In which way are diagrams for BL DC motors derived (and to understand)?

I'll start with a quote from another thread because I would like to dive deep in this special point (if possible) and don't want to shift the other thread.

Since years to no avail I looked out in the web to find a detailed description of a test stand and the testing method for electric motors to establish the motor characteristics and data (and the subsequent diagrams).

In web forums the diagrams are often used to underpin a certain point of view. In a lot of this cases - as far as I understand - there is a faulty interpretation in play.

I think, the test motor is fixed in the stand, has a DC source (battery) which will supply a constant voltage (say 48 V) with no relevant drop when under load (until max. current), and between both an appropriate motor controller unit. There is a adjustable brake (mechanical, hydraulic or electric) coupled to the motor shaft, which provides the measurements of the braking torque applied (preferably witat the otor The rotational shaft speed (rpm) is also measured.

The test starts (voltage "on") with no brake torque, so the motor spins after accelerating its rotor at its highest rpm. The measured current is low, because the power output is near zero (high speed, but no torque). The power input is used mainly to overcome friction of the bearings, air gap and electrical losses in motor and controller.

Then the brake ist set to apply in several steps a growing brake torque (replacing the load in practical motor use). The voltage is monitored (measured), the correspondent current and brake torque are measured. If there are no worries about killing motor or controller, this is done until the shaft stalls (rpm = 0). At this point the torque is at its maximum, but the rotational speed is zero, so the power output is zero.

My question is: will it be done this way or am I wrong with this idea about the tests?

Edit: The appendix is a diagram of a 48 V, 3000 W, BL DC motor (Golden Motor) and the corresponding data.

Edit2: Instead of fixing the motor (as I said above) it will be probably better to mount a lever to the motor housing and allow the motor housing to turn within a small angle and measure the force at the lever arm. So lever times force eaquals to torque.

 

If my perception of the test procedure is right, it is logical to draw the diagrams with torque at the x-axis and current etc. at the y-axis. Because torque is seen as the independend variable (it is set as (propeller) load the motor has to deal with). The respective current and rpm are seen as the results, so the dependend variables.

AFAIK all (or most of the) diagrams of electric motors (not only BL DC) are presenting the torque at their x-axis.

 

I don't see anything wrong your understanding. However, I don't know how much it really matters how you display the data as long as you understand what it is telling you. Regarding testing, the following comes from "Electric Motors and Their Applications", by Tom C. Lloyd, 1969.

 

Thank you @ziper1221

 

This link provides a typical set of dyno curves for a dc motor:
https://sep.turbifycdn.com/ty/cdn/yhst-129399866319704/ME0709data.pdf?t=1709531008&

This graph is for a brushed DC motor. Many BLDC motors have similar characteristics. Having designed and built a couple of dynamometers, I can say with reasonable certainty that the descriptions in this thread are generally correct regarding how motor output is measured. The process is similar for ICEs too, although it is common to start at low rpm and work upward, rather than starting with no load and full throttle.

(There are inexpensive chassis dynos for automotive use which use a heavy flywheel which the car or motorcycle under test accelerates under full throttle. (Greater torque accelerates the weight more quickly than less torque). The dyno does a little math, and in a few seconds out comes hp figures.

For electric motors of a few horsepower, it is pretty simple to connect the motor under test to another motor of known characteristics and of comfortably higher output... which we'll call the brake motor. Then the brake motor is connected to a high amperage load and the test motor is started. The brake motor becomes a generator. Knowing the characteristics of the brake motor allows one to calculate the output of the test motor, given voltage and amperage outputs of the brake motor into the load.

With a little care, one can get within 5 or 10 percent of actual test motor output figures using this method. I have an ME0709 motor in my shop and have used it to quickly test various other motors.

When I have nothing better to do, I may test the output of several of the small electric outboard motors, several of which have input values of about 1 kw, meaning that, at best, they have a shaft output of say 900 watts. 900 watts is 900/746 or 1.2 hp. Many of these motors are advertised as being "equivalent" to 3 hp, etc. which is, of course, untrue: a motor that produces an output greater than its input is a perpetual motion machine.

 

All the dynos I used were based on a hydraulic pump. A calibrated valve restricts the flow and the back pressure is measured as power.

 

No, it is not. Back pressure is measured in PSI (in the US), which is not an indication of power. It is a measure of torque. (Not torque of the engine under test, of course, because the pump is only sometimes connected directly to the engine's output: chassis dynamometers are far mor common.) PSI is a reliable analogue for torque.

The valve that restricts flow should not be (and never is, in a legitimate dyno) calibrated, because viscosity changes with heat, and changes in viscosity will change pressure, and therefore torque reaction forces. The thing that indicates pressure (and therefore torque, with a simple calculation) is a pressure gauge.

There are two common types of dynos that use hydraulic pumps as the brake. In one type, The pump is mounted so that its housing could rotate, but its rotation is restricted by a reaction arm with a scale. This gives a direct measure of torque. This type can be quite accurate and easy to understand.

In the second type (the type I designed and built) hydraulic pressure is used as an analogue for torque. One can reliably say that, even if viscosity has changed: if the output of a given pump produces 2200 psi, then the torque driving that pump has to be X lb ft. Pump HP is, in the US, GPM*PSI/1714. So a person could get a good direct estimate of HP just by measuring pressure and flow. This is not usually done, however, because inaccuracies creep in, and as the pump wears, the amount of HP required to drive it changes).

(The same thing can be done with one electric motor of known characteristics being driven by another -- the one under test. The "brake" motor is then loaded with dummy loads... and what may seem simple becomes a little tricky and not too accurate... especially when the dummy load starts to smoke.)

But no. Pressure is never measured as power. Likewise, in electric systems, voltage is never measured as power. Torque is not a measure of power: it is one of two things that multiplied together indicate power.

 

Incidentally, the slight difference in measuring an ICE vs an electric motor (slow to fast, vs fast to slow) on a dyno is a matter of tradition and safety. If an ICE is significantly oversped, the valves can float, contact the piston crowns, and damage ensues. Rev limiters are now more common, but even then, they can hunt, causing unstable rpm at the limit. So when you do a dyno run with an ICE in a classic dyno, you start at low rpm and work your way up, and by the time you reach the power peak, you have applied a very heavy load, and the possibility of overspeed (and damage) is low. But with a motor, with a good strong stable power supply, you can just turn it on, wait a second for it to reach full speed, and then start loading it. It this is done manually, it is already quick and easy, but if is under computer control, then the load can change very fast toward stall, and as long as rotor inertia is understood, the test can be over very fast, with all the recording being done automatically and a few times per second.

Back when I was doing this with motorcycles (in the days of kerosene-powered TVs) we'd have to write stuff down, and having a digital readout for speed was quite a new thing. Every thing would get hot, and the rear tire would begin to smoke, because it took a while to do a run.

 

BTW, I had intended to write earlier. Your description is both crystal clear and precisely correct.

 

The pressure is a torque equivalent (force) but there is a tachometer too.

The dynamometers are water cooled and the oil runs at constant temperature.