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The Right Motor for Tension Control
Tension control is a specialized area that can be quite challenging to implement effectively. Choosing the correct type of motor is the first crucial step towards achieving success. Contrary to popular belief, a servo motor isn’t always the go-to solution.
Take a look at this application image showing how two motors work 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 a precise tension level.
Applications such as film winding, stamping/pressing, or screw tightening often require a motor capable of controlling or limiting its torque.
For instance:
- **Film winding/unwinding** - The motor must adjust its torque according to the roll’s diameter to maintain consistent tension.
- **Stamping/pressing** - The motor needs to limit its torque to avoid damaging delicate items like eggs during labeling.
- **Screw tightening** - The motor must cap its torque to prevent exceeding the maximum tightening torque allowed for the screw.
While servo motors are frequently chosen for tension control applications, there’s a cost-effective alternative worth considering if you don’t need all the extra features.
## Torque Motors
Torque motors resemble standard induction motors but are uniquely designed to control torque and can handle continuous stalling or even back-driving without overheating—essential traits for tension control.
A torque motor achieves full torque at zero speed and is seldom used at full speed. Its design differs from an induction motor by using a balanced winding and a high-slip rotor instead of a low-slip rotor. This high-slip rotor enables the motor to act as a brake when back-driven. Despite its relatively small size (up to 20W), various gear ratios are available to amplify torque. Single-phase 110/115 and 220/230 VAC options are also available.
For example, with a given load, applying voltage V4 to the motor will limit its output torque at the intersection of the "V4" line and the "Load" line, causing the motor to run at "N4" speed.
Unlike a servo motor tension control system, a torque motor does not require motor feedback or additional sensors to control its torque. All you need is a device to vary 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 vary voltage to the torque motor. Alternatively, any device that can consistently output voltage to the motor, like a PLC with an analog output card, can be used.
There are three main ways to control voltage/torque:
1. Manual control
2. Automatic control via a PLC
3. Remote control
The torque motor can also operate on DC voltage. Instead of rotating, it provides braking torque and acts as a brake to offer brake tension. The braking torque can be adjusted with voltage.
## Avoiding Slack
Tension is essential to prevent “slack†in the material being wound or unwound.
When the diameter of the winding starts small, the motor might have lower torque but needs to rotate quickly. Conversely, as the winding diameter increases, the motor rotates slower but requires more torque due to the higher load.
## Fine Adjustment for Proper Tension
The torque motor can serve as an unwinding brake. It generates a braking force when rotated in the opposite direction to the winding motor by an external force. By using a torque motor on the unwinding side of the work, winding can be achieved while maintaining appropriate tension. Additionally, it does not wear out, eliminating the need for brake maintenance.
Rotate the motor in the opposite direction to the winding direction.
## Important Tips to Remember
- A torque motor heats up quickly when operated at 115 VAC. Therefore, a 5-minute duty cycle is recommended. For continuous duty, use 60 VAC or below.
- When winding with constant tension, remember that diameters are always changing. To maintain tension and feed rate, voltage needs to change accordingly.
- For proper tension control, you need two motors. One motor needs to be larger since it needs to pull material and overcome tension. The other motor is typically smaller, requiring sufficient torque to provide the tension. One motor is always "over-torquing" the other motor.
Here are some applications where the torque motor can be used:
- Fiber winding
- Tape winding
- Film winding
- Sheet winding
The torque motor can also be used for pushing/rejecting or screw tightening applications.
Last but not least, here's a video demonstration of the torque motor.
Ready to learn how to size a torque motor?
If you need a more advanced product than the simple operation offered by torque motors, here are some options:
| Options | Torque Control Accuracy | Total Cost |
|-----------------------|-------------------------|------------|
| AC Motors | ±10% | Low |
| Brushless Motors | ±10% | Moderate |
| AlphaStep AR/AZ Series| AR: ±10%, AZ: ±20% | Moderate |
| Servo Motors NX Series| ±5% | High |
You can expect much better "repeatability" when using the same motor, driver, load, and settings. For example, the repeatability for the NX Series is ±0.23%.
The right motor for tension control depends on how accurately you need to control the output torque (and how much you want to spend).