PMAC motor efficiency at low loads and or low RPM

Electric Motors and Controllers
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electroboater_captain
1 µW
1 µW
Posts: 1
Joined: Mon Oct 16, 2017 7:36 pm

PMAC motor efficiency at low loads and or low RPM

Post by electroboater_captain » Mon Oct 16, 2017 8:18 pm

Greetings all,

Excited to join this forum. I have a question that motor manufacturers have been tactfully avoiding and I want to know if it is an ignorant question to ask, or if they just don't want to spend the money, or if its just unnecessary.

I have a Motenergy ME1305 5kW PMAC motor and a Sevcon Gen4 size 2 controller on 48VDC in an small electric boat, inboard drive with V belt 2.8:1 gear reduction. It is a highly efficient system driving a 12" three bladed propeller of uncertain pitch and EAR. The boat is a displacement hull and 16.5' water line length, hull speed is about 6mph.

The current gear reduction and propeller configuration requires the motor to run at 1500RPM at 1kW input power, achieving 5.5mph and 170MPGe (0.2kWh/mi). This motor speed is well below the motor's performance chart:

I can see that the 48V efficiency curve of this motor peaks at the 3250RPM range, but this shows performance at full load of course.

So my question is, I need 1 to 2kW at cruising speeds, what is the most efficient motor speed to do this at? I want to adjust my gear ratios to optimize efficiency.

I asked Motenergy for a motor efficiency contour, but was told that was too expensive to run a test like this: https://www.researchgate.net/figure/224 ... tion-motor

I really wanted a torque/speed plot that shows shades efficiency like above. The regular "dyno chart" these motors come with really just shows me the top end of the motor RPM range where torque is dropping off fast.

Thanks for any advice.
Electroboater_captain!

major
10 kW
10 kW
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Joined: Sun Mar 10, 2013 10:12 am
Location: NW Ohio, USA

Re: PMAC motor efficiency at low loads and or low RPM

Post by major » Mon Oct 16, 2017 10:16 pm

electroboater_captain wrote:Greetings all,

Excited to join this forum. I have a question that motor manufacturers have been tactfully avoiding and I want to know if it is an ignorant question to ask, or if they just don't want to spend the money, or if its just unnecessary.

I have a Motenergy ME1305 5kW PMAC motor and a Sevcon Gen4 size 2 controller on 48VDC in an small electric boat, inboard drive with V belt 2.8:1 gear reduction. It is a highly efficient system driving a 12" three bladed propeller of uncertain pitch and EAR. The boat is a displacement hull and 16.5' water line length, hull speed is about 6mph.

The current gear reduction and propeller configuration requires the motor to run at 1500RPM at 1kW input power, achieving 5.5mph and 170MPGe (0.2kWh/mi). This motor speed is well below the motor's performance chart:

I can see that the 48V efficiency curve of this motor peaks at the 3250RPM range, but this shows performance at full load of course.

So my question is, I need 1 to 2kW at cruising speeds, what is the most efficient motor speed to do this at? I want to adjust my gear ratios to optimize efficiency.

I asked Motenergy for a motor efficiency contour, but was told that was too expensive to run a test like this: https://www.researchgate.net/figure/224 ... tion-motor

I really wanted a torque/speed plot that shows shades efficiency like above. The regular "dyno chart" these motors come with really just shows me the top end of the motor RPM range where torque is dropping off fast.

Thanks for any advice.
Electroboater_captain!
Hi,

I think I ran into you on another forum. But anyway here goes.

You can find a dyno curve on the Motenergy website for the motor and controller at 72V constant. Take your 1 kW at 1500 RPM and calculate torque. That's 56.4 lb.in. Draw a vertical line on the dyno curve at 56.4 lb.in. Put a hash mark at 1500RPM on the vertical line. Now on that line proportion the RPM at 72V with 1500RPM times 72V and you get about 26V. It will require 26 Volts to get 1500RPM at that load and that will be 1kW output. The current is read from that trace where it crosses your torque load line, about 45A. Input is then 45A * 26V = 1170W. Efficiency = output / input = 85.5%.

At that load at 72V it was like 87%. A bit higher load and peaks around 90%. What's wrong with that? A bit of interpolation involved so you're going to have some error, but I doubt more than a percent or two on the efficiency scale.

I hope you're able to follow that.

major

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