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#1
03-19-2017, 12:05 PM
 TheKid Junior Member Join Date: Mar 2017 Posts: 15
Horsepower confusion

I am trying to choose a suitable motor, but am getting confused along the way.
I require 5hp to move my vehicle at a constant 30mph, however when I look at motors power curves they always show 5hp at a really low RPM value. How do I know which motor to choose? Because the motors are always more powerful than they need to be?

#2
03-19-2017, 12:12 PM
 TheKid Junior Member Join Date: Mar 2017 Posts: 15
Re: Horsepower confusion

For example, I have found an AC motor with the following specs:
Rated power: 3kW
battery voltage: 48(VDC)
rated current: 78A
Rated speed: 2590 RPM
Max speed: 6800
Rated Torque: 8.18ft-lbs

This seems like a good fit. But I am also confused about the RPM value, from my understanding the rated speed is the speed at which the rated power is achieved?
My next task is calculating the gear ratio, but that leaves me with a ratio of 2.9.. isn't that really low for an electric vehicle? and for an AC motor in general?
#3
03-19-2017, 03:50 PM
 Duncan Administrator Join Date: Dec 2008 Location: Southland New Zealand Posts: 4,510
Re: Horsepower confusion

Hi

As far as your question is concerned that motor can develop 3Kw at 2590rpm

Which sounds about right - can you show us your calculations bout the power that you need?
Remember that you will need power to accelerate to your target speed and also to go up any slope as well as to simply maintain a speed

#4
03-19-2017, 04:38 PM
 TheKid Junior Member Join Date: Mar 2017 Posts: 15
Re: Horsepower confusion

I have attached the excel spreadsheet with my calculations on it (as a pdf). The method I am following is basically this: select the speed, select the motor for that speed, choose the rpm at which the motor delivers that hp, choose the gear ratio based on that rpm, THEN see if the motor provides the torque over the range of level and hill climbing conditions.
I have also read that the torque required values for inclines (that have been calculated in the excel sheet) correspond rather nicely with acceleration values. I.e. incline values from 5% to 25% correspond to the acceleration values for 1mph/sec to 5 mph/sec respectively, and they can be used interchangeably.

My selected speed is 30mph, and I have used the torque value on the worksheet (under "Total drag torque, level") to calculate the 'required' horsepower, I got a value of 4.5hp. Hence my original question about a 3kW motor because I had converted hp to kW. Underneath all the tables are some specifications that might be of interest to you. My dilemma still lies in choosing a motor to suit my application
Attached Files
 Torque Required_pdf.pdf (428.8 KB, 16 views)
#5
03-19-2017, 06:41 PM
 major Senior Member Join Date: Apr 2008 Location: Ohio, USA Posts: 7,330
Re: Horsepower confusion

Hi TheKid,

Suggest you take a look at the DIY garage on this site or the EValbum for EVs similar to what you want. Google search maybe. Also try talking to a vendor who sells EV motors. Any EV clubs in your area (EAA)? Or perhaps a school which has a program?

major
#6
03-19-2017, 08:57 PM
 Sunking Senior Member Join Date: Aug 2009 Location: DFW Posts: 1,680
Re: Horsepower confusion

Quote:
 Originally Posted by TheKid For example, I have found an AC motor with the following specs: Rated power: 3kW battery voltage: 48(VDC) rated current: 78A Rated speed: 2590 RPM Max speed: 6800 Rated Torque: 8.18ft-lbs
OK this sounds like a golf cart motor and if you note the bold face above are the numbers you need to work with.

The HP = 2590 RPM x 8.18ft-lbs / 5252 = 4.03 HP

It will use 48 volts x 78 amps = 3744 watts and that translates to 80% efficiency.

What you have to work out is a gear ratio that gives you the desired speed at 2590 RPM with 4 hp. With electric motors if it take 1 hp for desired speed the motor peak power needs to be 3 to 4 hp. Just guessing here but the motor are looking at is just a little shy of what you want.

However you would have to check, but if the motor can run at 60 volts might be perfect with a 200 amp controller.
__________________
Dereck BC, PE MSEE

Last edited by Sunking; 03-19-2017 at 09:11 PM.
#7
03-19-2017, 11:25 PM
 Karter2 Senior Member Join Date: Nov 2011 Posts: 1,304
Re: Horsepower confusion

Why design to a precise limit.
Oversize the power capacity of the motor to allow for unforseen situations.
You dont have to use the extra power, but it is much less hassle to increase a current limit setting than to have to change out a motor to a bigger one after the build is complete.
#8
03-20-2017, 06:52 AM
 TheKid Junior Member Join Date: Mar 2017 Posts: 15
Re: Horsepower confusion

Thank you all for your responses.

I have googled my heart out trying to research this! I will see if there are any EV motor vendors that I can contact for further help. I have actually contacted a couple of motor manufacturers to obtain performance curves.

Dereck, you have cleared up some of my confusion on what the values represent. As I mentioned before I am following a method for this, the bit where I am stuck at is looking at the performance curves and reading the RPM at which the motor delivers my required hp. From what you have said however, it seems that the rated speed IS the RPM at which the rated horsepower is delivered (from the HP calculation). This is very useful to me. How would you read this from a motors power curve though? I will give an example, the HPEV AC-20 is supposedly a good choice for lightweight vehicles, which mine definitely is. However, at 5hp on the curve the RPM is very low, which of course gives me a very low overall gear ratio. This is where the majority of my confusion has been..

http://www.hpevs.com/Site/power_grap...20imperial.pdf

In fact on the "peak" graph above (72 volts / 300 amps) it doesn't even measure 5hp at any point. However on the "continuous" plot for the same motor..
http://www.hpevs.com/Site/power_grap...nuous%20FC.pdf

I emailed HPEV and they said their ac motors "want" to run at 6000 RPM. As i said my main confusion is coming from the graphs..

The reason I have a precise limit is because of the method I am following, I am looking for a motor based on the required hp at my design center speed of 30mph. My next calculation is based on the RPM at which this horsepower is delivered, so that I can calculate the overall gear ratio:
overall gear ratio = RPM(motor) / RPM(wheel)

Anymore help to clear this confusion is greatly appreciated!
#9
03-20-2017, 08:01 AM
 major Senior Member Join Date: Apr 2008 Location: Ohio, USA Posts: 7,330
Re: Horsepower confusion

Kid,

Don't know what you mean by center speed. Maybe you're taking a wrong approach to motor selection. Also sounds like confusion interpretating motor performance curves. Here's another suggestion. Read: https://www.amazon.com/Build-Your-Ow.../dp/0830642315.
By Bob Brandt.

Regards,

major
#10
03-20-2017, 12:29 PM
 TheKid Junior Member Join Date: Mar 2017 Posts: 15
Re: Horsepower confusion

Funnily enough that is the book I am following!

What I meant by "design center speed" is simply the operating point for my vehicle. It is a car designed mainly for city use, so I have chosen to base calculations around this speed (30mph)

Last edited by TheKid; 03-20-2017 at 12:32 PM.

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