If the manufacturer says that the motor is rated for 2.3Hp, then it is customary--for motors--to convert that Hp rating to wattage via the formula you know. If you wish to know how much external work the motor can do, you must get that info from the manufacturer, possibly as an efficiency %. Multiply the motor input power (e.g. 2.3Hp) by the efficiency to get the available output power. The efficiency may not vary linearly with drive power; only the manufacturer (or datasheet) can give you that info. I do not know from where you got the 65-70% value and I have no knowledge as to whether it is relevant to this discussion.

From what I understand, PMDC motors have a wattage rating, but don't have a HP rating because of how they work. An induction motor can get very close to its wattage rating because the inner and outer coils are timed to operate to minimize the back EMF. However, a PMDC must use around ⅓ of the input to counter the back EMF it generates.

To my thinking, this means the 1700W rating of its core can only use ⅔ of the input to do the work.

I also understand that the Chinese only recently started using a HP rating to open up the US market for their magnet motors, because HP is a rating Americans understand, and that bigger is always better. Prior to that, a HP rating didn't exist for a PMDC.

It would be usual to rate a motor at its max power allowed output condition, assuming sufficient drive (electric input power) and appropriate load. It is my belief that if the manufacturer claims the motor is rated at 2.3Hp, then that input power (electrical) is what is required to achieve the maximum mechanical output power that the motor is rated to deliver. Due to inefficiencies, that output power will certainly be less than the input electrical power.

Exactly. Here is the information supplied by the crab pot puller manufacturer.

2.3HP PMDC motor wired for 12V using 8' of 6ga AWG wire, and an 80A fuse.

Since 2.3HP equates to 1700W, at 12V, that's 141A. Why the restriction of an 80A fuse?

Then, from the BlueSea site, ...


This graph shows that to maximize the 12V supply, they should be using a 4ga wire. The fact that they are using a 6ga wire lowers the voltage being supplied. Then to further restrict the supply with that fuse, ....what am I missing? Could this be the reason I can do more with my 600W puller than they can with all their beef?

BTW, to make it more complicated. Motors are not resistive loads; there is a "power factor" involved due to the phase shift between voltage and current in the inductive & resistive windings. The example you cited (230VAC, 4A) ignores that factor.

Is this true for a PMDC motor too?

Your question: "...to have to protect that winding by limiting the feed makes the rating moot." I do not understand why limiting the power to the rating poses any limitation. The protection is against a stall condition; that is not the condition at which motor power is specified. The data sheet should show the electrical drive and at what rpm and what external mechanical load (i.e. torque) ( OR the equivalent mechanical power) the motor is operating to meet its spec.

 

From what I understand, PMDC motors have a wattage rating, but don't have a HP rating because of how they work. An induction motor can get very close to its wattage rating because the inner and outer coils are timed to operate to minimize the back EMF. However, a PMDC must use around ⅓ of the input to counter the back EMF it generates.

To my thinking, this means the 1700W rating of its core can only use ⅔ of the input to do the work.

I also understand that the Chinese only recently started using a HP rating to open up the US market for their magnet motors, because HP is a rating Americans understand, and that bigger is always better. Prior to that, a HP rating didn't exist for a PMDC.




Exactly. Here is the information supplied by the crab pot puller manufacturer.

2.3HP PMDC motor wired for 12V using 8' of 6ga AWG wire, and an 80A fuse.

Since 2.3HP equates to 1700W, at 12V, that's 141A. Why the restriction of an 80A fuse?

Then, from the BlueSea site, ...
View attachment 184226

This graph shows that to maximize the 12V supply, they should be using a 4ga wire. The fact that they are using a 6ga wire lowers the voltage being supplied. Then to further restrict the supply with that fuse, ....what am I missing? Could this be the reason I can do more with my 600W puller than they can with all their beef?



Is this true for a PMDC motor too?

I have used only low power PMDC servo motors (and about 30+ years ago). Those motors were optimized for acceleration (low inertia armatures) and were well under 50watts ratings.

