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?

  • 2
    \$\begingroup\$ Using an inverter from 12VDC to 120VAC and then a controlled rectifier from 120VAC to 90VDC doesn't make any sense. \$\endgroup\$ Feb 26, 2017 at 20:12
  • \$\begingroup\$ Have you looked into electronic speed controls (ESC's) for electric vehicles? All you would need to do is rectify your AC, then feed it to the ESC. ESC has a throttle input. There are ESC's available for brushed and brushless DC motors. Sounds like you are using brushed motors. \$\endgroup\$
    – user57037
    Feb 26, 2017 at 20:29
  • \$\begingroup\$ It would be nice to eliminate the inverter, which you don't need at all. If you put 7 or 8 batteries in series, you could feed that voltage directly to the ESC (which you would have to buy). \$\endgroup\$
    – user57037
    Feb 26, 2017 at 20:31
  • \$\begingroup\$ Problem is the app itself. This is a tiny kayak. it has space and capacity for one, maybe two large marine batteries, tops... which can handle a draw like that. 7 or 8 much smaller batteries, try to pull multiple HP out of them, they'll blow. The inverter is necessary to produce a voltage high enough to make serious power. \$\endgroup\$ Feb 26, 2017 at 20:54
  • 1
    \$\begingroup\$ There is so much you don't understand. Every power conversion step you add makes your problem worse, not better, and places greater demand on the batteries, not less. For a given weight of battery, you will always be better off if the battery voltage is around 90V, not lower. When you convert 12V to 120, you need to supply 11 or 12 A at 12V for every amp you use at 120. High voltage battery packs have drawbacks, but efficiency is not one of them. \$\endgroup\$
    – user57037
    Feb 26, 2017 at 22:05

3 Answers 3


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.

  • \$\begingroup\$ 1: Doing conversion... seemed to work pretty well at the 2kw scale. \$\endgroup\$ Feb 27, 2017 at 2:22
  • \$\begingroup\$ 2: 48V still needs to supply a pretty serious current to get the wattages I'm trying to get, and gets much bulkier than a single or dual 800-1000 CCA battery. 3: 3.6 kW is about half what I'm hoping to apply, not full power. And I do know, that with this 10kW inverter, run off just 2 medium-sized batteries, I have about 83A@120VAC to work with. To do 48V, where the heck am I going to find a 2:1 DC-DC buck/boost converter that can handle over 100A? \$\endgroup\$ Feb 27, 2017 at 2:33

A small electric outboard ,You may find one second hand ,they are 12 volt DC with built in controller ,don't rob for parts use the outboard, use your motors for shop tools


10kW @ 12V, you do realize that is ~1,000 Amps! I smell car audio rating in play here.

The answer is probably to modify the inverter....

There will probably be an intermediate DC bus in the inverter fed by some sort of boost converter type affair (Actually probably some sort of forward converter, but whatever) outputting about 180V onto the intermediate DC link bus, then a set of chopper transistors to make AC at the output.

The hack is to use the DC link to feed the motor directly and then hack the feedback network to give you throttle control.

Do be careful, the voltages in play will make you stick to the wiring, especially as you will likely be wet at the time.

  • \$\begingroup\$ Now, THAT, is a solution that hadn't even crossed my mind, that's brilliant! Turns the inverter INTO my DC controller, and deletes the need for a DC management stage at all. I like it! I'll look into this. \$\endgroup\$ Feb 26, 2017 at 21:16
  • \$\begingroup\$ And, yeah, by the math that inverter can pull roughly 833 amps. Counting efficiency losses it's probably not far from 1000A. It's effectively a bottomless power supply as long as it's backed by a tough enough set of batteries. I just want to find out what happens if I punch 5 to 6 kw through that big fat motor. The boat was already seriously fast on 2kw. it'll fly with 5. \$\endgroup\$ Feb 26, 2017 at 21:20
  • \$\begingroup\$ I assume you are getting the hull up on the plane? Hull speed in a kayak operating as a displacement hull is not going to be that high.... Also, you might need to set the prop deeper to avoid just sitting there cavitating the thing. \$\endgroup\$
    – Dan Mills
    Feb 26, 2017 at 21:34
  • \$\begingroup\$ Almost. At half-power it was still displacement, but damn fast, whitewater wave in front, huge wake behind, faster than a man can sprint. Say what you will about too many conversion steps, that system worked FAR better than I expected. And it's a crude, handcarved 4-blade prop made of 80/20 extrusion and aluminum plate on a 7-foot longtail shaft. It was just starting to rise to planing when I hit the old inverter's limits@2kw and couldn't go any faster. Now that I have a bigger inverter, next bottleneck is the driver. If I can make your DC bus tap solution work, that solves THAT. \$\endgroup\$ Feb 27, 2017 at 0:43
  • \$\begingroup\$ Do watch the safety considerations with high power, high voltage DC, it is ah 'difficult' to fuse and will grab you if you get across it. Also you will probably find that there is a minimum bus voltage you can get the boost converter to do, so bottom end of the throttle may be limited. I would heavily advocate getting at least a dead mans switch lanyard so everything shuts down if you end up in the water. \$\endgroup\$
    – Dan Mills
    Feb 27, 2017 at 10:25

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