Commissioning VSDs

The purpose of commissioning

The main purpose of commissioning VSDs is to ensure the following:

• The AC converter and motor have been correctly installed and meet the wiring and safety standards such as AS 3000.

• The power and motor cables are correctly sized, installed, and terminated.

• All power cable shields have been correctly earthed at both ends to the PE terminal at the converter, at the motor, and at the DB or MCC.

• The control cables have been installed according to the control system design.

• All control cable shields have been correctly earthed at one end only, preferably at the process control system end ('cleaner' earth).

• There are no faults on the cables prior to energizing for the first time.

Correct application settings selection

Once all the basic checks have been completed, along with the commissioning test sheet, the VSD is ready for energizing.

It’s recommended that when energizing the converter for the first time, the motor cables should be disconnected until all the basic parameter settings have been installed into the converter. This will avoid problems like starting the motor in the wrong direction, starting with a high acceleration time, etc. There is no danger in running a PWM converter with the output side in a completely open circuit. Once all the initial settings and on-load checks have been completed, the motor cable is insulation-tested and connected for the final on-load commissioning tests.

Correct parameter settings selection

A VSD will only perform correctly, if the basic parameters have been correctly set, to suit the particular application. The following are the basic parameters that must be checked, before the VSD is connected to a mechanical load:

• The correct base voltage must be selected for the supply voltage and to suit the electric motor connected to the output. In Australia, this standard voltage is usually 415 V, three phase. This will ensure that the correct output Volts/Hz ratio is presented to the motor.

• The correct base frequency must be selected for the supply voltage and to suit the electric motor connected to the output. In Australia, this standard frequency is usually 50 Hz. This will ensure that the correct output Volts/Hz ratio is presented to the motor.

• The connections to the cooling fan should be checked to ensure that the correct tap on the transformer has been selected.

Thereafter, the remaining parameter settings can be selected as follows:

• Maximum speed: Usually set to 50 Hz, but often set to a higher speed to suit the application. Reference should be made to Section 6 to ensure that the maximum speed does not take the drive beyond the loadability limit.

• Minimum speed: Usually 0 Hz for a pump or fan drive, but often set at a higher speed to suit constant torque applications. Reference should be made to Section 6 to ensure that the minimum speed does not take the drive below the loadability limit.

• Rated current of the motor: This depends on the size of the motor relative to the rating of the converter. The current rating of the converter should always be equal to, or higher than, the motor rating. For adequate protection of the motor, the correct current rating should be chosen.

• Current limit: Determines the starting torque of the motor. If a high breakaway torque is expected, a setting of up to 150% will provide the highest starting torque.

• Acceleration time: Determines the ramp-up time from zero to maximum speed. This should be chosen in relation to the inertia of the mechanical load and the type of application. For example, in a pumping application, the acceleration time should be slow enough to prevent water hammer in the pipes.

• Deceleration time: Determines the ramp-down time from maximum speed to zero. This setting is only applicable if the 'ramp to stop' option is selected.

Other alternatives are usually 'coast to stop' and 'DC braking'. On high inertia loads, this should not be set too short. If the deceleration time is below the natural rundown time of the load, the DC voltage will rise to a high level and could result in an unexpected tripping on 'over-voltage'. The deceleration time can only be shorter than the natural rundown time if a dynamic braking resistor has been fitted.

• Starting torque boost: Can be selected if the load exhibits a high breakaway torque. This feature should be used cautiously, to prevent over-fluxing of the motor at low speeds. Too high a setting can result in motor overheating. Only sufficient torque boost should be selected, to ensure that the VSD exceeds the breakaway torque of the load during starting.

There are many settings commonly required on modern digital VSDs. The above are the most important and must be checked before starting. The remaining parameters usually have a 'default' setting which will probably be adequate for most applications.

However, these should be checked and adjusted for optimum operation.

This section has detailed installation and commissioning guidelines for VSD for AC motors. If these guidelines are followed in all aspects, then it will definitely help during installation, commissioning, and troubleshooting of VSDs..