Electrical motor efficiency is the ratio between shaft output power - and electrical input power.
Electrical Motor Efficiency when Shaft Output is measured in Watt
If power output is measured in Watt (W) then efficiency can be expressed as
ηm = Pout / Pin (1)
where
ηm = motor efficiency
Pout = shaft power out (Watt, W)
Pin = electric power in to the motor (Watt, W)
Electrical Motor Efficiency when Shaft Output is measured in Horsepower
If power output is measured in horsepower (hp), efficiency can be expressed as
ηm = Pout 746 / Pin (2)
where
Pout = shaft power out (horsepower, hp)
Pin = electric power in to the motor (Watt, W)
Primary and Secondary Resistance Losses
The electrical power lost in the primary rotor and secondary stator winding resistance are also called copper losses. The copper loss varies with the load in proportion to the current squared - and can be expressed as
Pcl = R I2 (3)
where
Pcl = stator winding - copper loss (W, watts)
R = resistance (Ω)
I = current (A, amps)
Iron Losses
These losses are the result of magnetic energy dissipated when when the motors magnetic field is applied to the stator core.
Stray Losses
Stray losses are the losses that remains after primary copper and secondary losses, iron losses and mechanical losses. The largest contribution to the stray losses is harmonic energies generated when the motor operates under load. These energies are dissipated as currents in the copper winding, harmonic flux components in the iron parts, leakage in the laminate core.
Mechanical Losses
Mechanical losses includes friction in the motor bearings and the fan for air cooling.
NEMA Design B Electrical Motors
Electrical motors constructed according NEMA Design B must meet the efficiencies below:
Electric Motors - Efficiency
Power
(hp)Minimum Nominal Efficiency1)
1 - 4
78.8
5 - 9
84.0
10 - 19
85.5
20 - 49
88.5
50 - 99
90.2
100 - 124
91.7
> 125
92.4
1) NEMA Design B, Single Speed 1200, 1800, 3600 RPM. Open Drip Proof (ODP) or Totally Enclosed Fan Cooled (TEFC) motors 1 hp and larger that operate more than 500 hours per year.
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.