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A look at the newest diagnostic tool for hybrid repair: The milliohmmeter

Thursday, February 1, 2018 - 09:00
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The digital multimeter (DMM) has been around for quite some time and is an invaluable tool for performing a variety of tests including available voltage, voltage drop, resistance, diode tests and more. With advancements in hybrid and electric vehicle technology, new testing equipment may now be required to perform advanced tests outside of the capabilities of the traditional DMM. The megohmmeter and milliohmmeter are two of the tools that you are most likely to see when diagnosing faults within the windings of motor generator units and the high-resistance, high-voltage wiring that carry the current for the motor generators. New tooling brings us to a familiar crossroad of the questioning of our diagnostic strategies and whether or not the hybrid repair business is one in which we want to venture. We will look at the ins and outs of this new tooling and help build a case for intelligent diagnostics. 

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(Image courtesy of Toyota Media) Diagnosing faults in the windings of a motor generator will require the use of a megohmmeter and a milliohmmeter.

Before we dive into specific tooling it may be helpful to refresh our knowledge of the units of measurement for resistance. The base unit of measurement is the Ohm, named for Georg Ohm, a German scientist who discovered and named the principle of resistance. Units of measure smaller than an ohm include milliohms and micro-ohms. Units of measure larger than the ohm are the kilohm, the megohm and the gigohm. A megohm is equivalent to 1,000,000 ohms while the base unit ohm is equivalent to 1,000 milliohms.  

The megohmmeter  

The megohmmeter, often known as an insulation meter, functions in a very similar fashion to the resistance or diode check feature on your traditional DMM. These features utilize a current that is sent through the leads to determine a voltage drop which is then displayed as a unit of resistance measured in Ohms. We have been taught that using the resistance function on the DMM does not give us an accurate indication of a circuit’s integrity due to the fact that we are not testing the circuit dynamically. However, that notion is misleading. We are testing dynamically but the meter does not produce enough voltage or current to truly load the circuit. This is why voltage drop testing a live circuit is so effective because the current within the live circuit Is providing the “dynamic” portion of the test. The DMM is simply reading the results. The lack of voltage and current is the reason that there are two different settings for resistance measurements and diodes to begin with. The resistance function utilizes approximately 2-3 volts and relatively low current. This amount of current is not enough to properly forward bias a diode while yielding an accurate voltage drop measurement.

(Image courtest of Fluke) The Fluke 1587 is a popular megohmmeter for hybrid insulation testing.

In plain English, when using the Ohms function to check a diode your resistance measurement will most likely be inaccurate. You will be able to see that the diode passes current in one direction and block it in the reverse polarity but the actual voltage drop reading displayed on the meter would be wrong. The diode test function provides enough voltage and current to forward bias the diode and will yield an accurate voltage drop measurement. When it comes to testing the windings of an electric motor such as those found in hybrid electric, electric and fuel cell vehicles the DMM simply does not have the ability to reach the voltage or current required to “dynamically” test the windings.  Additionally, using a voltage drop test for this type of circuit is dangerous and should be avoided at all costs. The megohmmeter allows the user to test these high voltage components in an isolated environment while the high voltage system within the vehicle is disconnected. Think of these tests as you would a pressure test of a hydraulic circuit. If you were to pressure test a hydraulic circuit you would most likely pressurize the system in excess of the typical system pressure in order to determine if there are leaks in the system. The insulation of high voltage systems is very similar to that of a hydraulic system and insulation of the wire is very much like the hose or pipe that carries the hydraulic pressure. If the pressure compromises the insulation, a faulty reading will be displayed on the meter. For the sake of your safety, do not attempt to test these circuits while the high voltage system is operational. Follow manufacturer instructions and perform your testing accordingly.  

Diagnosing with a meg or milliohmmeter requires the component or harness to be isolated from the HV circuit.
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