When testing the stall torque of small DC gear motors, an accurate and consistent method is the key to obtaining reliable results. Torque is a key performance parameter that reflects a motor’s ability to handle loads under different conditions. However, achieving consistent results can be challenging, especially when the test methods are different.

Here is a real case where different test methods lead to the different test results.  In the end, the customer agreed our testing results and designed our gear motors into their applications.

Let’s explore how to test the stall torque correctly.

Understanding The Stall Torque and How It’s Measured Torque

In our previous article, we learned how to calculate the DC motor torque, and we will briefly repeat it here.

(T) is a measure of rotational force, commonly expressed in Kg.cm, N.m, or oz.in.   

For small geared electric motors, torque can be classified into:

Stall Torque: The maximum torque the motor can produce when the output shaft is prevented from rotating.

Rated Torque: The torque a motor delivers at its rated speed and voltage under continuous operation.

Calculating Torque

The stall torque can be calculated using the formula:  

For a small geared motor, you also need to consider the gear reduction ratio, which amplifies the output torque but reduces the speed.

Measuring Torque

Torque is typically measured using a torque sensor or a spring-based torque meter (force gauge). To test stall torque, the motor’s output shaft is locked, and the force exerted is measured while the motor runs at its rated voltage.

Case study: Torque test differences

In this case, our engineers tested our micro DC gear motor samples using a professional torque meter and recorded a stall torque of more than 3.0 Kg.cm. However, The customer said there was too much difference testing results between them and us. They also used pulleys and weights to cross-check stall torque. And send us videos of their testing.

Our engineers disagree with the customer’s results for the following reasons:

Voltage Drop: The voltage dropped due to the long or thin wires, which increases resistance and affects the stall torque.

Limitations of the pulley and weight method: The pulley and weight method can be influenced by factors like weight swing or friction, which may reduce accuracy.

Then, we sent a video of our stall torque test to the customer, and they confirmed our test results.

How To Test The Stall Torque Correctly?

To ensure the consistency and accuracy of your torque test, follow these steps:

a. Ensure that the power supply is normal

Use a power supply that maintains a stable voltage over the motor rating.

b. Minimize voltage step-down and use short, thick wires to reduce resistance.

c. During the test, the voltage at the motor terminal is measured directly to ensure that it matches the rating.

Use a high-quality torque meter

A torque meter or force meter is ideal for direct measurement. Make sure the equipment is properly calibrated. For stall torque, lock the output shaft of the motor, measure the force applied at a known radius, and calculate the torque:

Avoid pulleys and weight methods although simple, this method is error-prone for the following reasons: A swaying or unstable weight. Friction in a pulley system. Set misalignment. If you insist on this approach, it is recommended to add a high-precision torque sensor for verification.

Zusammenfassung

To achieve accurate torque measurement and resolve disagreements: Ensure stable, accurate power supply with minimal voltage drop. Direct and reliable measurement with a professional torque meter. Avoid or improve the pulley and weight method to reduce errors.