Electric propulsion fundamentals

A motor datasheet packs many numbers into one page. This lesson explains the ones that matter most when you size a motor for a lift rotor or a propeller, and what to check behind each figure.

Continuous and peak power

Continuous power is the power the motor can deliver for an unlimited time without exceeding its temperature limits. It is only valid under stated conditions: ambient temperature, airflow or coolant flow, and coolant inlet temperature.

Peak power is available for a limited time, from a few seconds to a few minutes. Heating of the windings and magnets limits it, and so does the current the controller can supply. Always check the duration and the conditions behind a peak figure.

Size the motor so that hover and climb sit within the continuous rating. Keep the peak rating for short events, such as a gust, a go-around or the loss of another rotor.

Torque and speed

Power is torque times angular speed. In SI units, P (W) = T (N·m) × ω (rad/s), and ω = rpm × 2π / 60. A motor that gives 100 N·m at 3,000 rpm delivers about 31 kW.

Large rotors and propellers turn slowly and need high torque. A motor that matches the rotor speed can drive it directly and avoid a gearbox. Datasheets often give a torque constant (N·m per amp) and a back-EMF constant (volts per unit of speed). Together with the bus voltage, they set the current needed for a given torque and the top speed the motor can reach.

Look at the whole torque-speed curve, not only the rated point. Up to the base speed, the motor can deliver its full torque. Above it, the available torque falls, because the back-EMF approaches the supply voltage.

Efficiency

A single “maximum efficiency” figure applies to one operating point. Efficiency changes with torque and speed. Copper losses grow with the square of the current, so they dominate at high torque. Iron losses grow with speed and electrical frequency.

Ask for an efficiency map, or at least the efficiency at your hover and cruise points. Include the controller: the figure that matters is the efficiency of the motor and controller together.

Voltage

A higher bus voltage carries the same power with less current. This allows lighter cables and lowers cable losses. It also demands more insulation, larger clearances between conductors and attention to partial discharge. Partial discharge becomes more likely at altitude, because air at low pressure breaks down at lower voltages.

Battery voltage drops as the pack discharges. Check that the motor delivers the power you need at the minimum pack voltage, not only at the nominal one.

Power density

Specific power (kW/kg) is the figure most often used to compare motors, and the easiest to misread. Check whether it is based on continuous or peak power. Check what the mass includes: housing, cooling jacket, controller, cables and mounts. Compare like with like.

Key takeaways

• Size for continuous power. Treat peak power as a short-time reserve with stated limits.
• Power equals torque times angular speed. Direct drive needs high torque at low speed.
• Ask for efficiency at your own operating points, for motor and controller together.
• Check performance at the minimum battery voltage.
• Compare power density only on the same basis.

The rotor or propeller sets the load. Its size and speed decide how much torque the motor must deliver and at what speed. This lesson shows how to work out that operating point and how to check it under every condition the aircraft will meet.

Thrust, power and rotor size

In hover, a rotor produces thrust by pushing air downward. Simple momentum theory gives the ideal power needed for a thrust T with a rotor disk of area A, in air of density ρ:

P = T^1.5 / √(2 ρ A)

Real rotors need more power than this ideal value. The ratio between the two is the figure of merit, typically 0.6 to 0.8 for a well-designed rotor.

Take a rotor of 2 m diameter that must lift 1,000 N in hover at sea level (ρ = 1.225 kg/m³). The disk area is 3.14 m², the ideal power is about 11.4 kW and, with a figure of merit of 0.7, the shaft power is about 16.3 kW.

Rotor size matters a lot. For the same thrust, power falls with the square root of the disk area: doubling the diameter multiplies the area by four and halves the ideal power. Large, lightly loaded rotors are efficient in hover, but they turn slowly and need high torque.

Speed, tip speed and torque

Rotor speed is limited by the speed of the blade tips. Tip speed is the angular speed times the radius. When it approaches the speed of sound, noise and losses rise quickly, so designers usually keep the tip Mach number well below 1, and lower still for quiet operation.

In the example, the rotor turns at 2,000 rpm, which is 209 rad/s. The tip speed is 209 m/s, about Mach 0.62 at sea level. The torque is power divided by angular speed: 16,300 W / 209 rad/s ≈ 78 N·m. These two numbers, 78 N·m at 2,000 rpm, are what the motor must deliver in hover.

Direct drive or gearbox

A motor that turns at the rotor speed can drive it directly. Direct drive avoids the mass, losses, lubrication and maintenance of a gearbox, and removes a failure point. The price is a motor that produces high torque at low speed, which makes it larger and heavier for its power.

A gearbox lets a smaller, faster motor run at its most efficient speed. It adds mass, losses of around 1 to 2% per gear stage, wear and new failure modes. Compare both options with the whole drive in the balance: motor, gearbox, cooling and controller.

Hot, high and heavy

Air density falls with altitude and temperature. At 2,000 m on a hot day it can be 20 to 25% lower than at sea level. To make the same thrust, the rotor must turn faster, and the hover power rises by about 12 to 15%. Check that the motor has the speed, torque and thermal margin for the hottest and highest place where the aircraft will fly, at its maximum weight.

