How to use the Electrical Troubleshooter

This is your opportunity to practice the electrical diagnostic methods that this book has described.
A series of simulated faults have been created and you must use the test meter to diagnose and report the fault in each case. You must determine the fault from the meter readings since no other visual clues are provided on the schematics. Refer to the equipment nameplate just below the schematic for information such as rated voltage and rated RLA and LRA. AFTER you have determined the problem, submit your diagnosis in the chart at the bottom of the page to see if you are correct.

Measuring Amperage

To measure amperage, first you must put the meter into amperage mode by clicking the "A" button on the meter. Then click on any test point on the schematic. The amperage at that point will be displayed in the meter. The example above shows that the EFM is drawing 0.9 amps.

Measuring Voltage

To measure voltage, first you must put the meter into voltage mode by clicking the "V" button on the meter. Notice that the number 2 test point on the schematic automatically turns green indicating that it is selected. You require 2 test points to measure voltage potential between them and the number 2 test point (which is Neutral) is the appropriate place for one of the test leads. You must select any other test point and the voltage between the 2 points will be displayed in the meter. The example above displays that there is a 122 VAC potential between test point 2 and test point 3. In other words there is proper voltage to the EFM. If this is unclear review hop scotch troubleshooting techniques

Measuring Ohms

To measure ohms, first you must put the meter into ohms mode by clicking the button on the meter. Then click on any 2 test points on the schematic. The resistance between those 2 points will be displayed in the meter. The example above shows that the resistance between test points 2 and 3 is 15 ohms. In other words the resistance of the EFM windings is 15 ohms. You would realize from this measurement that since the windings have a MR (Measureable Resistance) they are not burnt open. As to whether or not the EFM is seized, that would take an amperage reading or a physical inspection.
Note: It is assumed that the power to the equipment is off when ever you place the meter in ohms mode. If you try to measure the resistance of a live circuit the fuse in the ohmeter will blow. It is further assumed that the component being measured has been isolated from the rest of the circuitry to elimimate unwanted parallel and series loads (resistances). If you wish to measure the resistance of a component just select the two closest test points. Otherwise, if you select points with many resistances between them you will be alerted that there are too many resistances to be meaningful.

On the other hand, if there is a direct circuit between 2 ohmeter test points then any parallel resistance loads that exist will become a non issue. This is because electricity always takes the route of least resistance. So sometimes the meter will display 0 ohms between 2 test points that are far apart because a closed circuit with no loads connects them. Or, if there is only a single load between 2 test points, even if they are far from each other, the measureable resistance will be reported. With a real meter, the actual resistance of a complicated circuit with many parallel and/or series resistances would be displayed. Since that type of data is unlikely to be of any value when troubleshooting, the virtual meter has been programmed to display an alert message under those circumstances rather than confuse the issue.

Helpful Facts

  • The FLA rating of a motor is not the actual amperage draw you should expect to see being drawn. For example, an actual amp reading might only be 75% of the rated FLA. A certain amount of amperage will be used just to make a motor operate with no loads being applied (such as moving air with a fan or refrigerant with pistons). When loads are applied the amperage draw will increase. The manufacturers FLA rating is the amount of amperage that a motor has been designed to sustain indefinitely. A motor subjected to amperage draws above this nameplate value are expected to fail. Motors are selected to be capable of matching the anticipated loads without going over the FLA rating. If a motor draws over the FLA rating with no loads applied there is obviously a problem with the motor. If a motor draws over the FLA rating with loads applied there may be a problem with the motor or the loads may be too excessive. A seized motor will by definition draw locked rotor amperage.

  • A seized compressor will draw LRA indefinitely or until an overload or breaker trips. A perfectly good compressor can do the exact same thing if there are certain start gear problems.

  • Electrical equipment is normally designed to be operated within 10% of the nameplate voltage rating. Therefore if a motor is rated for 115 volts, 10% plus or minus would give it a safe operating voltage range of from 104 to 126 VAC. Two common single phase voltages are 208 VAC and 230 VAC. Some equipment has been designed to operate with either voltage without having to re-configure the way it is wired. So a dual voltage 208/230 VAC rated apparatus could safely operate over the range from 10% less than 208 VAC up to 10% more than 230 VAC. In other words, it's acceptable operating range would be from 188 VAC to 253 VAC. Some motors can be re-configured to operate on 115 VAC or 230 VAC. The nameplate will show how to re-configure the wiring to allow this. Be sure you understand the difference between an apparatus that must be re-configured in order to be operated on a different voltage and an apparatus that that comes with a wider acceptable voltage range.

  • An open circuit has infinite resistance. A digital Ohm meter displays an open circuit with "0.L".

  • Measuring the voltage potential across an open circuit gives you the same reading as the voltage drop across a load such as motor windings. That's because the resistance of the windings have been designed to cause the specific voltage drop that the motor is designed to operate with. So you can not tell if windings or loads are open with a volt meter. Amps and/or ohm readings are required to determine if loads are open circuits or not.