Perplexed!

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Harold_V
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Perplexed!

Post by Harold_V »

Hey, guys!
I'm having a little trouble understanding what's wrong with the small electric lift I use with my shop gib crane. Here's the deal.

The unit operates on 120 volts and is single phase. It has two large capacitors (wired in parallel) that I assume are for starting, as the motor has no starter windings, nor does it have an internal starting switch. You likely understand by now that this motor will run in both directions.

The brake assembly has an air gap requirement of .3mm which I think I understand, and know how to adjust, although the manual suggests it should be adjusted only by an authorized dealer, of which there are none. The lift was made in Italy and is no longer produced. The company from which it came has no information beyond that which came with the lift when I made the purchase many years ago. It sat in storage, waiting for me to complete my shop, which many may understand.

To raise or lower the hook, one simply presses a toggle switch in the appropriate direction. The problem I'm having is that recently began failing to turn when operated in the down position. When it won't, the armature hunts and the motor may or may not eventually start. There is considerable brake chatter when the motor does not turn. If it starts, small chance it will start in the wrong direction. It will readily start in the down direction with just a hint of help by finger. It operates fine--it just doesn't want to self-start in that direction.

The brake assembly is quite interesting. On the end of the armature there's a machined pad on which the brake bears when the motor is activated. I suspect that the armature is the magnetic source that pulls the steel member of the brake assembly inward, releasing the brake. It moves freely and the degree of braking effort is easily adjustable.

I guess what I'd like to understand is why it starts in the proper direction, and why it does not in the opposite direction, which it did routinely until the past week. Any ideas?

H
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Steggy
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Re: Perplexed!

Post by Steggy »

Harold_V wrote: Fri Jan 27, 2023 12:56 am I guess what I'd like to understand is why it starts in the proper direction, and why it does not in the opposite direction, which it did routinely until the past week. Any ideas?

Disclaimer: I am offering a SWAG regarding how the hoist’s motor operates, since I don't have a schematic or the machine in front of me.

Based upon what you described, it sounds as though the motor has a split-winding stator and uses the capacitors to shift one winding’s phase relative to the other. This causes enough magnetic field rotation to start the motor. The (center-off) switch likely selects which stator section is to undergo phase shift, which then determines the direction of rotation.

I’d start by checking the part most likely to fail, which would be the switch. You need to verify continuity through the switch in both run positions. If the switch checks out, then there’s the possibility that a capacitor has gone bad or one of the stator windings has opened up. The stator can be checked with a suitable analog or digital multimeter (DMM). Checking the capacitors requires a DMM with capacitance-measuring capability. The DMM should be able to measure into the 100s of microfarads range to provide meaningful results.

Needless to say, verify that there is no line power to the hoist before you start testing. Also, be cautious around the capacitors. They can hold enough of a charge to give you an unpleasant shock if you make contact. I recommend you short them out with a jumper while poking around with the DMM, which can be damaged by a capacitor discharge.
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Bill Shields
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Re: Perplexed!

Post by Bill Shields »

Switch, capacitor, wiring are usual suspects....esp if it runs one way and not the other.

Old age gets caps (like people).

A corodwd connection or bad solder joint falls into concept of bad wiring.
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Harold_V
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Re: Perplexed!

Post by Harold_V »

Thanks for giving me some guidance. I have just enough experience and knowledge of electricity to be dangerous, but it appears that the mystery has been solved.

I had discounted the possibility of the switch being defective because the motor would operate properly in both directions. That was a mistake! One of the poles of the switch has no continuity, but due to the way the switch is wired, it powers a coil, just not the right one when that direction is chosen. A new double pole double throw momentary contact switch of acceptable capacity should put me back in business. Too bad I didn't arrive at that conclusion before I removed the hoist from service, which is no small chore. Routing the cable, alone, is a dreadful job.

Steggy, I suspect that your assessment of the winding configuration of the motor is spot on, but I was not aware of the science behind that kind of construction. I also appreciate your comment about the caps. I am aware of their potential, and, in fact, had shown Susan (my wife) their great potential by discharging them. Those two hold an amazing amount of energy!

Now to go online and find a switch.

Again, thanks, guys.

