Electrical motor maths: 25HP=8HP?

PWM as such is not a switching mode supply as I see it.

But in this case the PWM uses the inductance of the motor to make it work like a buck converter. http://www.4qdtec.com/pwm-01.html

Thus it can reduce the voltage, but not increase. And it short circuits the motor while PWM state is "off".

The PWM ratio needs to be very low in order to reduce the voltage to 2 V. . Still skeptical about good efficiency at 200 A 2 V. Any measurements or more detailed specifications available?

Joakim

 

You are in a very small minority if you think PWM DC-DC supplies are not switching mode. Last sentence in the attached might help explain.

Rick W

 

Uncertain why you included this, It is not related to my motor. My Mars motor is a 3-phase synchronous motor. So the switching mode DC-DC link is separate to the motor side. The motor is driven by a 3-phase bridge.

I have not checked the no-load losses in the controller at full current. The losses on the DC side will be low because the current in this side is low at zero speed condition even at current limit. The losses on the AC bridge side probably around 200W.

If I get around to it I will do some torque testing with the little batteries to see what is possible with 280W from these.

Rick W

 

I did the torque tests on one of my Mars motors. My calculations are very close. The best I could get from the little batteries before going into undervoltage control was 11A at 19.5V. This gave a torque of 11.2Nm.

I have attached a photo of the test set up that shows the peak result. I did a number of lower current test to get a good curve. The curve is almost linear as you would expect. Extrapolating indicates you would get twice rated torque of 28Nm with 600W.

There is also a video of the motor running up. If you take a look at current during creep speed the value is 0.27A with 23.6V. At full speed the current is 6A with 20.5V.

This gives a good idea of the losses in the motor and controller. They are not very high for something able to deliver 9kW on 48V.

Rick W

 

Ignoring the current debate right now, I will address the original poster.

I have heard many different numbers for horsepower conversions between electric and some sort of internal combustion engine.

The truth is, a horsepower is a horsepower is a horsepower. Watts is watts is watts.

It becomes a little more clear if you have ever tried to design a piece of industrial machinery. If you go through something like the Dodge power transmission handbook (not cars, industrial machinery) and calculate the rating for a belt or a gearbox, you will see where they take into account the smoothness of the power source and adjust power accordingly. In other words, the drive system reacts differently to different power impulse ratios.

Also, consider the torque curves and power curves for various engines and motors -- two stroke, four stroke electric, diesel, etc. Also consider that any application has curves for power and torque demand, and the lines go in all sorts of directions. The normal procedure is to find your required operating range, and then make sure your power source can supply the necessary torque at the necessary speeds, and that there is adequate reserve torque at any RPM up to and surrounding that range that all possible environment changes are accounted for.

Now, figure some guy is trying to get his pump working. It had a gas engine on it, and now he wants electric. He gets the handbook, and it's been decades since he ever used algebra. He was never good at it. He agonizes through the whole thing, and figures out that in his case the pump needs 1/3 the power of his gas engine. He gets his motor, sets it up and it all works.

Next time, this guy is not going to use the book. He needs a motor for an entirely different application, and rather than trudge through all that painful math, he just divides by 3 and goes with it, no matter how far off he is. Not only that, but everyone he talks to is going to hear about it, and some of them are just going to take his advice.

People always seem to think that horsepower is all there is. It's NOT TRUE, even if you are talking about gas engines in cars. Any number of variables come into it, even in one application like car engines.

 

Thanks Rick. Is this the same motor you said would have 27 Nm at 400 W? If so and it would really produce that 28 Nm at 600 W, the efficiency of the power system would be about 70%, which is more than I thought.

So it can produce almost the same torque (28 Nm) at 600 W electrical power compared to Yanmar 2YM15 (not 2YM20 as you wrote earlier), which can produce 32 Nm at 1800 rpm thus producing 6 kW of power and consuming probably about 20 kW worth of fuel.

I don't quite know how to compare these two motors. They have about the same maximum power (10 kW vs. 9 kW) and about the same maximum torque (or is this 28 Nm about the maximum?). For propeller applications you don't need a lot of torque at low rpm, thus this benefit of a electric motor (or more of its controller) is not much of use. Probably it revs up much quicker while manoeuvring, but that is really not a problem for most marine diesels.

Joakim

 

Joakim
There is not much point in a boat motor having good low down torque or fine speed control. The purpose in doing the test was to verify my quick calculations above and provide you and Matt with understanding about these motors and controllers. You both assumed there could be no current multiplication through the controller.

With hindsight the phase locked synchronous motor is overkill for a boat. I would have looked more closely at open loop controllers had I known they were readily available when I purchased the Kelly controllers. Irrespective the motor and controller are a nice combination. Very smooth power delivery and heaps of torque down low.

My aim is to swing a prop up around 0.5m diameter using about 4:1 gearing. It will usually operate at less than 5% slip. Even in severe conditions I will not have to worry about the motor bogging down. So I will have heaps of grip efficiently applied. Bollard pull will be over 1.2kN.

Rick W

 

If I read this correctly, you are saying an electric motor is more complicated than an ICE? One moving part versus dozens? I can see maybe the wiring a bit more complicated but nothing different than wiring all the accessorys on a vessel. From my knowledge, an ICE wastes anywhere from 60-70% of its energy in heat loss, where as electric motors are losing 5-10%.

