On Sun, 2006-06-18 at 16:58 -0700, Rideout Family wrote:
> Marsh, the reason fuses are required with motors is the short circuit
> current or AIC (ampere interrupting capacity). It would not be unusual
> for a three phase 40hp motor to fail and have a short circuit of 100,000
> amperes. The maximum rating of circuit breakers is 50,000 AIC so the
> breaker will explode. Fuses for 40hp motors would be required to have
> ratings of 250,000 AIC. Next time you visit Home Depot check their
> fuses you will find several price ranges which are related to the AIC.
> As for 400hp motors they sometimes use what looks like a circuit breaker
> but is only a disconnect and an integral part of the unit is a fuse.
> All large motors are protected by fuses.
>
> Best
>
> Art
>
Its NOT the motor that sets the short circuit current so much as its the
power system. The motor can contribute short circuit current to a fault
elsewhere for a few cycles and that contribution raises the fault duty
of equipment in the system, but its the low impedance of the large power
system that leads to the large interrupting capacity needed for breakers
and fuses.
I've been using regular blow 16 amp type ATC fuses to protect my Corsair
II for years and they have worked. You see, a plain fuse is rated to
carry 125% of nameplate current indefinitely. There are super fast fuses
made for semiconductor "protection" but most of them are more reliable
as indicators of fried semiconductors than protectors. They tend to use
silver wire as the fusible alloy and so are costly.
The circuit breaker protection of TenTec PAs is not for PA oscillations,
but for bad mismatches. At least in the Corsair II and before the PA is
quite sturdy and tolerates mismatched loads that many other makes of
radios would depend on PA protection based on reducing the drive from a
detected high SWR. TenTec didn't do that, they made the PA survive those
and the circuit breaker (or my fuse) acts as thermal protection when the
load reflects a low impedance to the active device outputs through the
internal low pass filters, coaxes, and transformers. That low impedance
load would have the PA produce more RF power which it is capable of
doing but the heat sinking isn't up to handling the extra power, so the
kind of fast acting circuit breaker or a moderately fast fuse ARE enough
to protect the PA. These are thermal, not instantaneous protection.
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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