Just checking to see if I understand joules.
1 Joule = 1 Watt * 1 sec, or J = 1Ws
If 1.5M Ws is the energy dissapated by a resistor (with a value 2
Ohms), does this mean that there is 866 Amps every second thru the
resistor?
Thanks for any push in the right direction.
Bill
Amps is a measure of the current flow. So 'amps every second' is not
appropriate. If a resistor is dissapating 1.5MWseconds, we need to know
over what time period it dissapates this energy. For example, it could be
1.5 MW of power for 1 second (one heck of a resistor), or it could be 1.5
watts of power over 1 million seconds. Both total 1.5MWseconds of energy.
With a 2 ohm resistor, 1.5MW of power would require P=I^2*R or I =
sqrt(P/R) = sqrt(1500000/ 2) = 866 amps. But if it was over a period of 1
million seconds, 1.5 watts would mean I = sqrt( 1.5/2) = 0.866 amps.
Hope this helps...
daestrom
Daestrom,
The ON time is a value that I wasn't told (well....sort of). The
person that I talked to stated that the resistor had be rated for 1.5M
Ws (at 2.3 Ohms) , which I interupted it as being a continous rating of
807 Amps (but that isn't the current rating required due to the fact
of it being a dynamic braking application of a 250HP drive). After a
few minutes of small talk, I asked how long the load would be applied,
he said 6 seconds.
So I decided before I call him back and say, "Hey, do you mean 250kW
second?", (250kW x 6s = 1.5M Ws)
The wattsecond rating for a DB resistor is the maximum amount of energy
that the device can absorb over a short time period without damage. The
energy is stored as heat and transferred to the environment as the resistor
cools off over a much longer period. Part of the heat energy is radiated,
most is transferred by convection to nearby air.
If the motor has a fixed field, DB current jumps to approximately armature
voltage divided by resistor ohms. It then decays toward zero along an
exponential curve. If this is a voltageandfield drive, DB current may be
constant until speed drops below base speed ... depending on how the field
regulator is set up. After that, current decays exponentially. In either
case, DB current changes with time during the stop and the resistor's
continuousduty current rating can be much lower than peak DB current.
The resistance is chosen to limit peak current at the beginning of the stop
and the wattsecond rating is set by how much kinetic energy must be
temporarily stored as tolerable temperature rise in the grid. The resistor
must be mechanically strong enough to withstand the magnetic forces
produced by the high current pulse as well.
Roby
 The ON time is a value that I wasn't told (well....sort of). The
 person that I talked to stated that the resistor had be rated for 1.5M
 Ws (at 2.3 Ohms) , which I interupted it as being a continous rating of
 807 Amps (but that isn't the current rating required due to the fact
 of it being a dynamic braking application of a 250HP drive). After a
 few minutes of small talk, I asked how long the load would be applied,
 he said 6 seconds.

 So I decided before I call him back and say, "Hey, do you mean 250kW
 second?", (250kW x 6s = 1.5M Ws)
What you need to find out is the actual current during the 6 seconds of
braking. That could vary with the motor speed, too, so the worst case
could be right at the start (when the resistor is cooler). Merely MWs
doesn't give the whole picture. The thermal environment can matter, too,
if the resistor will still have residule heat from previous application
as now it will start at a higher temperature and decrease your margins.
You need to know how often the brakes will be applied.
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