A little knowledge of some key factors and potential issues can help smooth the process of tuning an inverter to an electric motor.

Patrick Berkner, Systems Application Engineer
Jonah Leason, Electrification Product Manager
Kamal T. Wolly, Senior Systems Engineer
Parker Hannifin

With more companies attempting to electrify their equipment, a common challenge is properly tuning an inverter to an electric motor. To save time and create efficiency in this transition, it’s important to give proper consideration to a number of factors when tuning an inverter to an electric motor. These include control modes and limit functions, knowing the different types of tuning (current, velocity/speed loops, voltage), being familiar with some common tuning issues associated with the power source and/or mechanics, and how tuning is evolving into automation.

Control modes
When looking into purchasing an electric motor and inverter system, many parameters must be investigated, including the required system control mode.

Control modes are key tools in optimizing the tuning process for a specific application and represent the method used by the inverter to correct a performance error of the electric motor.  The appropriate control mode chosen is based on the motor type required for either a traction or work function. Other contributing factors include the specific application use and the machine’s architecture. Here’s a review of some of the more common control modes.

Speed Control
In an ePump application, the control mode would most likely be speed or velocity mode. This means your input command will control the motor speed.

A motor’s speed control mode provides a narrow window of acceptable speed, affecting the rotational speed of the motor, and it is an efficient method to manage motor output.

In ‘Speed Control’ mode, the speed loop’s proportional and integral (PI) controls adjust the motor’s torque demand, ensuring the measured motor speed matches the commanded speed.

The controller first calculates a speed error (speed demand minus speed feedback), which is then fed to the speed loop PI regulator.

Torque Control
A vehicle traction operation usually involves torque mode. Similar to how an automobile operates, pressing the accelerator applies torque to the wheels. The more the accelerator is pressed, the more torque is applied, making the vehicle go faster. Releasing the accelerator makes the vehicle coast.

In ‘Torque Control’ mode, the speed loop’s PI regulator is inactive. However, when stopping the inverter while in ‘Torque Control’ mode, the inverter automatically switches to ‘Speed Control’ mode and ramps the motor to zero speed.

‘Torque Control’ mode often has a speed limiter feature which reduces the torque limit to zero if the motor speed exceeds the set speed limit. This prevents motor acceleration beyond these speeds.

It’s highly recommended to set the control limit to ensure the torque does not exceed any system component’s torque limits.

Vdc Control Mode
If the system must generate electrical power, then Vdc control mode is selected. In applications that require ‘Vdc Voltage Control’ mode, the electric motor is driven from an engine (e.g., an internal combustion engine), which provides mechanical energy to the electric motor controlled by the inverter.

In “Vdc Voltage Control’ mode, the inverter automatically controls the motor’s torque demand to ensure that the measured Vdc bus voltage (feedback) matches the Vdc voltage demand.

A positive torque demand for a positive motor speed controls the flow of energy from the inverter dc link to the externally powered devices, thus reducing the dc link voltage. A negative torque demand for a positive motor speed will control energy into the inverter dc link from the motor (regenerating operation), thus increasing the dc link voltage.

The appropriate control mode chosen is based on the motor type required for either a traction or work function. Other contributing factors include the specific application use and the machine’s architecture.

Limit functions
Limit functions are a series of functional blocks for motors and drives that work together to provide a final current/torque limit at the motor’s output. In addition to these functional blocks, current and torque limits are internal protection methods with user-settable limits. Current and torque limiting is used to protect the inverter, motor, and the system components.

There are two types of current limits:

Nominal Voltage Current Limit
In an electric motor, the current limiting is derived first before being converted into a torque limit. Current limiting commonly occurs automatically to protect the inverter and motor.

Current limits are based on the temperature of the inverter, output switching frequency, output electrical frequency, and the rated device current. The output of this function is the maximum amount of continuous current that the inverter can produce at any moment in time. It may be read as a diagnostic if the commanded motor output is not attainable due to inverter device limitations.

