Home » Technical Resources » AB-032 : DC Motors – Voltage Vs. Output Speed Vs. Torque

AB-032

DC Motors – Voltage Vs. Output Speed Vs. Torque

The relationship between voltage, torque and output speed is a common topic of discussion between our customers and Precision Microdrives’ sales engineers.

The following article aims to discuss and elaborate on the relationship between these parameters and methods of using them together with other resources, to understand the full capabilities of our DC motors and gear motors.

Definitions of terms used in our data sheet can be found below with links generously distributed across the article for further reading.

Torque And Speed

Torque can be defined as a ‘twisting force’ that has a tendency to rotate an object about a fulcrum. In relation to DC motors and gear motors, we will typically refer to the ‘Rated Torque’ as the ‘Rated Load’ to avoid any confusion in our values, more information on this topic can be found here. Ultimately, the two terms represent the same value – the rotational force applied to the output shaft.

When discussing ‘speed’, we are typically referring to the angular velocity of the output shaft on our DC motors and gear motors (typically in revolutions per minute). Depending on the application, this parameter will affect the rate at which a particular function is executed and it may have a significant effect on the overall performance of the device.

Why Change Torque?

The most obvious benefit of varying the torque is to maintain a constant speed when the motor’s load varies, keeping in mind the interdependent nature of speed, torque, and voltage.

Although this example may be outdated, audio cassettes are a great way of explaining how some applications need to vary the torque to match a changing load. As the cassette plays and the audio tape moves from one spindle to the other, the driving motor will experience a change in load. However, the playback must remain at a constant speed throughout – otherwise the audio pitch would be affected.

There are also instances that the motor’s load is changed dramatically between operations, rather than a slow dynamic change like the cassette example. Pulleys and lifts often experience this, the motor stops at an extremity as the load is attached or removed. Here, keeping a constant speed is not as important as the motor being able to handle a range of different torque loads as moving a heavier object requires more output torque than a light object or no load.

Each of these applications has the common theme of a varying load attached to the motor. If your application involves a fixed load, then it is likely that you will be more interested in varying the speed.

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Why Change Speed?

The ability to vary motor speed whilst maintaining a steady torque is essential to many applications for a variety of reasons.

An example of an application that requires a variable speed and steady torque is an audio CD player as it is commonly observed that the CD will rotate faster at certain points than others. This is because the information is stored in spiralled circular tracks on the disk and the length/circumference of the track is directly proportional to the amount of information stored on them. This means that the speed must be decreased as the laser is reading from the outermost tracks because there is more information per revolution. Inversely, the speed is increased as the laser reads from the innermost tracks as the spiral circumferences are smaller and therefore contain less information per revolution.

Without the ability to adjust the motor speed (with the voltage) whilst maintaining this constant torque, it would be very difficult to read and play this information at a steady rate.

This same principle can be applied to a wide variety of applications and is often critical to their successful operation. Many of our DC motors and gear motors can operate across a wide variety of speeds and loads, this allows our customers to explore the possibilities of their project and usually reach a suitable solution with a single motor.

How To Read The Typical Performance Characteristics Chart

The Typical Performance Characteristics chart appears on the front page of each of our data sheets. This graph is an extremely useful tool that illustrates the typical behaviour of an individual motor.

As we have previously discussed, many of our customers are looking for a motor or gear motor that will operate at a given speed and load. One of the best places to find a solution is our online catalogue and we can always help to recommend suitable motors and discuss customisation options. As motor speed in DC motors and gear motors is primarily dictated by the load and driving voltage, the data sheet value for ‘Rated Speed’ is taken at a ‘Rated Voltage’ and a ‘Rated Load’. This means that the data sheet values for speed are taken under controlled and specific conditions and do not represent the full capabilities of any single motor. This is where the typical performance chart is a useful tool to view a wider range of the motor capabilities.108-106 Motor Performance Graph

108-106 Motor Performance Graph

The graphs for our DC motors and Gearmotors assume a fixed voltage and show how current draw, power, efficiency and motor speed are affected by a change in load. Each of the affected parameters has it’s own independent performance line and corresponding scale on the Y-axis.

