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Old 15th Oct 2020, 9:36 pm   #1
ricard
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Default Slow acting fuse in Uher Lehrgerät

I have in my collection a Uher Lehrgerät, a single speed version of the Uher 5000, intended for language studies. It has two tracks, called a student and a teacher track, where the teacher track is normally not recorded over, and the student track can be re-recorded by the student, and both tracks listened to together so that the student can ascertain how well he or she is matching the teacher's pronounciation.

I have a problem with it however: In series with the motor, which is a Papst external rotor type, with a 3.5µF motor run cap, there is some sort of fuse, shown below. I've never seen this type anywhere else, but if I understand it properly, current passing through the fuse will cause the bimetallic springs to open, and the sprung wire held in the resulting jaws will spring to the left, thus opening the fuse.

The problem is that after having the machine on for a couple of hours, the fuse suddenly opened. It now happens within 5 or 10 minutes every time. Prior to this it had been in storage for some 10 years, but there were no such problems when I used it prior to that.

The fuse is labeled 150 mA which I assume is some form of nominal current over which it will open after a while. I measured the current through the fuse which turns out to be about 225 mA, or 245 mA with the motor stalled. My first thought was that the run capacitor was faulty, although I can't really understand how that would cause this fault - it's just connected between the supply and one of the windings. At any rate, I tried replacing it with a 2.2 and a 1 µF capacitor in parallel, but the motor current unsurprisingly remains the same.

Interestingly, the fuse is not shown at all on the schematics I have, so I think it may have been added for more stringent safety requirements here in Sweden. As such, I don't have much of an issue about bypassing it altogether, although at the same time I'm not too comfortable disabling safety features such as this.

I am at a loss though at understanding the discrepency between the measured motor current and the fuse rating. One reason I suppose could be shorted turns in the motor, although there seems to be no loss of performance (perhaps there wouldn't be if few enough turns are shorted).

Calculating the apparent power, it comes out at 225 mA * 170V = 38 VA (the motor is supplied by a measured 170V using the mains transformer primary as an auto transformer), which seems a bit excessive compared to the 35 W that the whole machine is specified for. Interestingly enough, the apparent power is also specified on the type plate as '40 va' (sic) for the whole machine.
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Last edited by ricard; 15th Oct 2020 at 9:51 pm. Reason: Motor supply voltage is 170V, not 230V; va rating specified on type plate inside machine
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Old 15th Oct 2020, 10:26 pm   #2
DMcMahon
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Default Re: Slow acting fuse in Uher Lehrgerät

Wondered what was different about the Uher Lehrgerät, now I know.

I see from the schematics that the Uher Universal Lehrgerät and Uher Universal 5000 internal AC fuse is rated at 300mA for your mains voltage. So your 245mA is quite close to the 300mA fuse value, which may also indicate that it is actually higher than it should be for whatever reason.

Looks like there maybe different mains transformers between the Uher Universal Lehrgerät and 5000 machines, as the schematics show slightly different primary tapings, with the 5000 having a 165V tapping that connects to the motor, whereas the Lehrgerät has a 155V taping to the motor (different to your stated170V). The 5000 has a 2.5uF Run capacitor.

None of this of course is any help to your issue.

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Old 15th Oct 2020, 11:20 pm   #3
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Default Re: Slow acting fuse in Uher Lehrgerät

Quote:
Originally Posted by DMcMahon View Post
So your 245mA is quite close to the 300mA fuse value, which may also indicate that it is actually higher than it should be for whatever reason.
Although, remember that the 245 mA is at 170V, so the 230V current is a bit lower.
Quote:
... the 5000 having a 165V tapping that connects to the motor, whereas the Lehrgerät has a 155V taping to the motor (different to your stated170V). The 5000 has a 2.5uF Run capacitor.
They could be using two different motors, despite the machines' otherwise similar appearance.

One thing that certainly must be different is the motor drive chain. The Lehrgerät uses a belt between the motor and flywheel, which is all right with its single speed, while the 5000 has three speeds and I would have thought a movable idler mechanism (although I suppose there could be a pulley shift mechanism instead).

One thing I did notice tonight was that the mains voltage on the machine was set to 220V, and we seem to have about 235V in the outlets where we live, which would account for any voltage (and current) readings being on the high side. We recently moved, and in our previous location I'm pretty sure the mains voltage was a tad lower, like 225V or so. Actually, with the machine now set to 240V, the fuse hasn't blown with the machine operating for about an hour. So perhaps that's the explanation; I still think that the fuse seems precariously close to tripping!
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Old 16th Oct 2020, 8:14 am   #4
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Default Re: Slow acting fuse in Uher Lehrgerät

Thats sounds promising. Presumably the measured motor voltage is now lower than 170V ?
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Old 16th Oct 2020, 10:14 am   #5
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Default Re: Slow acting fuse in Uher Lehrgerät

The item pictured is a resettable thermal cutout, and more commonly found in such items as bread toasters and domestic irons. Be aware that the white insulator material is most likely asbestos based, so try not to disturb it or inhale any dust from it.
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Old 16th Oct 2020, 12:12 pm   #6
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Default Re: Slow acting fuse in Uher Lehrgerät

Dave, do you mean that its purpose is to cut out if the whole thing becomes too hot? I was thinking that it was solely because of the current passed (and consequent rise in head locally due to the winding heating up), partly because the base is made of isolating material, but mostly because of the fine winding seems there to heat the bimetallic strip.

Thanks for the point about asbestos. I had not considered that before (but I wasn't planning to dismantle it either).

