tailieunhanh - Industrial Brushless Servomotors Episode 3

Tham khảo tài liệu 'industrial brushless servomotors episode 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | The brushless machine 35 Figure Torque from a simple brushless motor The peak torque at positions 1 and 3 in Figures and is equivalent to the steady output torque of the two-pole brushed motor already described in Figure . The average torque constant of this particular brushless machine is therefore only half that of the brushed motor. The ripple in the output torque can be described as 100 and would be unacceptable 36 Industrial Brushless Servomotors in most servomotor applications. The problem arises when the poles are halfway between one side of the winding and the other where the net force is zero. We will see shortly how the use of three windings eliminates such nulls to produce a theoretically constant output. Figure Torque and backemf fora single-winding brushless motor Back emf Rotating flux sweeps across the stator winding to produce a back emf with an average half-cycle value of E Ke laJ where Ke is half the value of the voltage constant of the brushed machine of Figure . The back emf alternates in direction as the poles of the magnet change position as shown in Figure . It is important to note that the back emf at the input terminals of the brushless motor alternates in direction as does the direct current input. The motor has the same construction as an AC synchronous motor which normally has a sinusoidal rather than rectangular current waveform. The brushless machine 37 Although the single-phase brushless machine works correctly as a motor its output torque is lumpy and would be unsuitable for most industrial servomotor applications. The main uses occur at the low power end of the scale where the brushless motor is manufactured with a single winding in very large numbers for example as fan motors for the cooling of electronic equipment. These are normally exterior-rotor motors where the fan is mounted on a hollow cylindrical permanent magnet which rotates around a laminated cylindrical stator with slots for the winding.