Power tool

1. A power tool comprising:

a motor;

a switching device that switches over, in response to a conduction angle thereof, an AC voltage to be applied to the motor;

a rotation speed setting unit that sets a target rotation speed of the motor;

a rotation speed detection unit that detects an actual rotation speed of the motor; and

a controller that determines the conduction angle by comparing the actual rotation speed with the target rotation speed and that controls the switching device by use of the conduction angle based on a maximum conduction angle, the maximum conduction angle being set in accordance with the target rotation speed,

wherein the controller stops the motor based on the maximum conduction angle.

2. The power tool of claim 1 , wherein the controller detects an overcurrent state when the conduction angle exceeds the maximum conduction angle.

3. The power tool of claim 1 , wherein the controller has a storage unit, and

wherein the storage unit pre-stores a relationship between the target rotation speed and a corresponding maximum conduction angle at which an overcurrent is caused.

4. The power tool of claim 3 , wherein the relationship between the target rotation speed and the corresponding maximum conduction angle is set for each frequency of input AC power.

5. The power tool of claim 1 , wherein the rotation speed setting unit is a dial switch, and

wherein the maximum conduction angle is set in accordance with a manipulation of the dial switch.

6. The power tool of claim 1 , wherein the controller reduces the target rotation speed of the motor based on the maximum conduction angle.

7. The power tool of claim 1 , wherein the controller reduces the actual rotation speed by decreasing the conduction angle based on the maximum conduction angle.

8. The power tool of claim 1 , wherein the controller calculates the conduction angle by comparing the actual rotation speed with the target rotation, and

wherein the controller outputs a regulated conduction angle to the switching device, the regulated conduction angle being the same as the maximum conduction angle if the calculated conduction angle exceeds the maximum conduction angle.

9. A power tool comprising:

a motor;

a switching device that switches over, in response to a conduction angle thereof, an AC voltage to be applied to the motor;

a controller that controls the conduction angle, thereby controlling an actual rotation speed of the motor,

wherein, when starting the motor, the controller controls the conduction angle within a range of a first maximum conduction angle during a first period and within a range of a second maximum conduction angle that is smaller than the first maximum conduction angle during a second period, the first period starting at a start-up of the motor, the second period being continuous with the first period.

10. The power tool of claim 9 , wherein the controller has a storage unit, and

wherein the storage unit pre-stores the first maximum conduction angle and the second maximum conduction angle.

11. The power tool of claim 9 , further comprising:

a rotation speed setting unit that sets a target rotation speed of the motor; and

a rotation speed detection unit that detects the actual rotation speed of the motor,

wherein the controller determines the conduction angle by comparing the actual rotation speed with the target rotation speed.

12. The power tool of claim 10 , wherein the storage unit stores a plurality of sets of the first maximum conduction angle and the second maximum conduction angle for each target rotation speed.

13. The power tool of claim 12 , wherein the storage unit stores a relationship between the first maximum conduction angle and the second maximum conduction angle for each frequency of input AC power.

14. The power tool of claim 12 , wherein the storage unit stores a ratio of the first maximum conduction angle to the second maximum conduction angle for each target rotation speed.

15. The power tool of claim 9 , wherein the first period is defined from a time where the motor is started up to a time where a given time has been elapsed, and

wherein the second period is defined from a time where the motor is still not reached to a steady rotation speed to a time where the motor is reached to the steady rotation speed.

16. The power tool of claim 15 , wherein the controller performs a feedback of a deviation of the actual rotation speed from the target rotation speed through a proportional element, and

wherein a unit amount of the feedback of the motor is controlled to be larger in a time of starting and accelerating than that in a time of steadily rotating.