Dan
The graph you originally asked about is the product of brake dyno. It produces a chart which is opposite to an inertial dyno which is what you usually see for an car engine for example.
The motor supplied with full voltage so runs to maximum potential rpm – at which the motor draws a current equal to the torque needed to turn the motor + losses (resistance and induced losses). In this state the motor efficiency is about 5%. Or 95% of the energy that goes in is lost.
A brake is applied and the current draw goes up the rpm begins to fall. The key curve to watch is the efficiency line. You want max efficiency point as this is the highest energy return for input. Also for the voltage shown the rpm at which maximum efficiency is reached will always be the same at 100% throttle.
The brake is applied sometimes to 50-60% of free running rpm are reached. The reason for the partial curve is that below 60% the motor is extremely innefficeicnt – it does make more torque and consumes far more amps however the motor will only ever be run occasionally at those loadings. Constant running at max power output (50% of peak rpm) or toward maximum torque (0% rpms) will result in heat damage to the motor
In most applications the motor is setup so that at max throttle the motor can wind out to a few percent more rpm than those at which max efficiency occurs. You need a dyno chart to establish this accurately. In effect, at less optimal circumstances/higher loadings the motor will run at maximum efficiency
Yours,
Andrew Gilchrist
fastelectrics.com
Australia
0419 429 201
From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com] On Behalf Of Michael Mccomb
Sent: Tuesday, 30 October 2012 4:57 AM
To: electricboats@yahoogroups.com
Subject: Re: [Electric Boats] RPM vs Amps
so either his hull is much more efficient or his prop shape is much more efficient or the diameter of his prop is GREATLY influencing the efficiency..... unless there is some indicator to the contrary i'd have to bet on the latter, prop diameter makes a great deal of difference
From: James Lambden <james@toolboat.com>
To: electricboats@yahoogroups.com
Sent: Sunday, October 28, 2012 7:01 PM
! Subject: Re: [Electric Boats] RPM vs Amps
Hi Dan,
Welcome to the world of Electric Boating!
For PMAC (Permanent Magnet Alternating Current) and BLDC (Brushless Direct Current) motors, which are almost identical, current is torque with a constant, and rpm is voltage with a constant. When programming the controller, these constants are entered into the program.
As propeller increase in rpm, they demand exponentially more torque for rpm. So when looking at the motor, you need to look at the application to find out exactly what the curve will be. This is also dependent on the gear ratio, with lower ratios having a steeper curve than higher ratios.
For a good understanding of how these numbers all come together look at the website for our Catalina 30:
These are real world numbers for our Catalina 30. Please note that to establish a baseline between all boats, all measurements have been done in zero wind, zero waves, and at either high or low tide. Furthermore, the measurements are an average of two runs in opposite directions.
Only one boat that I know of has better efficiency than our Catalina, and that is Eric's Cheoy Lee which is running a four bladed prop of slightly larger dimensions. But we are still working on breaking Eric's record.
Cheers,
James
James Lambden
Propulsion Marine
325 Chapala Street,
Santa Barbara, CA
93101
805 455 8444
jlambden Skype
On Oct 28, 2012, at 5:21 PM, dan.randolph wrote:
Sorry, this is probably a really basic question, but I am confused so here you go.
From what I have read here, for DC motors, as amps go up, rpms go up in a linear fashion. However in looking at a variety of specifications for motors, they all show RPMs not starting at zero, but actually very high, and then linearly going slightly down with amp increase.
Efficiency usually does a rapid rise to a high point, then a slight drop. Torque is linear rise with amps. But why do the motor spec sheets show the drop from not near zero?
Thanks for indulging a newbie.
- Dan
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