Measuring Input Ripple

This article is a technical deep dive into the measurement and characterisation of the input voltage ripple to an ESC. As discussed in this blog post, input capacitors maintain a smooth voltage input to the ESC. Sizing the appropriate capacitance for a given setup requires a level of system characterisation further discussed below.

An important reminder: When operating close to power electronics and spinning systems, it is crucial to do so safely as these devices are dangerous. Use appropriate safety precautions to reduce the risk of physical harm and electrocution.

Importance of a Clean Bus

APD ESCs are typically sensorless, meaning that the back-EMF generated by the motor is used to determine the motor (and thus rotor) position. Further detail on the difference between sensored and sensorless ESCs can be found here. This information is used to control the timing of the motor’s power phase switching to ensure smooth operation.

If the input bus voltage is not clean and stable due to noise or voltage ripple, it will interfere with the accuracy of the rotor position information. This can lead to errors in the motor drive, causing the motor to run roughly or stall. Therefore, a clean and stable input is critical to ensure smooth and accurate sensorless control. This is where the external capacitors mounted on ESCs come into consideration.

Sources of Voltage Ripple

The two main contributing factors to voltage ripple are:

• Input lead length

• Motor inductance

Increased input lead length can lead to a larger voltage ripple due to the inductance of the wiring. Wire inductance can cause voltage spikes when the wires’ current changes, resulting in voltage ripple at the motor controller. As a general rule of thumb, APD recommends an additional capacitor for each 12cm length of the input cable. When choosing input capacitors, ensure that the capacitor specifications align with the system requirements. See further below for more detail.

On the contrary, the voltage ripple due to the inductance of the 3-phase motor is reduced for higher inductance motors. A higher inductance motor has a reduced output current ripple (the time constant of current change is proportional to inductance), which results in a smaller voltage drop across the winding. The phase-to-phase inductance can be measured with an LCR meter. For comparison, typical low-inductance (thus higher voltage ripple) motors have up to 10-20uH.

Measuring Voltage Ripple

Voltage ripple measurements should be performed with the assistance of an oscilloscope. It is important to set up the test with similar parameters to the final configuration (wire lead lengths, motors, loads), such that a similar voltage ripple will be generated.

APD recommends measurements to occur at the maximum load point of a given system while in a steady state condition (input duty cycle remains constant when the measurement is happening). The oscilloscope probe should be connected as close as possible to the ESC to minimise the effects of any additional wiring and should be grounded to the ESC power ground.

The target ripple level at the aforementioned load point should be less than 10% of the bus voltage, ideally less than 5%. For example, for a 40V bus, the voltage level shouldn't exceed 42V or drop below 38V. Image 1 illustrates a setup where the system ripple is outside the recommended range. 6.5V of ripple is measured, approximately 16% of V Bus. The measurement was taken with an oscilloscope setup to 10 V/div on the vertical axis and a timescale of 200 us. In such a case, further steps should be taken to reduce the ripple below the target threshold.

Image 1. Measurement across an ESC’s power terminals with unacceptable ripple.

Requirements for Additional Capacitance

After lead lengths have been shortened, capacitors should be introduced into the system until the voltage ripple is reduced to an appropriate level. Low ESR capacitors should be used, such as Panasonic FM/FS series or the Rubycon ZLJ series. The ripple current rating is a specific factor to check on a capacitor datasheet. APD selects capacitors that can handle a minimum of 2A ripple (at 105 degrees Celsius). The higher this number is, the better the capacitor will perform within a higher ripple application. The ripple current (alongside operating temperature) can be used to appropriately de-rate capacitor lifetimes from the endurance given on the datasheet.

Capacitors should be mounted as close as possible to the input terminals of the ESC. By placing the capacitors closer to the input of the ESC, the lead length between the ESC and the capacitors is reduced, resulting in lower impedance and voltage ripple. Capacitor leads should be kept as short as possible to reduce the risk of capacitor leads fusing out and failing.

Image 2. Measurement across an ESC’s power terminals with acceptable ripple.

Image 2 shows the same setup used for Image 1, however additional capacitance was installed to reduce the ripple below target thresholds. In this case, the ripple is 3.125 V, approximately 8% of V Bus. As shown above, the measurement and characterisation of the input voltage ripple is crucial for ensuring smooth motor operation. Sizing the appropriate capacitance for a given setup is the key to prolonged performance and unit lifetimes. Check out our other post on input capacitance.