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Which is the correct PWM frequency for a brushed DC motor?
Below are some oscilloscope traces of a small brushed DC motor at different PWM frequencies (upper trace in each image is voltage, lower trace is current). At 50Hz the PWM current follows the voltage, so the peak current is twice the average current and loss in the motor’s internal resistance is doubled compared to constant DC.
How does a PWM control the motor speed?
PWM controls motor speed by switching the power on and off rapidly with a varying ratio, so the motor is getting getting a series of full power pulses.
How does PWM ratio affect throttle linearity?
At lower PWM ratios the frequency has an effect on motor efficiency and throttle linearity – the lower the frequency the worse it gets. PWM controls motor speed by switching the power on and off rapidly with a varying ratio, so the motor is getting getting a series of full power pulses.
How does a brushed DC motor keep torque constant?
To keep a nearly constant torque on the rotor, we can do two things. First, we can reverse the current through the coil every half turn. So instead of an alternating torque like the one in the first figure below, the torque is always in the same direction. Also, additional coils can be used.
Which is frequency do I have to use on the PWM?
I’m using the L298 Dual H-Bridge to control a pair of 6V @ 1600mA DC motors for a line follower robot. I’m connecting “Enable A” and “Input 1” to Vcc and “Input 2” to uC with PWM. Which frequency do I have to use on the PWM? It’s not obvious in the ST L298N datasheet.
Do you need PWM for bidirectional motor control?
If you want bidirectional motor control using PWM then you need to supply the PWM to the inputs as shown in the ST application note. Using the EnA and EnB signal you can turn off all current to the motor. The motor is freewheeling, though if it’s speed is sufficient then one set of diodes will conduct and slow it because of the back emf.
Why does PWM affect the RMS of a motor?
At very low frequencies it has little effect so the motor current follows the PWM voltage. This causes extra heating in the motor because the rms (heating) current is higher than the average (torque) current.