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Old 15-04-2013, 17:46   #1
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Is an oversized electric motor more efficient?

I thought I read once that for slow speed, a larger electric motor actually(or might), uses less wattage per hour.

Such that a 2k watt motor might push a boat at 3 knots using x wattage, while a 4k watt motor might push that same boat at 3 knots with less than x wattage.

Does anyone know if this might be true, cause if it is, it would give more reason for me to consider what I consider to be an oversized system.
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Old 15-04-2013, 18:11   #2
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Yes and no. The trick with electric motors, which confuses the thinking of many, is some simple arithmetic and engineering is required to understand the operation. The fan boy myths are thick.

Very generally, the Watts you put in is the HP you get out. The trick is matching the load.

Larger motors are probably not any more efficient all else being equal.
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Old 15-04-2013, 20:41   #3
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Re: Is an oversized electric motor more efficient?

Quote:
Originally Posted by SURV69 View Post
I thought I read once that for slow speed, a larger electric motor actually(or might), uses less wattage per hour.

Such that a 2k watt motor might push a boat at 3 knots using x wattage, while a 4k watt motor might push that same boat at 3 knots with less than x wattage.

Does anyone know if this might be true, cause if it is, it would give more reason for me to consider what I consider to be an oversized system.
I don't know ... and I'm not sure there are all that many people who do, particularly for small boats. In other words, this is naval engineering question because there are a lot of factors -- wattage, prop size, prop rotation, drag when/if under sail, mass, charge and discharge rates being the obvious but then there ones like balance of mass, wire runs, conversion rates, prop design and loads. All this has been done for years on huge boats but to my recollection it only has been going in earnest for cruising boats since about 2005 with a major industry hit happening since then so I don't think there are many with the combined experience.

Probably the most experienced group of hobbyists are on the Yahoo Electric Boats group though most are experimenting with a large battery bank powering the system (hybrid) vs using an generator (or series of generators) to power the systems as is the case with the huge ships.
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Old 15-04-2013, 20:55   #4
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Re: Is an oversized electric motor more efficient?

Quote:
Originally Posted by SURV69 View Post
I thought I read once that for slow speed, a larger electric motor actually(or might), uses less wattage per hour.

Such that a 2k watt motor might push a boat at 3 knots using x wattage, while a 4k watt motor might push that same boat at 3 knots with less than x wattage.

Does anyone know if this might be true, cause if it is, it would give more reason for me to consider what I consider to be an oversized system.
I would think the wattage to move the boat at the same speed would be the same. The advantage of the larger motor is you could have a higher top end i.e. more power. Don't know if this will help you or not but, here are some numbers I did when I did a harbor test with my electrically propelled boat with 9 Hp electric motor.
THE BIANKA LOG BLOG: NOTES OF AN ELECTRIC SAILOR: Tests from the harbor 2011
As you can see my prop was not the cleanest it could have been. I'm hoping to do another test when I splash the boat later this spring.
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Old 15-04-2013, 21:24   #5
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Re: Is an oversized electric motor more efficient?

A quick look shows that larger electric motors reach peak efficiency at 40% load. But they don't loose too much at higher loads. Small electric motors like 0-1hp reach peak efficiency near or at full load.
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Old 15-04-2013, 21:33   #6
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Re: Is an oversized electric motor more efficient?

Motor efficiency is power out divided by power in.

Notice that further down the list are the efficiency standards for B motors where the more powerful they are the more efficient they are.
Electrical Motor Efficiency
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Old 15-04-2013, 22:09   #7
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Re: Is an oversized electric motor more efficient?

This is actually a pretty complicated subject. A larger motor will use roughly the same power at a hp point as a smaller motor at the same HP point, give or take a bit. The larger motor may be less efficient at part load due to flux "slippage" on the rotor, higher bearing loads, etc.

But it depends on many variables, such as drive efficiency, standard or High eff motor, power factor of the motor, etc.

If I was selecting an electric motor for a VSD system, I would want it's peak HP with power factor at or just above the required HP of what it was driving.
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