Unlike a conventional dc motor, a brushless dc motor has an “inside-out” construction, that is, the field poles rotate and the armature is stationary. The field poles consist of permanent magnets mounted on the inside of a steel cylinder and the armature is wound on a slotted laminated iron structure. The armature coils are switched by transistors or silicon controlled rectifiers (instead of the commutator) at the correct rotor position to maintain the armature field in space quadrature with the field poles.
Brushless DC Motor Working Principle
The terminology for describing brushless dc motors has not been standardized yet. They are being called even by various names such as ‘commutatorless dc motor’, ‘electronically commutated DC motor’, ‘self-synchronous machine’ and others.
Each type of motor is described by either the number of phases of the stator winding, current pulses delivered to the windings by the transistors or SCRs, or the number of poles on the rotor. The following classification of brushless dc motors is also useful to understand the brushless dc motor working principle:
One Phase, One-pulse Brushless DC Motor
The stator of this motor has only a one-phase winding which is energized by a transistor once per electrical revolution. The torque output of such a motor is totally Inadequate, because, at best, it can only produce a positive torque over 180 electrical degrees. The angular rotation remaining has to be overcome by the inertia of the rotor or by mans of auxiliary torques. (see Figure a)
One-phase, Two-pulse Brushless DC Motor
The stator of this motor has also a one-phase winding only but receives two pulses, that is, its winding is energized by two current pulses of opposite directions. The resulting torque distribution is, therefore, more favorable than with the one-pulse motor.
Still a continuous electromagnetic torque is not achieved. There are still small regions without torque which have to be bridged with stable auxiliary means. The advantage of this motor is its simple design, yielding a high utilization of the armature material. (see Figure b)
Two-phase, Two-pulse Brushless DC Motor
The stator of such a motor has two phase windings, which alternatively are energized by two current pulses. Therefore, the torque generated is basically the same as with a one-phase, two-pulse motor. Nevertheless, the winding will be utilized to 50 per cent only.
The advantage of this motor is to be seen in its simple control electronics. The gaps of the electromagnetic torque have to be bridged by suitable auxiliary means as with one-phase motor. (see Figure c)
Three phase, Three-pulse Brushless DC Motor
This motor has a stator with a three phase winding which is displaced in space by 120o electrical. Each phase winding is excited by one-pulse, that is, per electrical revolution; three current pulses are fed cyclically to the stator.
The fact that only three power transistors or SCRs are required is the main advantage of this motor design. One disadvantage is the relatively low utilization of the winding (on the average, nearly 33 per cent) as well as the necessity of three position sensors. (see Figure d)
Four-phase, Four-pulse Brushless DC Motor
The stator of this motor is wound with four-phase windings displaced in space by 90° electrical. The phase windings are energized cyclically with four currant pulses. This results in a torque without gaps and a utilization of the winding up to 50 per cent. However, the expenditure for the electronics is twice that of the two-pulse design. (see Figure e)
Three-phase, Six-pulse Brushless DC motor
The stator of this motor is wound with three-phase windings which can be connected in delta as well as in star. Generally, the neutral point is not used. The windings are excited with six pulses by six power transistors or SCRs in cyclic sequence.
Such a motor not only delivers an even torque output but also the utilization of the winding is at its optimum. Its disadvantage is the relatively high cost for the position sensors and the control electronics.
This leads to the most common brushless dc motor—a combination of three-phase permanent magnet synchronous motor, three-phase solid state inverter and rotor position sensor that results in a system producing a linear speed torque characteristic as in the conventional permanent magnet DC motor.
Figure illustrates the schematic representation of 3-phase, 6-pulse brushless DC motor using a transistor inverter as the DC to AC converter. Where high power requirements exist, SCRs are used instead of transistors.
Other trade of features like component cost, component reliability and simplicity of the inverter circuitry (relating to the need of commutation circuitry to turn off SCRs, a feature unnecessary for transistors) are of importance where SCRs and transistors of comparable power handling capabilities are available.
An Integral part of brushless DC motor system is the rotor position sensor. Although several methods are available for sensing angular position, the most commonly used ones are Hall effect sensors and electro-optical sensors.
Advantages of Brushless DC Motor
A brushless DC motors has several advantages over conventional dc motors or ac motors such as:
- absence of mechanical commutator and associated problems,
- high efficiency,
- high speed operation,
- less problems caused by radio frequency and electromagnetic interference and
- long life.
Applications of Brushless DC Motors
- In larger power applications including traction, brushless motors are fast replacing conventional dc motors.
- Typical small power application is in dc fans for cooling electronic equipment.
- Another important application is for spindle drives for disc memories.
- Other applications are in phonographs and tape drives.
- Brushless dc motors in the fractional horsepower range have been used in different types of actuators in advanced aircraft and satellite systems.
- Integral horsepower brushless dc motors have been developed for propulsion and precision servo systems.
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