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For the same inputs of torque (50Nm) and RPM (150), with a desired output of 35 watts, which would produce the power most efficiently, an alternator or a DC motor, and why?

I assume you are trying to compare an alternator to a brush type DC generator (or a motor being used as a generator). As to which is more efficient depends upon the construction details. This includes magnets, bearings, wire gauge, brush types, etc. These criteria in turn also depend upon the operating voltage.

Typically however, it is easier to make a small low voltage brushless electric machine more efficient than a brushed machine. The brush losses can be significant in terms of both friction and resistance losses. I would expect a low cost small low voltage (e.g. 12V) brushless DC motor would be about 80% efficient, while a brush type motor would be about 60%. I suspect that a generator would have similar characteristics. An alternator designed for efficiency (as opposed to low cost) might be 85% to 90% efficient (or more).

150RPM is an unusually low operating speed though. I wouldn't care to guess about the characteristics of something designed to work at that speed.

I'm not sure about your power calculations though. You have 150 rpm and 50Nm giving 35 watts. If I calculate this I get (150/60) * 2 * pi * 50 ~= 785 watts. You would have to have about 95% losses in the power system to end up with only 35 watts output.

If we assume 85% efficiency, and nominal speed of 150 rpm, then with a load of only 35 watts I would expect (35 / ((150/60) * 2 * pi)) / 0.85 = 2.62 Nm of torque.

Assuming we ignore my figures for the moment, are we saying that a brushed/brushless motor is 60-80% efficient in producing power? i.e. I input 1000 watts of mechanical power and can expect 600-800 watts of electrical power out? and 850-900 watts out of an alternator?

This is assuming that they both have the same load applied. Say, charging a 12V battery.

Also, is a brushless DC motor/generator acheived by having a permanent magnet rotor with fixed windings in the casing (stator)? Thus removing the need for brushes and slip rings/cummutator.

Or is this achieved by some other method?

The 60% - 80% values are typical for small (100 to 500 watt) motors used in automobiles. In these applications low cost and long life are considered to be much more important than efficiency. A typical permanent magnet brush type DC 12V small automotive motor would be about 60% to 65% efficient. A typical permanent magnet brushless DC 12V small automotive motor would be about 80%.

At 60% efficiency, you would have to put 833 watts of electric power in to get 500 watts of mechanical power out. At 80% efficiency you would have to put 625 watts of electric power in to get 500 watts of mechanical power out. You can reverse these numbers to get generating efficiency (i.e. at 60% efficiency, you would need to put 833 watts of mechanical power in to get 500 watts of electric power out).

Note though that these are given as examples only. You could use the brush type motor as a generator, but at 150 RPM the output voltage would be very low (typically less than a volt for a 12V motor).

An automotive brushless motor could not be used as an alternator without extensive modification as the windings are not accessible (the electronics are typically integrated into the motor case). You would need to cut the motor open, remove the electronics, re-terminate the windings, and install a rectifier (if you want DC out).

An industrial type brushless motor typically has the electronics mounted separately, with the windings brought out to a connector. You would still have to add your own rectifier though.

Small DC motors (including all small automobile motors) are normally permanent magnet. Industrial DC motors are either permanent magnet or wound field. Wound field motors are typically only for very large sizes or for special applications.

An automobile alternator uses a wound rotor because the field strength is automatically varied by a regulator to compensate for different engine speeds (since voltage output is proportional to speed). The field is in the rotor and the main windings are in the stator. The field is powered through slip rings.

With permanent magnet brushless motors or generators, the windings are always stationary (stator) while the magnets rotate (rotor). On a normal design, the rotor is in the middle with the stator arranged around the outside. There are some unconventional ones for special applications though where the stator is in the middle and the magnets are in the case which rotates around the core. In this design the case is in two sections, with the "front" rotating while the "back" stays stationary (which allows them to connect the power wiring).

For your application at 150 RPM, you either need a gearbox (or pulley system) to get the generator up to normal operating speed, or you need a special generator which is designed to operate at low speed. Since you are operating under water, you probably also want one which is designed to be submersible. I imagine that a brushless type would be most suitable for this. The windings would need to be encapsulated in epoxy (or some other material), and various other measures taken to keep the water out.

You might want to have a look at what OpenHydro (of Ireland) are doing. They have an underwater turbine which is intended to sit in tidal streams to generate power from tides. The generator is integrated into the outer rim of the turbine. The will be installing several of these in Nova Scotia soon to generate power from the tides in the Bay of Fundy (the highest in the world). Unlike a conventional tidal power plant (e.g. Annapolis Basin in Nova Scotia), they don't use a dam, but simply use the tidal currents which are allowed to flow through the turbine. The concept is similar to wind turbines, except they are using tidal currents instead of wind (the turbine design is also different of course).