If it has internal overload protection you are only protecting for a
short circuit and the 20a branch circuit breaker is usually
sufficient. (less than 2.5x the motor FLA)
Yes the 9a fuse may not be enough to hold the starting current over
time or it might be too much to protect from overloads. That is why
motor manufacturers will usually install internal protection on these
small motors. Then the overcurrent device only has to protect the
wiring up to the motor and protect against an internal short in the
Any reason that you didn't bother to disclose all the important motor
characteristics and details? Single phase or 3 phase? 1725 rpm or
something else? The nameplate of your 1/2 hp motor should specify the
operating current at 208v. What is the nameplate operating current?
Extra credit for the maker and model number.
The problem is that starting current can be 3 to 4 times the nameplate
oprating current. Your common 9A slow blow cartridge fuse isn't going
to work if your motor operates at about 4A.
You have some choices. Try plugging into this calculator and pick the
fuse type (non-time delay, dual element time delay, instant trip, or
inverse time trip) that you find appropriate.
Jeff Liebermann firstname.lastname@example.org
150 Felker St #D http://www.LearnByDestroying.com
How does thermal protection work? I understand the safety mechanism, but if a
motor is heating up under load, isn't it drawing over-max current? Shouldn't
the (carefully-chosen value) fuse blow?
I need to learn about this, I know...
The thermal is actually looking at the winding temperature and a
marginal overload that accumulates heat in the winding over time will
cook off the thermal where a fuse might not go.
It is fairly unusual for fractional HP motors not to have a thermal.
They are generally designed for connection to regular 15 or 20 amp
circuits. It should be indicated on the motor labels.
The phase angle of induction motors changes with varying load.
The current draw only changes a little with loads from zero
to the motor's rating. So, if the motor is rated
at 1/2 Hp, 208 V, the running current at rated load
will be something just over 2 A.
I = (745 W * 1/2 * 1.2 (efficiency))/208
At idle, the current may be 1.5 - 1.7 A with a very low power
factor (phase angle near 90 degrees lagging). At full load,
the current will be about 2.1 A with a phase angle of maybe
5 - 10 degrees. This, the motor draws more REAL POWER at
full load, but the current doesn't change much throughout the
range the motor is rated for. This is why external fusing
for motors is to protect the building wiring from fire, it
does not protect the motor from minor overload or overheating.
Internal thermal protectors actually measure the temperature
INSIDE the motor, and thus are a lot more effective.
There's usually a fair bit of heaving and grunting as a motor starts
spinning - especially if it starts under load.
Even a slow-blow fuse that can handle the second or two of what is pretty
much stall-current, will carry enough steady running current to let the
magic smoke out if a fault/overload develops.
A fuse is not a replacement for a properly-sized motor OLP overload
A fuse will protect wiring, as previously stated.. but you'll just keep
asking the question until you get the answer you want, right?
Overload protectors and TP thermal protection devices aren't the same, and
are intended for completely different situations.
The OLP will react rapidly to protect a motor in the event of a sudden
machine jam or other fault.
The application was previously stated to be a bandsaw which previously had a
smaller motor on it.
Generally, fuses aren't motor protection devices, and won't replace a proper
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