Monday, November 21, 2011

Brushless Motor vs Brushed Motor

A DC Brushless Motor uses a permanent magnet external rotor, three phases of driving coils, one or more Hall effect devices to sense the position of the rotor, and the associated drive electronics. The coils are activated, one phase after the other, by the drive electronics as cued by the signals from the Hall effect sensors, they act as three-phase synchronous motors containing their own variable frequency drive electronics.

Electronic commutation based on Hall position sensors
Less required maintenance due to absence of brushes
Speed/Torque- flat, enables operation at all speeds with rated load
High efficiency, no voltage drop across brushes
High output power/frame size.
Reduced size due to superior thermal characteristics. Because BLDC has the windings on the stator, which is connected to the case, the heat dissipation is better
Higher speed range - no mechanical limitation imposed by brushes/commutator
Low electric noise generation

Higher cost of construction
Control is complex and expensive
Electric Controller is required to keep the motor running. It offers double the price of the motor.

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A Brushed Motor has a rotating set of wound wire coils called an armature which acts as an electromagnet with two poles. A mechanical rotary switch called a commutator reverses the direction of the electric current twice every cycle, to flow through the armature so that the poles of the electromagnet push and pull against the permanent magnets on the outside of the motor. As the poles of the armature electromagnet pass the poles of the permanent magnets, the commutator reverses the polarity of the armature electromagnet. During the instant of switching polarity, inertia keeps the classical motor going in the proper direction.

PRO Brushed Motor
Two wire control
Replaceable brushes for extended life
Low cost of construction
Simple and inexpensive control
No controller is required for fixed speeds
Operates in extreme environments due to lack of electronics

CON Brushed Motor
Periodic maintenance is required
Speed/torque is moderately flat. At higher speeds, brush friction increases, thus reducing useful torque
Poor heat dissipation due to internal rotor construction
Higher rotor inertia which limits the dynamic characteristics
Lower speed range due to mechanical limitations on the brushes
Brush Arcing will generate noise causing EMI

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