Sizing and Commissioning Servo-Pump Systems for Plastics Machinery

Servo-driven hydraulics are now the standard in plastics machinery, replacing constant-speed, valve-controlled systems due to their improved efficiency, lower noise, and better performance.

With over 150,000 drive controllers installed worldwide and extensive experience in plastic injection molding and extrusion, KEB’s servo-driven hydraulic solutions utilize servo motors with high-performance drive control that integrates pressure and flow control directly in the drive.

This article provides and overview of how to size and integrate KEB servo motors and drives for usage in plastics applications that utilize servo-driven hydraulics technology.

Motor Sizing

In a servo-pump system, the motor is sized to match the selected hydraulic pump. The pump is chosen based on the required maximum flow rate and the machine cycle. The maximum required flow rate and maximum system pressure then determine the motor’s required speed and torque.

Speed and torque can be calculated using the following relationships:

This equation converts hydraulic pressure into the torque the motor must deliver, considering pump displacement and efficiency.

This equation converts the required flow rate into the motor’s corresponding rotational speed.

D = Pump displacement in [cm^3/rev]
P = pressure in [bar]
Q = flow rate in [l/min]
T = motor torque in [Nm]
𝜔 = motor speed in [rpm]
η = yield [%]

The following conditions should be taken into consideration during the motor selection process:

Motor torque at zero speed ≥ Maximum load torque

During pressure regulation (high pressure), motor speed approaches zero while torque demand remains high. The motor must be capable of delivering full required torque at or near zero speed.

Maximum motor speed ≥ Speed required for maximum flow rate

The motor must reach the speed necessary to achieve the system’s maximum flow demand.

Both conditions should be verified with appropriate design margins to ensure reliable operation across the full machine cycle.

KEB’s DL4 motor line is designed to meet the demands of plastics machinery such as injection molding and servo-pump applications.

KEB’s DL4 line offers high torque and low inertia motors, enabling fast response and precise control, which is critical for clamping, injection, and pressure regulation.

VFD Sizing

Once the motor has been sized for the required torque and speed, the next step is selecting the variable frequency drive (VFD). The drive must be sized in accordance with the motor’s electrical characteristics and machine duty cycle.

The following drive specifications should be taken into consideration during the selection process:

• Supply voltage
• Motor rated current and peak motor current
• Overload characteristics (duration, at low speeds, etc.)

Special consideration should be given to the drive’s low-speed overload characteristics. The machine cycle of many servo pump applications often requires high pressure at a low flow rate. For example, in an injection molding machine, plastic is injected into a mold and held at high pressure for a set period of time as the plastic begins to cool. This phase of the machine cycle causes the drive to output high current at low frequencies during the hold process. High current at low speed may cause the drive to issue a warning or trip on maximum current protection.

KEB’s servo pump drive offering is designed for servo-hydraulic applications in plastics machinery. KEB drives provide high overload capability, precise current control, and a wide power range to support dynamic pressure and flow requirements. KEB drives are available with a wide range of heat sink options, encoder options, and fieldbus options.

Servo Pump Commissioning

KEB’s COMBIVIS software provides guided tools for motor tuning, I/O configuration, and servo pump setup. These tools are intended to reduce commissioning time and ensure consistent integration across machine platforms.

Below is an overview of how servo pump control is implemented in the KEB S6/F6 drive platform and the key features used during commissioning.

Servo Pump Control Architecture

In the S6 and F6 drives, servo pump control is an integrated function designed to manage the pressure and flow rate of a hydraulic circuit driven by a servo motor and fixed displacement pump.

To regulate pressure, the servo pump control function calculates a speed command that is limited by the defined speed (flow rate) setpoint.

The control structure can be adapted to different hydraulic configurations and actuator types. Up to eight parameter sets are available, allowing OEMs to define separate hydraulic behaviors for different machine phases or machine models.

Hydraulic characteristics can be identified using the COMBIVIS tools, ensuring stable control behavior across different system setups. The servo pump control function also includes protective functions to prevent operating conditions that could shorten pump life, such as excessive speed or unstable pressure commands.

Below are the primary features used during startup.

Motor Identification – Measures key motor parameters required for optimal control.

Cogging Wizard – Compensates for torque ripple, improving smoothness at low speed.

Motor-Pump Rotation Check – Before pressurizing the system, the rotation direction must be verified. The drive allows safe validation of motor rotation to ensure correct pump direction and avoid reverse pressurization or cavitation.

Analog Input Configuration (Pressure Feedback) – The pressure sensor is typically connected to an analog input on the drive. This includes scaling the sensor signal, defining engineering units (e.g., bar) etc.

Pressure Profile and Pressure Feedback Loop Auto-Tuning – Defines the pressure profile and adjusts control gains to match the hydraulic dynamics of the machine. Proper tuning reduces pressure overshoot, oscillation, and instability during dynamic transitions.

Pump Protection Settings – Protective limits to protect the pump from damage during operation. This includes speed, acceleration, and pressure limits.

Servo-driven hydraulics are now widely used in plastics machinery because they improve efficiency, reduce noise, and provide more precise control than traditional constant-speed systems. Successful implementation requires correct motor sizing based on hydraulic pump characteristics. The drive must be selected to match the motor’s electrical requirements and handle the current requirements at different phases of the machine cycle.

KEB’s servo motors and drives are engineered specifically for these demands. With integrated pressure and flow control, along with commissioning tools that reduce tuning time and setup complexity, they give machine builders a straightforward path from design to a reliable, high-performing servo pump system.