What to check when commissioning variable speed drives

The purpose of commissioning VSDs

The main purpose of commissioning VSDs is to ensure that:

1. The AC converter and motor have been correctly installed and meet the wiring and safety standards such as IEC 60364 or Australian standard AS 3000.
2. The power and motor cables are correctly sized, installed and terminated.
3. 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.
4. The control cables have been installed according to the control system design.
5. All control cable shields have been correctly earthed at one end only, preferably at the process control system end (‘cleaner’ earth).
6. There are no faults on the cables prior to energization for the first time.

Selecting the correct application settings

Once all the basic checks have been completed and the commissioning test sheet completed, the VSD is ready for energization.

This will avoid problems with starting the motor in the wrong direction, starting with an acceleration time which is too fast, etc. There is no danger in running a PWM converter with the output side completely open circuit.

Once all the initial settings and on-load checks have been completed, the motor cable can be insulation tested and connected for the final on-load commissioning tests.

Selecting the correct parameter settings

A variable speed drive will only perform correctly if the basic parameters have been correctly set to suit the particular application.

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

1. Maximum speed

Maximum speed, usually set to 50 Hz, but often set to a higher speed to suit the application. Ensure that the maximum speed does not take the drive beyond the loadability limit.

2. Minimum speed

Minimum speed, usually 0 Hz for a pump or fan drive, but often set at a higher speed to suit constant torque applications. Ensure that the minimum speed does not take the drive below the loadability limit.

3. Rated current of the motor

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.

4. Current limit

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.

5. Acceleration time

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.

6. Deceleration time

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’.

The deceleration time can only be shorter than the natural rundown time if a dynamic braking resistor has been fitted.

7. Starting torque boost

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 over-heating.

Only sufficient torque boost should be selected to ensure that the VSD exceeds the breakaway torque of the load during starting.

Reference // Practical Variable Speed Drives and Power Electronics – Malcolm Barnes (Get hardcopy from Amazon)