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Field maintenance electricians and electronics bench technicians: Shared, standardized test procedures can improve understanding and performance

By Ron Auvil

January 2012

A bench test technician can check a variable speed drive

A bench test technician can check a variable speed drive while working with a field technician. Here a wireless Fluke 233 Remote Display Multimeter makes the job easier.

One of the hats that I wear is that of a “Performance Consultant” for different facilities. In that role I evaluate why problems are not corrected more quickly and why staff are not able to perform their jobs more effectively. My years of field technical experience combined with a training background help me put together a creative plan to address these deficiencies.

A common problem in many facilities is the divide between the field maintenance electricians and the electronics bench technicians. The field techs are called when a piece of equipment goes down. They troubleshoot the equipment in the field and then decide the best course of action. They replace the electronics and return the component to the electronics bench technicians. The electronics bench technicians perform the troubleshooting and repair of the electronics board.

Depending on the facility, the field technicians may know little about proper troubleshooting and repair of the electronics. In the same vein, the bench technicians sometimes do not know anything about what the field technicians do at the equipment level. It is worth noting that in many facilities the electronics boards are returned to the manufacturer for repair.

In general, the problems can be categorized as follows:

  1. Despite their best efforts, the field technicians were not equipped with the knowledge or testing equipment to perform field level troubleshooting.
  2. They simply replaced the main electronics board if it seemed to be the problem.
  3. The electronics technicians often found that the electronics board or other components were functioning properly. The actual problem was somewhere else in the system. This caused higher replacement and restocking fees.
  4. The two groups of technicians communicated very little and did not share information. In fact they had two different reporting structures and did not share any common goals.

A suggested solution was to equip the technicians with the training and equipment needed to perform adequate field troubleshooting of the equipment. The typical devices that needed troubleshooting were identified as power supplies, variable speed drives, building automation and PLC controller modules. High quality test instruments from Fluke should be purchased to equip the technicians. Then develop testing procedures, as described below.

Note that many problems occur under operating load conditions, where static testing procedures may not find any problems. For these situations more sophisticated troubleshooting or testing may be required with tools other than a digital multimeter or a clamp meter.

Power supply tests

Equipment and building power supply tests are some of the easiest to perform. This usually consists of the following:

  1. Check input current and voltage, preferably under a load. Usually this is done with a standard digital multimeter (DMM) or clamp meter.
  2. Check output current and voltage, preferably under a load. Getting accurate readings may require using a higher-end DMM that includes a special variable frequency drive (VFD) filter.
  3. When testing building power supply, be sure to use the appropriate personal protective equipment (PPE).

Variable speed drive tests

The vast majority of buildings today are equipped with variable speed drives (VSDs) for plant equipment ranging from fans and pumps to production equipment.

Visual test comes first

Before testing individual components, perform a thorough visual inspection. In 25 to 50 percent of failures the problem will be found here. Before performing this test, stop the drive and follow lock out/tag out. Allow the bus capacitors to discharge. Use a DMM to ensure that no voltage is present. Visually inspect the inside of the drive. Check dc bus capacitors for overheating, scorching, or fire. The dc bus capacitors may appear swollen due to overheating. Inspect the drive cooling fans as well. They must rotate freely and be free of any noise. The fan must also turn when power is applied this depends on the setting via the software. Sometimes fans switch on when a certain temperature is reached.

Initial test

Perform the initial test before any additional tests are done. Assuming you can power it down or take it offline, stop the drive using the keypad and then start up from 0%. Check the drive and load at 10%, 20%, and so forth, all the way to 100%. Check the programming to make certain that the proper parameters are entered. Check the load for proper speed and rotation. Monitor voltage and amperage at each phase during the ramp-up of the motor, using the VSD display. Note that the motor and the VSD display are not together, so you need another technician or wireless testing equipment to make the measurements. Proper safety procedures must be followed at all times.

Component testing

The electric motor drive component tests identify which components in a VSD are defective. The components to test are the dc bus capacitors, bus capacitor balancing resistors, cooling fans, converter semiconductors, and inverter semiconductors. The converter and inverter test uses the diode test mode of a digital multimeter. A good semiconductor reads about 0.8 vdc to 0.8 V dc when forward biased and OL (a reading of OL, or overload, means an open circuit or break in the electrical path) when reversed biased. A shorted semiconductor reads OL in both directions. Here are some notes about performing this test:

  1. Remove line and load conductors from the drive terminal strips to isolate the drive.
  2. Perform the tests at the power terminal strips of the drive.
  3. Since each drive manufacturer is different, specific testing procedures are different as well.
  4. The dc bus capacitors must be completely discharged before they can be tested. They can be dangerous before a proper discharge. Use a multimeter that has a capacitance test function to check the values of the capacitors.
  5. A DMM will charge the dc bus capacitors. This requires a few seconds, and then OL should appear. Some DMMs are incapable of charging large capacitors. Always consult the manufacturer in these cases.
  6. If semiconductors are paralleled in the inverter or converter sections of a VSD, tests may indicate that a group of semiconductors are bad, but not necessarily which specific component. In this instance the entire section is removed from the drive and replaced. This will usually be performed by removing the screws from the board that contains the components, removing any cables that attach it to other sections, then replacing them in reverse order.

