Brush Selection for Brush DC Motors

Decided that a brush DC motor is right for your application or device? Great! These motors are designed to provide high power density, low friction, low starting voltages, minimal iron losses, and superb speed-to-torque performance, making them ideal for applications in the medical, robotics, factory automation, and aerospace and defense fields.

Now it’s time to dive into the specific DC motor selection criteria, which include motor diameter, length, bearing type, winding options, and magnet type. Another key factor to consider is the motor’s commutation type – i.e. selecting between precious metal brushes or carbon brushes. Let’s explore this criterion in detail below.

How Do I Choose Between Precious Metal and Carbon Brushes?

This is a very common question when specifying the right brush DC motor for an application. Both precious metal and carbon (graphite-copper) brush commutation systems have advantages and drawbacks. Here are their main features and benefits:

Precious Metal Commutation

Precious metal brushes are the less expensive commutation system, being well adapted to low current density applications and providing low friction and high efficiency. This commutation system is well-suited for applications with continuous running operations, such as diaphragm pumps or tattoo machines, as well as battery-operated or portable devices like patch pumps or small dental hand tools. Table 1 provides an overview of precious metal brush’s technical characteristics and the corresponding application benefits.

Technical Characteristics	Application Benefits
Low friction	High efficiency (battery life)
Low brush pressure	Low no-load current (battery life) Low starting voltage (very low speed-operation ability)
Low audible noise and vibration	Quiet operation
Resistant to oxidation	Long shelf life
Constant contact resistance with collectors	Low electromagnetic interference (EMI)

 

Carbon Brush Commutation

Carbon brushes, while the more expensive brush option, are intended for high continuous/peak current and high load torque applications; large actuators are an excellent example. They are also ideal for use in highly dynamic applications that feature many start-stops, such as industrial hand tools. See Table 2 for carbon brushes’ technical characteristics and the associated application advantages.

Technical Characteristics	Application Benefits
High RMS current capability	High load torque ability
High peak current capability	Intensive start-stop ability, high acceleration ability
Higher friction	Higher no-load current Higher starting voltage
Higher audible noise and vibration	Louder operation
Less constant contact resistance with collectors	Higher electromagnetic interference (EMI)

 

Brush Options at Portescap

Small Portescap brush DC motors (under 13mm in diameter) are only available with precious metal brushes, as these motors draw very low current. Conversely, larger brush DC motors (above 30mm in diameter) are only available with carbon brushes because of the high current ratings; these brushes are also not easy to manufacture in a smaller size.

For motor sizes between 13mm and 30mm, both commutation options are usually available. An excellent example is our Athlonix™ brush DC motors platform, which are available in both precious metal and graphite commutations in Ø16/17mm and Ø22/24mm diameter.

Need assistance with selecting the ideal brush DC motor and commutation solution? Contact our engineering team here!