Brushless DC Motor Terminology and Motor Construction


Explanation of Terminology

  • Nominal Voltage: the applied voltage between two powered phases in block commutation.
  • No Load Speed: the speed at which an unloaded motor runs with the nominal voltage applied. It is approximately proportional to the applied voltage.
  • No load Current: is the typical current that the unloaded motor draws when operating at nominal voltage. It increases with rising speed owing to bearing friction and iron losses. No load friction depends largely on temperature. It decreases in extended operation and increases at lower temperature.
  • Nominal Speed: is the speed set for operation at nominal voltage and nominal torque at motor temperature of 25° C.
  • Nominal Torque: is the torque generated for operation at nominal voltage and nominal current at a motor temperature of 25° C. It is at the limit of the motors continuous operating range. Higher torques heat u the winding too much.
  • Nominal Current: is the current in the active phase in block commutation that generates the nominal torque at the given nominal speed (= max. permissible continuous load current).
  • Stall Torque: is the torque produced by the motor when at standstill. Rising motor temperature reduces stall torque.
  • Stall Current: is the from nominal voltage and the motor’s terminal resistance. Stall current is equivalent to stall torque.
  • Max. Efficiency: is the optimal relationship between input and output power at nominal voltage. It also doesn’t always denote the optimal operating point.
  • Terminal Resistance Phase to Phase: is determined through the resistance at 25° C between two connections.
  • Terminal Inductance Phase to Phase: is the winding inductance between two connections. It is measured at 1 kHz, sinusoidal.
  • Torque Constant: represents the quotient from generated torque and applicable current.
  • Voltage (Speed) Constant: indicates the theoretical no load speed per volt of applied voltage, disregarding friction losses.
  • Speed/Torque Gradient: is an indicator of the motor’s performance. The smaller the value, the more powerful the motor and consequently the less motor speed varies with load variations. It is based on the quotient of ideal no load speed and ideal stall torque (tolerance ± 20%).
  • Mechanical Time Constant: is the time required for the rotor to accelerate from standstill to 63% of it’s no load speed.
  • Rotor Moment of Inertia: is the mass moment of inertia of the rotor, based on the axis of rotation.

Motor Construction

  • Magnets_

Magnets are undeniable an extremely important component in the Brushless DC Motor. The motor gets its torque output from the strength of the magnet. What is not always considered by the engineer looking for a Brushless DC Motor is the type and grade of the magnet in the motor they are considering. Over the years motor manufactures specified Samarium cobalt magnets for high performance, Alnico magnets for good performance and Ceramic magnets for low performance applications. Today manufacturers typically use Neodymium Boron magnets for performance applications (servomotors) and ferrite for non-performance applications (fans, vending machines and etc.).

Neodymium magnets was invented in 1982 and have almost completely replaced the samarium cobalt magnet. The properties of the two are very similar in many respects and neodymium is much more available than samarium cobalt making it the idea choice. Neodymium magnets are permanent magnets, as they lose their magnetism, or degauss naturally, at approximately 1% per century. They generally operate within the temperature range of -215°F to 176°F (-138°C to 80°C). For applications that require a broader temperature range, Samarium Cobalt magnets are used. Neodymium magnets are susceptible to corrosion and will crumble. Because of this, they must have a protective coating before use. This coating is typically made of nickel to resist high temperature, high humidity, salt spray, solvents and gases. 

Neodymium is processed for motor magnets using two different type processes. Each has its pluses and minuses.  The two different types are Sintered and Bonded.

Sintered Neodymium magnets have the highest strength but are limited to relatively simple geometries and can be brittle. They are made by pressure forming the raw materials into blocks, which then go through a complex heating process. The block is then cut to shape and coated to prevent corrosion. Sintered magnets are typically anisotropic, which means they have a preference for the direction of their magnetic field. This type requires magnetizing the magnet in the preferred direction of magnetization for the Brushless DC Motor. If not done correctly up to 50% strength of the magnet can be lost.

Brushless DC Motor sintered magnets ranging from N35 (weakest and least expensive) to N52 (strongest, most expensive and more brittle). An N52 magnet is approximately 50% stronger than an N35 magnet (52/35 = 1.49). In the US, it is typical to find consumer grade magnets in the N40 to N42 range. In volume production, N35 is often used if size and weight are not a major consideration as it is less expensive. If size and weight are critical factors, higher grades are typically used. There is a premium on the price of the highest-grade magnets so it is more common to see N48 and N50 magnets used in production versus N52.

 

Bonded Neodymium magnets are typically about half as strong as sintered magnets but are less expensive and can be made into almost any size and shape. Raw materials are mixed with epoxy as a binder, pressed into a die cavity and heat cured. Bonded magnets are isotropic, which means they don’t have a “grain” or a natural preference for the direction of their magnetic field. Therefore, bonded magnets are easier magnetizing the magnet in the preferred direction.

Bonded Neodymium magnets have five different grades from the weakest BDM-4, BDM-6, BDM-8, BDM-10 and BDM-12 (strongest).

  • Laminations_
    When considering a Brushless DC Motor, the laminations are an important element in the manufacturing of the motor. If you are working with an unknown source of Brushless DC Motors and quality, reliability and performance is important in your application, we recommend you require the supplier to put on their drawing/specifications, under notes, the type of steel the laminations are made from. I would only accept motors made with Silicon Steel (Electric Steel). Like many components in a DC Brushless Motor, there are different grades and processes in manufacturing which can make your selection of a motor supplier a success or failure.

The motor manufacturer can select different grades of Silicon steel. Silicon steels are generally specified and selected on the basis of allowable core loss in watts/lb. The grades are called out, in increasing order of core loss by M numbers, such as M19, M27, M36 or M43, with each grade specifying a maximum core loss. The higher M numbers (and thus higher core losses) are progressively lower cost, although only a few percent is saved with each step down in performance.  M19 is probably the most common grade for motion control products, as it offers nearly the lowest core loss in this class of material, with only a small cost impact, particularly in low to medium production quantities. In addition to grade, there are a number of other decisions to make regarding silicon steels.  These are:
1. Semi vs. Fully processed material,
2. Annealing after stamping,
3. Material Thickness,
4. Surface insulation.
(For more detail, please search online under Silicon Steel Laminations.)

There are two lamination technologies when it comes to Brushless DC Motors.

    • The most common is where a stamping press is used to punch out laminations from flat steel sheets and later to be assembled together making up the stator lamination. This process requires maintaining a stamping tool quality to guarantee quality lamination pieces. This type lamination requires a winding machine to fish wire inside the slots of the assembled stator lamination. Because of this process it is impossible to obtain a dense fill of copper inside the lamination void.
    • Segmented laminations are the newest and most expensive technology used by the most advanced manufacturers of Brushless DC Motors. The segmented lamination is made using a progressive die tool and automatically stacked in the die. They are also made from Silicon Steel. The segmented lamination provides benefits to offset the high price. Segmented laminations require less raw material, increase magnetic properties and most important higher copper fill per segment increasing torque in an equivalent stamped lamination. The increase in torque is typically 20% to 30 %.                  
Stamped Lamination Stator Assembly Segmented Laminations
  • Bearings_

Bearings are the primary failure in Brushless DC motors. Altek Motion only provides ball bearings in our motors. For extra peace of mind, we only use NMB and NSK bearings in our motors. All our motors are tested after manufacturing to meet customers specifications and audible testing looking for any noise anomaly.

Brushless DC Motors are expected to last over 20,000 hours and when customer requires we will install ceramic bearing pushing longevity over 40,000 hours.


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Electric Motors