H
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Bill Shields
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Re: Perplexed!

Post by Bill Shields »

:mrgreen:
Too many things going on to bother listing them.
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Steggy
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Re: Perplexed!

Post by Steggy »

Harold_V wrote: Fri Jan 27, 2023 7:12 pmSteggy...I also appreciate your comment about the caps. I am aware of their potential, and, in fact, had shown Susan (my wife) their great potential by discharging them.

Since we’re talking about voltage, do I detect a pun in there? :D I’d be shocked if that weren’t the case. :shock:
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Harold_V
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Re: Perplexed!

Post by Harold_V »

Steggy wrote: Sat Jan 28, 2023 12:30 am
Harold_V wrote: Fri Jan 27, 2023 7:12 pmSteggy...I also appreciate your comment about the caps. I am aware of their potential, and, in fact, had shown Susan (my wife) their great potential by discharging them.

Since we’re talking about voltage, do I detect a pun in there? :D I’d be shocked if that weren’t the case. :shock:
Chuckle! Shocked, eh! :wink:

Only by coincidence, although I'm aware that voltage is referenced as potential. I'm not a very clever guy and rarely think in riddles. The world is pretty much black and white to me.

Maybe you can address something I noticed when I was checking voltages. While the lift operates @ 120V, when I checked the voltage at the caps it registered over 300. The reading was taken at the second cap, which is wired parallel with the first one. The resulting voltage reading was a complete surprise to me. I'm using an old Triplett analog meter, one I know and understand. It's been reliable, so I don't suspect it's responsible for a reading that makes no sense. What gives?

H
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Steggy
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Re: Perplexed!

Post by Steggy »

Harold_V wrote: Sat Jan 28, 2023 1:56 amMaybe you can address something I noticed when I was checking voltages. While the lift operates @ 120V, when I checked the voltage at the caps it registered over 300. The reading was taken at the second cap, which is wired parallel with the first one. The resulting voltage reading was a complete surprise to me. I'm using an old Triplett analog meter, one I know and understand. It's been reliable, so I don't suspect it's responsible for a reading that makes no sense. What gives?

Was your meter set to read DC or AC volts?
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Harold_V
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Re: Perplexed!

Post by Harold_V »

The meter was set for AC @ 250 volts. When it pegged the meter, I switched to the last voltage choice, one for 1,000-5,000 volts.

Doesn't make a lot of sense.

H
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Steggy
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Re: Perplexed!

Post by Steggy »

Harold_V wrote: Sun Jan 29, 2023 4:03 am The meter was set for AC @ 250 volts. When it pegged the meter, I switched to the last voltage choice, one for 1,000-5,000 volts.

Doesn't make a lot of sense.

Actually, it does.

The motor windings are likely wired in series with the capacitors, setting up what we electronics types would call a series-resonant circuit. Resonant action as the motor is running will cause voltages across the capacitors to be significantly higher than the applied line voltage, especially if the motor is lightly loaded. That’s the overly-simplified explanation, but should be sufficient to settle any worries you might have.
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Harold_V
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Re: Perplexed!

Post by Harold_V »

Thanks! While the explanation is a bit of an overload for my feeble mind, it does make sense now, and I thank you for that. The condition you described (light load) was precisely the condition (no load at all).

Great venue, this board, where those who know so willingly share their hard gained knowledge with others.

H
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Re: Perplexed!

Post by Steggy »

Harold_V wrote: Sun Jan 29, 2023 4:24 pm Thanks! While the explanation is a bit of an overload for my feeble mind, it does make sense now, and I thank you for that. The condition you described (light load) was precisely the condition (no load at all).

Something I meant to earlier mention was when using an analog meter to check a circuit carrying an unknown voltage, always begin with the meter set to its highest range. Pegging a d’Arsonval meter movement, the type found in most multimeters, may cause unseen mechanical damage that will permanently degrade accuracy. Once you have an idea of the maximum in-circuit voltage, you can safely reduce the meter's range to get a more precise reading. As a general rule, analog meters produce their best accuracy when the pointer is at or near full-scale.
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Music isn’t at all difficult.  All you gotta do is play the right notes at the right time!  :D
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