Variable speed drive control for chiller coast-through

1. A chiller system comprising:

a compressor, a condenser and an evaporator in fluid communication;

a motor connected to the compressor to drive the compressor;

a variable speed drive for powering the motor; and

a control panel configured to:

deactivate the variable speed drive in response to sensing that an input power to the chiller system is less than a predetermined threshold;

determine that the input power has been restored in response to sensing that the input power is greater than or equal to the predetermined threshold;

determine a motor rotation and motor rotational speed; and

in response to determining that the input power is restored and the motor is rotating in a forward direction, reactivate the variable speed drive.

2. The system of claim 1 , further comprising:

an oil heater and an oil pump system for circulating oil in the compressor; and

an uninterruptible power system (UPS) configured to power at least the oil heater, the oil pump, and the control panel during an interval wherein the input power is less than the predetermined threshold.

3. The system of claim 1 , wherein the control panel is configured to reactivate the variable speed drive by:

setting an output command to the variable speed drive to a predetermined motor rotational speed;

setting an output frequency of the variable speed drive to approximately equal the motor rotational speed; and

increasing the output frequency of the variable speed drive and the motor rotational speed to approximately equal the predetermined motor rotational speed.

4. The system of claim 3 , wherein the control panel is further configured to brake the motor rotational speed to zero, and then accelerate the motor rotational speed in a forward rotational direction from zero to the predetermined motor rotational speed, in response to determining that the motor is rotating in the reverse rotation when the input power is restored.

5. The system of claim 1 , wherein the control panel is further configured to determine at least one chiller capacity control parameter at a point when the variable speed drive is deactivated, and maintain the at least one chiller capacity control parameter while the variable speed drive is deactivated.

6. The system of claim 5 , wherein the control panel is further configured to release the at least one chiller capacity control parameter and reset the at least one chiller capacity control parameter via a capacity control portion of the control panel.

7. The system of claim 1 , wherein the control panel is further configured to control the variable speed drive to maintain a predetermined DC bus voltage for up to 10 seconds when the input power is less than the predetermined threshold using stored energy in a capacitor of the variable speed drive.

8. The system of claim 3 , wherein the control panel is further configured to control the increase in the output frequency of the variable speed drive by gradually increasing the output frequency.

9. The system of claim 3 , wherein the predetermined motor rotational speed is equal to the motor rotational speed at which the motor was rotating prior to sensing that the input power is less than the predetermined threshold.

10. The system of claim 1 , wherein the predetermined threshold is between 40% and 98% of a rated input power of the chiller system.

11. A control system for a chiller system comprising:

a control panel, the control panel configured to:

deactivate a variable speed drive in response to sensing that an input power to the chiller system is less than a predetermined threshold;

determine that the input power is restored when the input power is greater than or equal to the predetermined threshold;

determine a motor rotation and motor rotational speed; and

in response to determining that the input power is restored and the motor is rotating in the forward direction:

set an output frequency of the variable speed drive to approximately equal the motor rotational speed; and

increase the output frequency of the variable speed drive and the motor rotational speed to approximately equal a predetermined motor rotational speed.

12. The system of claim 11 , wherein the control panel is further configured to brake the motor rotational speed to zero, and then accelerate the motor rotational speed in a forward rotational direction from zero to the predetermined motor rotational speed, in response to determining that the motor is rotating in the reverse rotation when the input power is restored.

13. The system of claim 11 , wherein the control panel is configured to sense whether the input power is less than the predetermined threshold by sensing an input voltage and comparing the input voltage to a predetermined threshold voltage to determine whether the input voltage is less than the predetermined threshold voltage.

14. The system of claim 1 , wherein the control panel is configured to:

determine the motor rotation and the motor rotational speed by measuring a voltage and a phase of a first current at terminals of the motor while the variable speed drive is deactivated; and

in response to determining that the input power is restored and the motor is rotating in the forward direction, reactivate the variable speed drive to apply a second current having the voltage and the phase to the terminals of the motor.

15. A centrifugal chiller system comprising:

a motor configured to drive a centrifugal compressor of the centrifugal chiller system;

a variable speed drive coupled to terminals of the motor and configured to supply power to the motor; and

a control panel configured to:

measure an input power to the centrifugal chiller system;

in response to determining that the input power is less than a predetermined threshold, signal the variable speed drive to deactivate the motor by removing a first current supplied to the terminals of the motor;

in response to determining that the input power has been restored to greater than or equal to the predetermined threshold, measure a voltage and a phase of a second current at the terminals of the motor and determine a motor rotational direction; and

in response to determining a forward motor rotational direction when the input power has been restored, signal the variable speed drive to apply a third current having the measured voltage and the measured phase to the terminals of the motor to reactivate the motor.

16. The system of claim 15 , wherein the control panel is configured to:

in response to determining a reverse motor rotational direction when the input power is restored, brake the motor to a rotational speed of zero and then reactivate the variable speed drive to apply a fourth current to the terminals of the motor to accelerate the motor in the forward motor rotation direction.