Hoist brake problem

Hmmm odd one...my only guess is that there is some other circuitry involved (even just supression components)
The current flowing through the coil doesn't want to stop when the power is removed (due to the nature of it's inductance)...so maybe this current is continuing to flow through path which isn't obvious...
This begs the question...how is the power removed and what other components are there around that coil...? Capacitors diodes?.. Or maybe there is a breakdown or leakage path.

Good luck
Del

The motor is controlled via a set of reversing contactors. There is nothing else in the circuit past the reversing contactors besides the half wave rectifier and the brake coil (and the motor of course). The rectifier is in the motor junction box, there is only about 6 inches of wire coming out to the brake coil.

The schematic doesn't have anything else in that particular circuit. The part that I find really confusing is that there is so little in the circuit.

Sorry I've shot my bolt....that's all folks

Don't give it up, the problem must be somewhere there. Maybe the brake coil should be disconnected from the motor with an additional contact.

Yup apply the age old technique of dismantling all the parts involved, cleaning/refurbishing* and reassembling it may well cure the problem although it won't identify where it was unless you do the bits one at a time.

If you apply extra 'furbish' maybe they'll stay in good shape longer?
Del

I have seen it in the past where dirt contamination on a brake magnetic coil has "jumped" a flux gap allowing a "seal in" condition to happen that would cause the delay in the response time of the unit. Basically it stayed "pulled in" until the flux dropped off. Clean the components of the unit flawlessly and see what happens. This used to happen on cM hoists and Stearns brake units as well. Let me know if this helps at all.

we have had this problem with some hoists on site and found a couple of things to check. A build up of brake dust etc on the mechanism, the brake assemblies on our hoists have a 3 bolt and spring arrangement that wears and sticks resulting in a 2 - 3 second delay in the brake operating.

try following the clean up as suggested in reply #5 while your at it go back to the basics, scrape the contacts for the grounding parts of your circuits whistle clean re-attach them on their mating surfaces after you have whistle cleaned them as well.

one other wierd possibility is insulation on the wires circuits is breaking down allowing transient voltage to interfere with operations because the three seconds is a conistent brake on issue i would huess the idea of a condensor's voltage leakdown is tied in there somewhere as well.

after that we got no more rounds for the magazine.

'da ber

Ok, if i were you i will first let the members to know the operating voltage of the brake coil or any electronic circuit involed besides the half wave rectifier,you may also be right suspecting the coil being faulty but its better to first go through your control cicuit,check any defects in the circuit,if fine then you have all the rights to blame the coil.Compare the coil resistanc with the one working if you have any.

if you have a DC power supply, you may also have a capacitor in there to smooth it. When power is cut off, this capacitor might supply enough juice to keep the coil engaged. Solution, place the switch that controls the coil directly in series with the coil and have a snubber diode to bypass the reverses emf made by the stored field collapse.

I get the impression that the brake is turned on/off by upstream interruption off AC power and not by a series switch to the coil.

In addition, there may be a bleeder resistor that drains the power after power off, and this bleeder has failed 'open'.

I have done various projects involving magnetic locks. A dc current is applied to the lock to activate it. Over time it tends to "stick" after removal of power. A feature of the control cct is to momentarily reverse the current to de-gauss the device. This may help you in your quest.

Good morning,

Hope everyone is doing well today.

More than likely the intense magnetic field created by the DC voltage/current in the brake coil is "collapsing" back into the brake coil windings for a period of time after the voltage source is removed. This can be easily checked with an oscilliscope or by placing a "free-wheeling" diode across the brake coil leads attached in reverse polarity. Identify which brake lead is the positive lead and make sure that the cathode of the diode is attached to it. If the magnitude of current and voltage being generated by the collapsing DC field is very high it will be necessary to include a resistor 4.7K ohms @ 1 watt (or larger) to limit the current down to the rating of the diode you use. The diode will "bleed" the generated current from the collapsing DC field so that it quickly dissipates without causing any damage. Most of the time the voltage generated by collapsing DC fields is very high while the current is minimal but large enough to maintain a "hold-in" situation in equipment.

Good luck,

I haven't had a chance yet to try out some of your solutions. The engineer and production decided to run the hoist as is and won't let me have it back to troubleshoot it. Some extra information though is that the brake actually worked before. Also, there are no other items in the circuit besides the rectifier (460VAC to 208VDC) and the brake coil besides the reversing contactors and the motor itself. We have an option to hook a control relay up to the rectifier in order to completely shut off the rectifier after the motor is turned off, but the engineer didn't want to do that. He wants to just replace the coil with the same module and see if that fixes the problem.

And yes I know it is dangerous to run a hoist that freefalls when you let off on the input. And yes I did recommend that they don't run it, but the hoist is past a safety gate so they decided to run it. I've noticed many companies talk safety, but when faced with having production down, most will also run a piece of equipment they know is unsafe just so they can run production.

Update: Hoist brake is now working correctly by the expedient of changing the rectifier from a half wave rectifier to a full wave rectifier at 208 volts then dropping to a half-wave after the coil pulls in. When the circuit is deenergized a relay completely interrupts power to the output of the rectifier and the brake snaps closed. This has brought the response time of the brake down to about 12ms.

Now if only the operators can learn to not let the hoist pick up the entire mold with the pig things will be just fine for the hoist.

Nice to get the update....

Nice to see the fix.

I was surprised that so many people have forgotten just what is needed on every relay to prevent exactly this problem though.........

If you have an electromagnet pulling in an armature and making and iron to iron contact, you have a magnetic short circuit that takes a finite time to drop away, several seconds are normal.

To prevent this, all relays have a copper or other non magnetic "nipple" to prevent an iron to iron contact at some pint, this prevents a magnetic short circuit and allows the armature to drop off immediately the power is removed.....

It would be interesting to find out if the final "fix" involved a new relay which had a fresh copper nipple, not hammered flat (so allowing iron to iron contact) by years of usage!!!!

Where are the real (old) EE's here???????

The final fix added a relay that wasn't present in the circuit. The entire assembly was less than a year old. I'm thinking that the problem was either contamination, or that something happened with the plate that the coil pulls in to make it seal in.

I am 100% sure that you fixed the problem by putting a relay in that still had its tiny copper (usually) nipple working to break the magnetic short circuit that can occurr when closed.....

Also I get the impression that nobody today understands fully why they HAVE to be there AND in a good condition!!

I have searched on the web and can find no detail of this from any manufacturer, but when you have a complete iron to iron contact magnetized, it takes a finite time for the flux to disintegrate and let the relay release.....exactly the problem you had!!

May I ask you to examine the relay that was originally used and if possible energize it. Then remove the voltage source and see how long it takes to drop out. I would not be surprised to hear that it takes several seconds!!!

There was not a relay originally in the circuit. The brake was pulled in by the voltage applied to the motor. There is now a relay in the circuit. The brake works MUCH faster now. Actually it works so fast now we have to make some other adjustments in how the system works.

We had faced this problem when we shifted from AC Disc Type brake to DC Disc Type brake in hoists mfd by us. Advantage of DC brake is that brake coil does not draw more current with increase in air gap where as in AC brake coil use to draw more current more current and subsequently resulting in burning of brake coils. In DC brake there is residual magnetism due to flow of the current in the coil even after switching the brake. We solved this problem by providing additional contactor in brake coil circuit to cut off the flow of current. With this modification we could solve the problem of load slipping while operating the hoist.

Then the problem was to do with the motor generating "Back EMF" and "powering the brake to the off position in some manner......not having a circuit diagram of how it was before, makes diagnosis somewhat difficult....