DC Voltage Current Limit
DC voltage current limit and motor temperature current limits are configurable by the user and controlled to a performance band defined within the system hardware capability. The battery management system can limit available current as well, as it is responsible for providing available output current and input current (during regeneration). These limits can also restrict the input/output current if either the dc voltage or the motor temperature are outside the operation limits set at system commissioning.

The current limiting’s output is an important diagnostic, as the primary controller in any system will need to know what the inverter is capable of at any point in time. Finally, the primary controller also needs to be aware if any factors have resulted in a limitation of available performance.

Servo system tuning
A servo system typically consists of both a current and velocity loop. In this system, the current loop must be tuned first, followed by the velocity loop. In fact, a poorly tuned current loop can make an optimally tuned velocity loop impossible.

For systems that are run in torque control, with no speed limit, it’s only necessary to tune the current loop. In this scenario, be careful the motor does not run to its maximum possible motor speed, resulting in a run-away condition. Finally, when tuning a traction system with a speed limit, a tuned velocity loop is required.

Current loop tuning
To adjust the current loop in most inverters, there are two parameters, Kp, the proportional term and Ki, the integral term. As an example, with Parker’s GVI Inverter, Kp is used to adjust the rise time of the waveform’s initial segment and Ki is used to adjust the rise time of the second half of the waveform. The rise time signifies the time it takes for the current to go from the inverter to the motor windings in 2 msec.

The ideal scenario is to adjust the Kp and Ki terms to achieve a current rise time of 2 to 3 msec. This is achieved by starting the Kp and Ki with some initial values and then adjusting to reach the ideal rise time.

Velocity/speed loop tuning
While tuning the velocity loop, the goal is to keep the motor velocity consistent and independent of the shaft loading.

Tuning the velocity loop consists of adjusting the Kp and Ki to reduce any speed overshoot upon the release of pump press and any speed decrease due to the rapid application of hydraulic pressure.

With velocity loop tuning, Kp is used to reduce the over-and undershoot of the motor velocity, and Ki is used to reduce the amount of time it takes to recover from a velocity under- or over-shoot condition.

When the Kp and Ki parameters are optimally set, any external forces applied to the motor will have little, if any, change in velocity.

Common tuning issues
Common tuning issues associated with tuning servo systems are typically associated with two main areas; the power source and mechanics.

Frequent power source issues include:

• Not having a sufficient energy source
• The battery management system (BMS) limiting the available current, resulting in lower motor current or speed
• Insufficient battery system voltage, resulting in limited motor speed
• Insufficient instantaneous reaction time of the dc power supply relative to load spikes

Some common mechanical issues include:

• Shaft loading awareness for the different types of loop tuning
  • Current loop tuning does not require loads to be removed
  • Velocity loop tuning should be loaded for optimal results
• Compliance in the load train (chains, gears, belts etc.) resulting in instability, unnecessary vibrations and variations in speed and torque signals
• Identifying differences in system response times to reach an optimally dampened loop
  • Under-dampened loop in a velocity mode can cause slow oscillations “porposing”
  • Over-dampened loop is rigid with noticeable oscillations or overshoot where the loop appears unstable

The future of tuning
It’s likely that the days of manual tuning demonstrated above are nearing the end. Electric motor and inverter manufacturers are creating the next generation of inverters that can automatically tune an inverter to an electric motor. It’s now possible to simply select the motor part number and the motor parameters, limits, and tuning are loaded into the inverter for the speed and current loop settings.

This level of optimization can save valuable time and increase the efficiency of your electrified system significantly such that it outperforms a traditional diesel system and, potentially, a motor and inverter from a combination of suppliers.

Due to the complexity of modifying or implementing an electric system for optimal efficiency, a knowledgeable partner is critical to shorten your time to market, reduce the need for trial and error, help you achieve your system’s optimal efficiency faster, provide savings in implementation costs, and exceed your electrification system performance expectations.

Parker Hannifin
parker.com