Forward flight and propellers

A propeller’s efficiency depends on the advance ratio J = V / (n D), where V is the airspeed, n the speed in revolutions per second and D the diameter. A fixed-pitch propeller works well only in a narrow range of advance ratios, so it is usually a compromise between takeoff and cruise. A variable-pitch propeller widens that range. In both cases, the motor must deliver the torque needed at the static point and at the cruise point.

Control margin

Many multirotors control attitude by changing the speed of fixed-pitch rotors. The motor must then speed the rotor up and slow it down quickly. Rotor inertia grows fast with diameter, so large rotors respond slowly unless the motor has a torque reserve above the hover torque. Some designs use collective pitch control to avoid this. Size the motor for control as well as for hover.

Key takeaways

• Work out the hover point from thrust, rotor area and air density, then apply a realistic figure of merit.
• Larger rotors need less power for the same thrust, but more torque at lower speed.
• Tip speed limits rotor speed; torque is power divided by angular speed.
• Direct drive saves the gearbox but needs a high-torque motor.
• Check the hot and high case at maximum weight, the cruise point and the torque reserve for control.

Every motor turns part of its input power into heat. How well that heat is removed sets the continuous power rating, the life of the insulation and the risk to the magnets. This lesson covers where heat comes from, how it leaves the motor and how to read derating data.

Where the heat comes from

• Copper losses in the windings. They grow with the square of the current, so they rise fast at high torque.
• Iron losses in the stator core. They grow with speed and electrical frequency.
• Rotor losses, mainly eddy currents in the magnets and rotor parts, caused by harmonics in the current and the magnetic field.
• Mechanical losses in the bearings and from air drag on the rotor.

The controller adds its own conduction and switching losses. It needs its own cooling, and its limits can cap the system before the motor reaches its own.

Heat paths

Heat produced in the windings must cross the wire insulation, the potting or impregnation resin, the stator core and the housing before it reaches the air or the coolant. Each layer adds thermal resistance. The hottest point, usually in the slots or the end windings, sets the limit.

Two limits matter most:

• Insulation. Winding insulation is rated by thermal class, such as class F (155 °C) or class H (180 °C). Running hotter shortens its life.
• Magnets. Neodymium magnets lose strength as they heat up. Above their rated temperature they can be permanently demagnetized, especially under high current. Hot magnets also lower the torque per amp, so the motor needs more current and makes more heat.

Air cooling and liquid cooling

Air cooling is simple and light. It needs no pump, coolant or radiator, and lift rotor motors can use the airflow from their own rotor. Its weak points are hot days, high altitude, where thin air removes less heat, and conditions with little airflow, such as ground runs.

Liquid cooling removes more heat from a smaller area and keeps temperatures more stable. It allows more continuous power from the same motor mass. It also adds pumps, hoses, heat exchangers, coolant and possible leak points. Compare the two options at system level, including all of that mass and the power that pumps and fans draw.

Thermal time constants and derating

Windings heat up in seconds to minutes. The housing and the coolant take longer. This is why a motor can deliver peak power for a short time: its thermal mass absorbs heat before the limits are reached.

Derating means reducing the available power to protect the motor. Controllers do it automatically when temperature sensors in the windings approach their limits. Ask the supplier for:

• The conditions behind the continuous rating: ambient temperature, airflow or coolant flow, and coolant inlet temperature.
• Derating curves against ambient temperature and altitude.
• How long peak power is available from a cold start and from a hot motor.

Think in mission profiles

A single operating point is not enough. Model or test the full flight: takeoff, hover, transition, cruise, landing and reserve. Include back-to-back flights with short turnarounds, because a motor that starts hot has less margin.

Key takeaways

• Copper losses dominate at high torque, iron losses at high speed.
• The hottest point in the windings and the magnet temperature set the limits.
• Liquid cooling raises continuous power but adds system mass and complexity.
• Ask for the conditions and derating curves behind every rating.
• Check thermal margins over the full mission, including hot restarts.

A good motor can still perform badly if it is poorly mounted, wired or controlled. This lesson covers mechanical integration, the link between motor and controller, and how to design for faults.

Mounting

The motor mount carries the rotor thrust, the reaction torque, gyroscopic loads when the aircraft pitches or rolls, and vibration. It must be stiff and light at the same time.

• Check the natural frequencies of the motor, mount and boom against the rotor speed range and the blade-pass frequency, to avoid resonance.
• Use a locking method suited to vibration for every fastener.
• Account for thermal expansion where metal and composite parts meet.
• In axial-flux motors, the magnets pull the rotor toward the stator with a large axial force. The bearings and housing hold the air gap against that force, so follow the supplier’s limits for loads and deflection.

