MT03A Milliohm and Motor Tester

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Steve Smith
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MT03A Milliohm and Motor Tester

Post by Steve Smith »

Following on from Liviu’s forum post and Ben’s thorough reply here topic23168.html I thought it would be good to add a brief overview of the functionality of the MT03A which has opened my eyes to milliohm testing.

Traditionally, I had not given too much consideration to milliohm (mΩ) values thanks to using volt drop techniques instead!

For example, when starting an engine, we discover the cranking speed is low or laboured and may choose to measure the volt drop across the starter motor ground cable. Once we hit above 0.25 V (e.g., 1.4 V drop) we know we have an increase in resistance resulting in low current flow

We use this technique in our PicoDiagnostics Battery test and demonstrate using PicoScope here viewtopic.php?p=101584#p101584

Given we capture the current through the entire starter motor circuit whilst simultaneously measuring the voltage across our ground lead, we can determine resistance using ohms law. It is here where I have not paid too much attention to mΩ as my focus has always been on the recorded volt drop value.

If you have ever wondered about the resistance of a starter motor ground cable, (I know I should get out more) here are some real-world good examples calculated using ohms law
Image 1A
Image 1A
The average of all the above equates to 1.070 mΩ which does not sound like any resistance whatsoever!

Roll onto the present day with EV’s that may well draw current at a constant rate of 100 A under certain driving conditions, suddenly, milliohms become more than relevant

Every connection/terminal has a resistance regardless of the material used and the contact tension within.

Let us image a high voltage battery bus bar with a contact resistance of 10 mΩ (0.01 Ω) instead of the desired 1 mΩ (0.001 Ω) transferring a current flow of 100 A.

Volt drop = A x R
With our 10 mΩ (0.01 Ω) contact resistance we have 100 A x 0.01 Ω = 1 V drop
With our 1 mΩ (0.001 Ω) contact resistance we have 100 A x 0.001 Ω = 0.1 V drop

Can you now image the heat generated around the bus bar contact when 400 A is flowing (4 V drop)

Factor in “heat” whereby our 10 mΩ resistance will increase in proportion to rising temperature and we really do have food for thought. (Not to mention other HV connections within this circuit on route to the 3-phase motor)

As you can see from the figures above, trying to measure resistance with milliohm precision is going to require precise control over the test conditions along with the supplied current and voltage beyond the realms of a conventional multimeter

This is where our MT03A comes into play using the “Kelvin sensing” 4-wire measurement technique

More information on Kelvin sensing can be found here ... connection.

Below is a diagram highlighting how the 4-wire measurement principle is applied to resistance measurements. Note how the current flow via the resistor under test is a function of the measurement device via test leads “Amp 1 & Amp 2”
Image 1
Image 1
With a known voltage & current flow, resistance is measured via test leads “Volt 1 & Volt 2” at the contact point either side of the resistor under test. (Akin to how we measure volt drop across our ground lead using the starter motor analogy above)

The MT03A utilises Kelvin crocodile clips whereby each jaw is insulated from the other. A current lead (Amp 1 above) is connected to one jaw and a voltage lead (Volt 1 above) to the other. In this scenario one crocodile clip incorporates 2 leads (Amp & Volt) so reducing the number of clips and leads required to make a 4-wire measurement

With MT03A, both Amp & Volt leads are bound into a single cable terminating in a Kelvin clip at the measurement point and 2 x 4 mm banana connectors at the measurement device

In addition to the above, the MT03A triplicates the above test lead/clip design in order to deliver a highly accurate 3-point resistance measurement solution (ideal for 3-phase motor windings)

See image below for connection method for 3-phase motor winding test
Image 2
Image 2
Note above how the MT03A incorporates a temperature sensor which connects to the rear of the tester via a 3.5 mm jack plug (See below)
Image 3
Image 3
Incorporating temperature of the resistor under test allows for accurate compensation of the resistance measurement. For example, manufacturers will specify the phase winding resistance of their motors at 20°C for copper windings.

Given resistance increases with temperature, if we are measuring above 20°C the non-compensated measured value could appear out of specification, resulting in the incorrect replacement of a 3-phase motor (This could include motor & transmission combined!)

The MT03A and combined software will display both the measured and compensated resistance value to prevent such an error highlighted above

MT03A Software

The software for the MT03A will be available for downloaded here once released, but in the meantime, the following video summarises what to expect when carrying out a 3-phase motor winding resistance check

Note during the video above how current is reversed through each winding in order to remove the effects of Thermal EMF that can be generated between contact points of dissimilar metals (i.e., the Kelvin clip to the point of measurement) An average resistance value is then obtained using the formula:
Resistance with forward current + Resistance with reverse current / 2

The screen shot below is an example of a good motor winding test where the bar graph and “Phase resistance balance” summary, confirm a maximum deviance (deviation) between phases of 0.2 mΩ which equates to 1.1%
Image 4
Image 4
Notice above how we have 2 resistance values for each phase winding, “measured” and “temperature compensated”

