Servo Energy-Saving Injection Molding: How Permanent-Magnet Synchronous Motors Cut Idle Power by 90%

What Idle Power Means in a Servo-Hydraulic IMM

Idle power is the electrical input the machine draws when the hydraulic system is pressurised but no injection, holding, or ejector motion is taking place. On a conventional fixed-displacement pump setup, idle power stays at full pump load because the motor keeps spinning at constant speed. On a PMSM-driven servo system, idle power drops to the small fraction needed to maintain system pressure, because the pump displacement is throttled back to near zero by the servo loop. That is the source of the headline 90 percent idle-power reduction.

A real production cycle spends somewhere between 20 and 40 percent of its time in idle or low-flow phases — between the end of plasticating and the start of injection, during operator intervention, between cycles on slow heating, and during the cooling phase when only the holding pressure is being maintained. On an induction-motor machine, those phases all draw full motor power because the pump is still spinning at full displacement. On a PMSM-driven servo machine, the pump displacement tracks demand in real time, so the motor only draws the power actually needed.

For commercial IMM buyers comparing quotes, the idle-power figure rarely appears in the spec sheet. It does, however, show up on the electricity bill — and that is where the payback of a servo upgrade gets measured in practice. Engineering plastics moulders running medium-tonnage machines like the SK-170 typically see their monthly kWh drop by between 25 and 40 percent in the first full month after switching from a fixed-pump to a servo-pump hydraulic system.

Why PMSM Cuts Idle Power by 90 Percent

Permanent-magnet synchronous motors hold rotor flux from magnets rather than induced current, so they do not need magnetising current at low load. An induction motor, by contrast, draws magnetising current from the stator at all times whenever the rotor is spinning — that magnetising current is what holds the rotor field in place. On an unloaded or lightly-loaded induction motor, the magnetising current represents a substantial share of total input power.

Combined with closed-loop servo control of pump displacement, the PMSM only draws the power required to overcome system leakage. There is no fixed magnetising overhead, and the pump itself is throttling back to near zero displacement during idle. Field measurements on production servo-hydraulic IMMs typically show idle power falling from around 7 to 12 kW on an induction-motor setup to 0.8 to 1.5 kW on a PMSM setup — that is the 80 to 90 percent range referenced in the literature.

The other half of the savings comes from the drive electronics. A PMSM drive with closed-loop servo control can command the motor to a precise speed and torque setpoint in milliseconds. An induction motor with a variable-frequency drive is a few percentage points less efficient at low speeds and has a slower dynamic response, so the pump takes longer to settle into a stable low-flow state. The combination of motor-level efficiency and drive-level response is what produces the 90 percent idle-power figure.

How Idle Savings Roll Up to the 35 Percent Full-Cycle Number

The SUCCESSOR SK-170 product literature cites a 35 percent reduction in energy consumption compared to conventional machines. That 35 percent number is a full-cycle figure: it sums up the savings during injection, holding, cooling, plastication, and idle, weighted by the proportion of the cycle each phase occupies. It is not on top of the 90 percent idle figure — it is the roll-up of all the phase-level savings, including the idle phase.

To get from the 90 percent idle number to the 35 percent full-cycle number, the math is straightforward in principle. If a cycle spends 30 percent of its time in idle and 70 percent in active phases, and the PMSM cuts idle power by 90 percent but active power by only 10 to 15 percent, the weighted full-cycle saving works out to roughly 30 percent to 40 percent. The exact figure depends on the cycle profile — a thin-wall packaging cycle with a long cooling phase will show higher savings than a thick-wall engineering plastics cycle with a short cooling phase.

For commercial buyers doing a return-on-investment calculation on a SK-170 versus a conventional fixed-pump machine, the right approach is to take the published 35 percent figure, multiply by the local industrial electricity tariff and the expected operating hours per year, and treat that as the annual savings stream. The payback math then depends on the capital cost differential between the two machines, which is typically between 8 and 15 percent of the machine price for a mid-range servo upgrade.

4 Real-World Scenarios Where PMSM Pays Back Fastest

Not every production environment benefits equally from a PMSM servo upgrade. The four scenarios below are where Alex Wang's team at SUCCESSOR sees the fastest payback in the field, based on installed base across 40+ countries.

Scenario 1: Three-shift continuous automotive components. Automotive tier-1 and tier-2 suppliers running three shifts per day, five or six days a week, see the PMSM servo upgrade pay back inside 12 months. The machine spends most of its time in active injection or holding, but the 30 to 40 percent idle proportion is still meaningful, and at three-shift operating hours the absolute kWh saved is large.

Scenario 2: Packaging containers with long cooling cycles. Thin-wall packaging moulders running PP or PE food containers have long cooling phases where the hydraulic system is doing little more than maintaining clamp force. Idle and low-flow time can exceed 40 percent of the cycle. The PMSM upgrade on a SK-220 or SK-300 in this segment typically pays back inside 14 to 18 months.

Scenario 3: Engineering plastics with frequent grade changes. Moulders switching between ABS, PC, PA, and POM grades several times per day spend extra idle time during purging, barrel heating stabilisation, and operator setup. The PMSM servo pump handles those transients with much smaller energy penalty than a fixed-pump machine, because the pump displacement drops to zero the moment the machine pauses.