The blue line on the 108-106 Typical performance Chart (above) shows the speeds at which the motor will operate between a point of no-load all the way up to its stall torque (approx. 0.725 mNm) and allows us to examine the motor performance as well as understand the relationship between speed and torque for the individual motor.

For example; if a customer requires a steady speed and torque of 1900 RPM and 0.65 mNm respectively, the data sheet “key features” section (above) would indicate that the 108-106 was not suitable as it states:

Rated load – 0.15 mNm
Rated load speed – 12,600 RPM

However, after inspecting the performance chart, at a load of 0.65 mNm on the X-axis, the blue performance line (speed), indicates on the corresponding Y-axis that the speed will be 1900RPM. The image above illustrates this and demonstrates that the 108-106 is, in fact, suitable for the customer based on their fixed speed and torque requirements. This chart can also be extended to illustrate the range of capabilities for the motor if it is to be used with a dynamic load/speed.

The Relationship Between Speed, Torque And Voltage

Now that we’ve discussed how to read the performance chart, we can look at the relationship between speed and torque. In this section, we will outline the relationship between speed and torque and explain the limits of each before considering the further effect of voltage on these parameters.N/L speed and Stall Torque on Motor Performance Graphs

N/L speed and Stall Torque on Motor Performance Graphs

Assuming the motor is driven at a fixed voltage, there are two points that describe the peak performance for a motor at each extremity. “No load speed” (N/L) and “stall torque”

Our DC motors and gear motors can operate anywhere between these limits before reaching stall. If we take a look at the blue performance line, the relationship between speed and torque is quite easily understood – the torque is inversely proportional to the motor speed – starting at the point of no-load/full speed and, as the load increases, the speed decreases proportionally until the motor stalls.

Whilst the performance chart illustrates how speed is affected when applying various loads, it does not indicate that the speed of our DC motors is also directly proportional to the voltage applied. The theory behind this principle can be found here. In short, this means that we can control the speed of a motor independently of the torque and it allows us to maintain a steady speed for a variable load and also maintain a steady torque with a varying motor speed.

This principle is employed to ensure that our CD player and cassette tape are played correctly and would probably include a closed loop feedback system that will measure the motor speed and adjust the driving voltage to either maintain a steady speed for a variable load or provide a variable speed for a fixed load.

How We Can Change Motor Performance

There are several methods in which the performance of a motor can be customised, whether this is a highly customised solution tailored to a customer’s needs or a simple adjustment to how the motor is operated. Some common modifications are listed below:

Any combination of the above can be used together to achieve a wide variety of outputs from our gear motors. So, even if you cannot find a gear motor performance chart that meets your specification, please feel free to contact our engineers as there are a variety of ways that we can look to meet your requirements.

Limitations

As with all good things, there are limitations to what can be achieved. This section aims to outline some of the associated limitations that are encountered when modifying a gear motor.

In this section, we have discussed a few of the obvious limitations that present themselves when modifying a gear motor. In many cases, these limitations are surpassable if less critical parameters are more flexible. So, please do enquire with our engineers to assess what can be implemented within your application.

Conclusion

Throughout this article, we have discussed some reasons why a user will vary motor speed and torque and looked at specific examples for each situation. This prompted us to consider the limits of motor speed and torque for our gear motors when driven at a fixed voltage. Here we understood that, at a fixed voltage, our motors can operate across a wide range of speeds and torques between a point of no load (full speed) and the point of maximum load (stall).

We have also discussed how to read the “typical performance characteristics chart” to understand the full range of torque and speed capabilities of a given motor (at its rated voltage). From here we saw how the relationship between speed and torque is inversely proportional from a point of no – load to stall torque and discussed how we can adjust the driving voltage to maintain a steady speed or torque when the other variable is dynamic.

The final section of this bulletin was aimed at describing some methods of manipulating motor performance. Some of these methods are easily implemented and can be experimented with when testing, whilst others are permanent modifications that can be provided to a particular specification. If you would like to consider the options that you have for your project, please contact one of our engineers to discuss your requirements and the options that we can offer you.


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