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Old 11-22-2015, 04:17 PM   #2343 (permalink)
e*clipse
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Quote:
Originally Posted by P-hack View Post
I'm afraid the dual full-bridge idea for svpwm-ing a 2 phase motor is my own insanity.
(if you use 4 leads instead of 3, you can get full reversal of each coil)

edit: studying 2 phase also makes the field oriented control a bit more straightforward, no coordinate transform.
That's ok. I do really appreciate your suggestions.

Just as a starter, maybe now is a good time to figure out what a good boost-converter/inverter system would require.

For example, what boost ratio (difference between battery voltage and required bus voltage) is practical?

It seems it would be wise to look at stuff like **obtainable** EV batteries, and stuff like that. For example, the Leaf batteries I'm using have a 360V nominal pack voltage and can put out about 600A max. The pack's capacity is 24kWh. These are ORNL test numbers.

The pack's current can be reduced by increasing the voltage, with the difference between pack voltage and required bus voltage "fixed" by the booster.

The main down-side to adding more cells is cost and weight. If the pack's capacity is adequate for the intended use, then adding more cells will merely add weight and cost.

The catch is that the boost stage and battery pack must also supply enough current to meet the power requirements. For example, my setup will use four motors/controllers with a peak power output of about 75kW each. If I use the standard Leaf pack's numbers, the boost stage of each controller must supply 208A. ( 75kW/360V ) Since there are four motors, the poor battery must supply 4*208A = 833A !

So, in this example, the boost stage requirements are do-able, even with the un-optimized controller design. However, the battery cannot supply 833A. ORNL testing indicates it can supply about 600A. So, in this example I would **have to** either increase the battery pack voltage or add a parallel string to stay within the current limit, reduce the power requirements, or consider a different battery option.

In another example, say 1 motor can put out 150kW @ 650V. Using the Leaf pack, the boost stage would be required to provide 150kW/360V = 416A. In this example, the current design boost stage design would not work because three TO-247 packages can't handle the current. The options at this point are to re-design the controller or increase the battery pack voltage. If the boost stage's limit is 300A, then the system would work with a 500V battery pack. (150kW / 300A ) The 300A is well within the battery's capability, so the setup would work. The main issue would be the added weight/cost of the extra cells.

I'm just putting this out to start some conversation about this. Adding a boost stage definitely offers some opportunities. However, engineering is about compromise, and hopefully we can come up with something reasonable and useful.

- E*clipse
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