 

It's a matter of prespective and size, but it what I say is true.

Lets say you want to consider a small 8 kW shp main proplusion unit a 6 hour day boat with a maximum 6 amp hotel load.

You could just throw a 9.9 HP 4-stroke OB on it with a 5 gal tank so say 5 cubic feet and 200 lbs total including a 12 volt start/hotel battery.

For a comperable electrical system you need 2000 lbs of lead acid batterys or even 600 lbs of Li-ion. The battery weight alone can exceede the entire vessel weight of an ICE powered boat. Oh, and did I mention the 50Mw coal fired plant needed to charge those batteries (I won't even go into the energy-negative world of solar cells )?

Even self-produced electric proplusion plants for ships are much more complex and massive than straight direct coupled ICEs, so unless you have specific hotel load needs, or need it for noise reasons, electric is not the efficient way to power a small vessel

 

I see what you are saying, but if the energy storage weight can be reduced, would that tip the scales toward electric in your opinion?

 

Right now? Power generation in the way that the creator of the universe intended it...Nuclear.

Fission or Fusion, take your pick. All propaganda aside, from any dispassionate analysis, safer, cleaner, and more efficient in thermal and volume than any other soruce.

In the long term? Atmosphere supplied hydrocarbon fuel cell.

 

Many sailing boats carry more than 2000lb of lead just for ballast.

If you design right you can get a 10m boat to cruise at 8kts with a little over 1kW. This means for day cruising you can get away with 500lb of lead acid or half that with lithium. Batteries can be stowed to improve stability.

Add solar and wind energy collection and you have a boat capable of long distance cruising, only limited by human provisions.

You need to design from scratch not just replace the engine component.

Rick W

 

We were not discussing a sailboat, which doesn't need a motor at all, so electric is out also, not to mention the toxic and hazardous materials used in batteries don't make the vessel any more eco friendly.

And if I design it right, I can build a ICE powered boat that gets 10 knts on 1.0 kW without having to carry around a quarter ton of lead, and goes for a week at 8 knts in the same displacement. Your argument there is apples and oranges. You can only compare propulsion relative to other transport requirements the vessel must meet.

True, but why cause all the pollution and waste all that energy to dig up that copper, lead, and other rare earths for electric propulsion when you could just go to wind directly (i.e. natural fiber sails) and accieve the same result without pollution or wasted energy. FWIW, because of the material requirements, vehicle electrical propulsion systems never return any energy over ICE systems, they are always more enegry negative...it is always better to burn the fuel directly. If global energy effiency is the basis for your argument, then arguing for another process inbetween burning the fuel and applied power is counter to your tenent if you carry it to it's logical conclusion.

[rant] Most of what many people today is espouse as "energy effiency" is just a very NIMBY view of the world. They want thier electric cars, but not be downwind of the power plants that produce the electricty. They want the Li-ion battery or solar cells for thier Nano, but deride the strip mine in Bolivia that produces the rare earths.[/rant]

 

John
Most practical sailing vessels require an engine. Seems sensible to use electric. If you are going to carry lead ballast why not keep it in a form that can be useful when there is no wind.

Recycling of most metals achieves up to 80% in some cases. Could be better but it gets down to how much it costs to mine versus recycle. As cost of mining goes up recycle proportion increases.

Lithium is not rare, just hard to handle.
http://lithiumabundance.blogspot.com/

A boat using solar and wind collection does not rely on a power station.

Solar panels are already commercially viable against other forms of power production. Just no large scale applications yet.

Liquid hydrocarbons are diminishing resource and need to be kept for applications that are currently hard to substitute. You seem to forget that your little IC motor will be eating this precious resource with no chance of it being recycled.

Rick W

 

Yes, but of the flip side, why not make the power more dense so I have to carry less weight which means less hull structure which means less petrochemicals and offgassing and also which means less weight which allows me to reduce power which....etc, etc, etc. There is no single answer so electric only cannot be end all. (and we better stop this before the cat and foiler people jump in about the ballast comment... )

Another comment that is not really here or there, but if we look at energy expenduture to reform materials, I think you would agree that the more material required, the more energy expended. Battery lead is very hard to re-process BTW, it is poisoned with sulphates that are difficult to extract.

I never said Lithium was rare, in fact I was thinking of gallium, neodymium, and cadmium. These like lithium are not really "rare", they were called rare because they were difficult to extract, and still are.

Ok, but that wasn't my point. If you are concerned about energy efficient propulsion why have a motor at all? For the size of vessel that could main propulsion powered by wind or cells, I can row it for all the upwind work I would need.

Not for vessel main propulsion, which is the topic here. There is not acres of deckspace to place them, and the environment/motions are not conducive to optimum power generation, even setting the need for heavy batteries to store the night reserve aside.

That statement is absolutely wrong. Biofuels are far more viable today than solar for a marine prime mover. Plants are far more efficient converting solar energy than even 3rd generation cells as well as being carbon neutral. And existing ICE technology fully supports it. Liquid fuels are here to stay Rick, get with the program instead of being one of those toxic polluting electric dinosaurs... .