Converter Section

Test the converter section first. A general procedure is as follows:
  1. Verify the disconnect is OFF and locked out/tagged out.
  2. Test the diodes using a test matrix, which is provided by the manufacturer. It gives the correct reading for each diode.
  3. If a diode does not match the matrix it must be replaced.
  4. A diode that reads either 0 V dc or OL when either forward or reversed biased must be replaced.

Inverter Section

The inverter section usually consists of insulated-gate bipolar transistors (IGBTs).
  1. Verify the disconnect is OFF and locked out/tagged out.
  2. Similar to the inverter section, test the IGBTs using a test matrix provided by the manufacturer. It gives the correct reading for each IGBT.
  3. If an IGBT does not match the matrix it must be replaced.
  4. A diode that reads either 0vdc or OL when either forward or reversed biased must be replaced.
An electronic board

The field technician should check an electronic board before returning it to the electronics shop.

Building control systems

Many commercial buildings utilize a building automation system. A malfunction may occur that causes the HVAC or other equipment to operate improperly. Proper diagnosis and testing of these systems is important. The parts that can be tested include communications, inputs, and outputs.

Communications

While there are many different types of communications protocols, and all can be tested, many building control systems use RS-485 (also known as EIA-485 or TIA/EIA-485). Many RS-485 systems use a 3 wire communication bus that can be tested with a digital multimeter. RS-485 may consist of a positive, a negative, and a reference voltage. Typical voltages are as follows:

  1. RS-485 Bus signal between + and REF is 2.45 to 2.98 V dc
  2. RS-485 Bus signal between - and REF is 2.06 to 2.54 V dc

Often RS-485 problems may be due to duplicate addressing and/or wiring problems. The physical bus wire can be located at a controller and tested there. Broken segments between controllers can be located in this way.

Analog inputs

Analog inputs are usually temperature, pressure, or humidity sensors. Most building systems utilize resistance, current, or voltage sensor inputs. The reading at the sensor is represented by a signal at the controller. Most current sensors use a 0 to 20 mA or 4 to 20 mA sensor range. Milliamp process clamp meters, such as the Fluke 771 mA Clamp Meter, can measure mA signals without breaking the loop. Other sensors used include resistance and voltage types, 1000 or 10K ohm temperature sensors are common, along with 0-10 V dc sensors. These can be easily tested with a DMM.

Outputs (TRIACs)

The vast majority of building control systems today use Triodes for Alternating Current (TRIACs) as outputs. They can be thought of as solid state switches, with no moving parts.

An oscilloscope should be used when testing TRIACs on a test bench. In the field a digital multimeter using the ohmmeter function can be used to make an initial rough test with the TRIAC out of the circuit. Also, they should be tested under normal operating conditions. As always, PPE should be worn and all safety precautions followed. Here is an example of a TRIAC testing procedure:

  1. Set the function switch of the ohmmeter to a range of at least 100k ohms. Connect the leads to the meter jacks.
  2. Connect the negative test lead to main terminal 1 (MT1) of the TRIAC.
  3. Connect the positive lead to main terminal 2 (MT2) of the TRIAC. The ohmmeter should read infinity.
  4. Short circuit the gate to MT2 using a jumper wire. The ohmmeter should read almost zero ohms. The reading should be the same if the jumper wire is removed.
  5. Reverse the test leads from Step #2 on MT1 and MT2.
  6. Short circuit the gate to MT2 using a jumper wire. The ohmmeter should read almost zero ohms. The reading should be the same if the jumper wire is removed.
  7. Remove the ohmmeter from the TRIAC.

Beyond testing

In addition to cross-training on the test procedures listed above, it was also recommended that the job duties be changed so that the two groups of technicians spend one day per month working with each other to improve performance.

It was also recommended that the productivity goals of the two groups be aligned so that they complement each other.

By implementing some of the ideas above, the two groups of technicians should work more smoothly together and increase overall productivity while lowering costs.