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Permanent Magnet Synchronous Servo Motor vs. Standard Asynchronous Motor
Time:2026-04-17
In the field of plastic extrusion and compounding, the choice of drive system directly affects energy costs, process precision, and return on investment. Recently, more and more manufacturers are replacing standard asynchronous motors with Permanent Magnet Synchronous Servo Motors (PMSM) – especially in counter-rotating twin-screw extruder applications.
How does a permanent magnet synchronous servo motor differ from a standard asynchronous motor? This article compares them with real data.
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Four Critical Drawbacks of Standard Asynchronous Motors
| 1.Low power factor (only 0.75–0.85) | High reactive power places a heavy burden on the power grid. Company measurements show that the electricity meter over‑records by 10–15%, directly increasing electricity bills. |
| 2.Poor efficiency curve | Counter-rotating twin-screw extruders often run at 30–50% rated load. Under these conditions, asynchronous motor efficiency drops to only 75–82%, wasting substantial energy. |
| 3.Uncontrolled speed and severe low‑speed torque drop | Asynchronous motors have a slip of 3–5%. At low speeds, torque drops significantly, making it impossible to precisely match the low‑speed, high‑torque requirements of counter-rotating twin‑screw extruders. |
| 4.High inrush current during start‑up | Starting current is 5–7 times the rated current, causing large electrical and mechanical stress on the grid and the gearbox. |
Technical Advantages of Permanent Magnet Synchronous Servo Motors
The company’s technical specification clearly requires the use of permanent magnet synchronous servo motors. Their key advantages are shown in the following indicators:
| Technical Parameter | PMSM (Servo Motor) | Standard Asynchronous Motor |
| Rated efficiency | ≥94% | ≈89% |
| Full‑load efficiency | ≥90% | Only 75–82% at 30–50% load |
| Power factor | ≥0.95 | 0.75–0.85 |
| Speed range | 1:100 (rated torque even at 1 rpm) | Severe torque drop at low speed |
| Control method | Direct Torque Control (DTC) with automatic slowdown on foreign object detection | Open‑loop or simple VFD control |
| Speed accuracy | ±0.01% | ±2% (due to slip) |
Real production line comparison (75 kW motor)
| Parameter | PMSM (Servo Motor) | Asynchronous Motor | Advantage / Benefit |
| Full‑load efficiency | 95% | 89% | Saves 4.5 kW per hour |
| 40% load efficiency | 93% | 78% | Ideal for counter‑rotating twin‑screw extruders (often run at low load) |
| Reactive power loss | <5% | 15–20% | Significantly lower electricity bills |
| Speed accuracy | ±0.01% | ±2% | Much more precise coordination with feeders |
Deep Matching with Counter‑Rotating Twin‑Screw Extrusion Process
The core process requirement of a counter‑rotating twin‑screw extruder is constant torque speed control: torque must not drop at low speeds. The characteristics of a permanent magnet synchronous servo motor are a perfect match:
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Below base speed, the PMSM delivers constant torque output
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Even at a low speed of 30 rpm, it still provides 100% rated torque
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With Direct Torque Control (DTC), the motor automatically slows down upon encountering foreign objects, protecting the equipment from damage
In contrast, standard asynchronous motors suffer from severe torque decay in the low‑speed range and simply cannot meet the stable extrusion requirements of counter‑rotating twin‑screw extruders.
Real Energy Consumption Calculation Example: PP Fiber Line (300 kg/h)
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The company’s finance department performed a detailed calculation based on actual production line data:
| Item | Asynchronous Motor Solution | PMSM Servo Solution |
| Main motor power | 90 kW | 75 kW |
| Auxiliary motor power | 110 kW | 90 kW |
| Actual operating power factor | 0.8 | 0.95 |
| Active power draw | 160 kW | 145 kW |
| Daily electricity cost (24h) | 3072 RMB | 2784 RMB |
Daily savings: 288 RMB Monthly savings: 8,640 RMB Annual savings: 103,000 RMB
Conclusion
Why the Company Recommends Switching to PMSM Servo Motors？
Significant energy savings – Full‑load efficiency >90%, saving over 100,000 RMB per year in electricity costs
Perfect process matching – Constant torque at low speeds, ideally suited for counter‑rotating twin‑screw extrusion
Precise control – Speed accuracy ±0.01%, stable coordination with feeding systems
Built‑in protection – Direct Torque Control (DTC) automatically slows down on foreign object detection, reducing equipment damage risk
Fast payback – Premium cost recovered in less than six months
For companies operating counter‑rotating twin‑screw extruder production lines, upgrading from standard asynchronous motors to Permanent Magnet Synchronous Servo Motors (PMSM) is one of the most technically mature energy‑saving retrofits with the shortest payback period.
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