[GreenKeys] M15 How to disable unshift-on-space?

[Bruce Gentry]

There are two reasons I can think of, there are likely more. First, six 
amps would be about right for a 24 or 32 volt motor. I don't know if 
there were machines produced for this voltage, but it would seem likely 
there were. Secondly, the electrical codes have changed greatly. In the 
past, overcurrent protection was only to prevent fires. If the 
manufacturer could satisfy the testing laboratories that components 
failing would not start a fire, nothing more was required. The six amp 
fuse rating was probably to protect the wiring inside the equipment,  
switch, and power cord.  Damage to the equipment itself was of little 
concern so long as the fire hazard was acceptable. Codes for equipment 
intended for commercial use were also different from products intended 
for residential use. To protect the motor today, a thermostat on it 
combined with an appropriately sized fuse is probably the best idea.  
The difference in current between running and stalled is not that great 
for a permanent split capacitor synchronous motor, hence the large 
cooling fan, and the governed motor will usually only draw about twice 
it's rated current on 50/60 cycle AC when stalled.  On 25 cycles or DC, 
the current rise would be greater. A thermostat would also protect 
against overloads that do not stall the motor  but burden it beyond it's 
rating.

   Bruce Gentry, KA2IVY



On 9/19/15 5:32 AM, Dave Horsfall wrote:
> On Fri, 18 Sep 2015, drlegendre . wrote:
>
>> And while I'm at it, why does the circuit spec. a 6A fuse for a 1/40th
>> HP motor? 6A @ 117V is like 750W - and 1/40th HP is like... a tiny
>> fraction of that. How could a 6A fuse ever protect such a motor, even if
>> it went into 10X overload?
> Because the motor will blow in order to protect the fuse.
>
>
>
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