Automatic variable speed motor drive bypass

1. A heating, ventilation and air-conditioning system, comprising:

a variable speed motor drive configured to receive line power and output modulated power;

a blower motor configured to produce airflow in response to said modulated power;

a unit controller configured to bypass said variable speed motor drive to provide said line power directly to said blower motor in the event that a measured airflow is less than a predetermined value; and

a blower-proving switch configured to output an airflow-proving signal to said controller indicative of said measured airflow.

2. The system as recited in claim 1 , wherein said unit controller is configured to discriminate between a failure of said variable speed motor drive and a blower system that includes said blower motor.

3. The system as recited in claim 1 , wherein said unit controller is configured to send an automated message when performing said bypass.

4. The system as recited in claim 1 , wherein said blower motor, variable speed motor drive and unit controller are located in a rooftop unit.

5. The system as recited in claim 1 , wherein said controller is configured to disable operation of said motor in the event that said measured airflow remains less than said a predetermined value after said bypass.

6. The system as recited in claim 1 , wherein said controller is further configured to perform said bypass after at least two successive cycles that include stopping operation of said HVAC system and waiting for the expiration of a reset period.

7. A heating, ventilating and air conditioning unit controller, comprising:

a processor configured to execute program instructions stored by a program memory;

a VFD bypass module defined by said program instructions, said bypass module being configured to control a contactor to bypass a variable speed motor drive in the event that said controller receives an airflow proving signal indicating airflow produced by a blower motor powered by said variable speed motor drive is less than a predetermined value;

wherein said airflow proving signal is produced by a blower proving switch configured to sense said airflow.

8. The unit controller as recited in claim 7 , wherein said VFD bypass module is configured to discriminate between a failure of said variable speed motor drive and a blower system that includes said blower motor.

9. The unit controller as recited in claim 7 , wherein said VFD bypass module is configured to send an electronic message when performing said bypass.

10. The unit controller as recited in claim 7 , wherein said processor is located in a rooftop unit.

11. The unit controller as recited in claim 7 , wherein said VFD bypass module is configured to disable operation of said motor in the event that said airflow remains less than said a predetermined value after said bypass.

12. The unit controller as recited in claim 7 , wherein said VFD bypass module is further configured to perform said bypass after at least two successive cycles that include stopping operation of said HVAC system and waiting for the expiration of a reset period.

13. A method of manufacturing a heating, ventilation and air-conditioning system, comprising:

connecting first and second contactors to power inputs of a blower motor;

connecting an output of a variable speed motor drive to said first contactor such that said first contactor is electrically connected between said variable speed motor drive and said blower motor;

configuring a unit controller to control said first and second contactors to bypass said variable speed motor drive to provide said line power directly to said blower motor in the event that a measured airflow produced by said blower motor is less than a predetermined value; and

configuring a blower motor proving switch to output an airflow proving signal to said controller indicative of said measured airflow.

14. The method as recited in claim 13 , further comprising configuring a blower proving switch to output an airflow proving signal to said controller indicative of said measured airflow.

15. The method as recited in claim 13 , further comprising configuring said unit controller to discriminate between a failure of said variable speed motor drive and a blower system that includes said blower motor.

16. The method as recited in claim 13 , further comprising configuring said unit controller to send an electronic message when performing said bypass.

17. The method as recited in claim 13 , wherein said blower motor, variable speed motor drive and unit controller are located in a rooftop unit.

18. The method as recited in claim 13 , further comprising configuring said controller to disable operation of said motor in the event that said measured airflow remains less than said a predetermined value after said bypass.