate the drive outside of the exp area.

ice

Harmless flakes working together can unleash an avalanche of destruction

 

[waross]

I have seen air brakes in that size range. The braking may be controlled by controlling the air pressure. jraef's warning concerning heat from the friction is still operative.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[elfman]

Thanks for the comments. I like the idea of air-brakes, but we don't have, and will not be supplying compressed air. Air latches or brakes are not a possibility.

I also like the idea of using a VFD, BUT, with the disconnect at the motor, it would be possible for the motor to be running at full speed and the operator could kill it with the disconnect, if that is done, then the VFD braking is null-and-void. So, what I am trying to achieve with the mechanical solution would have to stop the motor, only in the advent of the disconnect being used. Otherwise, I can just use a VFD. Comments, roasts, ideas, questions?

"To be or not to be, that is the question" - William Shakespeare via Hamlet

 

[iceworm]

I also like the idea of using a VFD, BUT, with the disconnect at the motor, it would be possible for the motor to be running at full speed and the operator could kill it with the disconnect, if that is done, then the VFD braking is null-and-void.

Yep - that's a problem. One I worked, a 5000hp motor-pump, pushing the stop button would pull the rpm down in just a few seconds. Hitting the emergency stop would open the contactor feeding the VFD and the motor would take 10s of seconds to wind down. I suggested to the design group that perhaps we should consider having the E-stop activate the STOP for several seconds then open the contactor. It would safe-out quicker. Like talking to a brick wall.

In your case you can't verify the crew waited for the motor to stop after opening the disconnect, before they opened the cover. Even with a good, consciencious crew, you want to verify.

Following that, how would you verify the crew hit the stop button before opening the disconnect and then immediately opened the cover? I agree, a VFD driven stop is likley not a solution for this.

ice

Harmless flakes working together can unleash an avalanche of destruction

 

[waross]

A kirk interlock may work. The crew must operate one lock to activate the VFD stop. Then they are able to remove the key and open the second lock. The cover must be locked closed before the key may be returned to free the stop button so the VFD may be restarted.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[iceworm]

I could not think of any mecnaical interlock for the Stop/Disconnect. Trapped key interlock is an excellent choice.

ice

Harmless flakes working together can unleash an avalanche of destruction

 

[jraef]

So if you use a VFD, would it be in a different non-hazardous location? Because if not, that can be problematic, especially if using dynamic braking resistors. If so, then that may work great with the trapped key system, but with a catch: set it up so that the local disconnect needs the key, but the key is captive at the VFD, and use a solenoid interlocked trap system that does not allow the key to be removed until the solenoid is released. Then electrically interlock the trap solenoid to engage whenever the drive is commanded to run, and released only when the motor is at zero speed (meaning it is done braking), most modern VFDs are capable of doing this with internal programming, but depending on your safety integration, you may need to use a safety relay type shaft speed monitor.

If the VFD must be in the same classified atmosphere, the cost to enclose / purge and cool the VFD and braking resistors may prove to be prohibitive, depending on what size you are talking about (you never said). It might be less expensive to just create an overly complex mechanical interlock system that takes longer to implement than it takes for the blade to coast to a stop. For example place the key trap behind another trapped key system separated by a distance. I had to do that once on a production bread slicer which, for various reasons, could not be braked to a stop, so workers were opening the access hatch when the blades were still moving and basically invisible. We made them go through 4 steps to get access to the hatch, which we timed out to be 3x the spin down time of the blades.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[mikekilroy]

replace the 5hp INDUCTION motor with a 5hp SYNCRONOUS motor? They are made in xpln proof too (ac servo, no brushes to spark)...

then your disconnet opens the power leads and shorts them together; bingo, stop. Add resistor(s) for complete 'soft stop' decel rate....

http://www.KilroyWasHere<dot>com