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The Right Motor for Tension Control
Tension control is a specialized application that can be challenging to implement effectively. Choosing the correct motor type is the crucial first step toward success. Contrary to popular belief, a servo motor isn’t always the go-to solution.
Take a look at this application image showing two motors working together to maintain tension and feed a material. The smaller brushless motor in the center is responsible for winding, while the larger torque motor ensures the specific tension needed.
Applications such as film winding, stamping/pressing, or screw tightening require the motor to regulate or limit its torque. Here are some examples:
- **Film winding/unwinding**: Requires the motor to adjust its torque depending on the roll diameter to ensure consistent tension.
- **Stamping/pressing**: Requires limiting torque to prevent damaging delicate products, such as eggs during labeling.
- **Screw tightening**: Requires limiting torque to avoid exceeding the maximum tightening torque allowed for the screw.
While servo motors are commonly chosen for these applications, there’s a cost-effective alternative worth considering if you don’t need all the advanced features of a servo motor.
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### **Torque Motors**
Torque motors resemble regular induction motors but possess unique characteristics that make them ideal for tension control. They are designed to control their torque and can endure continuous stalling or even back-driving without overheating—essential traits for tension control.
A torque motor achieves full torque at zero speed and is rarely used at full speed. Its design differs from an induction motor due to its balanced winding and high-slip rotor instead of a low-slip rotor. This high-slip rotor allows the motor to act as a brake when back-driven. Although the motor size only goes up to 20W, various gear ratios are available to boost torque. Single-phase 110/115VAC and 220/230VAC options are also available.
For instance, if the load is set and voltage V4 is applied to the motor, the motor will adjust its output torque where the "V4" line meets the "Load" line, and it will operate at "N4" speed.
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### **Simple Configuration**
Unlike a servo motor tension control system, a torque motor doesn’t require motor feedback or additional sensors to control its torque. All you need is a device capable of varying the input voltage into the motor, such as the **TMP-1 power controller**.
The TMP-1 power controller uses the triac phase control method to adjust the voltage supplied to the torque motor. Alternatively, you could use any device that outputs a stable voltage, like a PLC with an analog output card.
There are multiple ways to control voltage/torque, including:
- Manual potentiometers
- Analog signals from PLCs
- Digital controllers
The torque motor can also operate on DC voltage, providing braking torque instead of rotation when back-driven. The braking torque can be adjusted via voltage.
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### **Avoids Slack**
Tension is essential to prevent "slack" in materials being wound or unwound. For example, when winding starts with a small diameter, the torque of the motor may be low, but it needs to rotate quickly. Conversely, when the diameter increases, the motor rotates slower but requires more torque due to the higher load.
To maintain proper tension and feed rate, the voltage must change in sync with the changing diameter.
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### **Fine Adjustment for Proper Tension**
A torque motor can function as an unwinding brake. It generates braking force when rotated in the opposite direction to the winding motor by an external force. This setup allows winding to occur while maintaining appropriate tension, eliminating the need for brake maintenance.
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### **Tips to Remember**
- A torque motor heats up quickly at 115VAC. For optimal performance, operate it at 60VAC or lower for continuous duty cycles.
- When winding with constant tension, remember that diameters are always changing. Thus, voltage must adjust accordingly to maintain tension and feed rate.
- Proper tension control requires two motors: one larger motor to pull material and overcome tension, and a smaller motor to provide sufficient torque for tension. One motor is always "over-torquing" the other.
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### **Applications**
Torque motors can be used in various applications, including:
- Fiber winding
- Tape winding
- Film winding
- Sheet winding
- Sheet tension
- Lead wire tension
- Packaging tension
- Film tension
- Seat tension adjustment
- Lead wire tension adjustment
- Wrapping tension adjustment
- Film tension adjustment
They are also suitable for pushing/rejecting or screw tightening applications.
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### **Video Demonstration**
Check out this video demonstrating the torque motor in action:
[Insert Video Embed]
In the video, a torque motor is positioned on the left, and a speed control motor is on the right. A rope and pulley are attached to each motor shaft to showcase the tension effect. As the rope extends and retracts on the left side, it remains taut with tension. On the right side, slack builds up on the rope.
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### **Products Available**
If you’re looking for more advanced options beyond the simplicity of torque motors, consider the following:
| **Category** | **Product** | **Torque Control Accuracy** | **Total Cost** |
|--------------|-------------|---------------------------|----------------|
| AC Motors | Torque Motors | ±10% | Low |
| Brushless | BXII Series | ±10% | Moderate |
| Stepper | AlphaStep AR/AZ Series | AR: ±10%, AZ: ±20% | Moderate |
| Servo | NX Series | ±5% | High |
For example, the NX Series offers exceptional repeatability with ±0.23% accuracy.
The right motor for tension control depends on how precisely you need to control the output torque and your budget.
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Let me know if you'd like to learn more about sizing a torque motor!