Home Forums General Discussion Technical Questions Single Speed AC Otis

We have been called in on a unit and thought that we had it done.
Several companies have looked at this and told them they needed a mod.(a whole different subject please)
It is a 1958 circa Single speed AC unit, no selector. It was blowing the mainline fuses when it took off. The problem we thought, and still do,was the brake was not releasing fast enough and the motor was trying to pull through it. You can see the brake pick just a little, the motor hums pretty good and then the brake will pick all the way. We first cleaned all contacts, adjusted the resistors, to get full brake voltage from the rectifier. It worked for about a week and then started blowing the fuses again. We then did a complete brake teardown and adjustment. We pulled apart the brake pistons, cleaned them, turned the sleeve, lubed it with graphite and replaced the coil to be on the safe side. The new coil resistance was the same as the old one. It worked this time for two weeks and today it blew one of the mainline fuses. They are 30 amp, with 25 amp fuses inside the controller. The rectifier was supposedly replaced a couple of years ago. The rectifier while not under a load will produce 160 volts. Once under a load it drops to 127 volts, which we are getting to the coil. It will only blow the fuse at take off, once the brake has lifted it will travel the entire 60 foot of shaft with no problem.

Any ideas?

Does this car apply full voltage to the brake instantly? Or does it keep the brake partially applied to prevent rollback?

is it possible that a corner of the brake lining is not picking off of the pulley, and then starts heating the pulley up as the day goes on, and at some point the pulley has expanded enough that it's not clearing the lining. If the problem doesn't happen until the car has run for a while, it could be worth a look. Also are the fuses in the controller dual element or slow blow? They should go before the main line. Have you checked the connections in the main line for tightness, sometimes they loosen up and cause this situation. Just some thoughts, I have had all these happen to me. One last one, make sure you have oil in the gear box, and the gears aren't bottoming out.

add oil in the motor bearing cups and check the sliggers. had a 1926 2 speed ac motor do the same thing. added a teaspoon of oil and the thing hasent done it since

I would also look at running temporary brake wiring, along the floor away from other wiring, to the brake just in case your getting a leak either thru other cabling(induced ac voltages) or your having a under load break in the wiring so your voltages drop to the coil.

How did you go mate?

It has not blown the fuse again. I am not holding my breath. Will keep you informed.

UPDATE

We received a call back out this weekend. Two of the fuses in the disconnect were blown. This is the first time we have been back since my posting. When we replaced the fuses it ran fine while we were there. Got a call yesterday and the fuses were blown again. This time we changed the oil in both bearings of the motor, and the repair tech noticed that the contacts were arching, spent about an hour filing and adjusting the contacts to keep it from arching and not blowing the fuses again. (it blew several times while they were there) No call today. I guess we will keep on it until we get under control. They are not calling anybody else, and seem to be happy with us.

Just thinking out loud...

How many amps are you showing in each direction with a empty car? With capacity? With a balanced load? You should see a balanced load between 40 and 50 % capacity. Remember when checking for a balanced load that your amps and your car speed should be the same in both directions.

Also have you meggered out the motor? Have you opened up the motor and checked the rotor. Broken bars will cause problems.

I've bumped this up as I think this should be your start maybe after you have tested the motor like JKH just stated. It appears that you have 2 phases shorting or overloading and those 2 are the ones id look at. Also make sure your arc shields are in place as they can track across them.

Any chance someone changed the stationary and movable contacts prior to this trouble starting? Otis used two types of materials for their stationary contacts. Could someone have replaced the metallized (copper) stationary contacts with carbon contacts? Like a voltage drop, switching to a carbon contact would create a drop in current.
Check and see that the carbons are even and the copper paddles have plenty of wipe.

Is this a 2 of 3 phase unit?

Sorry SolidState

I was trying to ask is this unit a 2 OR 3 phase unit...

Any conclusions?

It is a three phase unit. Just as it ran for a while after replacing the brake coil and brake tear down. It has ran now for a week without blowing the fuses. It was blowing different fuses in the mainline, not the same ones each time.

We have opened up the motor and looked at it. We have made sure the contacts are even, wipe good, and the shields are in place. Your comment about the material is a likely possibility, did not even consider that.


When we go back out there again, we will look at the contact material, check the amps with a loaded car, and try uppo's suggestion about the brake wires.

Thanks for the follow up.

hey solid, this is really weird. Is the motor hum because of the brake holding or because of a motor issue? have you meggered it? By the way ive never seen a 2 phase motor!! I'm trying to remember the drawing and would imagine the reverser cct as u or d and br then all we are doing is reversing 2 phases for direction. By having different fuses blowing indicates its not the same phases overloading which makes this harder. Do you know where the lift is in the shaft? I wonder if the slipper guides are tight on the rails at a specific point in the shaft? Im just spit balling here. When you run temp brake wiring I would also look to tong test the 3 phases going to the motor as well and even run temp wiring for these too. This type of intermittent faults can be a prick to find. Ultimately you have an over current situation and you have to isolate everything with this in mind.

Solidstate
Have the overloads ever tripped? Are you sure they work? Are they adjustable? I would think if your brake doesn't open the overload would trip. You could do a current injection test. Although most of us probably don't have that type of equipment. You could ask you local motor repair shop if they can test the overloads. But it would be less expensive to just replace them with solidstate overloads.
Here's my concern...
If you block out the break from lifting and the overload doesn't trip then the motor will take a big hit.
How do the stator windings look? What have they meggered out too?

Brett
I will send you an old print from a tear out. Nothing like a print to refresh your memories.

thanks mate.

I would seriously consider bring your motor rewinders in to test the ac machine properly(they can test the motors better than we can with their equipment and knowledge). if they check out ok then at least you've isolated that from your fault finding.

I second having the motor tested.

When you're done that mod it out

So what was the end result ??