Insulation meter(mega ohm meter) with DVM

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fjpompeo

Joined: 29 Nov 2003
Posts: 306

22 Oct 2004 23:55 mega ohm meter

Hi,

I need to build an add-in insulation meter ranging from 5000 Mohms up to zero (or almost zero) with 400VDC to measure galvanic insulation of 3~ or DC electric motors and cables. This circuit will use a Fluke 87III DVM (in any range) to inform the insulation resistance.

Regards,

Fernando

Mr.Cool

Joined: 20 Jun 2001
Posts: 637
Helped: 24

23 Oct 2004 0:40 make isolation meter

so what exactly is your question?

one thing you have to keep in mind about dielectric testers is that it needs to react as fast as possible to arcing or short circuits. also it matters the charge up and charge down rates because there could be leakage capacitance between the two nodes you are measuring and you do not want to hit these caps with huge dV/dt.

Mr.Cool

IanP

Joined: 05 Oct 2004
Posts: 6490
Helped: 1542
Location: West Coast

23 Oct 2004 3:30 megaohm build meter

Here is the priciple of a megger:
1. HV generator, most meters have 2 ranges 500Vdc and 1000Vdc. It is done using small F-transformer, rectified by HV diode (eg. BY159) and smooted by HV capacitor (about 47nF).
2. Refference resistor used to measure voltage drop of leakage current (in some FLUKE meggers this value is 100k/1%)
3. Choose your own resistor so your meter will display direct value

Don't forget about momentary On/Off switch to activate HV generator for only short period of TEST. Cool

fjpompeo

Joined: 29 Nov 2003
Posts: 306

23 Oct 2004 20:43 minimum insulation megaohm

Hi there,

Very thank you for the above information. I need to mesasure the insulation of motor winding to make sure they are working properly. Yokogawa it is the leader in manufacturing these type of instrument but here in Brazil is very expensive so was looking for something a litle less expensive just because I do not use very often. The motors I usualy test are ~3 x 380VAC or ~3 x 440VAC and sometimes from 2,2Nm up to 20,0Nm 400VDC.

Regards,

Fernando

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23 Oct 2004 20:43 Ads

XNOX_Rambo

Joined: 13 Jul 2002
Posts: 437
Helped: 87
Location: Far out, man!

24 Oct 2004 0:17 megaohm principle

Fala bicho! Wink

Maybe this is useful as a start:
http://www.piersononline.com/tips/may2000.cfm?srcID=1

/Rambo

fjpompeo

Joined: 29 Nov 2003
Posts: 306

26 Oct 2004 5:40 dc insulation meter

Very thank you RAMBO but to measure motor winding insulation I nead at least 500V, this circuit is to 9V.

Mesmo assim valeu!!

Fernando

XNOX_Rambo

Joined: 13 Jul 2002
Posts: 437
Helped: 87
Location: Far out, man!

26 Oct 2004 11:30 insulation meter circuits

Yes, but they use several 9V batteries to get 50V. You might use the
same principle and "just" increase the voltage? Not using batteries though...
You could build a boost-converter, as IanP suggested (I presume he did).

Here in Sweden I could probably find an old, cheap hand-crancked megger,
but it doesn't help you way over there in Brazil... Sad