# How can this BLDC motor have constant torque over its RPM range?

I was going through this catalogue of BLDC motors for my EV project. I found two motors in it, one rated at 2.5 kW of peak power, and the other 6 kW of peak power.

My questions are as follows:

1. It is clear to me that torque depends on the current drawn, and since the graph of the 2.5 kW motor doesn't list the torque supplied at the peak current, I'm guessing the maximum torque rating occurs when the current drawn is 120 A. However, in the 6 kW motor, the torque drawn seems to stay the same no matter how much current is drawn. How is this possible? How can the maximum torque given be achieved if it stays constant?

2. I'm guessing question 1 would make sense if I knew what the PH I column in the 6 kW motor graph stood for, but I tried looking for it and couldn't find any clue as to what it is.

I am clear now that I still don't understand BLDC motors quite properly yet, hence I won't stop researching it as it's a new topic for me. However, some info would really help out.

• Rated torque 19 and max torque 57. So how, with your analysis can the torque be constant? Sep 5, 2021 at 18:19
• @SolarMike Well, the graph sheet shows the torque stays the same across every value of current, I'm just wondering how that is possible, I was under the assumption it varies across the range as a characteristic of BLDC motors themselves. Also, how is peak torque achieved then? Sep 5, 2021 at 18:43
• So image 3, 19 and 57. Stated clearly. Sep 5, 2021 at 18:45
• I think there is an error in there. Sep 5, 2021 at 18:51
• So, you can't trust the accuracy of the datasheet. Can you trust the supplier, or the motor?
– user16324
Sep 5, 2021 at 18:53

One possible meaning is that the test conditions are that the torque is set to 19Nm by the brake and the controller is adjusted to give the speeds defined by the test shown in the table.

The current shown is not the current flowing through the windings of the motor but the current into the controller.

At low speeds, the current out of the controller can be much higher than that going into the controller - it acts as if it was a buck converter feeding the motor. The torque is not linearly related to the current into the controller.

The "PH I" column could mean phase current (ie current into the windings) although that is not obvious and has a wide range for each row.

• Thanks for the help @Kevin White, the website is www.techimperial.in/bldcmotors . The datasheet is on the link techimperial.in/wp-content/uploads/2018/05/…. Sep 5, 2021 at 19:44
• @JameelAhamed - the website doesn't seem to be accessible to me in California. Sep 5, 2021 at 20:25
• the link isn't accessible for me either at the moment, I think the whole server is down or something. Really bad timing Sep 6, 2021 at 5:10

It is clear to me that torque depends on the current drawn,

Yes, it is correct.

and since the graph of the 2.5 kW motor doesn't list the torque supplied at the peak current, I'm guessing the maximum torque rating occurs when the current drawn is 120 A.

It is not correct. Some area of the curve, we may extrapolate unknown data, but very limited. Meantime, there are many factors to have the torque already saturated before the peak current. You need to see the full scale torque-current chart/graph.

However, in the 6 kW motor, the torque drawn seems to stay the same no matter how much current is drawn. How is this possible?

That 4.5KW (not 6KW) chart is showing torque locked speed-current relation. Not only torque, but speed depends on the current due to control and mechanical factors.

I'm guessing question 1 would make sense if I knew what the PH I column in the 6 kW motor graph stood for, but I tried looking for it and couldn't find any clue as to what it is.

Yes, I am with you about that we need to contact the manufacturer.

• Thanks for the help @jay, I guess the supplier hasn't done a great job in listing the motor specifications. However, the competition I'm participating in (ESVC 3000) only allows teams to buy from them. So I guess I'll just go with this and do my own testing, and add a comment here explaining what I found. Sep 5, 2021 at 19:43
• @JameelAhamed , Dang! It sounds like so fun. Since your motor spec is fixed, it probably is easier to start, since the motor performance is not a concern anymore, really(?). :-)
– jay
Sep 5, 2021 at 19:49
• not exactly fixed, we still have the freedom to choose to go for 2 motors for each wheel, or one motor with an open differential, stuff like that. I needed these values for some calculations to do different tests like the hill climb and acceleration run, but now since the chart is off I can only guesstimate things Sep 5, 2021 at 19:58
• @JameelAhamed "two motors for each wheel"? why would you do that? driving 2 rear wheels would need 4 motors... Or did you mean one motor per driven wheel? Sep 5, 2021 at 20:13
• @JameelAhamed, Reminds me this. EV motor manufactures, usually, supply complete data set thorough Dyno test, I thought.
– jay
Sep 5, 2021 at 23:08

The last motor ratings are merely steady-state values with zero acceleration at a max temp rise controlled by your controller and load to regulate speed.

