Importance of Continuous Duty vs Limited Duty for NX servos

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Importance of Continuous Duty vs Limited Duty for NX servos

Postby om_tech_support_JT » Tue Apr 23, 2013 5:15 pm

Q: What is the difference between the continuous duty region and the limited duty region in your NX servo motor's speed-torque curves?

A: When sizing a servo motor system, we would need to greatly consider required torque (load torque + acceleration torque), RMS torque (root means squared) and acceleration time. Even though the required torque may meet the requirements, RMS torque would still need to be calculated as we need to analyze the overall power a servo system may require.

Below is a formula to calculate RMS torque:

Servo Motor Sizing RMS Torque Calculation Formula.jpg
Servo Motor Sizing RMS Torque Calculation Formula.jpg (13.91 KiB) Viewed 1978 times

Ta = Calculated Acceleration Torque
TL = Calculated Load Torque
t1 = Acceleration / Deceleration time
t0 = Positioning time
ts = Stopping time

RMS TORQUE
As you can see from the image below,there are two distinct regions within the torque-speed curves. One of the curves is for continuous duty and the other for limited duty.

NX series servo motor speed torque curve, characteristics, definitions.jpg
NX series servo motor speed torque curve, characteristics, definitions.jpg (124.08 KiB) Viewed 1978 times

The required torque, at maximum velocity is needed to select a proper motor for continuous duty. However, if the motor waits between moves, this is known as “dwell time” and can be used to calculate Root Mean Square (RMS) Torque. This lowers the required continuous duty torque and a smaller motor can often be selected.

The calculated RMS torque shall not exceed the continuous duty region. If it does, you would need to select a larger wattage motor, or perhaps increase dwell time between moves.

How and when can we use the limited Duty Region?

Limited Duty region is meant to be used momentarily during acceleration and deceleration purposes. The chart below will provide information how long can a motor operate in limited duty region before the driver generates an alarm and stops motion.

NX servo motor limited duty region, overload detection time.jpg
NX servo motor limited duty region, overload detection time.jpg (65.65 KiB) Viewed 1978 times

From the chart above, at 100% of continuous output torque we would be able to operate continuously. However during acceleration and deceleration, we may need to use the limited duty region. Referencing the chart above, limited duty region can operate for an X amount of time. For example: If you would an output toque of 250% for acceleration, we would have approximately 1 sec to accelerate up to speed or else driver will alarm out. If acceleration time is less that the overload detection time, we would be able to select this motor system.

Online Web-Sizing

An FREE Online Web-sizing tool is available to assist you if you need help calculating these motor requirements for specific motor applications. Please click the link below to try it out. If you have any questions or need further assistance to select an exact product model, please feel free to contact our knowledgeable Technical Support group at 1-800-468-3982, or e-mail us at techsupport@orientalmotor.com.

http://www.orientalmotor.com/support/motor-sizing.html

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