It is not the purpose of a fuse to restrict supply, although any fuse does indeed have some resistance (that's what heats it up to blow) that is wasted energy during normal operation. The goal is certainly to make that fuse loss as small as possible yet (of course) sufficient to blow the fuse for currents above its rating. In many cases, the fuse resistance is significant and cannot be ignored. That's why technology has improved so that electronic switches (primarily MOSFET transistors) can provide lower losses and sharper distinction between "okay" and "too much" current.

Crab pot puller? Hah! That's new to me (although I had heard of lobster traps/pots). In any case, the puller manufacturer seems to have chosen to limit motor drive to less than the 1700watts max allowed by the motor manufacturer...and certainly with only 80A max (rather than 140A) current the motor will not be accepting 1700watts nor producing the expected mechanical output power. Beyond that I cannot commit; you need to discuss those issues with the puller manufacturer.

I question your assertion that "this means the 1700W rating of its core can only use ⅔ of the input to do the work." BEMF is a voltage due to rotation of the armature; it is not a power loss and does not cause power loss. Additionally, if the armature is moving slowly (e.g. under heavy load), there will be very little BEMF (that is proportional to rpm). BEMF is significant mostly for its effect at high rpm when it then limits how much drive can safely be given to a motor under load. That is, due to BEMF a higher drive voltage is required to produce the same current (torque) when the armature is rotating rapidly (compared to slowly). If you have a system with essentially no load, then BEMF may not be much of a problem; you just raise the drive voltage if you want higher speed. However, if your system must operate under varying load, then driving with high voltage at light load AND driving with high voltage at heavy load would cause a problem (too much drive at heavy load when BEMF has dropped; more than motor is rated for).

I believe that I have offered all that I know about this subject. Others may still contribute comments. If you believe it worth your while, you can find sufficient info about motors (both theoretical & practical) online to keep you busy for years. Good luck!

 

I have worked with both DC and BLDC motors and generally go by the torque curves issued by the manufacturer.
Here is a typical torque graph for both continuous torque and the momentary torque, where if used in the latter range for any appreciable length of time can result in the destruction of the motor, especially if not protected by drive feature or overload protection.
Max.

 

My puller works great, so good that the more I learn about the others, the more I have no reason why it works when others are struggling. I simply worked on the simple principle of minimizing the weakest link by oversizing everything. I just don't understand how mine works and their bigger units don't.

 

You appear to have a worm and pinion gear reduction as well as different pulley ratio's, this has to be included in any torque calculations in order to size the motor accordingly.
Apparently you have a 60x increase in torque over the actual motor output.
Max.

 

You appear to have a worm and pinion gear reduction as well as different pulley ratio's, this has to be included in any torque calculations in order to size the motor accordingly.
Max.

Yes, all PMDC tarp motors are offered with 2 different gearing ratios; 60:1 and 90:1. Mine is 60:1, and theirs are 90:1. My pulley diameter is 11.5" and theirs are around 10". With their bigger motors, lower gear ratios and smaller pulleys, they should easily be able to out pull my little unit, but based on the problems they are having, exactly the opposite is the case.

If you would like to look them up, two friends have the Ace LineHauler Brutus version. The manufacturer says they can plug it into a Scotty downrigger system, which uses a 14ga feed line with a 30A fuse. They recently started offering a bigger motor with a larger feed wire, but still plug it into the Scotty DR wiring harness. Little wonder they are having issues. (One friend just informed me that he put a meter on the motor terminal, and he is only getting 10V at the motor, and he had already switched to the new larger motor.)

The other is the Discovery Bay PowerHauler. They offer two sizes, a 2.3HP and a 1.8HP, both with a 90:1 gearing, using about a 10" pulley. They have 6ga AWG wire, an 80A fuse, and a very beefy switch. They claim 110fpm line speed.

A fellow crabber just informed us, that after having issues with DBPH unit, he had the builder come aboard to trouble shoot while pulling pots. Granted, this is third hand information, but the builder said he needed to change the attitude of the boat so the puller wasn't fighting the current too.

Based on experience, this is complete BS. I have no problem pulling in any tidal flow, and my unit is ⅓ of what they claim theirs is.

I was hoping to understand why.

 

I would say for a definitive answer, torque measurements would have to be done at the motor input shaft of each under operating conditions, as the sizing and gear ratio's used for the present calculations obviously do not presently make any comparative sense.
Max.