The Issue
For a number of decades all industries using electric motors in their devices urged motor manufacturers to make their motors smaller and lighter without giving up any performance. This is particularly true in the automotive industry where more and more motors are used to perform functions once undertaken by the vehicle driver. Space is always at a premium and excess weight reduces fuel efficiency.
There has also been an increasing requirement to suppress electrical noise from the motors to avoid noise transfer into electronic circuitry and disturb other functions. These requirements are posing greater challenges as motors become smaller. The commutators and brushes inside the motors become smaller, thereby increasing current densities in brushes and generating more electrical noise.
The Solution
Engineers at Johnson Electric recognized that the closer the suppression circuit is to the source of the noise, the more effective it will be. Gradual increases in the energy-density of ceramic magnets have allowed designers to produce smaller motor designs without loss of performance. This creates thermal problems, since loss of motor mass also reduces thermal mass, reducing the time it takes to get overheated.
For a great number of applications, such as door locks, mirrors, antenna lifts and flap actuation in air conditioning, the average usage time of the motor is very short, allowing the motor to be used at maximum power point. Misuse, however, in many applications, occasionally occurs. As a result, it becomes necessary to prevent any damage to the motor or the application that it is serving. Adding an overload protective device such as a positive temperature coefficient (PTC-thermistor) chip in the motor circuit will switch off any large and damaging overload currents before failure occurs.
The result has been the use of much smaller motors in many applications in which both suppression and protective devices are needed and mounted in close proximity to the brush gear and motor terminals.
The Design
Various components are used in suppression circuits to reduce electrical noise. Manufacturers of these components have also worked successfully at miniaturization. Such small components require careful handling particularly since they need to be connected to one another and the motor to create a complete circuit.
| The drawing above (Fig. 1) illustrates a typical suppression/protection circuit. Click here to enlarge |
It can be seen that the positive motor terminal (1) is connected to the negative terminal (2) through a capacitor (3). In series with the positive terminal is a choke (4) that is connected to the brush (6) resting on the commutator (7) that connects to the motor windings (8). The negative terminal (2) is connected in series through the PTC thermistor (5) to the other brush (7). The negative terminal is also grounded to the motor housing.
Engineers have established a protocol to mount all of these components within the same motor end cap that carried the brush gear so noise generated in the motor leads was instantly suppressed.
| The end-result of this study was the motor end cap design illustrated in the following drawings (Fig. 2): Click here to enlarge |
Positive and negative motor terminals (1 and 2), a chip capacitor (3) having connecting ends (13), in a groove (12), a choke (4), a PTC thermistor (5), two brush assemblies (9 and 10), and a bearing (11), are all placed in their respective pockets and connected to
completethe circuit by means of spring contact and pinched contact with the spring brass blades. Each motor terminal (1 and 2) is of the female type and is located around the terminal posts (14), which protect the motor from damage if the male terminal is connected with too much force.
| The end-result of this study was the motor end cap design illustrated in the following drawings ( Fig. 3): Click here to enlarge |
Conclusion
It is important to mount suppression circuits and thermal protective devices in close proximity to the source of noise and heat. This requirement becomes challenging when it is also important to reduce the overall size of motors to satisfy users.
Johnson Electric
www.johnsonelectric.com
