Need Some Advice Regarding a Handicap Lift, Odd Transformer Issue

This is about as far from audio as you can get! I need some help with an odd electrical issue and I'm not getting it from the company that built the lift or any techs that service this thing.

My Mom moved in during the pandemic and it was obvious she shouldn't be on her own. I purchased an old wheelchair lift and cut down the platform to install it in our home so she didn't have to use the stairs. It lifts about 21"

It is very basic, switches, relays, a delay timer, a transformer and a reversible 120 vac motor. Not even a PC board (I told you it was old!)

The 120 vac line goes directly to the transformer that drops to 24 vac. Before that voltage goes anywhere, it goes to a switch that completely disconnects the 24 vac from all other electronics. That switch is turned on manually when you want to operate the lift and turned off after. In a day, it is on for maybe 4 lift sessions that last about 1 minute each.

The 120 vac continues to relays as the motor runs off 120 vac.

My issue: After about a month of the unit being plugged in (constant 120 vac to the transformer), the lift will not operate. I see the operation lights go on in the relays and then sort of dim like a car trying to start with a partially dead battery. I can push the operation button on the relays (manual activate) and the lift will operate.

The solution is to unplug for about 12 hours, plug back in and it will work fine for another month.

I have 2 of every part for this unit. (3 relays and their sockets, the delay timer and the transformer). I figured I should have spares as this is old and I may not be able to find them in the future. When this first happened, I swapped every part (except the transformer) with no luck. I gave up for the night and when I came back, it was working. The same issue has happened a few times since and unplugging overnight seems to do the trick. I did replace the transformer after the first occurance.

The transformer always buzzes and is always pretty hot to touch even though the load is usually switched off. I replaced the transformer once already (it is a 40 va unit). With the switch off, the draw on the secondary side of the transformer is 4.3 mA, the resistance of the load on the secondary with the switch off is so high, my meter just calls it OPEN.

I'm guessing the transformer is overheating and that drops the voltage or current so it can't operate the relays?

Any suggestions?

Thanks,

Eric M.

Lift in operation:
 

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Why guess, hook up the voltmeter and see what the transformer secondary AC voltage is.
Since this is so important, you really should also draw out a complete schematic.
Shouldn't take more than an hour or so.

In the meantime, just switch off the incoming AC line with a switched wall outlet, or a power strip,
when not in use. Sounds like that might prevent the overheating problem for now.

The transformer may be heating up because it is operating with no load most of the time.
There may be an internal thermal switch inside the power transformer. Maybe you need a bigger transformer.

Here's a new, identical transformer.
https://www.amazon.com/WHITE-RODGERS-90-T40F3-Transformer-Output/dp/B00QW2AW5S
 
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It's a new transformer.
That's a good idea, test the output when it stops working. I should have thought of that. :sick:
I can't really do a power strip (She needs to be able to operate the lift when upstairs or down), but if I can't get this figured out, I'll probably look for some sort of remote power switch and just velcro the remote to the inside of the lift.

Thanks,
Eric
 
The transformer has a thermal circuit breaker built in. I suspect your AC line voltage is a bit high or has a DC offset. The circuit breaker is on the secondary since it trips from prolonged being energized not actual use, it will stay tripped as the primary is still energized and creating the heat that trips it.

Turning it off to cool down will work for a while, but I suspect the unit will eventually fail. So measure the AC line voltage from several different outlets and if it is high call the power company. If the outlets have different voltages, call an electrician to balance the circuits.

If the AC mains voltage is not high, just buy a better transformer.
 
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+1 on bigger transformer.
And try to put a staircase light circuit (you can switch on and off from top and bottom), that means no need to keep it on all the time, switch on only when needed.
And see the cap (if any) on the low volts side is okay or damaged...not visible after a glance at the photo.
Add one, after the seniors here telling you yes.
 
The 120 vac line goes directly to the transformer that drops to 24 vac. Before that voltage goes anywhere, it goes to a switch that completely disconnects the 24 vac from all other electronics. That switch is turned on manually when you want to operate the lift and turned off after.
Why not use a mains rated switch (125/250V) replace the existing switch and rewire it to disconnect the transformer primary instead of secondary. Also add a in line fuse for safety.

