According to one embodiment, there is provided a control device including a detecting circuit, a control circuit and a drive circuit. The detecting circuit detects changes in a current flowing between a node and a smoothing circuit, the node being to which a DC brush motor, a power supply circuit and the smoothing circuit can be electrically connected via respective different lines. The control circuit generates a control signal to control rotation speed of the DC brush motor according to the detected changes in the current. The drive circuit drives the DC brush motor according to the control signal.

The invention relates to an arrangement for identifying an electrical line interruption during operation of a drive system (20), which comprises two parallel-connected DC motors (21, 41), which are mechanically coupled to one another by means of a part that is to be driven, a driver for driving the two parallel-connected DC motors (21, 41) and a measuring device for detecting different electrical variables of the drive system (20). The measuring device comprises a rotational speed determination apparatus (32) for determining the rotational speed of the two DC motors (21, 41) and for supplying a piece of rotational speed information representing the rotational speed. The measuring device further comprises a current measuring apparatus (27; 28) for determining a summation current flowing in the two DC motors (21, 41) and for supplying a piece of current information representing the summation current and a voltage determination apparatus (60; 61) for determining the terminal voltage of the two DC motors (21, 41) and for supplying a piece of voltage information representing the terminal voltage. A computation unit (70) is designed to ascertain a motor parameter (k.sub.) for the drive system (20) from the voltage information, the current information and the rotational speed information and to compare said motor parameter with a setpoint value (k.sub._set) of the motor parameter (k.sub.), wherein the presence of a line interruption to one of the two DC motors (21, 41) can be inferred from the result of the comparison.

An object of the present invention is to provide an electric power tool capable of automatically changing the rotating speed of an electric motor in accordance with operation conditions. The electric power tool 11 which carries out an operation of polishing, peel-off, cutting, or the like by swinging a tip tool 13 attached to a tool retaining 31 part within the range of predetermined angles by the drive power of the electric motor 12, wherein the electric power tool has a control device configured to indirectly detect the load of the tip tool from a current value, and to control the rotating speed of the electric motor 12 to a maximum rotating speed and a minimum rotating speed on the basis of the load of the tip tool 13.

According to one embodiment, there is provided a control device including a detecting circuit, a control circuit and a drive circuit. The detecting circuit detects changes in a current flowing between a node and a smoothing circuit, the node being to which a DC brush motor, a power supply circuit and the smoothing circuit can be electrically connected via respective different lines. The control circuit generates a control signal to control rotation speed of the DC brush motor according to the detected changes in the current. The drive circuit drives the DC brush motor according to the control signal.

The invention relates to an arrangement for identifying an electrical line interruption during operation of a drive system (20), which comprises two parallel-connected DC motors (21, 41), which are mechanically coupled to one another by means of a part that is to be driven, a driver for driving the two parallel-connected DC motors (21, 41) and a measuring device for detecting different electrical variables of the drive system (20). The measuring device comprises a rotational speed determination apparatus (32) for determining the rotational speed of the two DC motors (21, 41) and for supplying a piece of rotational speed information representing the rotational speed. The measuring device further comprises a current measuring apparatus (27; 28) for determining a summation current flowing in the two DC motors (21, 41) and for supplying a piece of current information representing the summation current and a voltage determination apparatus (60; 61) for determining the terminal voltage of the two DC motors (21, 41) and for supplying a piece of voltage information representing the terminal voltage. A computation unit (70) is designed to ascertain a motor parameter (k.sub.107 ) for the drive system (20) from the voltage information, the current information and the rotational speed information and to compare said motor parameter with a setpoint value (k.sub.107 .sub._.sub.set) of the motor parameter (k.sub.107 ), wherein the presence of a line interruption to one of the two DC motors (21, 41) can be inferred from the result of the comparison.