# Comparison of the Effects of Sine Waves and Square Waves on BLDC

**Comparison of the Effects of Sine Waves and Square Waves on BLDC**

Taking a certain model of permanent magnet motor as an example, it can be driven by either square wave or sine wave. The specific motor parameters are shown in Table 1.

Indicator name | Technical indicators | Indicator name | Technical indicators |

Rated voltage | 24V | Maximum no-load speed | 1200rpm~1300rpm |

Rated current |
≤4A | Normal temperature no-load current |
≤0.3A |

Rated power | 60W | Cogging torque | ≤50mNm |

Rated speed | 900rpm | Torque ripple | ≤17% |

Rated torque | 0.64Nm | Work system | S1 |

Figure 1 Main flux closed magnetic circuit

Detect the air gap magnetic density under a pair of stator magnetic poles, the stator tooth magnetic density, the stator yoke length magnetic density and the rotor yoke length magnetic density. The yoke length is the stator yoke or rotor yoke part along the magnetic path direction of a pair of magnetic poles. The average length is shown in Figure 2.

Figure 2 Stator and rotor yoke length

Draw a thin line at the air gap, stator teeth, stator yoke and rotor yoke respectively. The magnetic density curve and magnetic density cloud distribution are shown in Figures 3 and 4.

(a) Air gap magnetic density (b) Stator tooth magnetic density

(c) Stator yoke magnetic density (d) Rotor yoke magnetic density

Figure 3 Magnetic density distribution inside the motor

Figure 4 Magnetic density cloud map

Whether it is a square wave drive or a sine wave drive, ensure that the output torque and output speed under rated operating conditions are the same, that is, the output speed is 900rpm and the output torque is 0.64Nm. Compare the electromagnetic performance of the same motor under different drive modes. .

The output torque under rated condition is 0.64Nm, as shown in Figure 5.

Figure 5 Rated output torque

**The output torque under sine wave drive is much smaller than the torque ripple under square wave drive.** After calculation, the torque ripple under square wave drive is 14.5%, and the torque ripple under sine wave drive is 6.1%. The torque ripple is better than the square wave. The current under rated conditions is shown in Figure 6.

Figure 6 Rated current

It can be seen from Figure 6 that the effective value of the current under square wave driving is 3.1A, and the effective value of the current under sine wave driving is 2.7A.

The self-inductance and mutual inductance under the two driving modes are shown in Figure 7.

(a)Square wave drive (b) Sine wave drive

Figure 7 Self-inductance and mutual inductance

It can be seen from Figure 7 that **there is no difference in the self-inductance and mutual inductance under the two driving modes**.

The induced voltages under the two driving modes are shown in Figure 8.

(a)Square wave drive (b) Sine wave drive

Figure 8 Inductive voltage

**It can be seen from Figure 8 that the back electromotive force driven by a square wave is not a standard square wave, but the back electromotive force driven by a sine wave is a standard sine wave. **

The three-phase flux linkage under the two driving modes is shown in Figure 9.

(a)Square wave drive (b) Sine wave drive

Figure 9 Three-phase flux linkage

The copper loss, iron loss and eddy current loss under the two driving modes of loss and efficiency are shown in Figure 10.

(a)Copper loss (b) Iron loss

(c) Eddy current loss

Figure 10 Loss

Figure 10 Efficiency

As shown in Figure 10 and Figure 11,** the motor efficiency under sine wave drive is 83.6%, and the motor efficiency under square wave drive is 75.5%.**