Screenshot 2023-05-22 at 21-41-46 In-line-fuse-holder-for-6-3x32mm-fuses-rated-to-at-least-7A-...png
 
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Have to agree it seems likely there is a thermostat inside the transformer. Have you actually measured the line voltage coming into the transformer? Line to Neutral, Line to ground, DC volts, AC volts, etc? Reason I ask is because there can be various reasons a transformer heats up, which it shouldn't do if there is no load or only if its only lightly loaded. If no load or a light load then the load impedance should be reflected to the primary making its impedance high too. Sometimes serious problems can occur where a neutral wire feeding a fuse box or breaker box develops a loose connection. That can be dangerous and its not that uncommon. Electricians always need to be on the lookout for such problems. If a loose neutral, the voltage at a load device may depend on what other devices connected to the same power panel are switched on or off.

Basic point is this: A transformer shouldn't overheat if you are operating it within its specs. If it does heat up enough to trip an internal thermostat then something is wrong somewhere and it should be investigated.
 
The issue with rewiring so the switch on the carriage controls the 120 vac primary is that flexible wires exit the control panel and ride along the frame to the switch. I'm a bit uncomfortable not have line voltage wires properly protected, especially while flexing up and down with the carriage very close to moving parts. That is probably why the manufacturer dropped the voltage to 24 and used relays.

Some sort of remote control like this would work:
https://www.amazon.com/VONVOFF-Wireless-Control-Switches-Security/dp/B07CTL3TG6?th=1

Darn, I really should have just gone with that and a light to indicate on / off on ther carriage.
Oh well, transformer is on it's way. We'll try that first.

Thanks,

Eric
 
I'm not sure there is any sort of thermal protection going on in the transformer. The model # is: 90-T40F3
Below is the cut sheet for the transformer. I don't see any mention of thermal protection on this unit.

chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/https://s3.amazonaws.com/s3.supplyhouse.com/product_files/White%20Rodgers%20-%2090-T40F3%20-%20Brochure.pdf
or this
https://www.supplyhouse.com/sh/control/product/~product_id=90-T40F3
and click the Brochure link

At the plug I measure 117.5 VAC. I get .5 mV DC

I don't think we have any loose neutral issues. I've dealt with those before. You'll have a lot more issues with other items on the same circuit.
 
Good.

Brochure describes the transformer as 'energy limiting.'

From: https://patents.google.com/patent/US5561576
Regarding an 'Energy Limited Transformer:'
"Typically the transformer is protected against a shorted secondary output either by a fuse or by an energy limiting construction which avoids unacceptable temperature buildup under a shorted output condition. A continuously shorted energy limiting transformer will get very hot and eventually will suffer irreparable damage."

As to why the transformer is getting hot in the first place, is it possible its not getting enough ventilation?

How much primary current does the transformer draw with no output load? IOW, how much magnetizing inductance does it have, enough to keep primary leakage current low and thus avoid self-heating?

When the lift is carrying someone upstairs, what is the transformer secondary voltage and current?

EDIT: Just found some more info: https://www.irjet.net/archives/V6/i3/IRJET-V6I382.pdf
Also: https://ieeexplore.ieee.org/abstract/document/1067663
 
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Thanks, very interesting. So there is a form of protection built into the transformer.
There was never any means of ventilation in the design. The picture you see is with the cover off of the electrical component box AND the cover off of the unit.
Although when I got it, ther cover on the electrical box was missing and I replaced it. I wonder if that was by design and not just a loss over the years?

When I get home tonight, I'll check the no-load primary current and the loaded voltage at the secondary.

Thanks
 
OK, the secondary voltage is 28.15 vac unloaded, the switch right at the secondary output turned off.
The voltage remains the same when the switch is on.
The voltage drops to 27.5 vac when the lift is in operation (All the secondary does operate relays)

The current draw at the 120 vac primary is .42 amps when the secondary is unloaded, the switch right at the secondary output turned off.
That increases to .58 amps when the switch is on and the lift is in operation.

Another note:
I had a lift company come out today to look at installing a second lift to access our backyard. While there I asked about this issue. He said it was a common problem in those lifts. Some had that problem, some did not. They never knew of a permanent fix but did swap out transformer's on many (just the original speced units, never an upgrade). If someone called with a similar issue, they told them to unplug it for a few hours and plug it back in. If it still didn't work, call back and they would schedule an appointment.
 
