Electrical power systems are critical to a UAV’s design and operation. But they don’t operate in a vacuum. Their weight, operating temperature, and efficiency of the power components have a major influence on performance parameters such as flight duration, payload, operating ceiling, and range.
Below is a short introduction to UAV power systems, what’s involved, and why it all matters.
The basics
Electrical power in a UAV is generated by a starter generator, which is a critical component for ensuring the electrical system can meet the design objectives of the aircraft. The starter generator is mounted on the engine so that the drive shaft turns the rotor to produce electricity.
If the electrical machine is only used to generate power for the onboard systems, it may be referred to as an alternator. A starter generator is also capable of starting the internal combustion engine (ICE) itself if paired with an electronic engine starter (EES). The EES is a power electronics component that provides the commutation to the starter generator to rotate the ICE shaft to the cranking speed and torque.
A battery pack, which can also be used to start the engine, stores energy to ensure its continuous supply over the required minimum operating time.
Power generation
An electrical generator converts mechanical energy to electrical energy. Its basic parts are a rotor and a stator. The rotor is the magnetic field source while the stator contains the conductor where the electric motive force is induced (generally the conductor is a coiled wire called windings). All generators operate on the same basic principle: a magnetic field cutting through conductors, or conductors passing through a magnetic field, to produce electricity. An electrical generator is the same basic design as an electric motor because its construction and working principles are based on the mutual induction between the rotor and the stator windings.
Windings are the wire wound on the stator armatures to work as a conductor. The only physical difference between a single-phase generator and a multiphase generator is the additional coils with accompanying parts in the stator. But a multiphase generator is smaller, lighter, and less expensive than a single-phase generator with equal power.
The rotor houses a permanent magnet made from a hard ferromagnetic material that is magnetized in a manufacturing process, creating a persistent magnetic field. Permanent magnets do not require any power source and usually produce a powerful magnetic field compared to their size. Most starter generators use rare earth magnets for high power density.
The performance of a generator is a complex combination of the material used, the physical design, and the manufacturing processes. An ideal generator will be lightweight but able to produce power reliably and efficiently to meet the mission parameters.
Electronics
Given the increased demand for electrical power for more complex missions, system designers have to provide higher currents and multiple supply rails. New and innovative power supply topologies are necessary to improve efficiency, reduce weight, minimize heat dissipation, and lower overall costs.
UAVs can have onboard or outboard starters that must be able to create enough torque and speed for the ICE while the battery must provide enough voltage and current. An onboard electronic engine starter is a good option to solve this.
Some designers rely on ground-based starters to minimize onboard requirements and thus keep overall weight down. However, an onboard system has the advantage of being self-contained and may provide greater flexibility in the field.
Batteries
For platforms that run on internal combustion engines, energy storage is necessary to ensure a reliable supply of electrical power for onboard systems. This means batteries. But designers need to understand their options in battery technology such as LiHV, LiFePO4, Li-ion, NiCad, NiMH, and lead-acid batteries.
Regardless of the battery used in the UAV, its charge and its use are going to influence mission parameters and final performance. Therefore, when designing UAVs, the importance of a battery management system cannot be overstated.
What’s more, a reliable battery management system will:
- continuously monitor important battery parameters,
- handle the varying power demands of the many aspects of the UAV’s operation, and
- optimize the battery usage.
The battery management system can also protect the battery during charging, safeguarding against conditions such as overcurrent or overvoltage.
Hybrid operation
Most large UAVs are still powered by internal combustion engines, while electric propulsion systems are more common in civilian micro and mini UAVs. Increasingly, high-performance electric motors make electric propulsion possible for larger commercial UAVs.
However, electric propulsion systems may be limited by a reasonably-sized battery pack. What’s more, the noise produced by internal combustion propeller-driven UAVs can be a drawback, which can be partially solved by installing an electric motor to drive the propeller.
Below are some hybrid applications:
- Internal combustion engines can be sized for steady state flight rather than sized exclusively for peak power requirements at takeoff. We refer to this as power assist.
- Battery power can be used to extend flight time. This provides a mechanism to use the battery power to land the aircraft in the event of an emergency.
- Converting to electrical power temporarily for reduced noise.
The choices involved in selecting an electric power system are dictated largely by the duration of continuous and peak power requirements. Unlike many other UAV subsystems, the power system supports both the platform and the payload. Depending on the mission, the payload will require electrical power while in flight. This power demand can be tens or hundreds of watts for sensors or communications or tens of kilowatts for more for complex payloads.
This is why so many components need to be considered when it comes to electric power systems for UAVs. To learn even more about this subject, download our Guide to UAV Power Systems.
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