Selecting electric motors once power requirements are calculated

In summary: W motor would be capable of pulling 300kg at 5mph.In summary, an electric car designed to meet minimum range and top speed requirements can use a power requirement of 6.5 kW. The energy required for this car to reach its min speed and range is 10.4 kWh. To determine the gear ratio needed to operate around this speed, the car's motor controller needs to be matched to the power rating of the motor.
  • #1
darthapple35
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So for a school project I got to design an electric car that can meet minimum range and top speed requirements for how heavy it is. I have calculated the amount of power needed(kW) for its heaviest load situation to reach the min speed and range, along with the necessary energy(kWh) required for this. Now when it comes to selecting a capable motor for this what kind of factor of safety should be used in terms of power?
 
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  • #2
If you've factored in wind and rolling resistance, you shouldn't need much of a safety factor... wind resistance is the harder to calculate since it's hard to know the frontal area of the car and the coefficient of resistance without wind tunnel testing, and it is increases with the square of velocity... If you're not going very fast, I'd go with a 20% safety factor and select then next larger motor.. IE.. you calculate your requirement as 10kw, add 20% = 12kw, next readily available size is probably 15kw.. that would give a 50% safety factor, which I would think is plenty... Of course the motor controller has to be matched to that power rating too.
 
  • #4
Rx7man said:
If you've factored in wind and rolling resistance, you shouldn't need much of a safety factor... wind resistance is the harder to calculate since it's hard to know the frontal area of the car and the coefficient of resistance without wind tunnel testing, and it is increases with the square of velocity... If you're not going very fast, I'd go with a 20% safety factor and select then next larger motor.. IE.. you calculate your requirement as 10kw, add 20% = 12kw, next readily available size is probably 15kw.. that would give a 50% safety factor, which I would think is plenty... Of course the motor controller has to be matched to that power rating too.

Only calculated rolling resistance since minimum top speed needed is only 4 mph. Which the power req. at the wheels to do this is only 6.5 kW and the energy req. for that speed and required range was 10.4 kWh. Thanks for the helpful reply. My next question is how would you determine the gear ratio needed to operate around this speed? A lot of the motors with the power req. i needed have much higher rated top speeds than needed and currently somewhat lost on how to determine necessary gear ratios for the speeds of operation. Thanks
 
  • #5
OK, let's say you have a 12" tire, or about 36" circumference, and since your minimum top speed is 4mph, I'll round it up to 5 mph.

5280 feet per mile, or 26400 feet per hour or 8800 tire revolutions per hour, or ~150 RPM.
If your motor speed is 2000 RPM, you need a 2000/150 reduction, or 13.3:1.
Most differentials have a reduction ratio of about 3.5:1 (though you can get others), so you'd need a primary reduction of 13.3/3.5 or 3.8:1

How heavy do you figure this vehicle will be?, what is the maximum steepness of the slope it must climb? 6.5kW @ 4mph is a lot of pulling power
6.5kW = 8 3/4 hp
4mph = 440ft/min or 7.3 ft/s
8 3/4 Hp = 4812 ft*lb/s
4812/7.3 = 650 lbs of pulling power @ 100% efficiency and 4mph.

Is this a tractor?
 
  • #6
6.5kW is about 8HP. I might be wrong but that seems a lot for 4mph? Is it an electric car or more like a go-cart?

On this website it details a 10HP electric bike that can do 65mph (see #4).
https://www.electricbike.com/10-fastest-electric-bikes/

I think the Wright Brothers calculated that around 8HP was enough for them to fly although their motor actually produced nearer 12HP if I remember correctly.
 
  • #7
Rx7man said:
OK, let's say you have a 12" tire, or about 36" circumference, and since your minimum top speed is 4mph, I'll round it up to 5 mph.

5280 feet per mile, or 26400 feet per hour or 8800 tire revolutions per hour, or ~150 RPM.
If your motor speed is 2000 RPM, you need a 2000/150 reduction, or 13.3:1.
Most differentials have a reduction ratio of about 3.5:1 (though you can get others), so you'd need a primary reduction of 13.3/3.5 or 3.8:1

How heavy do you figure this vehicle will be?, what is the maximum steepness of the slope it must climb? 6.5kW @ 4mph is a lot of pulling power
6.5kW = 8 3/4 hp
4mph = 440ft/min or 7.3 ft/s
8 3/4 Hp = 4812 ft*lb/s
4812/7.3 = 650 lbs of pulling power @ 100% efficiency and 4mph.

Is this a tractor?
its a low speed transportation/cargo car
 
  • #8
Does it need to go up any inclines? I think if you could sacrifice some acceleration you may increase battery life significantly.

