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Torque
Torque is defined as the force that can cause an object to rotate around an axis.
When torque is applied to the input shaft of a transmission, it sets motors in motion. Torque enables a machine, like an air compressor, to run and maintain the required pressure. Understanding torque can help you choose the right compressor for industrial applications.
Effectively matching torque to your application requirements reduces the mechanical stress on your machine during operation, keeping the motor and compressor running longer. Proper torque also reduces energy waste and overall operating costs.
FAQs
Here’s a formula used to calculate torque:
τ = r F sin(θ)
Here’s what each part of this formula means:
- τ: torque (in newton-meters)
- r: the distance between the pivot point and the point where the pivot force is applied
- F: the force acting on the object
- θ: the angle between the force vector and lever arm, typically equal to 90°
To calculate torque for a motor, you can use the following calculation:
Torque (Nm) = (Power (kW) × 9550) ÷ RPM
Here’s what each part of this formula means:
- Power: The rated power of the motor in kilowatts
- 9550: A metric constant relating power, torque and RPM
- RPM: The rotational speed of the motor (revolutions per minute)
To use this formula, obtain the actual output power and RPM. Consider an air compressor with an output power of 7.5 kW and a motor speed of 3,600 RPM. Multiply the output power by the constant for a value of 71,625. Then, divide 71,625 by the RPM value of 3,600. This calculation would result in a torque of about 19.9 newton-meters.
Motors need adequate rotational force to start up and operate under various load conditions. Different motor control methods can impact the level of torque required. Here are a few common control methods:
- Fixed speed: Motors using this control run at a constant speed. This method is also known as the across-the-line start. The biggest challenge is providing enough starting torque to overcome the initial load.
- Load/unload: This control method causes the motor to cycle between loaded and unloaded states. It requires robust torque delivery during the loaded stage, but it uses minimal torque during the unload stage.
- Variable speed: Motors with this control method adjust the motor speed to match demand. For example, when air demand is low in an air compressor, the motor speed slows. This control method reduces unnecessary high-torque operation, which improves efficiency and decreases wear.
If you’re trying to find the right air compressor for your needs, avoid oversizing or undersizing the system. Both scenarios create inefficient torque use and increase your energy costs. For example, if you choose a compressor with too much power for your application, it will use a high torque level that will ultimately be wasted.
Determine the level of pressure your process needs and choose a compressor that operates within that range, not above or below. Also, consider advanced motor controls like variable speed drives or load/unload controls, which minimize unnecessary high torque. Regular compressor maintenance can also help prevent torque loss through friction, wear, misalignment or other issues.