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On this page: Functional description | Hall sensor phasing and alignment | Graphing & logging | Product resources

UAV Engine Starter

UAV Engine Starter

UAV Engine Starter

The UAV Engine Starter is designed to start internal combustion engines up to 150cc in size. It is small, lightweight, and delivers rapid and reliable engine starting without the use of decompression valves.

The engine starter drives the BLDC alternator as a motor in order start the engine. Once the engine is running the starter disconnects itself from the BLDC alternator to allow electrical power generation. A suitable BLDC alternator fitted with Hall sensors is required.

Engine starting is initiated locally by push-button or remotely using CAN or RS232 commands. In-flight restarts are possible with CAN or RS232 commands.

Why not just use an ESC as an engine starter?

Absolutely an ESC can be used to start an internal combustion engine, but there are some major disadvantages and design challenges to overcome. Here are just a few:

  1. Torque at zero speed is the #1 requirement for successful engine starting - all of the action happens in the first couple of hundred milliseconds. Sensored operation is the only way to guarantee torque at zero speed, and not all ESCs support this. Although a sensorless ESC may be able to start an engine under some circumstances, it is essentially a lottery - it will never be as reliable or as fast as a sensored solution.
  2. Back-EMF tolerance. If the engine starts then the BLDC motor becomes an alternator, generating 3-phase power. Regardless of the type of engine starter used, unless a 3-phase isolation circuit is present the generated voltage must never exceed the battery voltage or uncontrolled battery charging occurs. This is obviously catastrophic. To avoid this, the Kv of the BLDC alternator must be chosen very carefully and this limits the range of alternators that can be used. Most of the commonly available BLDC alternators intended for UAV applications generate over 100V at maximum RPM, ruling them out of contention.
  3. Poor efficiency when generating electrical power. Because the generated 3-phase voltage cannot be greater than the battery voltage, it is not possible to simply rectify the 3-phase output and obtain a useful bus voltage. Boost conversion must be performed to step-up the voltage, which is an additional step incurring additional losses. Boost conversion can be performed by operating the ESC in its regenerative braking quadrant, but this is electrically noisy and inefficient.

This engine starter suffers from none of these problems. It can produce maximum torque at zero speed. It can operate from battery voltages as low as 20V and is able to withstand generated 3-phase voltages of up to 140VAC. It can use a wide range of off-the-shelf BLDC alternators.


The UAV Engine Starter is distributed internationally from Europe by VISIONAIRtronics.

If you are located closer to Australia, you can choose to purchase from Millswood Engineering.

Functional description

UAV Engine Starter wiring diagram

Wiring diagram

The Engine Starter allows a 3-phase BLDC alternator to be used as a motor to start an internal combustion engine. It requires a source of power capable of delivering high-current, and a BLDC alternator fitted with Hall sensors.

When requested to do so, the starter energises the windings of the BLDC alternator to accelerate the engine up to starting speed. Once the engine starts, the starter disconnects itself electrically from the BLDC alternator. This allows the engine to spin freely and the BLDC alternator to generate electrical power without interference from the starter.

Hall sensor phasing and alignment

UAV Engine Starter configuration software screenshot

Checking Hall sensor phasing and alignment

The engine starter comes with several features that reduce the engineering effort required to achieve successful engine starting.

One of the most common problems with setting up sensored BLDC controllers is getting the phasing right. There are 36 ways of wiring up 3 windings and 3 Hall sensors: 6 of these will work reliably, 12 of them will almost work but not reliably, and the remainder will not work at all and potentially break stuff. Phasing between Hall sensors and windings is difficult to analyse and can take a long time to get right. The engine starter addresses this problem by providing a test mode to verify that each of the Hall sensors is phased and aligned correctly.

It is very simple to do, and it works like this.

Pressing each button in the configuration software energises the windings to align the chosen Hall sensor with the middle of its range. The corresponding Hall sensor LED (on the engine starter itself) then turns on if the Hall sensor is wired up correctly. This allows verification (or correction) of the Hall sensor wiring, and it also allows an assessment to be made of the positional error of each Hall sensor.

Graphing & logging

UAV Engine Starter configuration software screenshot

Graphing & logging

Another feature designed to reduce engineering effort is the graphing and logging available from within the configuration software.

This is trivial to get up and running because of the USB connection to the engine starter. Graphs are created dynamically in real-time, and show all of the key variables. All data is synchronised to the starter's internal state, so there is no aliasing from a secondary sampling process.

If a more permanent record of starting performance is required, then all of the same data can be written to file.

The graph on the right shows a 70cc engine being cranked at 1500RPM. (It has no ignition system fitted, so it's not actually going to start.)

Interesting things that can been seen at a glance:

Less obviously (but derivable from the logged data), the peak torque applied to the engine is 15.7Nm, and the average torque required to maintain 1500RPM at steady state is 1.8Nm.

This is a fairly easy engine to turn over, and so the starter is not working particularly hard.

Product resources

More resources are available here.

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