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Non-bridge device for large motor winding resistance readings

Non-bridge device for large motor winding resistance readings

Non-bridge device for large motor winding resistance readings

(OP)
Historically we have used bridges for winding resistance readings.

Our metrology people tell me these are no longer available, and we are having problems finding parts to maintain the ones we have... so they want to find a new device for winding resistance readings.


I am looking at two options:

1 –DLRO such as Megger DLRO 10 shown here
http://www.megger.com/us/products/ProductDetails.php?ID=77&Description=

2 – Transformer "Ohmmeter" such as Megger MTO210 shown here:
http://www.megger.com/us/products/ProductDetails.php?ID=183&Description=

Both look to have accuracy which is more than sufficient. (I'm thinking we need something like 0.5% or 0.5 milliohms, whichever is smaller when reading windings in the range 50 – 500 milliohms.)

The DLRO is preferable since we already have them on-site for other purposes.    The question is how long will it take the device to stabilize when reading an inductive load.   Megger says the DLRO has an "inductance" model which makes them stabilize faster, but they can't say exactly how fast.  Also I'm not sure how long battery lasts when using this feature. We would use the Kelvin clip probes instead of the spring-loaded prong probes.

The Transformer Ohmmeter seems better suited to the application, but I'm not sure it is required to buy this additional equipment if the DLRO will work.

Any comments or suggestions?  Have you had luck using equipment other than bridge for winding resistance tests?  Or found bridges that are still readily available ?
 

=====================================
(2B)+(2B)'  ?

RE: Non-bridge device for large motor winding resistance readings

I'd say the DLRO is a good bet, the reading should be evidence itself that the instrument is reached a stable state. Settling time should be a few tens of seconds, if that.
  

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If we learn from our mistakes I'm getting a great education!
 

RE: Non-bridge device for large motor winding resistance readings

(OP)
Thanks, that's encouraging.  I've ordered a set of the Kelvin clip probes, and I'll try our DLRO out on one of our warehouse spare motors when those probes come in.

Any other experience or comments on the general question of what works well for this test?

=====================================
(2B)+(2B)'  ?

RE: Non-bridge device for large motor winding resistance readings

Watch for induced voltages, especially on the high current ranges. A big DC motor can store enough energy in the field to make a heck of a spark when you disconnect on the high current ranges. Don't ask how I found that out. blush It's an obvious hazard I know, but don't forget to switch the instrument off before disconnecting and allow time for the current to collapse. smile
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: Non-bridge device for large motor winding resistance readings

Does this help?
MEGGER DLRO10 and DLRO10X
Digital Low Resistance Ohmmeters
• Accurate results in under three seconds  

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Non-bridge device for large motor winding resistance readings

(OP)
That doubtless is a general statement, not intended to apply to inductive load.  Here is perhaps more relevant statement from the link labeled "data sheet":

Quote:

Inductive mode is intended for use when measuring inductive loads. When measuring inductive loads it is necessary to wait for the voltage to stabilise. This means that the measurement could take a few seconds or several minutes. The test leads are firmly connected to the item to be measured and the Test button is pressed. The instrument will pass a current through the sample and wait for the voltage to stabilise. If possible the current will be increased. This procedure will be repeated until the voltage detected falls into the range 15 mV to 200 mV. The instrument will then continue to take readings, which will gradually decrease to the true value as the voltage stabilises further. The operator decides when the result is stable and presses the Test button to terminate the test. Measurement is made with forward current only.

=====================================
(2B)+(2B)'  ?

RE: Non-bridge device for large motor winding resistance readings

I don't see any problem with a DLRO. A typical synchronous motor field winding has enough inductance that it takes 2 or 3 seconds to reach the regulated current when the field is applied. Induction machines have much less inductance compared to a synchronous field so I would not expect the meter to take more than a few seconds to stabilize.
 

RE: Non-bridge device for large motor winding resistance readings

I have used a Ducter to measure tranformer winding resistances.
As has been pointed out by other members, beware of damaging the instrument when disconnecting leads because of inductive kick - refer to Manual regarding the order of disconnecting test leads.
I Seem to remember that potential leads should always disconnected
first before disconnecting current leads. Please refer to Ducter Instruction manual!  

RE: Non-bridge device for large motor winding resistance readings

I have seen many a DLRO ruined by using it for winding resistances when disconnecting the leads.  

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