Cabling and controllers

The phase cables between the controller (inverter or ESC) and the motor carry fast-switching currents. Keep them short and of equal length, route them away from sensors and avionics, and shield them where needed. Long cables can cause voltage overshoot at the motor terminals, which stresses the winding insulation. Choose connectors rated for the current, temperature and vibration.

Match the controller to the motor: voltage range, phase current, number of poles and maximum electrical frequency. Decide how the controller will know the rotor position. Sensorless control saves parts but can struggle at very low speed and at startup. A resolver or an encoder gives a reliable position at every speed. Finally, check the data interface, for example CAN, and the telemetry you need: current, speed, temperatures and fault codes.

Redundant windings and fault tolerance

A single failure in a motor or controller must not cause loss of the aircraft. There are two main approaches, often used together:

• At aircraft level. Use enough rotors that the vehicle stays controllable after losing one, with the remaining motors able to supply the extra thrust for as long as it takes to land.
• At motor level. Split the motor into two or more independent winding sets, each driven by its own controller and, ideally, fed from a separate battery bus. If one channel fails, the motor keeps running at reduced torque.

Faults must not spread. In a permanent-magnet motor, a short-circuited winding keeps generating current and heat while the rotor turns, because the magnets cannot be switched off. The design must keep the channels electrically, thermally and magnetically isolated, so a fault in one does not disable the others. The controllers must detect faults fast and isolate the failed channel.

Verification

Plan the evidence early. Typical steps are a failure mode and effects analysis, bench tests with one channel lost, tests of fault detection and response time, and electromagnetic compatibility tests. For aircraft that need certification, agree with the authority on which requirements apply and what each test must show.

Key takeaways

• Check mount stiffness and natural frequencies against rotor speed and blade-pass frequency.
• Keep phase cables short, matched and away from sensitive electronics.
• Match the controller to the motor and choose how it will sense rotor position.
• Combine redundancy at aircraft level and at motor level.
• Make sure a fault in one channel cannot spread to the others.

Datasheets and models only take you so far. This lesson covers the tests that confirm a motor does what its datasheet says, what to check on every unit you receive, and how to keep motors healthy in service.

Bench tests on a dynamometer

On a dynamometer, the motor drives a load machine or a brake while instruments measure shaft torque and speed, and the voltage and current at the input of the controller and at the motor phases. From those measurements you get:

• Efficiency maps of the motor, of the controller and of both together, across the torque and speed range. Check your hover, climb and cruise points.
• Thermal tests. Run at the continuous rating until temperatures stop rising, with sensors in the windings, to confirm the rating and find the hot spots. Then run the peak rating from a cold motor and from a hot one, and record how long it lasts.
• Back-EMF constant. Spin the motor with the load machine and measure the open-circuit voltage against speed. It confirms the magnet strength, and a later drop is a sign of demagnetization.

Thrust stand tests

A thrust stand runs the motor with the real rotor or propeller and measures thrust, torque and electrical power against speed. Compare the results with your predictions from the previous lessons. Differences usually come from the rotor’s figure of merit, the installation or the controller tuning.

Electrical and environmental tests

• Insulation resistance and dielectric withstand (hipot) tests check that the winding insulation separates the windings from the frame without leaking or breaking down. At high bus voltages, a partial discharge test shows the voltage at which discharges start, which falls at altitude.
• Environmental tests cover temperature, humidity, low pressure, vibration and shock. RTCA DO-160 is the usual reference for airborne equipment. Ingress protection against dust and water is rated with the IP code (IEC 60529).
• Electromagnetic compatibility tests check that the motor and its controller neither disturb nor suffer from other equipment on board.

What to ask the supplier

• Test reports that state the conditions behind each figure.
• Efficiency maps and derating curves, as covered in lessons 1 and 3.
• Bearing life data, for example the L10 rating, which is the life that 90% of bearings reach.
• Failure rates and failure modes, for your safety analysis.
• Their quality system, for example AS9100 for aerospace suppliers, and traceability of materials such as magnets.

Acceptance of each unit

Agree a short acceptance test for every motor delivered: visual inspection, winding resistance (balanced between phases), insulation resistance, back-EMF at a set speed, no-load current and speed, and a vibration check. Record the serial number and the results. The record gives you a baseline for later inspections and traceability if a problem appears.

Maintenance and health monitoring

In a well-cooled motor, the bearings are usually the main wear item and set the maintenance intervals. Winding insulation ages with temperature: as a rule of thumb, its life roughly halves for every 10 °C it runs above its rating.

Use the data the controller already sends. Trends in winding temperature, vibration, current imbalance between phases and efficiency show wear before it becomes a failure. Inspect connectors and cable fixings at every scheduled check, because vibration loosens them over time.

Key takeaways

• Confirm efficiency, thermal ratings and back-EMF on a dynamometer, and thrust with the real rotor.
• Test insulation and, at high voltage, partial discharge; cover environment and EMC.
• Ask suppliers for the conditions behind their data, bearing life and failure rates.
• Run a short acceptance test on every unit and keep the results.
• Bearings set the maintenance intervals; monitor temperature, vibration and current trends.