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Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714
Mads Barnkob wrote ... See page 1 for a overload curve, %overload vs. time ,%20M5%20,%20M10,%20and%20M20%20Variac%
20Autotransformers.pdf
Great reference there, Mads. It's for a series of 400 Hz products, which is how a 3 kVA autotransformer can weigh in at only 13 pounds. I bet the overload vs. time curve is reasonably valid for 50 and 60 Hz designs.
Reminds me of one company promoting 400-Hz motors (and associated 3-phase alternators) for applications like concrete drilling. Here's a 7.5 HP (5.6 kW) motor weighing 16 lbs (7.3 kg):
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
klugesmith wrote ...
Mads Barnkob wrote ... See page 1 for a overload curve, %overload vs. time ,%20M5%20,%20M10,%20and%20M20%20Variac%
20Autotransformers.pdf
Great reference there, Mads. It's for a series of 400 Hz products, which is how a 3 kVA autotransformer can weigh in at only 13 pounds. I bet the overload vs. time curve is reasonably valid for 50 and 60 Hz designs.
Reminds me of one company promoting 400-Hz motors (and associated 3-phase alternators) for applications like concrete drilling. Here's a 7.5 HP (5.6 kW) motor weighing 16 lbs (7.3 kg):
The 400Hz rating slipped my eye, it was the first curve I found :)
I remember seeing other curves where it was more conservative, as in 300% maximum overload at 30 seconds, 200% for 5 minutes .
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
klugesmith wrote ...
It's for a series of 400 Hz products, which is how a 3 kVA autotransformer can weigh in at only 13 pounds. I bet the overload vs. time curve is reasonably valid for 50 and 60 Hz designs.
Reminds me of one company promoting 400-Hz motors (and associated 3-phase alternators) for applications like concrete drilling. Here's a 7.5 HP (5.6 kW) motor weighing 16 lbs (7.3 kg):
450Hz is commonly used on aircraft, for this very reason.
50/60Hz was only chosen for mains supply because it was the limit of technology (3000-3600RPM) at the time the decision was taken.
(I live in 50Hz land, but regularly crank my 7.5kVA 50Hz generator up to 3600RPM, 60Hz@300V)
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714
220A, where 30A is intended, would set a lot of fires in 3 long seconds. yea?
Nay. The rate of temperature rise is limited by the thermal mass of the copper wire.
Suppose the circuit uses AWG 10 copper wire, which in ordinary applications can carry 30 amps forever. For short circuit calculations where the wire is initially at 75 degrees C, and can briefly withstand 150 degrees C, the I-squared-t rating for AWG10 is about 300,000 ampere-squared-seconds. For example:
100 A for 30 seconds
1000 A for 0.3 seconds
220 A for 6 seconds.
Protection device is supposed to interrupt the circuit sooner than that.
Registered Member #54278
Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367
klugesmith wrote ...
220A, where 30A is intended, would set a lot of fires in 3 long seconds. yea?
Nay. The rate of temperature rise is limited by the thermal mass of the copper wire.
Suppose the circuit uses AWG 10 copper wire, which in ordinary applications can carry 30 amps forever. For short circuit calculations where the wire is initially at 75 degrees C, and can briefly withstand 150 degrees C, the I-squared-t rating for AWG10 is about 300,000 ampere-squared-seconds. For example:
100 A for 30 seconds
1000 A for 0.3 seconds
220 A for 6 seconds.
Protection device is supposed to interrupt the circuit sooner than that.
This was discussed at 4hv about a year ago:
I have always wanted to learn about that (I^2)t stuff. All I need is a tiny little push. I can easily handle the rate of temperature rise (dT/dt), thermal mass of materials, and can do the thermal calculations for proper heat sinks, given the thermal resistance (deg/W), in steps from junction to ambient).
In fact (kludgesmith), your comment made a big difference. A lot of this started when I was trying to learn about the so-called ("ACTION INTEGRAL") usually associated with SCR's.
In fact, this "action integral", mostly what i saw by -electroholic-, way before I joined, was reason for my VERY FIRST post here!! and a main reason I joined--Of course, now I see there is so much more here!!
EDIT, OK--got it--thanks kludgesmith. Now for that 'action integral' application.
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How cruical is a VARIAC?