Scenario 4: Plants in high electricity-tariff markets. Industrial tariffs in parts of Europe, Japan, Australia, and California routinely sit above USD 0.18 per kWh. At those tariff levels, even modest kWh savings translate to large absolute dollar amounts, which compresses payback periods. Moulders in those markets frequently see 10 to 14 month payback on a mid-range servo upgrade even on single-shift operation.

When the Servo Upgrade Does Not Pay Back

Honest framing matters here. There are production environments where the PMSM servo upgrade takes longer to pay back, and there are a few where it never quite pays back on energy savings alone. Buyers evaluating servo energy-saving injection molding machines should size the decision against their actual operating profile.

Low utilisation single-shift operations. A moulder running one shift per day, four days a week, on short cycles will see the payback stretch to 30 to 36 months. At that point the decision is more about lifecycle operating cost and brand positioning than pure payback.

Very short cycles with negligible idle. Ultra-fast thin-wall packaging cycles where the active phase dominates and the idle phase is only a few percent of total time will show much smaller absolute kWh savings. The servo upgrade is still worth it for noise reduction, oil-thermal stability, and process repeatability, but the energy payback alone is weak.

Very low electricity tariffs. Plants in regions where industrial electricity is below USD 0.06 per kWh will see much longer payback periods. The servo upgrade still delivers process benefits, but the pure energy economics are slow.

Maintenance Reality: PMSM vs Variable-Volume Pump

PMSM motors have no brushes, no slip rings, and the rotor magnets are sealed inside the rotor body, so there is no wear item inside the motor itself. The servo drive does need to be kept clean and dry, and the cooling fans on the drive cabinet are the only consumable parts in normal operation. For most production environments, PMSM-based servo systems actually show lower total maintenance cost than older variable-volume pump systems because there are fewer hydraulic components under continuous cyclic load.

The variable-volume pump — sometimes called a load-sensing pump — was the older generation of energy-saving hydraulic design. It is still in service on many production floors, but its swashplate mechanism is a wear item that eventually drifts out of specification, requiring pump rebuild or replacement. The PMSM-driven servo pump eliminates that wear surface by replacing the mechanical swashplate control with electronic pump-displacement command. The pump itself still wears, but the energy-control mechanism does not.

For buyers evaluating a SK-170 against an older fixed-pump or variable-volume-pump machine, the maintenance savings show up as longer service intervals and fewer pump rebuild events. Alex Wang's team at SUCCESSOR routinely sees 30 to 50 percent lower hydraulic maintenance spend on PMSM servo machines compared with the same tonnage of variable-volume-pump machine over a five-year operating horizon.

How SUCCESSOR Specifies Servo Across the SK Series

SUCCESSOR Machinery, based in Ningbo, China, designs and manufactures the SK series injection molding machines — from the SK-110 up through the SK-560 and larger. The full series spans 110 tonnes to 560 tonnes of clamping force, with the SK-170 sitting in the mid-range sweet spot for engineering plastics and consumer-durables production. The mid-range SK-170 delivers 1700 kN clamping force and a 312 to 442 cm³ shot volume range, suitable for PP, PS, PE, ABS, and PC production runs.

The standard configuration across the SK series ships with PMSM-driven servo pump as the default energy-saving setup. Detailed SK-170 servo machine specifications — including the full injection unit, clamping unit, hydraulic unit, and electric unit data — are published on the product page, and the servo pump spec sheet is available on request. Buyers evaluating the SK series should request a technical consultation to confirm the servo configuration matches their local electricity tariff and cycle profile before placing the order.

The SK series is sold across 40+ countries through direct sales and qualified agents. Two factory locations support the global installed base — the office at No.199 Changxing Road, Jiang Bei, Ningbo, and the main production facility at No. 30 Kaiyuan Road, Jiang Bei, Ningbo. After-sales service and spare-parts supply run through the same Ningbo operation, with regional service partners in major markets.

Frequently Asked Questions

Source Citations and Reference Material

The engineering references below informed the PMSM servo and energy-saving analysis above. All sources are open-access or published standards from independent third-party organisations.

• EUROMAP 67 Interface Standard for IMM and Host Controller Communication — independent third-party reference.
• EUROMAP General Industry Standards — independent third-party reference.
• Siemens Electric Motors Product Range — independent third-party reference.
• Fanuc Injection Molding Machine Solutions — independent third-party reference.
• Engel Austria Injection Molding Machine Reference Designs — independent third-party reference.
• Arburg All-Electric and Servo-Hydraulic IMMs — independent third-party reference.
• KraussMaffei Injection Molding Technology — independent third-party reference.
• Husky Injection Molding Systems — independent third-party reference.
• Wittmann Group Servo-Hydraulic Robot and IMM Integration — independent third-party reference.
• Sumitomo (SHI) Demag Injection Molding — independent third-party reference.
• NEMA Premium Efficiency Motor Standard — independent third-party reference.
• BSI Industrial Machinery Certification — independent third-party reference.
• Intertek Machinery Testing Services — independent third-party reference.
• SGS Industrial Equipment Certification — independent third-party reference.
• TUV Rheinland Industrial Inspection — independent third-party reference.
• UL 508C Industrial Control Equipment Standard — independent third-party reference.
• CE Marking for Industrial Machinery — independent third-party reference.
• AMI Plastics Industry Intelligence — independent third-party reference.
• Plastemart Injection Molding Knowledge Base — independent third-party reference.