Understanding Variable Speed Drives (Part 4)

Jun 1, 1999 12:00 PM, Turkel, Solomon S.

A carefully organized start-up procedure assures better equipment operation and system reliability while greatly simplifying future troubleshooting.

Proper start-up procedures for variable frequency drives (VFDs) are of paramount importance to the ultimate success and effective operation of the VFD installation. In fact, successful installation of nearly any electrical equipment or system benefits from an orderly, well-planned start-up process similar to the procedure presented here.

In addition to a fast and smooth start-up, other major benefits of an organized start-up procedure are as follows.

* A better understanding of all component and system functions.

* Documentation of data for warranty purposes.

* Accumulation of baseline information and values to speed future troubleshooting and proper maintenance.

Of course, during initial design stages and long before start-up, you'll have developed a good understanding of the application and its variations to assure the best possible performance of the load. Also by this time, you'll have a good knowledge of the VFD's features and adjustments for fine tuning it to the application.

Prior to start-up

First, read and follow all caution notes and warnings provided by the VFD manufacturer. These will be found in the equipment manual as well as labeled on the VFD equipment itself.

Next, read the entire VFD manual and highlight the features and adjustments you expect to use. Pay particular attention to connection terminals for power and control and locate these within the VFD control enclosure.

Important note. Before energizing the VFD make a physical inspection of it and look for the following.

* Any moisture or debris (metal shavings for example) inside the equipment.

* Damage or dents to the enclosure, damaged or loose components and wires, and disconnected terminal connectors.

* Possible restrictions to air flow at the cooling fans or heat sink.

* Unremoved shipping blocks or tapes at power contactors, relays, etc.

In addition to the VFD itself, you should also make a visual inspection of the entire system, including motors, disconnect switches, circuit breakers, controls, load components, control devices (limit, float, pressure switches), etc.

Finally, you should make an intense and thorough check of the following items.

* Connections (line, load, and ground).

* Motor (horsepower, full-load amps, voltage, and rotation).

* VFD (input/output voltages, maximum output current).

* Protective devices (circuit breaker, fuses, overloads, thermal devices).

* Disconnects (are they in place and sized correctly).

* Incoming line power voltage measurements to the VFD (A-B phase, B-C phase, C-A phase).

Start-up guide sheet/report

You should use a VFD start-up guide sheet/report in your start-up procedure. A sample of a report is on page 62. This document verifies all parameters prior to power up; documents the installation for warranty claims; and aids in troubleshooting for future problems. Even if you are not doing start-up, you should require this report from whomever performs the start-up.

Start-up instruments

To help you have a start-up that is effective and efficient, we recommended that you have available the following instruments.

* True rms multimeter capable of reading AC/DC voltages up to 750V.

* True rms clamp-on ammeter capable of reading the VFD's maximum current output.

* Photo tachometer to verify shaft output speed at load.

* Current/voltage signal generator to generate a reference analog signal to VFD (4-20mA or 0-5V). (This is extremely useful on HVAC applications where the building automation system designed to control the VFD is not ready at time of start-up.)

* Oscilloscope to check wave shapes of VFD output to motor. These wave shapes can be compared to those provided in the start-up manual, or recorded (via Polaroid camera) for future comparison during troubleshooting or maintenance. The scope also can be used to check volts/hertz ratio.

Make up a complete final check, via a check-off list, of all electrical and mechanical components to be sure that they are set correctly. This includes valves, dampers, limit switches, steady-state voltage and current values, etc.

Station people at key locations (motor, controller panel, load(s), etc).

A proper start-up can be considered complete only when the VFD is operated at full load. This is important because you then can make meaningful drive adjustments. You can verify this by actually checking the FLA and comparing the value to that on the motor nameplate.

When the start-up command is given, watch, listen, and smell for anything unusual. Once start-up has been accomplished, allow the system to run a few hours before taking test readings for future comparison.

Solomon S. Turkel is Senior Instructor and Course Author for ATMS (Advanced Technologies Marketing and Services) Inc. Baltimore, Md.