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Old 06-30-2012, 11:16 PM   #128 (permalink)
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Salvaged batteries for testing

Why am I limiting things to 225 or 250A?

First, those kinds of large number scare me a bit.

Second, I got a REALLY good deal on electronic fuses that are rated at 300A so that leaves me a bit of room for safety before the (normally EXPENSIVE) fuses start to blow.

How do you take 18A at 450V and do low-speed tests on a motor that needs 37.3 amps and 575VAC?

The key is 'low-speed'. If you have 18A at 450VDC (about 320VAC), you can roughly use 450 VDC * 18A and get 8100 Volt-Amps or VA. Now that power (energy delivered per unit time) can be delivered by the VFD as a variable voltage at a maximum current. For example, I can use up to 36A at 225VDC, or 72A at 117VDC. I guess you can look at it as trading maximum voltage for higher current.

I'd like to deliver 37.3 amps or the motor's rated current. That should give me the motor's rated torque, or 120 foot-lbs. 8100 VA / 37.3 gives me 217 VDC or 153VAC - you divide the VDC by the square root of 2 to get the VAC. 153 VAC is just over 1/4 of the 575 VAC that the motor is rated for, so I should be able to get rated torque out of the motor up to about 25% speed, or just under 430 rpm. That test will prove out that I can get rated torque at rated current. This will be the first test of the 40 HP 575 VAC motor.

In another test, I'd like the motor to use 3 times rated current, or 3X for short, which is about 111.9A. Since that's 3 times the current I can only get 1/3 the voltage we discussed above (153) or just over 50 VAC. And the speed won't be very fast. 1770 rpm is rated speed. 575 / 50 = 11.5, so 1770 / 11.5 gives maybe 150 rpm or so. That test will prove out my theory that 3X current will give me around 220% torque or 264 foot-lbs.

In the last interesting test, I'd like the motor to use 3.5 times rated current, or 3.5X for short, which is about 130.5A. Working the numbers through, 8100 / 130.5 = 62VDC, or 44 VAC. 575 / 44 = 13. So 1770 / 13 = 135 rpm.

BUT - this is a practical limitation - since the motor 'normally' only needs 37.3 amps to generate 120 foot-lbs of torque, the 'resistance' of the motor (actually called impedance) will limit the amount of current that the motor can use ... this may be limited to 2X or perhaps a bit higher than that. In practice, the voltage may have to go higher to get higher current. So the last test may only get 100 rpm instead of 135 rpm.

The testing will show me how high the voltage needs to go to get 3X and 3.5X current, how much torque the motor will put out, and where it does not matter how much extra current you have - there is no more torque to give. I expect that the motor will heat up quite a bit during this test as well.
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