There is no definition or meaning of PHI $$\\phi\$$ but my research is as follows;

$$\V=IR + kφω\$$ , $$\T = kφI\$$ (torque), k = back emf constant φ=Torque coefficient, I= amps $$φ=T/kI$$

It will be at some unspecified hotspot temperature rise above 25'C like 60'C. You have to add solar heating in your ambient climate and package to find how to derate this power loss (heat) with speed. There is also a loss of magnet strength with temperature rise and TBD cooling thermal resistance parameter, such as open-air convection-cooled. Forced air external cooling may make a difference above 1 m/s before it starts to plateau from internal thermal resistance. You want laminar flow for low-drag and turbulent air flow for air over the heat radiators for cooling to get over 2 m/s thermal efficiencies.

In applications where a jerk would be too stressful from excessive torque for machine control, it is useful to use a high torque value at maximum temperature rise and RPM then repeat that value down to a very low RPM for max T and report the electric and mechanical values to indicate efficiency.

Max Torque is always at 0 RPM and the current is V/DCR for the applied voltage and coil resistance. This is not really useful to have whiplash and excessive stress on mechanical and electrical parts, so although max torque is given for max. acceleration the efficiency is zero at zero RPM and only increases when moving as Power = Torque x RPM.

For aerial EV drones, no matter how big you make the vehicle battery and blades, you can't expect more than a 30-minute flight. This is due to the current technology for Joules/kg for batteries, motors, and energy required for 30 minute flight / kg. (Perhaps Toshiba may exceed this with their \$160m JV investment into aerial EV-taxies)

For ground based EV's, there will be a similar cost range limit for the energy density of storage and weight/size/drag coefficient of the vehicle with speed. Of course there will be an optimal speed for E to M efficiency, but your task must have some goal with renewable solar energy and the added cost, size/area of the vehicle and resulting drag coefficient of the aerodynamic design.

It could look like a pigskin Wilson J5V football, or Tesla or a Formula 1 race car. Your goal is to start the design by optimal specifications to achieve the goal, whether it is distance, speed, efficiency these must be worked on first with all the variables, and estimated coefficients of power, efficiency, weight and drag and optimized in a spreadsheet.

Your first step is to research every similar competition around the world, find the best, and make it better in some way without critical compromise. . Read about all the schedule, cost, and obstacles that had to be overcome.

Bigger and faster is more efficient for motors, and lighter and slower is more efficient for vehicles. Knowing how to estimate the unknowns and aging rates of batteries, how to maximize charge storage but suffer long term charge cycles and minimizing acceleration, maximizing coasting and minimizing braking are all your “Green goals” for driving habits. Controls can be specified to smoothen these controls to minimize current and torque surges with an algorithm using this table for steady state, using the output power to match your gear ratio and vehicle drag vs speed. Make a big list of all the variables and definitions and create the best EV in your work.

Everything is a tradeoff from Comfort to Power to efficiency to cost. So find the edges of these trade-offs and find which balance gives you the competitive edge.

An Engineer I grew up with did this for a motorcycle power tri-wheeler with 2 seats that had super high (?>100) MPG and a custom 3D printed body and welded frame with rear-wheel self-centering steering. It was called the Urbee 2 If you want to use some of his designs, I might be able to put you in touch, or just mention my name. He is a Prof. now in his retirement from Industrial amazing vehicle designs for super peddle bikes in tunnels, super-tractors and ultra-efficient gas vehicles.

For supercar blondie and sexy car fans

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