This one is certainly resettable, though not automatically; one must manually reposition the spring in the opening formed by the bimetallic strips.
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Old 16th Oct 2020, 1:08 pm   #7
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Default Re: Slow acting fuse in Uher Lehrgerät

I would do an insulation test on the motor run capacitor.
Also make sure the motor is not gummed up with dried up grease.
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Old 16th Oct 2020, 2:51 pm   #8
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Default Re: Slow acting fuse in Uher Lehrgerät

The device works by passing current through the wire windings which act like a heating element. I would guess that the wire is nichrome or similar. The heat from the wire is transferred through the insulator into the bi-metal strip, which in turn if hot enough will bend and therefore break the circuit. Some of the domestic appliances using this method would auto reset, such as thermostats, whilst others had a small reset button to remake the circuit. It would need to be exceedingley hot to make it trip by ambient air temperature. Given that it's tripping after a period of time suggests a slow increase of temperature, possibly caused by a slight over current. Perhaps due to a leaky motor run capacitor or other component that is likewise becoming leaky. Things to check are the supply voltage ratings, is it designed for 250, 240, 230 or 220v mains supply etc. If it is designed for 220 operation, then 230 might be enough to upset it over time. Also what frequency was it designed to work on 50 or 60HZ. It's possible that the equipment is not actually faulty, just being over run supply wise. This is just my thoughts about it.
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Old 16th Oct 2020, 4:11 pm   #9
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Default Re: Slow acting fuse in Uher Lehrgerät

Will be interesting if you can detect any obvious audible playback speed difference on old recordings done when machine was 220 volt configured, compared to now at 240 volts.

David
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Old 20th Oct 2020, 12:53 pm   #10
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Default Re: Slow acting fuse in Uher Lehrgerät

I'd be tempted to wire in a 500mA (say) conventional fuse in case the motor does develop shorting turns.
Interesting to see the Made in England label. I trust that's just the part causing trouble (!)
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Old 21st Oct 2020, 7:35 am   #11
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Default Re: Slow acting fuse in Uher Lehrgerät

Well, in a way, yes, but on the other hand since it's apparently rated for 150 mA I can't blame it for blowing when subjected to 225 mA.

The thing I can't understand is why they've chosen that component in the first place. If it had been a DC motor, the starting current would have been very high compared to the running current, so even a slow-blow fuse might have been too fast, but it's not, so why not just have an ordinary fuse if they wanted the extra protection?
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Old 21st Oct 2020, 10:45 am   #12
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Default Re: Slow acting fuse in Uher Lehrgerät

The only reason I can think of is if the motor stalled for some reason, maybe a jammed tape that could be quickly put right. That would give you some breathing space while it was sorted out which a fuse obviously wouldn't do.
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Old 24th Oct 2020, 10:54 pm   #13
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Default Re: Slow acting fuse in Uher Lehrgerät

Yes, that would make sense, if the motor current significantly increased when it stalled. It doesn't seem to, but on the other hand, if it for whatever reason there was something in the drive train offering a significant braking force, that would explain why the initial motor current was as high as 225 mA, increasing by a mere 25 mA to 250 mA when stalled. It doesn't behave as if there is any specially high load though, the rotor turns freely with all belts in place.

I really don't know how these induction motors behave when stalled. In a DC motor there is a significant back EMF when the motor is running, keeping the motor current down, which is absent when the motor is stalled, with only the DC resistance in the windings remaining. Conversely, the coils in an induction motor are always in the circuit the same regardless of the state of the rotor, however, the rotor must have some effect on the magnetic properties of the laminations as seen by the coils I would think. For instance, I remember reading that a syncronous motor, running unloaded, behaves largely like a capacitor in terms of the electric properties, and are (were?) used as phase compensators for industries that have large inductive loads.

I finally got around to measuring the speed of the machine, using a speed check / azimuth test tape I have. According to the measurements, the machine runs about 0.4 % fast when set to 240V, and about 1% fast when set to 220V. So these figures don't point to any unduly high load on the motor, if anything they indicate that the machine has a bit of reserve when it comes to motor torque.

Last edited by ricard; 24th Oct 2020 at 11:01 pm. Reason: Thoughts on the properties of stalled induction motors.
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Old 24th Oct 2020, 11:47 pm   #14
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Default Re: Slow acting fuse in Uher Lehrgerät

Induction motors exhibit saturation if the voltage gets too high, and the current they take increases rapidly. The difference in voltage you're talking about does seem too small for a massive current increase, but it will be non-linear and accelerating.

Back in the older days at HP we used papst outer-rotor induction motor cooling fans. 115v ones with the mains transformer primaries serving as an auto transformer for them.

An inevitable error is someone plugging a box set to 110 or 120v into a 240v socket. We had to test this to show that the fuse would pop with no further damage. The transformer climbed into saturation and took more current, but the fans were the star performers in pulling the fuse. They really did saturate deeply and quickly

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Old 25th Oct 2020, 12:00 am   #15
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Default Re: Slow acting fuse in Uher Lehrgerät

Did we ever get round to doing an insulation test on the run capacitor while it is out of circuit?
Checking its value out of circuit would be a good move too after is has been discharged obviously.
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Old 25th Oct 2020, 2:15 am   #16
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Default Re: Slow acting fuse in Uher Lehrgerät

With several watts of power going AWOL, there ought to be some extra heat appearing somewhere.

Do you have another machine to compare against? Language lab stations used to be in groups of several.

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