Given the motor temperate was 13.3°C at the time of measurement, and the reference temperature for the resistance of copper winding is stated at 20°C we apply the following compensation

Temperature coefficient of copper = 0.00393 @ 20°C
Copper has a positive temperature coefficient (PTC) where resistance increase with temperature
E.g., Copper wire resistance at 20°C = 1 Ω
What is the resistance at 22.7° C?
22.7°C – 20°C = 2.7°C (Temperature change)
Temperature change x Coefficient of Copper x Measured resistance (Ω) = Change in resistance
2.7°C x 0.00393 x 1 Ω = 0.010611 (this is 2.7°C above 20 °C so add to 1 Ω)
1 Ω + 0.010611 = 1.010611 Ω (This would be the resistance of our 1 Ω copper wire at 22.7°C)

Moving onto our test results for U to V phase above @ 19.2 mΩ (0.0192 Ω) at 13.3°C
What is the resistance value at 20°C? (Temperature compensation value)
Once again:
Temperature change x Coefficient of Copper x Measured resistance = Change in resistance
(20°C – 13.3°C) * 0.00393 * 0.0192 Ω = 0.000505 Ω
This value needs to be added to 0.0192 Ω measured at 13.3°C (Added due to PTC)
0.0192 Ω + 0.000505 Ω = 0.019705 Ω = 19.71 mΩ
19.71 mΩ is our temperature compensated resistance value for our winding measured at 13.3°C to display what the resistance value would have been if measured at 20°C

The displayed Deviance (Deviation) and percentage values are calculated as follows:

Deviance (Deviation)
Max value – Min value (We will use the temperature compensated value)
19.7 mΩ - 19.5 mΩ = 0.2 mΩ

Percentage maximum deviance
100 – (Min value (mΩ) / Max value (mΩ) x 100)
100 – (19.5 / 19.7 x 100)
100 – 98.98 = 1.1%

Customer copies of the test results can be saved using the “Print” function and “Microsoft print to PDF”
Image 5
Image 5
Below is the PDF print out from our initial motor test at 13.3°C
Image 6
Image 6
A word to the wise, like all tests, our results are only as good as our connection and with milliohm testing this has never been more relevant. Just think about continuity testing where we approve 0.5 Ω as being “acceptable” if we assume we have 0.1 Ω resistance in our test leads and point of contact

To now put 0.5 Ω into perspective, this is 500 mΩ and in the world of high current transfer this value is huge! I guess the message I am trying to get across is we need to think in and not Ω so we can grasp the enormity of low resistance (I hope that makes sense)

Moving onto Milliohm testing

As you will be aware from the title “Milliohm and Motor tester,” we can also use the MT03A to measure resistance across items such as earth boding leads and bus bars to name but a few applications.

Using your choice of 2 x Kelvin clips/leads from the MT03A we can accurately measured between any two points on a vehicle to obtain a highly accurate resistance value. This may be between a Hybrid transmission assembly and chassis ground, or, across a bus bar in the example below.
Image 7
Image 7
The real beauty of the MT03A is via USB connectivity only, we comfortably achieve the legislated test current required to carry our such tests as earth bonding: (See ECER100 extract below)

ECER100 Earth bond test specifications The resistance between all exposed conductive parts and the electrical chassis
shall be lower than 0.1 ohm when there is current flow of at least 0.2 amperes.

The MT03A meets these specifications (200 mA below 100 mΩ) whilst also continuing to deliver sufficient current up to a maximum resistance of 2 Ω (2 Ω, the maximum resistance value the MT03A can measure)

Below we are logging the resistance between our chosen test leads (U & V) to determine our bus bar resistance at 0.1 mΩ (0.0001 Ω or 100 µΩ)

Note in the image above how the Kelvin clips are attached to bolts passing through the eyelets of the bus bar. If I now slowly loosen one of the bolts, we can see how the resistance changes dramatically (This is ideal for wiggle testing)
Image 8
Image 8
The following video covers the 2 tests described above and help clarify the process of connection, set up and obtained results

Information on the purchase of the MT03A can be found here ... tester-kit

A further in-depth video testing a faulty EV motor beyond milliohm testing can be found here and will feature in an up-and-coming video case study in May 2023

I hope this helps, take care……Steve

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Re: MT03A Milliohm and Motor Tester

Post by ben.martins »

Motor winding resistance - Star or Delta?

When Pico introduced the MT03A Milliohm and Motor tester, it’s predominant role was to be used for testing the motor windings of 3 phase motors found in battery electric vehicles. With the exceptional quick setup and with a test completed in 60 seconds, it was clear that this device has uses outside of the automotive world and could be used in the majority of industries where 3 phase motors are found.

For those that have a MT03A or watched it in action you will have seen that the UI features a Star configuration for the motor windings. Typically this is the most common configuration used for automotive 3 phase motors but in other applications it is possible to find Delta wound motors. So what is the difference and how does it affect the measurement?

By no means am I an expert in industrial 3 phase electrical motors and the principles of operation, but following a recent support ticket there are some key points to look out for when measuring winding resistance based on the configuration.

