We Claim:
1. A driving device to drive a motor having coils, the driving
device comprising:
a converter to generate a bus voltage;
an inverter to convert the bus voltage to an AC voltage and supply the AC voltage to the coils; and
a connection switching unit to switch a connection state of the coils;
wherein the bus voltage generated by the converter is switched in accordance with the connection state of the coils.
2. The driving device according to claim 1, wherein the
connection state of the coils has a first connection state and a
second connection state, and
wherein the inverter outputs a lower line voltage in the second connection state than in the first connection state.
3. The driving device according to claim 2, wherein the bus
voltage of the converter is a first bus voltage VI when the
connection state of the coils is the first connection state,
wherein the bus voltage of the converter is a second bus voltage V2 when the connection state of the coils is the second connection state, and
wherein the second bus voltage V2 is higher than the first bus voltage VI.
4. The driving device according to claim 3, wherein the coils
are three-phase coils, and
wherein the first connection state is a state where the three-phase coils are connected in a Y connection, and the second connection state is a state where the three-phase coils are connected in a delta connection.

5. The driving device according to claim 4, wherein a first
rotation speed Nl and a second rotation speed N2 of the motor
satisfy N2/N2 > V3;
wherein when a rotation speed of the motor is the first rotation speed Nl, the connection switching unit sets the connection state of the coils to the first connection state; and
wherein when the rotation speed of the motor is the second rotation speed N2, the connection switching unit sets the connection state of the coils to the second connection state.
6. The driving device according to claim 5, wherein the first bus voltage VI, the second bus voltage V2, the first rotation speed Nl and the second rotation speed N2 satisfy V2 > (V1/V3) x N2/N1.
7. The driving device according to claim 3, wherein the coils are three-phase coils connected in a Y connection or a delta connection,
wherein the first connection state is a state where the three-phase coils have serially-connected coil portions in a first number for each phase, and
wherein the second connection state is a state where the three-phase coils have parallely-connected coil portions in the first number for each phase.
8. The driving device according to claim 7, wherein the first
number is expressed as "n";
wherein a first rotation speed Nl and a second rotation speed N2 of the motor satisfy N2/N1 > n;
wherein when a rotation speed of the motor is the first rotation speed Nl, the connection switching unit sets the connection state of the coils to the first connection state; and
wherein when the rotation speed of the motor is the second rotation speed N2, the connection switching unit sets the

connection state of the coils to the second connection state.
9. The driving device according to claim 8, wherein the first bus voltage VI, the second bus voltage V2, the first rotation speed Nl and the second rotation speed N2 satisfy V2 > (Vl/n) x N2/N1.
10. The driving device according to any one of claims 5, 6, 8 and 9, wherein the motor is used in an air conditioner;
wherein the first rotation speed Nl corresponds to an intermediate heating condition for the air conditioner; and
wherein the second rotation speed N2 corresponds to a rated heating condition for the air conditioner.
11. The driving device according to any one of claims 5, 6, 8,
9 and 10, wherein the motor is used in an air conditioner;
wherein the first rotation speed Nl corresponds to a condition that yields a highest ratio in an annual performance factor (APF) for the air conditioner;
wherein the second rotation speed N2 corresponds to a condition that yields a second highest ratio in the annual performance factor for the air conditioner.
12. The driving device according to any one of claims 3 to 11,
further comprising a power source to supply an AC power supply
voltage to the converter;
wherein the first bus voltage VI is the same as V2 times an effective value of the power supply voltage.
13. The driving device according to any one of claims 2 to 12,
wherein the connection switching unit switches the connection
state of the motor to the first connection state in a case where
a rotation speed of the motor is less than or equal to a threshold,
and switches the connection state of the motor to the second

connection state in a case where the rotation speed of the motor is greater than the threshold.
14. The driving device according to any one of claims 1 to 13, wherein the inverter performs field-weakening control in accordance with a rotation speed of the motor.
15. The driving device according to any one of claims 1 to 14, wherein the converter has a SiC element or a GaN element.
16. An air conditioner comprising:
a motor having coils;
a compressor driven by the motor; and
a driving device to drive the motor,
wherein the driving device comprises:
a converter to generate a bus voltage;
an inverter to convert the bus voltage to an AC voltage and supply the AC voltage to the coils; and
a connection switching unit to switch a connection state of the coils;
wherein the bus voltage generated by the converter is switched in accordance with the connection state of the coils.
17. A method for driving a motor having coils, the method using
a converter to generate a bus voltage and an inverter to convert
the bus voltage to an AC voltage and supply the AC voltage to the
coils, the method comprising the steps of:
switching a connection state of the coils, and switching the bus voltage in accordance with the connection state of the coils.