Load meter on 36 inch blanchard gtinder

[rock]

I have a 36 inch blanchard grinder running on 240 3 phase. Our shops incoming 3 phase has 2 legs of 110v and 1 wild leg that is around 255v. Volts are ref to ground. Phase to phase between any two shows 236-238.

This machine was set up in an industrial plant running on either 440 or 480 before I got it several years ago. I switched it over from high to low voltage, if memory serves it was delta wound. It had been used on low voltage before I think because low volt heaters were in the cabinet in boxes.

Anyway the one thing I never changed was the load meter. Just the wheel running free shows around 70%. When grinding heavy its not unusual to peg meter at 150% which I realize is not accurate because of the voltage switch. I have run other blanchards over the years and know the non grinding load should show maybe 20%-30% on the load meter. I suspect this is set up with a coil around a lead someplace and is reading an induced amperage or voltage on the meter.

So my question is what do I do to adjust this reading to something more in line with reality for the less experienced guys in the shop to see. I run it by sound and feel vs the meter myself but I have 40 years time in doing this stuff.

Can a resistor be put in series or parallel with the meter?
Do I need a different pickup coil?
Do I need a different meter?

Keep in mind this is a visual reference thing more than anything else.

 

[Just a Sparky]

Replace the CT/shunt.

Meter should have a current or voltage rating on it. Determine the right shunt rating or current transformer ratio to achieve 100% scale reading at rated motor FLA.

E.g. 75mV meter, 50 FLA motor - use a 50A, 75mV shunt.
E.g. 5A meter, 50 FLA motor - use a 10:1 current transformer.

DO NOT power up the machine without a load (meter) connected to the current transformer secondary. This can destroy the CT.

If meter has a 0-150% scale and has ratings listed at full-scale value only, do math to account for the difference between 100% and 150%.

If you just want to ballpark 50% of the existing reading on the cheap, you'll need to determine the meter's internal impedance with a good degree of accuracy. Pull up the datasheet on it if you can because this will often be a milliohm rating in the case of AC ammeters. Install a resistor of equal value in parallel with it - with sufficient wattage rating of course.

 

[CarbideBob]

Is the power lead looped multiple times through the CT?
A somewhat common practice on Blanchards for voltage conversion.

 

[SomeoneSomewhere]

In general, I would expect current at 240V line to be exactly double that at 480V line. A straight doubling of the scale or, as CarbideBob suggested, doubling the number of turns on the CT primary.

 

[JST]

Sparky has it. Those are the best choices.