Selecting a Motor

As I mentioned previously, I need to find a motor manufacturer that posts more than a picture of their product on an electric bike. I need torque curves and CAD models. I am surprised at how few of these manufacturers exist, or at least have websites that I can find. Maybe I just don’t know how to search for them.

Part of the problem is that I’m looking for a motor that has power output on the level of a small riding lawn mower. Finding smaller BLDC motors has been pretty easy, but finding something that operates efficiently at lawn mower speeds while still outputting decent torque has been challenging. Also, it has to cost no more than a few hundred dollars, because I’m cheap and I need some money left over for an RTK GPS module down the road.

So I’m starting out from behind the 8-ball to begin with. I wasn’t sure such a motor existed until I found the folks at Golden Motor. These guys are a little sketchy, and they need to hire a web designer from the 21st century. But they appear to have a wide selection of motors in some pretty epic sizes, with fairly comprehensive performance data, too.

But before we go looking at individual motors, let’s review our requirements.

Motor Requirements

At the moment I am leaning toward a single mower blade design. I’ve learned the hard way that complexity is not your friend, and simple designs are generally more robust.

To maximize the deck width, I am considering a 30in long blade. A 30in blade needs to rotate no faster than 2400RPM to be safe. Riding mowers with one single 30in cutting blade are equipped with engines that output about 10ft-lbf of torque.

The plan currently is to have four 12V 35Ah sealed lead acid batteries wired in series to power the motor. So it must be rated for 48V.

Evaluating the BLDC-108 Motor

This is a 1500W motor that Golden Motor makes. It’s also the only one that doesn’t have  a nice chart for all the relevant performance parameters, but they did give an excel spreadsheet with several data points. I graphed the data and here’s what it looks like.

BLDC-108 speed torque power
The speed and power versus torque curves for the BLDC-108 motor. The blue curve is speed and the red curve is power. Excel only lets you plot two separate y-axes, which is very frustrating. Get it together Microsoft, this is 2018!

The efficiency of the motor is greater than 85% between 1 to 3.5ft-lbf torque with a maximum efficiency of 88% at 3325RPM and 2.6ft-lbf torque. The motor consumes about 40A of current at the torque shown on the chart.

To achieve the maximum safe blade rotation speed of 2400RPM, we need to use two pulleys of different diameters on the motor shaft and the blade spindle. Taking the maximum speed the motor operates at of 4072RPM and dividing it by 2400RPM gives us the pulley ratio we’ll need, which in this case is 1.7.

If we use the motor with a pulley ratio of 1.7, we could obtain 5.9ft-lbf of torque at 1800RPM. Reducing the speed increases the torque.

Some thoughts about this motor:

  1. The torque is a little on the low side, even after you factor in the pulley ratio. I suppose you could go with a higher pulley ratio, but that reduces your speed even further, which I’m told affects the quality of your grass cut.
  2. I like that the current draw is only 40A. With four 35Ah batteries, you could in theory get 3.5 hours of operating time, neglecting current consumption from other electronics and motors. I like that number.
  3. The picture on the website is nothing like the CAD model you can download. So that’s something I’ll need to investigate if this is the motor I choose.

The motor is priced at $142 with $60 for shipping. BLDC motors require a separate motor controller, so that will be another $95 with $30 for shipping. So a system built around this motor will cost a total of $327. Not horrible. I wish I knew why the shipping was so high…

Next time I’ll evaluate the 3000W HPM3000B motor.

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