Pulse Width Modulation vs Variable Voltage Control

When controlling the speed of a DC motor, like the ones found in our electric linear actuators, it is possible to use both Pulse Width Modulation (PWM) and Analog Voltage Control however there are large benefits to one (PWM) over the other (Analog Voltage). When varying the voltage of a DC motor, it results in large amounts of power dissipation. Sometimes it may even cause the motor to stall due to the loss of torque. This is why it is considered a highly inefficient circuit and the main reason why PWM control is the industry standard.

PWM is able to supply full rated voltage to the motor at all times without having to worry about stalling issues or torque loss. The speed control aspect comes into effect as you vary the duty cycle of the PWM. The diagram below shows how this works.

figure 1

 

Induction of the motor will cause the current to average which will keep the torque in proportion to the current, unlike variable voltage control. By changing the duty cycle you can achieve top quality speed control while maintaining torque and efficiency in the process.

figure 2

 

PWM is not without its faults though, specifically in relation to the frequency of the signal. If the frequency of the pulses is too high or too slow then proper averaging will not happen inside of the motor windings. The figure below shows how important the rise time of the voltage is.

figure 3

 

To make sure the voltage has enough time to reach its steady state calculations can be made in regards to the values of the system. This may be a bit much for most applications, as most frequencies in the range of a few KHz will work just fine, but for applications where precision is key then this calculation will be very important.

If you found this article helpful be sure to look through our blog section for other informative articles like this. If you have any questions about this article or our products please contact us and we'll make sure to help with whatever you need.

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