There are typically two types of 3 phase motor winding Star, or Y, and Delta. Each have different properties allowing users to configure a motor to ensure the best efficiency for its application.
Delta winding.png
Delta winding.png (14.68 KiB) Viewed 4297 times
Star winding.png
A Delta configured motor will allow for higher torque and maximum current as the overall resistance of the windings is lower as we have resistors in parallel. Given the voltage in phase is the same as the line voltage, this means the phase can pull more current on startup. There is a downside though as this can cause extreme stress on the motor, any couplings and the drivetrain as well as a huge current inrush that could trip breakers or blow fuses. This is where many industrial motors will start up in a Star configuration, or soft start, before switching over to Delta for higher torque capabilities.

In comparison, Star configuration sees the line current being the same as the phase current but the phase voltage will be different to the line voltage. Due to windings now being in series, the overall resistance is higher which in turn leads to a lower current draw.

Going back to the Delta, if we wanted to measure the blue phase resistance in Delta, we can rearrange the phases to looks like this.
Delta in parallel.png
We can now see that as all phases are connected there is always a path through the motor. This is important and a reason why Delta wound motors are used in industrial settings as they have a higher reliability. If one phase went down, there are still two others that can work. When it comes to resistance measurement we now have to factor in parallel resistors. A general rule of thumb is that the resistance will always be lower than the lowest resistor in the circuit. Using the formulas we can see this in theory. First though we need the individual winding resistances which can be discovered if disconnecting all the phases so we have both ends of the coil open and measuring each individual winding. For example I have a motor that I’ve opened the coils and measured each winding as follows:

R1 = 171mΩ
R2 = 171mΩ
R3 = 169mΩ

If we work on the basic principle then the measured value when testing any of these phases is going to be under 169mΩ. Let's see if that works if we reconnect the phases in Delta configuration.

As we can simplify our phases to two resistance values we will use the following formula to calculate two resistors in parallel.

Rt = (R1 x R2) / (R1 + R2)

Where R2 will be adding two of the phases we need to add R2 and R3 together which we will refer to as R23.

R23 = R2 + R3 = 171 + 169 = 340mΩ

Now we have the values of the two resistors in parallel, we can use the formula to calculate the blue phase

Blue Phase = (R1 x R23) / (R1 + R23) = 58140 / 511 = 113.777mΩ

Doing the same for the other phases gets the following results:

Green Phase = (R2 x R13) / (R2 + R13) = 58140 / 511 = 113.777mΩ
Yellow Phase = (R3 x R12) / (R3 + R12) = 57798/ 511 = 113.108mΩ

Here we can see that the measured resistances are always going to be lower than the actual phase resistance. There is a slightly quicker way in that measured resistance should be ⅔ x winding resistance.

MT03A is measuring resistance and then compares the phases to create a balanced output. Assuming all phases are equal then we should be able to see if there is an odd one out. Below we have the results from the motor configured in Delta and apart from some connection allowances I believe it’s not far off. Please note that for the purposes of illustration the screenshots below do not completely reflect the software.
Delta resistance.png
So let’s compare the same motor but configure it to Star which MT03A is designed to be used.

When measuring a phase we are now including 2 windings rather than just one hence in the software we have UV, VW and WU.

The theory here is much simpler as there is only one route through the winding and with resistors in series we simply add them together. Using the individual windings from above we would expect to see the following

UV = R1 + R2 = 342mΩ
VW = R2 + R3 = 340mΩ
WU = R3 + R1 = 340mΩ

Star resistance.png
So, if you are working with motors which can be configured to either Star or Delta depending on the motor application, MT03A will still calculate the resistances between phases. With Delta though, even if you have a phase which is open, there is still that path through the windings to the kelvin clips so the software will proceed with the test. However, there will be an imbalance in resistance between the windings and so the software will highlight this as shown below.
Delta with open.png
If this was a star configuration and a winding was open, the kelvin clip precheck test will pick this up before the test is carried out.
Star with open.png
This is a very long winded way to say that MT03A can be used on both Star and Delta configured motors! Personally though, if the motor can be reconfigured I would make it into Star - mainly so that it matches the images in the software!

I hope this helps.

Steve Smith
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Pico Staff Member
Posts: 1633
Joined: Sun Aug 25, 2013 7:22 am

Re: MT03A Milliohm and Motor Tester

Post by Steve Smith »

Just a quick “heads up” to inform users of the MT03A Milliohm & Motor Tester here ... tester-kit the accompanying software has just been updated to display milliohm values to two decimal places
MT03A software version 1.0.2 can be found here
In the earlier posts above you will notice the previous version of our MT03A software displayed resistance values to 1 decimal point (See below)
Whereas in Ben’s “Star or Delta” forum post above we have two decimal places (See below)
Going forward, our MT03A software will display resistance values to two decimal places so delivering the specified resolution of 0.01 mΩ (10 µΩ or 0.00001 Ω)

I hope this helps, take care……Steve

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