Understand the Effect of PWM When Controlling a Brushless DC Motor
Designers of motion systems often face challenges when selecting or developing electronics using pulse width modulation (PWM) to drive brushless DC motors. To avoid unexpected performance issues, it’s important to understand some of the basic concepts.
Commutation of a Brushless DC Motor
Brushless DC motors are electronically commutated. For a three-phase brushless DC motor, the driver is composed of six electronic switches — typically transistors — usually called a three-phase bridge. Opening and closing the transistors in a specific sequence energizes the phases of the motor to maintain the optimal orientation of the magnetic field induced by the stator versus the rotor magnet.
The speed and torque of the application, also known as the working point, can vary. An amplifier varies the supply voltage or the current, or both, to achieve the desired motion output. You can vary the voltage or the current using:
|•||Linear amplifiers to adapt the power delivered to the motor by linearly changing the voltage or current.|
|•||A chopper amplifier, which modulates the voltage and current by switching the power transistors on and off. This helps conserve battery life and produces less heating from the electronics. Most of the time, chopper amplifiers use a PWM method.|
The PWM method varies the duty cycle at a fixed frequency to adjust the voltage or current within the desired target value. With this method, the switching frequency is a fixed parameter. When the transistor of the PWM is open 100 percent of the time, the voltage applied to the motor is the full bus voltage. When the transistor is open 50 percent of the time, the average voltage applied to the motor is half the bus voltage. When the transistor is closed 100 percent of the time, no voltage is applied to the motor.
A DC motor is characterized by an inductance L, a resistance R and a back-electromotive force (back-EMF) E in series. The back-EMF is a voltage caused by the magnetic induction that opposes the applied voltage and is proportional to the motor speed.
When applying voltage or switching off voltage to an RL circuit, the inductor will oppose the change of the current. Applying a voltage to the RL circuit, the current will follow a first-order exponential rise whose dynamic depends on the electric time constant equal to the ratio L/R. It will asymptotically reach the steady state value after five-times the time constant. The same exponential behavior can be observed when the RL circuit will discharge.
Limitations of PWM
PWM will lead to current rise and fall at each cycle. The variation between the minimum and maximum value of the current is called the current ripple. The torque of a DC motor is proportional to the average current, which must be considered for the motor torque.
For brushless DC motors, high current ripple is not a problem for the lifetime itself. The current ripple will cause joules losses and iron losses as well as unnecessary heat.
To minimize the current ripple, consider reducing or adapting the power supply voltage, increasing the PWM frequency or increasing the inductance.
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