Author: Owen

Simple DC machines

Simple DC machines includes a DC motor with permanent magnet field and wound armature with commutator. The permanent magnet DC motor is a good case to study.

In simple DC machines, the difference between being a motor and generator is often simply a matter of rotational speed. The motor develops an induced voltage in its windings by virtue of its rotational speed, and current flows in the winding if that voltage is different to the terminal voltage… the direction of current determined by which voltage is higher and the direction of current determines whether the torque assists or resists the rotation.

The counter torque from reverse current is often referred to as regenerative braking as the retarding effect of the current driven by the induced emf of rotation slows the motor, and current flows into the source.

If a simple DC machine is powered from a simple rectifier circuit, the rectifier will block the flow of reverse current, and so there is no regenerative braking… the rotation induced emf simply raises the terminal voltage of the motor (possibly dangerously), but no current flows and there is no counter torque.

If a simple DC machine is powered from an electronic regulated power supply, the situation is a little different. The regulator will commonly block reverse current, and it may sense that output voltage is greater than desired and shut down, it may even be damaged by the excess terminal voltage.

Brushless DC motors

Brushless DC motors use some form of electronic driver to provide commutation of current in the coils, whether derived from sensors fitted to the motor or sensed from the undriven coil at any instant.

Some driver configurations provide a path for regenerative current to flow to the power source. If the power source blocks the regenerative current, the terminal voltage of the motor and power supply may increase, possibly to levels that may damage the motor driver and damage or disrupt the power supply. Electronic power supplies do not usually contain provision for regenerative current.

An example sensorless brushless DC motor used in UAVs

This example illustrates the nature of regenerative current in a particular application where rapid response of the drive is very important.

Above is a supply current graph for a test scenario that subjects the drive to a number of acceleration / deceleration scenarios. The current sensor does not measure negative current, its output is clipped at I=0. Continue reading Regenerative braking and electronic power supplies

I purchased an inexpensive BLDC for some tests on a 6S battery pack. The RCTimer 4215-530Kv could be loaded up to about 20A at 24V (the limit of my bench supply) with an 11×4.7 SF propeller (in stock). The motor is a low pole count motor, 12N14P. On 24V, the no load speed should be almost 13,000rpm, and fully loaded perhaps three quarters of that.

Above is the motor as supplied. I used an ordinary M6 propeller nut so that it was easy to remove and replace without wearing out the nyloc nut supplied.

Above, the induced voltage waveform at 940rpm, somewhat the result of the 12N14P configuration.

The ESC was a Hobbyking 40A ESC 4A UBEC 9261000003, SimonK commit 02bd8e4ca36a06722efe51bc7cd5130d72a184b8 with COMP_PWM.

On a steady test on the 24V bench supply, the drive drew just on 20A cold and was clocked at 9200rpm with the Gemfan 1147 SF. Winding speed up and down slowly (to avoid degenerative braking which is incompatible with the power supply), motor starting, acceleration and deceleration were always smooth and without any sign of sync loss.

Tests were conducted with a script that I use consistently with asrg and eLogger to capture current, altitude is 700m. Continue reading RCTimer 4215-530Kv BLDC motor checkout

I bought a flight controller described as a CC3D F4 on eBay. It is basic in facilities, but with gyro, acc and F4 it would be a good test base for floating point PID loops (F4 series chips have floating point hardware). Continue reading CC3D F4 fraud

This article documents a case study in use of Complementary PWM (COMP_PWM) to improve quadcopter stability at very low throttle.

An observation of two quadcopters of 450 size running several releases of Cleanflight and now Betaflight 3.01 is a loss of stability at very low throttle opening.

This is not uncommon for several reasons, and there is ‘airmode’ in both firmwares to address the problem that motors at minimum speed cannot be slowed further. Experience with airmode on Cleanflight up to v1.14.1 was that it raised throttle so much that descents were extremely slow sometimes, certainly never quick, and its use was discontinued.

I have since abandoned Cleanflight due to unresolved flight problems, lack of migration facility from version to version, and the quiet removal of the backup and restore facility.

The objective of this study was to explore the effect of enhanced motor braking with COMP_PWM on basic angle mode loop stability at low rpm.

Test scenario

The study uses a BC3530 1100Kv motor with 11×4.7 SF propeller, F-30A ESC with SimonK (1e4c01782eff85da3971f628a3bd599b7a0725eb) with COMP_PWM enabled.

Tests were conducted with a script that I use consistently with asrg and eLogger to capture current and rpm, and all tests conducted at similar pressure, temperature and humidity, altitude is 700m.

Test results

One of the effects of COMP_PWM is stronger braking of the motor when throttle is reduced. In multi-rotor application, the motor braking under COMP_PWM is dwarfed by the propeller load at maximum rpm, but propeller torque falls as the square of rpm and at lower speeds motor braking becomes more significant.

Above is a graph of the drive response with a non-COMP_PWM response feint overlay. It can be seen at 13s, that under rapid deceleration, the COMP_PWM response differs, lets zoom in on that. Continue reading Improving quadcopter stability at very low throttle using Complementary PWM

This article documents a comparative thrust test of a stock Hobbywing X-Rotor 40A, Hobbywing X-Rotor 40A with BLHeli firmware and a Hobbyking 9261000003 40A with SimonK firmware on a medium sized motor at wide open throttle (WOT).

Battery is 3S fully charged.

Motor is a Turnigy Propdrive 28-26S 1100kV;

Propeller is a 9×4.7 SF;

Stock Hobbywing configuration

The X-Rotor 40A out of the box then a throttle cal for 1030/2000 performed.

BLHeli configuration

The X-Rotor 40A is configured with BLHeli v14.8 MULTI, default BLHeli config, then a throttle cal for 1030/2000 performed.

SimonK configuration

The Hobbyking 9261000003 40A is flashed with SimonK’s tgy Hobbyking 9261000003 40A (1e4c01782eff85da3971f628a3bd599b7a0725eb) 15/10/2015.

Above, the motor and prop used for the test.

Maximum measured thrust results:

• X-Rotor 40A with stock Hobbywing firmware: 850g;
• X-Rotor 40A with BLHeli: 870g;
• Hobbyking 9261000003 40A with SimonK: 950g