Selecting the Proper Brushless DC Motor

You may have an application that requires high-speed, but quiet operation with low EMI generation and long operating life. For such applications, BLDC or brushless DC motors are what you must be looking at. Among many advantages of these motors, high-speed operation is a special one. As there are no brushes or commutator in the motor, the bearing friction is the only factor limiting their rotational speed.

Absence of brushes and commutator also means there is no arcing within the BLDC motor to cause erosion or EMI. The last factor makes these motors suitable for use in RF applications. With windings on the stator, BLDC motors show superior thermal characteristics over conventional motors and are consequently more efficient. Because the stator is connected to the case, heat dissipation is fast. All such factors means a BLDC motor has virtually non-existent maintenance problems.

The major downside to all the above good characteristics of BLDC motors is their higher cost. BLDC motors can easily cost about twice as much as simple brushed motor and this puts the BLDC technology out of reach for many applications. Apart from the cost of the basic motor, there is the added cost of the control or drive electronics. If not integrated within the motor itself, you will need to find space for mounting the electronics outside, but nearby. You cannot separate the drive and the motor with long cables, as the noise introduced will cause malfunctioning.

A brushless motor also must overcome starting friction, just as brushed motors do. Again, starting friction does not depend on speed, but is the sum total of torque losses. Dynamic friction, proportional to speed, defines the torque losses in BLDC motors. Viscous friction in the ball bearings cause dynamic friction and eddy currents in the stator, originated by the rotating magnetic field of the magnet, adds to it. Nevertheless, the speed-torque curve of a BLDC motor demonstrates excellent linearity.

Directly connecting to a DC supply will not operate a BLDC motor, unlike a brushed DC motor. This is because brushless technology makes use of electronic commutation. Although there is no physical commutator or brushes, the operating principal remains the same, with the permanent magnet rotor initiating motion by chasing a revolving magnetic field induced by a current in the stator windings. A PWM or pulse width modulated signal is necessary to create the on/off signal, which actually creates the motion.

A comparator normally generated the PWM signal, which is a voltage generated because of a sinusoidal command signal superimposed with a saw toothed carrier or chopper frequency. If the command is greater than the carrier frequency, the PWM signal will be high. This is because the low chopping frequency gives the current more time to gain amplitude. The current density governs the rate at which the motor accelerates or decelerates.

To avoid ripples and a shortened motor life, it is important that the switching frequency is high enough. This is usually done by controlling the discrete on/off steps with six semiconductor switches. These send the amplified current through the correct phases, with the necessary switching being done by the semiconductor switches.