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DC Motor RPM vs. Voltage: Relationship Between Motor Speed and Voltage

May 18, 2021

Though the AC induction motor is increasing in popularity, many industries still use DC motors. They're best suited for applications requiring low-speed and high-torque and where the load stays relatively consistent. The DC motor's simple design allows for a lower purchase price for smaller HP’s, simple operation, higher efficiency, and precise operational control.

DC Motor Operating Principles

A DC motor's operating principles are reasonably straightforward. A coil is placed in a magnetic field, that's the armature placed within the permanent magnets, and a current is passed through the coil. The current passing through the coil produces torque (a rotational force) that turns the motor's shaft and, in turn, the attached load. Since the whole operation depends on applying a current to the coil of the motor, the voltage of the source power is directly related to the motor's output speed.


(Image sourced from: https://www.precisionmicrodrives.com/content/dc-motor-speed-voltage-and-torque-relationships/)

Motor Speed Explained

Motor speed is the rate at which the motor can perform a function. Specifically, the angular velocity of the output shaft rated as revolutions per minute (RPM). A DC motor's speed is determined by the relationship between the input voltage and the load of the driven object.

Motor Torque Explained

Motor torque is the ability to rotate an object around a fulcrum. Torque is a twisting force rated as Newton-meters. A DC motor's torque is determined by the relationship between the input voltage and the motor's speed.

Motor Voltage Explained

Motor voltage is the pressure from the power source that pushes a current through a circuit. Measured as the electrical potential between two points and rated in volts (V). A DC motor's voltage is determined by the power source and is generally constant unless manipulated by a motor control unit.

DC Motor Speed vs Voltage

A DC motor's speed is directly proportional to the input voltage. The higher the input voltage, the faster the output speed. The lower the input voltage, the slower the output speed.

We can control the speed independently of torque by manipulating the supply voltage using a DC motor control unit. This allows the operator to maintain a steady torque over varying speeds or maintain a constant speed over a variable load.

The Relationship Between Speed, Torque, and Voltage

Speed, torque, and voltage are all interdependent in the operation of a DC motor. To better understand their interdependent nature and what that means for your DC motor, we need to understand the Torque/Speed Curve.

For the sake of this explanation, we'll assume that the motor is operating on a fixed supply voltage. To determine the Torque/Speed Curve, we need to calculate two peak performance factors for the motor:

Stall-Torque

The motor's rated stall-torque is the maximum operational load at which the motor's shaft stops turning. It is best to procure this rating directly from the manufacturer, as bringing your motor up to this rating will likely lead to your motor failing.

No-Load Speed

When the motor runs with no load, the No-Load Speed rating is the maximum speed it can reach at a given voltage.

When these two ratings are plotted on a graph and a line drawn between the two, it's easy to see the direct correlation between a DC motor's torque and speed. Starting at the point on the graph of no-load/full speed, we can see that as the load increases, the speed decreases proportionally up until the moment the motor stalls.


(Image source: https://www.precisionmicrodrives.com/content/ab-032-dc-motors-voltage-vs-output-speed-vs-torque/)

Voltage adjustments can be made to manipulate the Torque/Speed Curve when needed. As noted above, a DC drive can be used to control speed without making changes to the torque. A DC Drive will maintain a steady torque over varying speeds or maintain a constant speed over a variable load.

Gear Reduction for DC Motors

Another way to manipulate your DC motor's speed and torque without making any changes to the input voltage is to use a gear reducer. Mounted between the motor and the driven load, the gear reducer contains a gearing sequence that reduces the motor's output speed while increasing the torque.

Operating Motors Below Base Speed

All motors come with a base speed rating. The manufacturer provides this rating as a warning as to the limitations of the cooling system. Certain enclosure types include an internal, shaft-mounted fan. If the shaft spins too slowly, the fan loses its ability to move the hot air away from the motor's internals.

A motor operating at a higher torque produces more heat. If you are decreasing your motor's speed and increasing the torque, you'll come to a point where the motor is making more heat than it can dispel. So, manufacturers provide a rating called the "Turndown Ratio" that tells you the lowest speed a motor can run at efficiently.

Summary

Your DC motor's speed, torque, and voltage are all interconnected. Understanding how manipulating one factor can affect another allows you to comprehend the different ways you can control your electric motor to complete the functions you require. However, understanding the limitations around manipulating speed, torque, or voltage will help you keep your motor running efficiently and avoid any failures due to overheating issues.

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