Could use some help with a DC motor application.
Note: A number of factors here are determined by the available hardware. I'm sure this might work better with a more specific AC drive, but this is a budget, fun, experimental project, just trying to make best use of what I have without spending much to build it.
The setup: An experimental electric kayak. Very tiny boat, (8 ft one person)
with a 1/2 hp Baldor 90VDC motor I acquired as scrap, nailed to the back.
Last season's system was a single marine battery, a 2kw Harbor Freight inverter, the 1/2 hp motor and a KB DC motor drive, 90 VDC, 16A max.
I also have a 3/4 hp DC motor, just a bigger copy of the 1/2 hp model.
The observation was that the motors were very, very heavily underrated, and the motors might be able to take more than they say… a lot more.
This was fine, and a lot of fun, but topped out at 2kw when the inverter would cry uncle, alarm out and shut down, right when the kayak was hitting some serious speed. Even using the smaller 1/2 HP motor it was obvious that this was nowhere near the motor's limits… and I was itching to try the 3/4 HP in its place, but lacked any means to supply enough current to drive it properly.
I just acquired a much bigger 12VDC to 120VAC inverter good to a whopping 10,000 watts, for cheap. So now I can drive just about any level of current I want once I put together enough batteries in parallel to feed it. Problem is, my available DC motor controller tops out at 90V, 16A. With a little forced cooling I might manage to pull 20A out of it, but that's as far as it can go. Trying to find a DC motor controller that can handle in the 4-10 HP range, the only ones that even come close are 180VDC and run on 220VAC.
I understand why, (under normal industrial circumstances, no sense trying to toss around that kind of power and run it on 120VAC when most 120V hardware tops out at 15-20 amps, most serious power apps you'd run on 220, half the current, lot more power available…)
But the bottom line is, I'm trying to do this taking advantage of the cheap inverter and free motors, so I have to work around what I have.
I'm considering building a monster rectifier, which, if I remember my high school audio amp builds right, would give me about 180VDC coming off the diodes, give or take a bit, which would be awesome, and allow me to drive those motors harder on less current, but I'd have no means of modulating or applying it.
So I'm looking for, either: A truly massive 120VAC to 90VDC controller similar to the KB line but 2-3x the current, able to apply 20-40A or more at 90VDC, which doesn't seem to exist,
Or,
A relatively crude way to slap a rectifier and a couple of caps onto the output of the inverter, giving me 180VDC with as much as 55A backing it, and then a means to dial it down, PWM or some form of chopper.
I'm considering maybe an intermediate stage, something like a very, very heavy triac lamp dimmer, followed by the rectifier feeding directly into the motor… use the triac to turn it down before it even reaches the DC stage. But I'm not all that sure how a rectifier would handle being fed the chopped-up output of a triac. I've never tried such a combo before and I have no idea if that would even work.
Not sure which way to go with this. There is probably a much better way to do this than I know about, but so far, no luck finding it. Anybody got any suggestions?
Best Answer
Doing a conversion from 12 V to 120 Vac and then back to DC is not the way to go.
You mention that you will be putting multiple 12 V batteries in parallel to get your total power storage (AH capacity) and current capability ...why not put them in series and get the DC voltage you have up to a more sane level.
I'd aim for at least 48 V DC from you battery system.
Using your figures: 40 A @ 90 VDC is 3.6 kW At 48 VDC you will consume about 75 A at maximum power. Not insignificant, but perhaps more manageable than the 300 A you'd need at 12 V.
From there you could use a large DC-DC convertor to boost the voltage to 80 - 100 V without having to deal with insanely high battery currents.
There are lots of 48 VDC electric bike motor controllers, those with BLDC motors would seem a much better bet.