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KEB Technology

Injection Molding Hydraulic Pump Control

Jonathan Bullick | August 31st, 2015

If you’ve been reading our blog then you know that KEB is a leader in drive technology for Plastic machines.

 

One application that KEB has a lot of experience in (10+ years & 100,000+ units installed) is operating hydraulic pump motors on injection molding machines. The post gives an overview of the application and some of the advantages KEB brings to the table.

Good HMI interface design ideas
HMI software and hardware for plastics machinery


Operating the hydraulic motor from the mains

Historically, injection molding pump motors were connected to the mains power and operated at a fixed speed with a simple on/off control. With the motor running at a constant speed, the fluid volume and pressure were regulated through hydraulic valves. While this control method is very straightforward and easy to implement, throttling with valves can be extremely inefficient and can create excessive heating in the system.

Studies have shown that an injection molding machines hydraulic drives can account for nearly 50% of the machine’s energy usage – So improving the energy usage of the pump goes a long way in improving the overall machine efficiency.


KEB’s Solution – ASCL Pump Control

It has been well documented that a pump’s speed and flow can be regulated by adding a VFD to the pump’s induction motor and controlling the motor speed with simple V/Hz control. V/Hz control has the added benefit that it soft-starts the motor. This is a big improvement over line-starting which causes large inrush currents and excessive mechanical wear.

However, basic V/Hz control has some performance limitations – for example, it does not make any adjustments for the induction motor’s slip because there is no feedback mechanism. Additionally, V/Hz control will have torque limitations at low speeds.

KEB’s ASCL motor control is the answer. ASCL uses an advanced motor model and continuously measures the VFD’s output current and motor back EMF to precisely calculate the motor speed. The drive is able to make small current adjustments on the microsecond level to provide the optimal operating point, even at reduced speeds.

servo pump vfd
F6-K drive running a hydraulic servo pump


Retrofit Opportunities

Because ASCL does not require any feedback and can be used with any manufacturer of induction motor, it can be easily added to existing installations.


Servo Pump Control

For even more responsive control, a servo motor can be used instead of an induction motor. A PM servo motor will provide increased power density, responsiveness, and efficiency compared to an induction motor. KEB’s SCL software functions much like ASCL except it is intended to be used with a Synchronous PM motor.

 

Again, the software does not require a feedback device and expensive encoder cabling but still provides exceptionally accurate speed regulation across a wide range. All of KEB’s drive platforms support ASCL/SCL software.

injection molding servo pump VFD

Internal PID control

There are many ways the pump control can be handled. Historically, the PID control was handled in the PLC or a dedicated pump controller and the VFD was simply given a speed command. But another implementation is to let the VFD manage the PID control for the hydraulic drive. In this arrangement, a PLC or machine controller would provide the drive with an analog input for the desired pressure setpoint. A 2nd analog signal which measures the actual pressure is used as feedback in the control and is also fed to the drive.

KEB’s F5 drive, for example, is perfect for this implementation as it has an internal PID function reserved for controlling an auxiliary process. The advantage of this configuration is that it offloads some processing from the PLC and does not require a special dedicated pump controller.


Other KEB drive features

In addition to ASCL/SCL motor control and an internal PID function, KEB drives are ideal for injection molding hydraulic pump applications because they offer SIL3 rated safety functionality like Safe-Torque-Off.  Additionally, high speed Ethernet bus options like EtherCAT can be used which boast 500µs (F6-K) scan times and can be easily integrated with the machine controller.

In addition to the standard fan cooled design, liquid cooled heatsinks are available which allow the drives to output more power for a given frame size.

servo pump vfd
Boy35 E injection molding machine using KEB servo pump drive

 

In summary, here are the features and benefits with using a KEB drive on your hydraulic pump:

Feature Benefit
ASCL/SCL

 

Precise speed control without feedback

Improved system efficiency

 

Lower operating costs

Lower upfront costs (no feedback/cable)

Solutions for induction and servo pumps

Less heating in the system Smaller cooling system required

 

Longer operating lifetime of components

Less frequent coolant changes

Internal PID controller Used in place of pump controller or PLC program
Less audible noise Improved worker comfort
Potential for field weakening operation Reduced cycle times

 

Improved throughput

Standard KEB hardware/software Off-the-shelf product

 

Better Machines with Better Drive Technology

KEB’s success running hydro pumps is not limited to injection molding machines. Similar hydraulic applications for balers, rams, presses, shears, and lifts all could benefit to adding a VFD.

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