Tension control is a specialized application that can be quite challenging to implement effectively. Selecting the appropriate motor type is crucial for achieving success. Contrary to popular belief, you don’t always have to use a servo motor for this purpose.
Take a look at this application image showing how two motors work together to maintain tension while feeding material. The smaller brushless motor in the center is responsible for winding, while the larger torque motor provides the specific tension needed.
Certain applications, such as film winding, stamping/pressing, or screw tightening, require the motor to regulate or limit its torque. For instance:
- **Film winding/unwinding**: The motor must adjust its torque based on the roll’s diameter to ensure consistent tension.
- **Stamping/pressing**: The motor must limit its torque to prevent damaging delicate products, like eggs, during labeling.
- **Screw tightening**: The motor must limit its torque to avoid exceeding the maximum tightening torque for the screw.
Several motor types can handle these tasks. While servo motors are often chosen for tension control applications, there’s a cost-effective alternative worth considering if you don’t need all the extra features.
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**Torque Motors**
Torque motors look similar to standard induction motors but are designed differently. They are capable of controlling their torque and can be stalled or back-driven continuously without overheating—key characteristics 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 in that it uses a balanced winding and a high-slip rotor rather than a low-slip rotor. This high-slip rotor allows the torque motor to function as a brake when back-driven. Despite their relatively small size (up to 20 W), torque motors offer various gear ratios to increase torque. Single-phase 110/115 VAC and 220/230 VAC options are also available.
For example, given a certain load, applying voltage V4 to the motor will cause it to limit its output torque where the "V4" line intersects the "Load" line, resulting in a motor speed of "N4."
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 to vary the input voltage to the motor, such as the **TMP-1 power controller**.
The TMP-1 power controller uses the triac phase control method to vary the voltage supplied to the torque motor. Alternatively, you could use any device that outputs a consistent voltage, such as a PLC with an analog output card.
The torque motor can also operate on DC voltage. Instead of rotating, the motor will provide braking torque and act as a brake to provide brake tension. The braking torque can be adjusted with voltage.
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**Benefits of Torque Motors**
**Benefit #1: Simple Configuration**
Unlike servo systems, torque motors don’t need complex feedback mechanisms or sensors. You just need a device to control the voltage input, making the setup straightforward and cost-effective.
**Benefit #2: Avoids Slack**
Tension is essential to prevent "slack" in materials being wound or unwound. When the diameter of the winding starts small, the torque motor may have lower torque but needs to rotate quickly. As the winding diameter increases, the motor rotates slower but requires more torque due to the increased load.
**Benefit #3: Fine Adjustment for Proper Tension**
The torque motor can serve as an unwinding brake. When rotated in the opposite direction to the winding direction by an external force (like another motor), it generates a braking force. This setup enables winding with proper tension and eliminates the need for brake maintenance.
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**Tips to Remember**
- A torque motor heats up quickly when operated at 115 VAC. For this reason, a 5-minute duty cycle is recommended. For continuous operation, use 60 VAC or below.
- When winding with constant tension, remember that the diameters are always changing. To maintain tension and feed rate, the voltage needs to change accordingly.
- For proper tension control, you need two motors: one larger motor to pull material and overcome tension, and one smaller motor to provide sufficient torque for tension. One motor is always "over-torquing" the other.
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**Applications for Torque Motors**
Torque motors are versatile and can be used in various applications:
- **Fiber winding**
- **Tape winding**
- **Film winding**
- **Sheet winding**
- **Sheet tension**
- **Lead wire tension**
- **Packaging tension**
- **Film tension**
They can also be used for pushing/rejecting or screw tightening applications.
Watch this video demonstration of a torque motor in action. On the left, you see a torque motor, and on the right, a speed control motor. A rope and pulley are mounted to each motor shaft to show the effect of tension. As the rope extends and retracts on the left side, you can see that the rope remains taut. On the right side, however, slack builds up on the rope.
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**Conclusion**
The right motor for tension control depends on your accuracy requirements and budget. If you’re looking for simplicity and cost-effectiveness, torque motors are an excellent choice. However, if you need more advanced functionality, consider brushless motors, stepper motors, or servo motors.
Ready to learn how to size a torque motor? Check out our resources for sizing guidelines.
If you need a more advanced solution than the simple operation provided by torque motors, consider the following options:
| **Motor Type** | **Example** | **Torque Control Accuracy** | **Total Cost** |
|--------------------|---------------------------------|----------------------------|----------------|
| **Torque Motors** | 3W Torque Motor | ±10% | Low |
| **Brushless Motors**| BXII Series | ±10% | Moderate |
| **Stepper Motors** | AlphaStep AR/AZ Series | AR: ±10%, AZ: ±20% | Moderate |
| **Servo Motors** | NX Series | ±5% | High |
For instance, the NX Series offers excellent repeatability, with a reference value of ±0.23%.
In summary, the ideal motor for tension control depends on your precision requirements and budget.
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