Speed control of variable-speed multiple-phase motors

1. A method of controlling a variable-speed motor, the method comprising:

defining a control duration as a predetermined number of cycles of a multiple-phase power supply;

defining each speed in a range of speeds by a corresponding number of the cycles of the control duration; and

providing power to a motor from the multiple-phase power supply at a selected one of the speeds, by enabling an input from the multiple-phase power supply for the number of the cycles of the control duration corresponding to the selected speed;

wherein:

the method is performed to control motors of first and second blowers, the first blower positioned above the second blower inside a drying unit in which a condenser is positioned between the first and second blowers and a damper is configured to admit air into the drying unit; and

the method further comprises:

alternating operation of the first or second blowers to alternate a direction of circulation in the drying unit; and/or

substantially simultaneously operating one of the first or second blowers in a forward direction and the other one of the first and second blowers in a reverse direction.

2. The method of claim 1 , wherein enabling the input from the multiple-phase power supply comprises driving a signal to switch the input from lines of the multiple-phase power supply between “on” and “off” states at zero-crossing points of the cycles of the multiple-phase power supply.

3. The method of claim 2 , further comprising, for the control duration, causing the input from the lines to be in the “on” state only for the number of the cycles of the control duration corresponding to the selected speed.

4. The method of claim 1 , wherein:

the cycles are full-wave cycles; and/or

the control duration is defined as eight cycles of the power supply; and/or

the method further comprises determining the duration of the cycles of the power supply; and/or

the motor is included in a blower.

5. A method of controlling a variable-speed motor, the method comprising:

using a frequency of a multiple-phase power supply to define a control duration having a fixed number of cycles;

defining each of a plurality of speeds as a corresponding number of the cycles of the control duration; and

providing power to a motor from the multi-phase power supply at a selected one of the plurality of speeds, by driving a signal configured in accordance with the frequency to enable an input from lines of the multi-phase power supply for the number of cycles of the control duration corresponding to the selected speed;

wherein:

the method is performed to control motors of first and second blowers, the first blower positioned above the second blower inside a drying unit in which a condenser is positioned between the first and second blowers and a damper is configured to admit air into the drying unit; and

the method further comprises:

alternating operation of the first or second blowers to alternate a direction of circulation in the drying unit; and/or

substantially simultaneously operating one of the first or second blowers in a forward direction and the other one of the first or second blowers in a reverse direction.

6. The method of claim 5 , wherein:

the cycles are full-wave cycles; and/or

the control duration is defined as eight cycles of the power supply; and/or

the method further comprises determining the duration of the cycles of the power supply; and/or

the motor is included in a blower.

7. A tobacco curing barn including at least one blower having a motor controlled by a system for controlling a variable-speed motor, the system comprising:

a controller configured to provide a driver signal configured in accordance with a frequency of a multiple-line, multiple-phase power supply to a motor; and

an interface circuit between an input and load of each line, each interface circuit configured to detect zero crossings in the corresponding line, and to enable or disable power through the corresponding line at the zero crossings based on the driver signal;

wherein:

the at least one blower comprises first and second blowers, the first blower positioned above the second blower inside a drying unit in which a condenser is positioned between the first and second blowers and a damper is configured to admit air into the drying unit; and

the system is operable for controlling motors of the first and second blowers, including:

alternating operation of the first or second blowers to alternate a direction of circulation in the drying unit; and/or

substantially simultaneously operating one of the first or second blowers in a forward direction and the other one of the first or second blowers in a reverse direction.

8. The tobacco curing barn of claim 7 , wherein the controller is further configured to provide power to the motor at each of a plurality of speeds, each speed defined by a corresponding number of cycles of a control duration of the driver signal.

9. The tobacco curing barn of claim 7 , further comprising a bidirectional switch between the input and load of the corresponding line, the interface circuit further configured to enable or disable power through the corresponding line at the zero crossings by triggering the bidirectional switch.

10. The tobacco curing barn of claim 8 , wherein:

the control duration comprises eight cycles; and

the speeds range from a zero speed up to and including a full speed defined by the eight cycles of the control duration.