Driving DC Brushed Motors
To control a DC motor, a variable, and controlled power supply is needed. There are three types of methods that can be used:
- Voltage-controlled linear supply
- Current linear supply
- PWM (Pulse Width Modulation)
We will discuss PWM briefly.
In many applications, the DC motor needs to provide various load points. PWM is a method that can provide variable controlled power to the motor to adjust to various load points in the application. If the application is battery-driven, PWM offers an efficient way to drive the motor and extending battery life.
Portescap brushed DC motors have very low inductance and inertia. With this design, the motor can be used in applications where dynamic behavior and fast response are required. This design with the use of PWM enables current control in the windings. Output torque is linearly proportional to the average current in the windings and can be accurately controlled.
Designing a PWM power supply does require some considerations. A quick summary of these would be as follows:
The current ripple in the design should be small. What does small mean? Typically, you want this to be less than 10% of the total current draw in the system.
Reasons small current ripple is desired is to minimize the dissipated Joules power which lowers the iron losses.
If using a precious metal brushed motor, this also helps in reducing the electro-erosion of the brushes and commutator.
Taking the above considerations in designing the PWM circuit can help in extending the life of the motor. The typical life of a DC brushed motor is a complex combination of factors of the various components in the motor. In most applications, the life of a DC Brushed motor is dependent upon the aging and wear of the brushes since this is a mechanical commutation system. When the brushes are worn to the point of not contacting the commutator, the motor will no longer function.
Some common factors that can influence the life of a motor are:
- Running the motor at the higher current draw in the commutation will accelerate the wear of the brushes. It's always good to understand the maximum current rating of the motor, and if possible, try to limit it to 70% of the motor rating to help extend motor life.
- If the application has multiple start/stops with high starting current peaks, this will accelerate the wear of the brushes.
- Working points of the application that require high temperatures can also work on exercising the properties of the various lubricants. High temperatures can have an impact on drying out the lubricants and causing excessive heat and wear.
PWM driving of a DC motor is a relatively inexpensive way to run the motor in many applications. We invite you to review our more detailed information white paper Controlling Brushed DC Motors Using PWM.