Pulling a number out of my arse for the weigh of this thing, I'll switch to metric measure, and say 300kg (660 lb)... and I already figured the pulling power at 650 lbs... pretty much 300kg or 3000N
F = MA, so A = F/M
A = 3000/300 = 10m/s2

This means that it can climb vertical walls given the traction

So even at 650 lbs this thing will accelerate at a rate equivalent to gravity...
So how long does it take to get to top speed?
Vf = Vi + AT
We know the final velocity to be 4 MPH, or 1.8m/s, and acceleration to be 10m/s2

so T = 1.8/10, or .18 seconds to go from standstill to top speed...

if the thing is 10 times heavier (6500 lbs, 3 metric tonnes) we have a 1/10th the acceleration (1m/s2), meaning it would achieve top speed in 1.8 seconds... still not bad!
 
  • #9
x
Rx7man said:
Does it need to go up any inclines? I think if you could sacrifice some acceleration you may increase battery life significantly.

Pulling a number out of my arse for the weigh of this thing, I'll switch to metric measure, and say 300kg (660 lb)... and I already figured the pulling power at 650 lbs... pretty much 300kg or 3000N
F = MA, so A = F/M
A = 3000/300 = 10m/s2

This means that it can climb vertical walls given the traction

So even at 650 lbs this thing will accelerate at a rate equivalent to gravity...
So how long does it take to get to top speed?
Vf = Vi + AT
We know the final velocity to be 4 MPH, or 1.8m/s, and acceleration to be 10m/s2

so T = 1.8/10, or .18 seconds to go from standstill to top speed...

if the thing is 10 times heavier (6500 lbs, 3 metric tonnes) we have a 1/10th the acceleration (1m/s2), meaning it would achieve top speed in 1.8 seconds... still not bad!

Aight so I started from square 1 again and here's all the needed requirements-
-Transport car
-low speed
-minimum range 5 miles
-minimum top speed 5mph
-loaded minimum top speed 3
-unloaded weight=1000lB
-max loaded weight 1500lB
-must be able to climb 30 degree gradient

-So I re did all my calculations and found that the power required for this was 3.96kw, this was calculated by using the force necessary to move the fully loaded weight of 1500 lb up a 30 degree incline at a top speed of 3.6 mph(F.S. = 1.2)

-Next I calculated the rpm of the tire needed for 3.6, which turned out to be about 67.4 rpm with an 18" diameter tire.

-So now I am back to selecting a motor. When selecting motors, would I select a motor based on peak hp numbers?

-Then once I select a motor, my gear reduction/ratio needed would the rpm of the motor divided by the rpm of the tire.
 
  • #10
Selecting the motor will depend a little on the duty rating.. how often it will need to be climbing that slope, and how often it's starting and stopping.. For the details of that you will have to look at the data sheet for each model of motor.
I run some 3ph AC motors at above their rated power by running them at a higher frequency... this can mean I can gear them lower (thus requiring less startup amperage) and revving them higher can also improve fan cooling efficiency, though they usually lose some electrical efficiency if you go much above their rated speed...

Yes, you would need to select your motor, and then calculate the reduction ratio from it's max speed.. you should be pretty close and can add a fudge factor one way or another.. more gear ratio for more power or less gear ratio for higher speed...
 
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1. What factors should I consider when selecting an electric motor based on power requirements?

There are several factors to consider when selecting an electric motor, including:

  • Power requirements: This includes the required output power and voltage of the motor.
  • Speed and torque: The motor should be able to provide the necessary speed and torque for your specific application.
  • Size and weight: The motor should be compact and lightweight enough to fit your space and meet weight limitations.
  • Efficiency: Look for a motor with high efficiency to minimize energy consumption and reduce operating costs.
  • Environmental factors: Consider the operating environment, such as temperature, humidity, and exposure to dust or water, and choose a motor that can withstand these conditions.

2. How do I calculate the power requirements for an electric motor?

To calculate the power requirements for an electric motor, you will need to know the required output power and voltage. You can determine the required output power by considering the load that the motor will be driving and the required speed and torque. The required voltage can be determined by the power supply available in your application. Once you have these values, you can use the formula P=VI to calculate the power requirements, where P is power in watts, V is voltage, and I is current.

3. What type of electric motor is best for my application?

The type of electric motor that is best for your application will depend on several factors, including the power requirements, speed and torque needs, and environmental factors. Some common types of electric motors include AC induction motors, DC motors, and servo motors. Consult with an expert or do thorough research to determine the best type of motor for your specific application.

4. Can I use a motor with a higher power rating than what is required?

It is generally not recommended to use a motor with a higher power rating than what is required for your application. This can result in the motor running inefficiently, consuming more energy, and potentially overheating. It is best to select a motor with a power rating that meets the exact requirements of your application.

5. What are some common maintenance practices for electric motors?

Proper maintenance is crucial for the longevity and efficient operation of electric motors. Some common maintenance practices include regularly checking for wear and tear, ensuring proper lubrication, cleaning the motor, and checking connections and wiring for any issues. It is also important to follow the manufacturer's recommended maintenance schedule and seek professional help if any problems arise.

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