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Tech Tips: Brushless DC Motor Basics

This Tech Tips video explains the basics of Brushless DC Motors: how they are designed and how they operate.

Video Trascript: Groschopp Tech Tips – Brushless DC Motor Basics

Hello!  This is Brent with a Groschopp Tech Tip.  Today we will be discussing Brushless DC Motor Basics.

First, let’s discuss how a Brushless DC motor operates.  It works in a system consisting of two parts: the motor and a control, which is required to operate the motor. The BLDC motor has three main parts. First is the rotor, the part that rotates. Our brushless motor’s magnets are located on its rotor core which consists of a four-pole permanent magnet and a smaller four-pole sensor magnet.  The sensor magnet provides excitation to the second part, the sensor board which is an array of hall effect transistors.  The on/off state of the sensors is how the control determines the rotor’s angular position.  The sensor board can be replaced with a commutating encoder to provide higher resolution position information.  Third is the stator.  This is the part that holds the windings.  The three phase windings are placed such that when current is supplied to the coils making up the winding it creates North and South poles that attract the magnet.

Let’s put it together: when the control and motor are powered on, the position of the rotor is read and the control will allow current to flow through two phases.  The selected phases are predetermined based on an input/output table.  When current is flowing through the stator magnetic poles are created that attract the rotor and rotary motion begins to happen.  As the rotor turns the states of the sensors will change signaling the control to energize the next set of phases in the stator while the rotor continues to follow the generated magnetic field. The current is always switched before the magnets catch up insuring continual motion. The speed of the motor corresponds to the current switching rate. Two windings are always energized at a time with the third one off; combining the torques of two phases at once, increasing the overall torque output of the motor.

Brushless DC motors, in general, have characteristics of 3 phase AC induction motors in their construction.  BLDC motors have a rotor core that rotates within a stator.  The winding pattern of both can be the same.  The difference is really in the rotor core.  The induction rotor is made of a stack of laminations and then the slots of the laminations are cast with aluminum or copper.  The BLDC rotor is constructed using permanent magnets. Both designs are considered maintenance free because the only wear item is the bearings which are lubricated for life.  There is quite a bit of difference in performance between the BLDC and AC induction motors.  AC induction motors hold a fairly constant speed while torque increases to the breakdown point.  BLDC motors have in general a linear speed vs. torque curve.  This means as torque increases that speed will go down at a rate equal to the change in torque.  The BLDC motor also has a much higher starting torque.

The PMDC brush type motor actually has the same speed vs. torque characteristics as the BLDC motor, but from a construction stand point, the only thing in common is that it uses permanent magnets.  The largest difference is the maintenance free design of the BLDC motor.  The brush type motor uses brushes in order to commutate the coils. The brushes are normally the first thing to wear out, which can happen between two and three thousand hours.  This is much less than the 20,000 hour life of the BLDC.  Because the windings are on a BLDC are in the stator as opposed to the armature on the PMDC, the BLDC can dissipate generated heat easier allowing higher continuous duty power output in the same motor size.

This has been a Groschopp Tech Tip.  For more information about any of our products or to view other Tech Tips please visit us online at www.groschopp.com.

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