# using a strong(9-10HP) brushless DC motor

48755 Views 13 Replies 6 Participants Last post by  PStechPaul
Hi,
I am new at this field of EV,I have a background in RC airplanes if that helps
The RC field tend to use very bleeding edge technolgies(like LiPo and Brushless motors) before they were wildly used everywhere else

I found this 7KW peak,1.8Kg,180RPM/V Brushless DC Motor it cost 99\$! (if you leave the page open for a while it will offer you for 96\$ (video)

also there is a 300\$ motor,2.5Kg,150RPM/V which has about 20HP!(video)

My idea is to do a conversion with 2 of the smaller engines providing 15-20HP,i saw some conversions(Geo Metro,VW Beetle) done using a 50Kg 20HP forklift engine so i guess this pair of engines(weighting only 3.6kg!) can power a light car.

My questions are:
1.Is this possible?are this engines going to be strong enough to power a EV,15-20HP for 200\$ sounds really cheap am i missing something?

also can some one give me a estimate of how much power(HP/KW) i need to move a car at X speed of W weight?
just simple function(a estimate of course) that given a standard Drag coefficent(let say 0.38 like a smart ForTwo) i put the weight and speed and get the amount of HP/KW i need
1 - 14 of 14 Posts

#### major

· Registered
Joined
·
7,793 Posts
Hi,
I am new at this field of EV,I have a background in RC airplanes if that helps
The RC field tend to use very bleeding edge technolgies(like LiPo and Brushless motors) before they were wildly used everywhere else

I found this 7KW peak,1.8Kg,180RPM/V Brushless DC Motor it cost 99\$! (if you leave the page open for a while it will offer you for 96\$ (video)

also there is a 300\$ motor,2.5Kg,150RPM/V which has about 20HP!(video)

My idea is to do a conversion with 2 of the smaller engines providing 15-20HP,i saw some conversions(Geo Metro,VW Beetle) done using a 50Kg 20HP forklift engine so i guess this pair of engines(weighting only 3.6kg!) can power a light car.

My questions are:
1.Is this possible?are this engines going to be strong enough to power a EV,15-20HP for 200\$ sounds really cheap am i missing something?

also can some one give me a estimate of how much power(HP/KW) i need to move a car at X speed of W weight?
just simple function(a estimate of course) that given a standard Drag coefficent(let say 0.38 like a smart ForTwo) i put the weight and speed and get the amount of HP/KW i need
Hi Uriel,

Power is power. So if it requires 20HP to move your car at 50mph, then is really doesn't matter how that 20HP is produced. However this does not account for the acceleration to get the car up to 50mph. And that will be a big problem for these hobby prop motors.

They are impressive machines and carry crazy power to mass ratios. But this is undoubtedly peak power and can be sustained only with the massive air blast over and thru the motor.

Here's a thought experiment for you, or maybe a real experiment. Get yourself two of those motors and the propellers. Mount them on top of your car and see if they will move the car and how fast it can go

Regards,

major

#### Uriel Katz

· Registered
Joined
·
6 Posts
Discussion Starter · ·
I don't think that if they can move my car using props,that is also a question what is more efficient? using a motor to move a wheel or to create trust?

From doing a little research on the Internet they can easily do 4-5 KW continuously,that means 10-13hp for a pair of those

Can you give me a estimate function for power needed to move a car at X speed using a 0.38 drag coefficient?

#### major

· Registered
Joined
·
7,793 Posts
I don't think that if they can move my car using props,that is also a question what is more efficient? using a motor to move a wheel or to create trust?

From doing a little research on the Internet they can easily do 4-5 KW continuously,that means 10-13hp for a pair of those

Can you give me a estimate function for power needed to move a car at X speed using a 0.38 drag coefficient?
I won't do that. But there is such information on this forum. Look in the EV Information section. Also there are threads devoted to EV calculators and such.

#### Uriel Katz

· Registered
Joined
·
6 Posts
Discussion Starter · ·
According to the formula,the powers that oppose the car movement of a 1000kg,0.38 drag,0.633 m^2 drag area,0.015 rolling resistance at 90kmh(25m/s) is 3.6kw(4.8hp) is that correct?
power = ((1000) * (9.8) * (25) * (0.015)) + ((0.6465) * (0.38) * (0.663) *(25^3))

does this means a 10hp continues power can pretty much drive a 1000kg car at 90kmh?

