Remember Iron Losses During BLDC Motor Selection

When engineers seek to improve critical components like miniature electric motors, efficiency is always a point of emphasis. Defined as the ratio of the mechanical power and electrical power, efficiency directly impacts the motor’s heating and power consumption. That means machine designers must be sure that the motor they select will make the most of the available space. Here are three factors relevant to BLDC motor efficiency and losses and how they play a role in motor design and selection:

1.	Friction Losses. These losses are caused by the ball and bush bearings, and they depend on the application’s use plus the bearing’s intrinsic characteristics like material, wear, lubricants and sealing.
2.	Copper losses. Also called joule losses, copper losses are caused by the coil’s resistance. Torque is directly linearly proportional to the current (T=k*I), so the more torque a motor provides, the higher the copper losses. This follows the quadratic function: Copper Losses = R*I2. The resistance will increase when the motor is heating, which will reduce the efficiency.
3.	Iron Losses. The losses depend greatly on the frequency of the variation of the magnetic flux into a material, so more losses are generated the faster a motor rotates. Iron losses can be explained by Lenz’s law, which states that a change of magnetic field induces current that flows in a direction that opposes the change in the magnetic field that produced it. These losses are caused by two phenomena: eddy currents and hysteresis. a. Eddy Currents. Faraday’s law states that a magnetic field interacting with a conductor will induce a current through it. And, since the material has a specific electrical resistance, it creates some iron losses (R*I2). b. Hysteresis. When the magnetic flux is reversed into a ferromagnetic material, energy is lost as the material magnetizes and demagnetizes. To remove the remaining flux density, an opposite magnetic flux should pass the point of coercivity. Iron losses also depend on material properties — such as permeability and volume — and the frequency of flux variation. Therefore, be sure to select the right material for the right speed.

Different Electric Motor Designs to Achieve High Performances

Different losses produced by an electric motor will limit its maximum power, so remember to select the right motor based on the specific torque-speed working point. Joule losses are mainly caused when creating torque, and iron losses usually occur at high speed.

Changing the number of poles of a magnet can have a big impact on motor performance. Typically, long motors are 2 poles and can run at high speeds. Although this can increase the maximum torque of a motor, it will also increase the iron losses and thus reduce the maximum continuous speed.

Since iron losses depend highly on the frequency of variation of magnetic flux for a similar speed, increasing the number of poles increases the number of variations for one motor turn. For eddy current losses, this happens with the square of the increased frequency and can quickly reduce the motor’s efficiency.

Conserve Energy and Improve Device Designs

Because the motor’s limits are thermal, iron losses play a role in the motor’s efficiency, especially at high speeds or in multipolar, high-torque designs. Optimizing the ratio between joule and iron losses will conserve energy and improve device designs. Portescap understands these characteristics and designs motors to deliver high performance with lower losses.

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