Power tool for performing soft-start control appropriated for motor load

1. A power tool comprising:

a motor;

a power supply unit that supplies power to the motor;

a trigger unit that causes the power supply unit to start applying a voltage to the motor;

a control unit that controls the power supply unit to increase the voltage to the motor at a constant increasing rate; and

a motor load detection unit that detects a motor load, wherein the control unit changes the constant increasing rate in accordance with the motor load,

wherein the control unit comprises a determination unit that determines whether the motor load is heavy or light, wherein the control unit increases the constant increasing rate if the determination unit determines that the motor load is light.

2. The power tool as claimed in claim 1 , wherein the power supply unit comprises a switching unit that is controlled by Pulse Width Modulation (PWM) to supply power to the motor.

3. The power tool as claimed in claim 1 , wherein the power supply unit comprises a switching unit that is controlled by Thyristor Phase control to supply power to the motor.

4. The power tool as claimed in claim 1 , wherein the voltage applied to the motor is an effective value.

5. A power tool comprising:

a motor;

a power supply unit that supplies power to the motor;

a trigger unit that causes the power supply unit to start applying a voltage to the motor;

a control unit that controls the power supply unit to increase the voltage to the motor at a constant increasing rate; and

a motor load detection unit that detects a motor load, wherein the control unit changes the constant increasing rate in accordance with the motor load;

a detection unit that detects a rotational speed of the motor; and

a determination unit that determines whether the rotational speed of the motor exceeds a threshold within a first time period after a beginning of power supply to the motor, wherein

the control unit increases the constant increasing rate if the determination unit determines that the rotational speed of the motor exceeds the threshold.

6. The power tool as claimed in claim 5 , wherein the control unit has a plurality of thresholds, the control unit increases the constant increasing rate every time the detected rotational speed exceeds the plurality of thresholds in ascending order.

7. The power tool as claimed in claim 5 , wherein the threshold is used to determine whether the motor load is heavy or light,

if the rotational speed exceeds the threshold within the first time period, the control unit determines that the motor load is light, if the rotational speed does not exceed the threshold within the first time period, the control unit determines that that the motor load is heavy.

8. The power tool as claimed in claim 5 , wherein the power supply unit comprises a switching unit that is controlled by Pulse Width Modulation (PWM) to supply power to the motor.

9. The power tool as claimed in claim 5 , wherein the power supply unit comprises a switching unit that is controlled by Thyristor Phase control to supply power to the motor.

10. The power tool as claimed in claim 5 , wherein the voltage applied to the motor is an effective value.

11. A power tool comprising:

a motor;

a power supply unit that supplies power to the motor;

a trigger unit that causes the power supply unit to start applying a voltage to the motor;

a control unit that controls the power supply unit to increase the voltage to the motor at a constant increasing rate; and

a motor load detection unit that detects a motor load, wherein the control unit changes the constant increasing rate in accordance with the motor load,

wherein the motor load detection unit detects a rotational speed of the motor within a first time period from a beginning of power supply to the motor, and the control unit determines whether the motor load is heavy or light in accordance with the detected motor load, and wherein

if the detected rotational speed exceeds a threshold within the first time period, the control unit determines that the motor load is light and then increases the constant increasing rate, and

if the detected rotational speed does not exceed the threshold, the control unit determines that the motor load is heavy and then maintain the constant increasing rate.

12. The power tool as claimed in claim 11 , wherein the power supply unit comprises a switching unit that is controlled by Pulse Width Modulation (PWM) to supply power to the motor.

13. The power tool as claimed in claim 11 , wherein the power supply unit comprises a switching unit that is controlled by Thyristor Phase control to supply power to the motor.

14. The power tool as claimed in claim 11 , wherein the voltage applied to the motor is an effective value.