Arithmetic device and motor drive device

1. An arithmetic device performing calculation regarding a three-phase motor that is driven by a pulse signal in a square wave, and is receiving an electric power from a power supply through power supply lines via a power conversion unit that converts the supply of the electric power and that has a plurality of switches being turned ON and OFF according to the pulse signal, the arithmetic device comprising:

a zero-cross current acquirer acquiring a power supply current as a zero-cross current value, the power supply current flowing in the power supply lines at a corresponding timing corresponding to zero-cross of a phase voltage of the motor in an ON period of the pulse signal, the ON period and an OFF period of the pulse signal respectively defining an ON state of a switch having an ON level of the pulse signal and an OFF state of the switch having an OFF level of the pulse signal; and

a motor current calculator calculating a motor current value indicative of a motor current flowing in the motor by using the zero-cross current value acquired by the zero-cross current acquirer.

2. The arithmetic device of claim 1 , wherein

the zero-cross current acquirer has

a timing setting unit for setting the corresponding timing according to a duty ratio of the pulse signal.

3. The arithmetic device of claim 1 , wherein

a waiting period from a start of the ON period to the corresponding timing is ½ of the ON period.

4. The arithmetic device of claim 1 , wherein

the zero-cross current acquirer has

a power supply current unit that acquires the power supply current as a power supply current value each time the ON period arrives, and

a current selection unit that selects an immediately-before current value already acquired immediately before the zero-cross by the power supply current unit as the zero-cross current value.

5. The arithmetic device of claim 1 , wherein

the motor current unit calculates an effective value of the motor current as the motor current value.

6. The arithmetic device of claim 1 , wherein:

the zero-cross current acquirer acquires, in view of zero-cross of the phase voltage in three phases, a first zero-cross current value as the zero-cross current value corresponding to the zero-cross of a first phase voltage, a second zero-cross current value as the zero-cross of a second phase voltage, and a third zero-cross current value corresponding to the zero-cross of a third phase voltage, and

the motor current calculator has an average calculation unit for calculating an average value of the first zero-cross current value, the second zero-cross current value, and the third zero-cross current value.

7. The arithmetic device of claim 1 , wherein:

the zero-cross current acquirer acquires the zero-cross current value each time the zero-cross of the phase voltage occurs in one of three phases, and

the motor current calculator has an electric cycle unit for calculating an average value of a plurality of the zero-cross current values acquired by the zero-cross current acquirer in one cycle of an electric angle.

8. The arithmetic device of claim 1 , wherein

the motor current calculator has a correction unit that corrects the zero-cross current value by a correction coefficient.

9. A motor drive device for driving a three-phase motor with a pulse signal in a square wave, the motor drive device comprising:

a power conversion unit that has a plurality of switches being turned ON and OFF according to a pulse signal, and that converts electric power supplied from a power supply to the motor via a power supply line;

a zero-cross current acquirer acquiring a power supply current as a zero-cross current value, the power supply current flowing in the power supply lines at a timing corresponding to zero-cross of a phase voltage of the motor in an ON period of the pulse signal, the ON period and an OFF period of the pulse signal respectively defining an ON state of a switch having an ON level of the pulse signal and an OFF state of the switch having an OFF level of the pulse signal; and

a motor current calculator calculating a motor current value indicative of a motor current flowing in the motor by using the zero-cross current value acquired by the zero-cross current acquirer.

10. An arithmetic device comprising:

a processor, and

a non-transitory computer-readable storage medium,

wherein the arithmetic device is configured to:

acquire a duty ratio;

calculate a waiting period based at least partly on: (i) an on-time, and (ii) a waiting coefficient;

determine that an elapsed time is greater than or equal to the waiting period;

detect a present shunt voltage;

calculate a present shunt current; and

determine whether a zero-cross occurred.

11. The arithmetic device of claim 10 , wherein, upon a determination that the zero-cross occurred, the arithmetic device is further configured to:

determine that a previous shunt current is a zero-cross current;

calculate an average zero-cross current based on at least three zero-cross currents respectively from three phases; and

correct the average zero-cross current to generate an effective current.

12. The arithmetic device of claim 11 , wherein:

correcting the average zero-cross current is based at least partly upon a motor rotation number and an instruction voltage.

13. The arithmetic device of claim 10 , wherein, upon a determination that the zero-cross did not occur, the arithmetic device is further configured to:

determine that the present shunt current is now the previous shunt current;

detect a new present shunt voltage;

calculate a new present shunt current; and

determine whether the zero-cross occurred.

14. The arithmetic device of claim 10 , wherein:

the waiting coefficient is approximately 0.5, and is used for calculating waiting period.