Generally speaking it is far better to fit a bigger motor than required, rather than run one continuously at close to its rated power. It is true that the frictional and windage losses for a bigger motor will be very slightly higher, but the resistive loss (which is far and away the biggest power loss factor at normal operating rpm) will be much lower.
For example, let's say you need 5kW with a 50V motor voltage. Your current will be 100A. Let's also assume that resistance scales linearly with motor size (it doesn't really, in practice bigger motors have lower resistance than linear scaling would give).
If a 5kW motor has a resistance of 0.06 ohms then it will be losing around 600W as heat from resistive loss (about 12% of the total power being provided to the motor). This power loss comes from this formula: P = I²R, where P is in watts, I is in amps and R is in ohms.
If you use a 10kW motor, with a resistance of 0.03ohms (half that of the 5kW motor, because its bigger and has thicker wire) then the power wasted as heat in the motor will drop from 600W to 300W.
The penalty of using too big a motor is the extra size and weight, the small amount of extra bearing and windage drag, and the extra cost. If you can tolerate these downsides then bigger is pretty much always better.
Jeremy
--- In mailto:electricboats%40yahoogroups.com, Michael Mccomb <mccomb.michael@...> wrote:
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> my original thoughts centered around no belts to break, being able to decouple the motors in the case of a problem 50% of the time and direct connecting because of the constant torque that a motor provides.... others reminded me that the motors would operate more efficiently if they were allowed to reasonably spin up and that cooling would be better served by spinning up.... just thought it interesting that motors had been doubled up
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> as a sort of corollary question to my original one, IF an electric motor can, for arguments sake, utilize 100 amps at a given voltage but never uses more than 30amps because that provides so acceptably close to hull speed then isn't that motor over sized by more than two times? Wouldn't it be better to have half the motor as such a motor only only being able to absorb 50amps would be no detriment at all at the same power level of 30amps. Isn't 30amps at a certain voltage delivering the exact same power regardless of whether the motor could ultimately handle 50, 100 or 500amps. I realize that inertia and other factors would have a part to play but don't these other factors all tend to point to the use of the smaller motor rather than the larger?Â
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