Permanent Magnet DC motors are used in a wide variety of applications, and particularly with power tools such as drills, saws, sanders, etc. Such motors are used with cordless power tools that are powered from a rechargeable DC battery. With cordless power tools, a wide variety of tasks often need to be performed that require different motor performance characteristics to best perform the work task. For example, when drilling a hole with a cordless drill in a soft piece of wood, the amount of torque required, and thus the power output required from the motor, may be only a small fraction of what would be needed to drill through a piece of hardwood or pressure treated lumber. However, designing the motor for high power applications is inefficient, from a power standpoint, if the same drill will frequently be used with tasks that involve only light duty drilling, where only low torque is needed for the work task.
In permanent magnet DC motors, the operating characteristics of the motor can be significantly changed by varying the wire size and the number of winding turns of each of the coils that are wound onto the armature lamination stack. For a permanent magnet DC given motor, doubling the number of winding turns making up each coil cuts the no-load speed of the motor roughly in half, and the stall torque of the motor increases significantly. The actual stall torque will be greatly influenced by the source impedance. For example, a typical battery/power tool combination may result in a 50% increase in stall torque for the motor. Also, motor efficiency will increase, but at the same time the maximum power output of the motor will decrease. Thus, simply doubling the number of winding turns for the coils, while providing significantly increased stall torque and greater efficiency at low power, will alter the operating characteristics of the motor in a way that will make the motor less suitable for work tasks requiring a greater power output. However, designing a permanent magnet motor to provide a higher constant power output will result in the motor drawing additional battery current that may not be needed for many drilling tasks (i.e., light duty drilling tasks). For a given motor, this will reduce the run time of the battery powering the tool, compared to the run time that could be achieved with a motor designed for a lower maximum power output.
Accordingly, it would be beneficial to provide a DC motor having a plurality of distinctly different operating modes that provide varying degrees of motor speed, torque and power output, to better match the needs of specific work tasks. For example, it would be highly beneficial if a motor and associated control system was provided that could automatically sense when additional motor power is required when performing a given task, and the motor automatically switched to a specific operating mode to either increase or decrease the torque and/or operating speed of the motor. Alternatively, it would be desirable if the different operating modes of the motor could be selected by a user via a control on the power tool. This would give the user control over implementing the various available operating modes. Optimizing the motor performance for a given work task would also help to make most efficient use of the available battery power, in view of the work task(s) being performed. This in turn could serve to significantly extend the run time of the battery, for a given DC motor, on a given charge.
USPTO Backgrounds
USPTO Backgrounds