Let's measure the torque of a DC motor (Motors #3)

Summary

In this video, we explore the performance of a 3-pole DC motor, focusing on how gear ratios and added mass affect its speed and torque. By using a gear train with a 5.6 ratio, the motor’s speed is reduced, and its torque is increased. We measure the motor's speed using a tachometer and observe how adding screws to a bucket affects the motor's RPM. By converting mass to torque and RPM to radians per second, we plot a linear speed vs. torque graph, which provides insights for designing systems that use this motor.

Takeaways

• 😀 The motor is spinning fast, but its speed is being reduced by a gear train with a ratio of 5.6 to increase torque.
• 😀 The gear train consists of a small gear with 10 teeth and a larger gear with 56 teeth, resulting in the motor spinning 5.6 times faster than the pulley, but with 5.6 times more torque.
• 😀 The pulley is connected to a thread that reels in a paper bucket, and screws are added to the bucket to observe the effect on motor speed.
• 😀 As more screws are added, the motor speed decreases, and this change is monitored with a tachometer that measures the motor's RPM.
• 😀 The motor speed without screws in the bucket is 204 rpm. As screws are added, the speed gradually decreases (e.g., 177 rpm with 1 screw, 172 rpm with 5 screws).
• 😀 The motor continues to slow down with each added screw, reaching 48 rpm with 26 screws, after which it stalls and can't turn with 27 screws.
• 😀 The relationship between the number of screws (or weight) and motor speed is expected to be linear for a DC brushed motor.
• 😀 A scale is used to weigh the bucket with screws, and the bucket weighs 54 grams with 27 screws.
• 😀 To convert the weight of the screws into torque, the mass is multiplied by gravitational acceleration, and the radius of the pulley is measured as 17 millimeters.
• 😀 A speed vs. torque graph is generated by converting speed from rpm to radians per second, and the resulting curve is approximately linear, which is typical for brushed DC motors.
• 😀 The final graph provides important insights for designing equipment that uses the motor, allowing designers to refer to the standard speed vs. torque relationship for their projects.

Q & A

What is the purpose of using a gear train in the motor setup?

The gear train is used to reduce the speed of the motor and increase its torque. The smaller gear has 10 teeth, and the larger gear has 56 teeth, resulting in a gear ratio of 5.6, meaning the motor will spin 5.6 times faster than the pulley while increasing torque by the same factor.

How does the number of screws in the bucket affect the motor's performance?

As screws are added to the bucket, the load on the motor increases, causing the motor to slow down. The relationship between the number of screws (which increases mass) and motor speed is inversely proportional, meaning more screws result in slower motor speeds.

What causes the voltage drop when the motor is turned on?

The voltage drop occurs because the DC power supply is not perfect. When the brushes make contact with the commutator of the motor, it causes a momentary short circuit, which disrupts the power supply and causes the voltage to drop temporarily.

How is the motor's speed measured during the experiment?

The motor's speed is measured using a tachometer, an optical sensor that uses a laser. A piece of black tape is placed on the pulley to provide contrast, allowing the tachometer to detect the pulley’s rotations and calculate the motor's RPM (revolutions per minute).

What happens to the motor's speed as more screws are added to the bucket?

As more screws are added, the motor's speed decreases. For example, the motor starts at 204 rpm without screws, and the speed drops progressively to 48 rpm with 27 screws, at which point the motor stalls.

What is the significance of the tachometer's laser and the black tape on the pulley?

The tachometer uses a laser to detect the rotation of the pulley. The black tape on the pulley creates a high contrast for the optical sensor, making it easier to detect the pulley’s movement and calculate the motor’s speed accurately.

Why does the motor stall at 27 screws in the bucket?

The motor stalls at 27 screws because the load (torque) has become too great for the motor to overcome. Despite a high current, the motor cannot generate enough torque to keep rotating, indicating it has reached its maximum capacity.

How are the mass and torque related in this experiment?

The mass of the screws in the bucket is converted into force (weight) using gravitational acceleration. This force is then used to calculate the torque on the motor by multiplying the force by the radius of the pulley (lever arm).

What is the relationship between speed and torque in a brushed DC motor?

For a brushed DC motor, there is a linear relationship between speed and torque. As the load increases (more screws), the motor speed decreases, and this relationship can be plotted on a graph showing a straight-line decline.

How can the data from the experiment be used for motor design?

The data, particularly the speed vs. torque graph, is crucial for designing equipment that uses the motor. Engineers can refer to the graph to understand the motor’s behavior under different loads and optimize its performance in various applications.