From another engineer: The controller is outputting an output pulse width to the motor that is proportional to the throttle position. The motor, being largely inductive, integrates those pulses and sees an average voltage proportional to that pulse width. The speed then is determined by this effective voltage, the load and the motor characteristics.
Mathematically, for a DC permanent magnet motor:
Vm = Im*Rm + Kb * N ; where Kb = Motor BackEMF constant in V/RPM; N is RPM and the rest is motor electrical
So N = (Vm – Rm*Im)/Kb
And remember that Vm ~ Kthrottle * Throttle Position
So, speed drops as current (i.e. “torque”) increases for any given Voltage or throttle setting.
Go away from the wind, speed increases some and power drops.
Go into the wind, speed decreases some and power increases.
Close the loop on speed: Speed stays constant, but the power trends are as above.
Close the loop on power: Power stays constant, but the speed trends are as above.
So by having constant voltage, it’s kind of a middle of the road between the two possibilities above.
-MT
From: electricboats@
Sent: Friday, May 21, 2010 3:23 PM
To: electricboats@
Subject: Re: [Electric Boats] Pontoon Boat Update
On 22/05/10 03:26, greenpjs04 wrote:
>
> However, being an engineer, I am never satisfied that things work well - I always want to understand *how* they work. That brings me to a question about the Torqeedo. I started off assuming that a particular setting of the throttle coresponded to a particular power input. For example, if I set the throttle such that power input was 800 watts, the power usage would stay at 800 watts. However, that is clearly not the case. If I am heading with the wind at 800 watts and then turn around to head into the wind, the boat slows as expected, but the power usage also rises significantly. For example, the speed might slow from 4.5 to 3.8 mph while the power input rises from 800 to 1100 watts. Can I assume the controller is trying to maintain a certain prop speed?
>
The controller is adjusting voltage thus indirectly adjusting speed.
The motor draws what ever power it can (with in certain design limits of
the motor) to get to this speed. (Over simplifed, for example brushless
motors don't work like this, but are usually set up with controllers
that make them appear the same).
So you select a certain setting and the motor runs up to that speed
and settles down to a steady state, and you see a nice reading say 800W
on your power meter. Then the wind changes. The motor controller is
effectively set for a certain speed but the apposing wind means this now
takes more power to maintain so the power consumption might jump to over
1000W with out you having touched a thing.
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