quick and dirty way to determine proper fuse?

[BrianS]

Today I had to rewire a Leslie 860 speaker that had long ago had its amplifier removed.  I upgraded and "properly" wired the switches that change the speeds of the 2 motors. 

I added a master on/off switch and a fuse to the circuit, and this is how I determined the fuse rating: I plugged the unit into a Kill a Watt meter that I have mounted to my Variac and set it to measure current.  I thought I put a high value fuse in the holder, but it turns out I put 1 amp Slo-Blo fuse in there instead.  This turned out to be what I think would be the correct fuse anyway.
When I fired up both motors (individually switched), they drew around 1.2 amps of current (going off memory here) and leveled off at .93 amps.  Running one or the other motor individually was either .43 or .69 amps (give or take...).  The fuse never blew after being turned on and off several times.
Obviously this wouldn't work in many situations and I'm sure engineers are rolling their eyes, but in this instance, it seemed like a reasonable way to quickly determine a fuse rating.  The unit worked before I rewired it, the wiring was just very "hacked" and unsafe.
Not sure if this is useful info or not...I'm always up for learning something, though.  I wonder if I'm running too close to the maximum current rating of the fuse? 

[shooter]

I'm sure engineers are rolling their eyes

always fun to play with engineers 

Kill a Watt meter

I killed my Kill-O-Watt 
they are great, but really don't like modified sinewave invertors much

[PRR]

A motor's running current is always less than the Starting Current.

0.93A run current on a 1A fuse, even SloBlo, may be skating close to the edge for long term reliability. I'd round-up to 2A already.

Motors also have a Stall Current. Get your wood saw JAMMED in twisted oak, the lights dim from the excess current, and the motor gets HOT. While a saw has to be protected from this, smaller motors are often sized to run "stalled" for many-many minutes. Leslie could have seen that some drunk could get his arm jammed inside and stall the motor, and engineered it to survive a while. And the player would prefer not to have to replace a fuse every time this happens. If you can do it safely (leather glove), stall the motor for a second and see how it reads.

The #1 goal of the fuse is not to let the line-cord burn (set fire to foam panels, burn the joint down). At this point the #2 goal IS to protect the now-precious motor. #3 is to rarely/ever blow "for no reason". A fuse can never provide 100% protection, especially to a motor where 80% of the input power goes to the load (spinners) and normally only 20% heats the motor.

My Kil-A-Watt died on good utility power. May not have been your bad waveform (I don't see how it is sensitive to that). I think it is such an iconic product that they no longer care about build quality, and they fail randomly.

[shooter]

+1 on the 2A or split the difference at 1.5A slo

they no longer care about build quality

sadly that's the industrial standard now
the boutique, craft, n job shop still appear to be good finds for quality

[Ritchie200]

See also:


https://el34world.com/Forum/index.php?topic=24921.0


Check out the two links posted.


Jim

[trobbins]

Good to see you used a kilawatt-style meter - they are indeed very handy, and worth being on every bench as a test tool imho.

Assuming you are in north america, then your general purpose fuses are designed to operate with no more than 80% operating current of the fuse rating.  So 1.2A is probably as low a rating as you should go.

The other aspect is whether the fuse is standard, or fast blo or slo blow.  Given you have a start-up surge type of load, and it is noticeable on the meter, then slo-blow is also an appropriate spec.

[BrianS]

Thanks for the insights.  I'll bump the fuse rating up.   I did read the '56 article posted by PRR, but did not see the other link in the thread until now...thanks Jim!
It seems like this Leslie is not very well regarded, but it does sound really good for guitar anyway. 

Thanks again.