#### major

· Registered
Joined
·
7,793 Posts
does this means a 10hp continues power can pretty much drive a 1000kg car at 90kmh?
However this does not account for the acceleration to get the car up to 50mph.
I didn't check your math, but the steady state power requirements for vehicles do often calculate surprisingly low because we are used to seeing quite high powered gas engines in automobiles. However, like I mentioned before, it will require more power to accelerate the vehicle in a reasonable time. Also, you will need to account for conditions less than perfect (as assumed in the equation), like headwind and going uphill.

Regards,

major

#### Uriel Katz

· Registered
Joined
·
6 Posts
Discussion Starter · ·
I was indeed surprised to find that you don't need that many HP to keep motion at highway speeds.

I understand that acceleration needs more power,but all the power that doesn't go to oppose the friction/drag is used for acceleration right?

Let say a hypothetical car, 1000kg 15hp/20hp peak modest drag/rolling restistance ,how much time it will take it to get to 50kmh and to 90kmh? will it be usable as a city car + 10km of highway per day?

#### major

· Registered
Joined
·
7,793 Posts
I was indeed surprised to find that you don't need that many HP to keep motion at highway speeds.

I understand that acceleration needs more power,but all the power that doesn't go to oppose the friction/drag is used for acceleration right?

Let say a hypothetical car, 1000kg 15hp/20hp peak modest drag/rolling restistance ,how much time it will take it to get to 50kmh and to 90kmh? will it be usable as a city car + 10km of highway per day?
Hi Uri,

Sir Newton tells us that F = ma. Force (F) relates to motor torque. Mass (m) of course. And acceleration (a), dV/dt. So you need some calculus to figure out the power and energy for your acceleration event. Basically, kinetic energy needed to change the velocity of the mass. KE = ½mV²

Here again, I am not going to do these calculations. But I recall several members posting threads which give you either web site or downloads of calculators which will run the numbers for you.

Maybe someone else can chime in and give us a reference or you can search this forum to find it.

Regards,

major

#### Uriel Katz

· Registered
Joined
·
6 Posts
Discussion Starter · ·
Thanks Major! you are very helpful

#### matt1023

· Registered
Joined
·
1 Posts
Hello folks. Looks like you're on the right track.
What you need to consider is the torque/horsepower ratio. You want the load torque to be about 5% to 10% higher than the horsepower; in other words take a look at truck engines, if you have an engine that produces 230 horsepower at say 2300 rpm, under load, then you want the engine to produce about 245 ftlbs of torque. Under load means with the truck in motion.
So, look at the torque output of your motor, and this will determine if the motor will drive the car.

matt

#### Uriel Katz

· Registered
Joined
·
6 Posts
Discussion Starter · ·
Thanks! unfortunately I never had time to do it maybe in the future

#### Hollie Maea

· Registered
Joined
·
1,021 Posts
Hello folks. Looks like you're on the right track.
What you need to consider is the torque/horsepower ratio. You want the load torque to be about 5% to 10% higher than the horsepower; in other words take a look at truck engines, if you have an engine that produces 230 horsepower at say 2300 rpm, under load, then you want the engine to produce about 245 ftlbs of torque. Under load means with the truck in motion.
So, look at the torque output of your motor, and this will determine if the motor will drive the car.

matt
Not sure that ICE rules of thumb are very useful for electric motors, which have a completely different power band. Power is torque and speed, and you can always swap one for the other with gears. For instance, some AC motors can spin up to 10k rpm, with flat torque up to 6k. Obviously it will have a low torque to hp ratio, but you can boost the torque up by gearing it down.

#### Karter2

· Registered
Joined
·
1,666 Posts
Keep calculating if you wish,..
..but you will be very disappointed in the performance of those motors in a 1000kg vehicle.

#### PStechPaul

· Registered
Joined
·
3,143 Posts
I was originally very excited to find such motors with amazingly high power/weight and power/size ratio, but they are designed for very high RPM and lots of air cooling, and they are not expected to last very long in hobby applications. More realistic performance may be obtained for reasonable cost using surplus or used three phase industrial motors and VFDs or forklift motors. A reasonable size motor for a small 2-4 passenger on-road EV is 15-40 HP. Figure on about 10 lb/HP. Here's a handy calculator I made:
http://enginuitysystems.com/EVCalculator.htm

You may want to make a small EV like a tractor or bike to get a feel for what's required at a lower cost.

1 - 14 of 14 Posts