Dynamic Braking under Power Failure

[jander]

We have to make some electrical engineering changes in a portal crane. The best solution in terms of efficiency, manteinance, etc, involves using AC motors. The question is, is there any commercial AC system capable to provide dynamic braking (to dissipating resistors) in the event of a complete (emergency) power failure?

Hoist drive motors are around 35 Kw.

 

[BJC]

No
Never heard of it. Most hoist ( all of them I have worked on that is ) motors have a mechanical brake that must be held open for the motor to run. Loose power the brake goes on and the load stays where it is.
I think any kind of dynamic braking would just let the load down slowly, which could be dangerous.

 

[jraef]

There are a few electronic braking systems for AC motors available that work off of battery power in the case of a complete line power loss. They are used quite frequently in the oil well industry to prevent backspin of screw pumps after a power failure. If the power fails, the screw pump "untwists" in the well and allows the oil to flow back from the pipeline down the hole. Once it starts flowing backwards, the shear weight of the oil keeps the shaft spinning and the oil flowing backward. If they hold the shaft for a few seconds with the brake, the shaft untwists and stops, preventing the oil flow. Mechanical brakes fail after one or two of these power failures, so they began using VFDs with battery backed dynamic braking some years ago with great success. VFD manufacturers who specialize in the oilfield industry can usually supply this, try Toshiba, Omron/IDM etc.

That said, it is very dangerous to rely upon any electronic brake method for use on a hoist application unless there is a mechanical brake available to hold the load when braking power is gone. I have seen several manufacturers use a DC injection brake to stop a hoist, then set the mechanical brake to hold it. The DC Injection brake just serves to lengthen the lifespan of the mechanical brake. The problem with this is that when the motor starter is opened, triggerring the DC injection brake to energize, there needs to be a time delay of at least 25-50ms to allow the motor field to colapse before energizing the DC, otherwise the SCRs will short. If the hoist has a high gear ratio, the hoist load can drop during that delay. I have only seen it work on a hoist with a 300:1 gear ratio. That which does not kill me, makes me stronger... and pissed off!

 

[jbartos]

Sggestion to jander (Industrial) Jun 28, 2002 marked ///\\We have to make some electrical engineering changes in a portal crane. The best solution in terms of efficiency, manteinance, etc, involves using AC motors. The question is, is there any commercial AC system capable to provide dynamic braking (to dissipating resistors) in the event of a complete (emergency) power failure?
///As the above posting indicates, there is a system capable to provide dynamic braking. However, the crane application will require an increased safety. The safety can be increased by redundancies. Therefore, the braking system redundancy is appropriate.\\
Hoist drive motors are around 35 Kw.

 

[jander]

Crane has an electro-mechanical emergency brake, which holds the load in the event of a power failure. But our customer wants increased passive safety similar to autoexcited DC motors passive braking (not hold, but slow the load).

 

[tmahan]

Once the incoming AC power is gone I don't think the dynamic braking will last much past the motor field collapses. (IE not long enough to be useful)

Wouldn't a battery UPS system be a more useful solution.

 

[jbartos]

Suggestion: The braking system redundancy is meant to implement a dual breaking system, rather than to look at some complementary solution, although that would not be a bad idea either.