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I have recently purchased a 24 V brushless DC motor set (underwater thruster) along with its controller. Here is the link to the product.

The description says it's 1000 W but when I tested, it consumes around 720 W at full throttle. I have connected the controller to DIY lithium battery pack of 24 V and 30 A is measured on the clamp multimeter.

Seller has no information about the original manufacturer's technical parameters about the DC motor, nor there is any sticker or label attached to the controllers about the voltage and power ratings of controller.

My findings about the motor and controller

1. Brushless Motor

enter image description here

After googling about the motor using part number (7822.277.297) printed on the motor it was found that this motor is specifically designed for power steering systems in cars. But I could not find the datasheet of the motor as most of the results shows the complete steering rack system and not about the specified part number.

2. The Controller

enter image description here

The controller has no sticker labels about the technical specifications on the heat sink aluminium box. When I opened, it has 15 MOSFETs which I guess 5 MOSFETs for each phase wire and two large capacitors of 80 V.

I am just curious what are ratings of motor and the controller? and if I can connect the controller to a more higher voltage like 36 V or 48 V battery pack to draw more power?

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    \$\begingroup\$ I assume you tested the motor under load. You could very roughly guess the current from the motor's wire gauge or guess power by comparing mass with motors of similar mass and construction (i.e. magnet type). The controller is probably not 80V. The caps have just been derated. Derating from 80V to 24V seems excessive though. You can try looking up the MOSFET datasheet. \$\endgroup\$
    – DKNguyen
    Commented Feb 14, 2023 at 16:36
  • \$\begingroup\$ The motor's three phase wire thickness is 12 AWG each and the controller is drawing around 30 amperes from the main battery load. The weight of the motor alone is about 3 kilograms. But what most confuses me is that I have seen drone motors that are rated 1000 watts which is very much smaller than this. \$\endgroup\$ Commented Feb 14, 2023 at 16:47
  • \$\begingroup\$ The drone motors probably use better magnets and are expected to run better cooling. Many move at high speed through the air. Then again, does your motor not run submerged in water? On the other hand, your motor might be running on a different part of the power curve. Most motors produce most of their power in terms of speed, not torque. Air propellers spin much in thinner air than water propellers in thicker water so if both are direct drive your motor is not running on the same part of the power curve. \$\endgroup\$
    – DKNguyen
    Commented Feb 14, 2023 at 17:25
  • \$\begingroup\$ Under what conditions were you testing it? Using electric outboards I find the current they consume decreases as the boat speeds up. If you are finding 720W consumption in a moving boat, 1000W is probably realistic from a static thrust test. \$\endgroup\$
    – user16324
    Commented Feb 14, 2023 at 17:53

1 Answer 1

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The numbers written on a motor are called nominal i.e. by name. The numbers are how the manufacturer of the motor communicates how they designed the motor. If the motor says 1000W on it, that means they designed it for 1000W, but it doesn't mean you have to use it at 1000W. Unlike computer chips, say, which are often treated as black boxes so you better follow the specification exactly, motors follow the motor laws of physics and you can take this into account when designing.

Mainly, you should know that voltage is proportional to speed, current is proportional to torque, and the thing that limits the power is usually overheating (but insulation degradation can also be a thing at higher voltages, and brushes in brushed motors can wear out).

Mechanical systems have speed/torque curves much like electronic systems have I/V curves. At full speed, the amount of torque used by the propeller might be different in air, still water, and moving water, depending on how fast the water is going. Air is much easier to move than water, so if you tested it on your workbench, this explains the discrepancy. And less torque means less current used. That's a good thing, really, as it doesn't waste battery power.

On the other hand, you bought from Ali Express, so who knows - the rating could have been inflated.

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  • \$\begingroup\$ Another seller has listed the same bldc motor "7822.277.297" set as 1800 watt with very marginal cost difference, so I am sceptical about the power ratings. Is it possible that the seller has sold this set with 1000 watt controller and another is selling with 1800 watt controller? What I am looking for is the original manufacturer's datasheet about the motor ratings. \$\endgroup\$ Commented Feb 15, 2023 at 2:24
  • \$\begingroup\$ @DipendraGurung yes, possible. as I did say, the power limit is often to do with cooling. It is also possible that they just lie. When using no-brand cheap parts from Ali Express you may find yourself having to guess instead of relying on what the manufacturer says. And don't make it into an underwater scooter that could kill you if it stops working. \$\endgroup\$ Commented Feb 15, 2023 at 12:44

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