Electric motors are one of the largest sources of energy consumption in electromechanical systems. Optimally converting electrical to mechanical energy has a far-reaching impact on both mechanical output and operational costs.
Different types of electric motors have their own natural limits on energy conversion, which makes motor design one of the most fundamental factors of system efficiency and power output. In this blog, we will compare the electrical/mechanical conversion rates of the following electric motors:
- Brushed DC motors
- Brushless DC motors
- AC induction motors
- Synchronous motors
Factors Affecting Electric Motor Efficiency
The efficiency of electric motors depends on how well a motor can convert current into mechanical energy. For all types of electric motors, this conversion comes down to the amount of heat they generate, which indicates how much electric power is lost and fails to convert to mechanical motion.
Generally, motor efficiency is expressed as a percentage of the electrical energy that becomes mechanical force (or torque), and the remainder is roughly equivalent to the amount of heat also produced. The main factors influencing this three-way dynamic are:
- Steel magnetism
- Conductor materials
- Thermal management
- Aerodynamic design
- Manufacturing processes and quality controls
The goal is to achieve maximum motor output with as little heat generated as possible. Reducing energy loss in the form of heat not only improves motor efficiency but protects various motor components from unnecessary wear and system malfunction.
Brushed DC Motors
A brushed DC motor drives the rotor using a brush system integrated with the commutator plate to facilitate direct current flow between the winding and the commutator.
Of the four types of motors compared here, it's the most inefficient design, converting only 75–80% of electrical power to mechanical energy. Higher motor speeds translate to greater efficiency — which is true of almost all types of electric motors — creating even more demand for total energy consumption.
Brushless DC Motors
A brushless DC motor (BLDC), or electronically commutated motor, uses electronic controllers to convert current to the motor winding via a magnetic field. The field itself rotates and moves a permanent magnet rotor with it.
By omitting rotor windings, a permanent magnet rotor dramatically reduces slip between the rotor and stator. The result is much higher efficiency than brushed DC motors, with electromechanical conversion rates between 85% and 90%.
AC Induction Motors
An AC induction motor, or asynchronous motor, drives rotor movement through electromagnetic induction originating in the stator winding's magnetic field. Induction involves some inherent slippage between the applied current and the magnetic field, resulting in an asynchronous lag between the rotor and the stator.
Depending on speed variability and the number of stator poles, an induction motor can achieve 90–93% efficiency.
Synchronous Motors
Synchronous motors, or switched reluctance motors (SRMs), eliminate the need for current to flow into the rotor at all. This is possible due to the synchronization of current frequency and the magnetic field generated by the winding. The shaft and magnetic field rotate in lockstep with current oscillation, driven by the stator's sophisticated electromagnet geometry.
This reduces internal rotor resistance, regardless of its position, increasing flux availability for maximum conversion. It also breaks the dependency on fast rotor speeds to attain higher efficiency. Synchronous motors are capable of producing near-perfect conversion of electrical and mechanical energy, making up to 99% efficiency rates possible. Synchronous motors can also provide higher power with more compact designs, as well as superior torque at lower speeds.
Learn More with Pelonis Technologies
The efficiency of electric motors depends on numerous design factors, but for all types of electric motor designs, system inefficiencies contribute significantly to the amount of heat the motor consistently generates. Because the total output of any motorized system is much less than the efficiency of the electric motors themselves, total system efficiency depends on maximizing the conversion of electrical to mechanical energy.
Pelonis Technologies is continually advancing the study and application of electric motor efficiency for our partners across numerous global industries, including the following:
- Medical equipment manufacturing
- Aerospace and defense
- Heating and air conditioning
- Automotive
- Appliance manufacturing
To learn more, contact us or request a quote, and tell our experienced technicians about your electric motor system's needs.
The biggest and best reason…
Paul @PDWhite listed ‘truely zero torque’ as the first reason for using multiple motors, but didn’t really elaborate on it. A lot of pilots, especially those with previous experience of paramotors, will get that straight away but it occurs to me that many will of not noticed the ramifications. So here’s a VERY simplistic explanation of why zero torque is such a big deal, aimed not so much at most of the people here but at those passing through or only just starting to look into becoming flyers.
There are two ways to turn a paraglider. Pulling on a brake line, those are the lines attached to the trailing edge of the wing and held in the pilots hands, or weight shifting. In weight shift the pilot leans over increasing the wing loading on that side. (Though a paraglider looks like one skinny parachute it’s actually two inflatable wings connected by a bellows section. All the lines on say the right wing collect together through some straps called risers and are fixed to the right side of the harness slightly higher than the pilots hip. The left ones all go to the left side. The brakes are lines connected to a wings trailing edge, the back, again left hand to left wing only). When the wing loading is increased that wing slows thus if the pilot leans hard to the right the right wing slows a little and the left wing speeds up by a slightly greater amount resulting in the paraglider turning direction to the right. Note that no brake has been imputed which means that the wing has not been distorted resulting in a more efficient turn.
Now add a motor. In the diagram below the black arrow represents the turning propeller. Its beating the air with around 12hp. Remember back in school when they said every action results in an equal and opposite reaction? The prop is not just pushing air behind it resulting in a reaction forwards (which we want) but is also spinning against resistance which causes the motor and its mounting to the harness trying to twist the opposite way, red arrow (which we don’t want). This force translates into a pull through the risers on that side, green arrow. Effectively it’s a weight shift input resulting in a turn, yellow arrow.
Historically, (and I’m an old fart who started paragliding in 2000) there have been several systems to try and negate the effect including: Variable hang points (the point at which the risers are fixed to the harness). Reflexed wings that respond poorly to weigh shift. Motors mounted to produce offset thrust. Trimmers on the risers to cause one wing to be a bit faster than the other or to effectively raise or lower the relative hang point. All of these methods are ways of causing the craft to turn by about the same amount, but to the other side, that the motor is causing a turn thus negating each other. Aside from the loss in efficiency this produces another problem. Generally a paraglider flies at one speed, it’s trimspeed (yes I know about speed bar etc but this is a Janet & John article when the throttle is opened a paramotor does not fly faster, it climbs. When the throttle is closed it does not fly slower, it descends. So if the torque compensation is set to fly straight and level at a certain power setting then it follows that when you want to take off it’ll try to turn one way and when you come into land it’ll turn the other!
On Paul’s design two of the propellers are turning clockwise and two anticlockwise so any resultant torque effects are balanced within the motor framework and transmitted no further. This also means that the harness does not need to be so rigid and can be tuned to allow the pilots (and only the pilots) weightshift input to cause more control (lower and wider set hangpoints). It used to be very difficult to fly a paramotor as a paraglider, gaining altitude on the motor then thermalling XC, but with a torqueless setup it should be possible.
Wow. This got a lot wordier than I’d intended! Apologies to anyone who feels I’ve been stating the bleeding obvious and missing out a few important bits, but as I said previously there will be people hitting this forum who share the dreams but don’t fly… Yet.