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Power Supplies for Motion Control

Monday, October 23, 2017

DC motor controllers are nearly all non-linear switching (PWM) topologies. As such their power requirements can be difficult to calculate. Analysing the motion profile will help us understand the power requirements of the motion control system. These non linear motor drivers and the torque profile of the application will dictate the fluctuating demands on the system's power source. One must find out the peak requirements and the average requirements of the system.

Motor duty - RMS averages and Peak loads

Let us consider a simple profile of motion; accelerate at constant rate to constant speed, maintain speed and decelerate to a stop with the same rate as the acceleration. This can be represented graphically in terms of velocity and torque (acceleration).

In the first section of the profile we are accelerating the most, because not only are we increasing in speed but we are also fighting the frictional forces on the rotating load, which will demand the most torque from the motor (angular acceleration = torque / moment of inertia). Since torque is proportional to current for a DC motor this will also mean that the highest current draw will be in the first phase of this motion.

Once the load accelerates to the desired speed, although there is not more acceleration to do, the motor must still deliver a minimum of torque to overcome the relentless friction on the load. So it is very likely that in this phase of the motion profile the motor will be drawing the minimum current.

In the final stage of the profile the motor decelerates, or applies torque in the reverse direction, in order to slow the load. Since the frictional forces are still in play, they actually assist in the deceleration of the load, as such the torque required to decelerate is less than the acceleration torque.

Once the profile has been broken down into these components we can isolate our peak acceleration/torque/current and also our average RMS values, in a similar way peak loading is analysed for the power supply please see this article.


What does this mean for the power supply load?

The power supply to the motor controller will have to supply the necessary power such that the motor has the resources to complete the profile. Since the switching PWM motor controller has its reserve of energy stored in the charged DC bus capacitor, not all of the instantaneous current comes from the power supply. For very short spikes in current demand the controller is served from its local resource found in the DC bus capacitor:

The power supply is therefore not feeding the motor directly, but “topping up” the capacitor losses as the h-bridge switches in accordance to its required duty to achieve the desired motion. This means that the current flowing from the power supply is not related to the current that the motor is pulling from the drive, be careful not to fall into this trap! Despite this when choosing a power supply, we should not rely on this capacitor because often the DC bus capacitance in the drive is not publicised and also it is not so easy to predict how much energy is left in the capacitor and how quickly the power supply is able to replenish the capacitor all the while the motor is still running. As the capacitor discharges the potential difference between the source and the output of the capacitor changes which will affect the charging rate of the capacitor:


Desired power supply characteristics for motion controllers

Peak power to handle overloads can be very useful and save space and money. If the peak loads are only for short duration (a few seconds, limitations dependent on power supply model and the degree of overloading required) a smaller power supply could be used.
Please see "Peak power & power supply selection" for more details

Regenerative loads occur during the breaking parts of the duty cycle, when a 4 quadrant drive is used. Some ancillaries may be required to handle these types of loads. For more information on this topic please read "Overvoltage and regenerative failure of 4 Quadrant servo drives"


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