Not quite sure what 'in operation' means? The current draw of the motor should depend on how much work the motor is doing. Lifting 200lbs upstairs should draw more current than running an empty lift up or down.

Regarding when the transformer secondary is unloaded, if the volts and amps are in RMS values and if they are in phase with each other, then that would be about 50 Watts of power. Maybe enough to get kind of hot in an enclosed box. Real power is volts x amps x cos(phase angle): https://www.js-technik.de/en/power-calculation-ac-single-phase-motors-p-v-x-i-x-cos-f

Depending on how the transformer responds to heat, self-heating might leave it in a current limiting state.

Some more info on possible transformer construction attached.
 

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In operation is when the lift is moving up or down. But, remember, the motor is 120 VAC, all the secondary voltage does is operate relays. The line voltage splits before the transformer, going to the trans. and the relays. There is no additional load on the transformer other than the magnetic relays which switch on or off the 120 volt line to the motor. I measured the current draw at the primary side of the transformer based on the load the transformer was seeing. The draw of the motor should not play a roll in this. When it fails, the motor is not engaged or has not been engaged.

50 watts does seem like enough to cause heat and issues. It also seems pretty high for a transformer with the secondary disconnected. I did replace the transformer with the identical model #, maybe there is a flaw in the design? Back to the idea that a bigger or different transformer may be the solution?
The larger 250 VA transformer is showing up today. I'll need to wire it up and see if it does better.

Thanks for the info., I'll look it over tonight.
 
Okay, got it. It does seem unlikely that transformer voltage and current are exactly in phase, but we don't have a number for phase angle. That the transformer gets hot is the main clue. Other than that, enough current is being drawn with no load to make self heating of a normal transformer potentially plausible. A bigger transformer could turn out to be worse, if the causal mechanism is self-heating without a load of a normal energy limited transformer. Would depend on how the specific transformer is designed.
 
Well let us consider the data and options.

First off a 24 volt transformer is putting out 27.5 volts. It is possible the transformer is mis-spec’d, but very unlikely. However a 24 volt transformer would put out more voltage under load if the AC input voltage was high. There is one other specification that would come into play and that is called transformer regulation.

A poor transformer would have the output voltage go up 20% from full load to no load. A perfect transformer would of course not change. To accomplish that would require no core losses or winding resistance which unsurprisingly is not possible.

So if we had 10% regulation a 24 volt transformer would output 26.4 volts unloaded.

From your unloaded output measurements I would suspect your 120 volt outlet is really at 130 volts. That is actually within AC power line specifications. Thus implying a mis-design in the control box.

However the more likely explanation is that the AC mains are fed 240 volts into the residence and this is split into the nominal 120 volts in the circuit breaker box. You need to measure several different AC outlets. I suspect you will find some are at 110 volts and a few are at 130 volts. If that is the case a competent electrician can move the loads in the circuit breaker box to balance the loads and get the outlets to the nominal 120 volts.

However as the electric utility company may actually deliver more than 240 volts to the circuit breaker box and still comply with the applicable standards. In that case you will need to change the transformer.

Of course the other method is to remove the cover that you previously replaced. Of course that does reduce the designed in safety. The upside of that method is you can increase the life insurance of the handicapped resident. After all a fire at the base of the chair lift will be quite dangerous!

So in short, first check the AC outlets to see if the power is unbalanced. If the power is balanced but high, then a request to the company providing the power to lower the voltage would be in order. The final effort would be to change the transformer. Note that any transformer in this application will require a thermal cutoff.
 
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You mentioned 0.42A primary current when the transformer is idle, and 0.58A, not knowing anything about power factor, but it is quite possible this transformer is dissipating more than 40W just sitting around doing nothing. That primary current is just excessive for a transformer not delivering current to a load.

A quicker and very cheap experiment would be to get an Antek toroid to power the relays. Idle current will be around 20 - 30mA or less. (Note no thermal cutoff in windings you will need fuse on primary to provide short circuit protection.)

https://www.antekinc.com/as-0524-50va-24v-transformer/
 
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