What is a Torque Speed Curve and Why Is It Important?

A torque/speed curve

plots the torque of a motor against the speed the motor is going at. The relationship between speed and torque varies dramatically across different types of motor and also between manufacturers. In part this is a result of the mechanical design of the motors in question but in other cases, choice of materials and manufacturing tolerances may be relevant.

The relationship between the torque and the speed is critical to choosing the right motor for your application.
 

Given that the amount of torque available changes with a change in speed it is important to think about the performance point you need when selecting a motor and controller. 

A common mistake is to buy a motor based on its rated or holding torque (thinking this is perfect for your application) only to find that this torque is not available at the required speed.

The ZD Series of Stepper Motor Controllers can be run using a range of external inputs, offers 25600 potential positions (on a standard 200 step stepper motor) and can be programmed to do virtually anything! However, as the above torque speed graph highlights, an increase in speed leads directly to a drop off in the torque available from the motor and controller.

Depending on the torque and speed that you require for you application, we can supply the ZD Series of Stepper Motor Controllers with a range of stepper motors giving you plenty of choice!

What happens to the available torque as the speed increases can be very different from motor to motor

As you can see in the example above the torque at lower rpm is very high but as the speed builds within the motor, the torque begins to drop away. In many respects this can be seen as a resultant trade-off given that the power going into the motor is the same. By using the power to generate higher speeds it is not possible to maintain the same level of torque. Bear in mind that the torque speed curve displayed on a datasheet is the maximum possible and that it is possible to use an intelligent controller to maintain fixed torque over a set speed range 

It is important to bear in mind that different motors will have unique torque speed curves and also that with stepper and bldc motors, the controller used will also directly affect the torque speed curve. The key to using torque/speed curves is therefore to understand the exact or minimum requirements that you need from a motor and use the torque speed curve to make a much more accurate assessment of the motor and controller that you need.

The importance of the stepper motor driver or stepper motor controller when using a stepper motor torque speed curve

When viewing a stepper motor torque speed curve such as the one above it is common to see an explanation at the top of the diagram which will explain what controller was used in the measuring of that particular curve.

Different stepper motor controllers can output different levels of power and it may be the case that the motor you are looking at could output more power if it was used with a higher powered stepper motor controller. Equally it is also important to bear in mind that a stepper motor controller set on full step will typically generate a higher torque than a stepper motor controller running a microstepping sequence so if you are thinking of utilising a microstepping programme then it is worth contacting us first to discuss.

Conclusion

The torque speed is a fundamental guide to what you can expect to get in terms of mechanical performance but it is important to understand the specific situation in which a torque speed curve was created in order to ensure it meets your expectations. If you have any questions at all about torque speed curves and how they might affect your choice of motor then please don’t hesitate to contact us via online chat or phone and we will be more than happy to help.

Categories: Brushless Motor Controller FAQs, Commercial, Stepper Motor Controller FAQs
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