ebay 500 amp ammeter inaccruate at part throttle

sportcoupe

I bought a cheap eBay ammeter with 500 amp shunt. It is only accurate at full throttle, steady cruise. Any readings at part throttle under any condition results in crazy, impossible numbers. I have it wired up like the diagram I created below. Any ideas? Do I need an external dummy load in parallel with the shunt or maybe filter the power going to the display?

Attached Images

ammeter shunt wiring.jpg (60.1 KB, 0 views)

BGW

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Volt_Ampere

You can fix that by adding an RC filter to the measuring circuit after the shunt. Your problem comes from the fact that the current isn't DC. It's a very nasty looking chopped waveform. The meter is grabbing a snapshot of it. I would try a time constant of about 100mSec (1/RC) That should smooth out the readings for you.

JohnnieB

That is roughly the same Ammeter I have. I don't have it installed in the dash yet, but have played with it some. Mine seemed to act normally at all times will a stock PDS controller and 36V.

The voltage drop across the shunt is only 75 mV @ 500A, so the meter input is fairly sensitive and EMI from the controller may be an issue.

Try connecting the shunt between the two batteries that are physically located the furthest from the controller and motor.

You could also use shielded cables between shunt and meter. I'd use high quality microphone cable and connect the shields to B- on the 72V battery pack.

You could also add some external filter caps between the B- and B+ terminals on the controller and on the motor (between A1 &A2 terminals and S1 & S2 terminals)

I'm not sure what the PWM frequency is for a SMP, but it is probably in the 15PPS to 18PPS range and the major EMI component would be the third harmonic.

FWIW: I have a set of earphones with a built in AM/FM radio that I wear while on my riding mower. I tuned the radio to an unused AM frequency wore them while taking my cart for a drive. The EMI was strongest during acceleration and going up hills, and weakest when cruising at a steady speed. However, when the radio is tuned to a station, no interference is noted, so my controller is reasonably quiet.

Since you are running at 72V, your set-up would be more prone to generate electrical noise and transmit it through the air as well as through the high current cables.

Hope this helps.

JohnnieB

I'd give this a try first, if it doesn't clear the problem, start looking for EMI.

sportcoupe

How would I determine the values for the resister and capacitor for the RC filter?

JohnnieB

http://www.referencedesigner.com/rfcal/cal_05.php
Note: Enter Time in seconds, Capacitance in Farads and Resistance in Ohms.
The suggested TC was 100mS, so the time entry would be .01
Pick a Capacitance value and calculate Resistance value or pick a Resistance value and calculate the Capacitance value.

Personally, I'd go with a 1uF (100V or higher) ceramic disc and just let the input impedance of the meter drain it.

The PWM frequency isn't known, but most likely in the high audio range, so I'm suggesting a capacitance value an order of magnitude higher than what is typically used to bypass ICs that operate in the low RF ranges.

sportcoupe

And I would put the cap in parellel with the meter shunt wires I'm assuming?

I did use twisted pair shielded wires for shunt to meter connection.

sportcoupe

What do you know, I just happen to have several .1uF, 100v caps laying around. How convenient.

Attached Images

2A104K.jpg (12.8 KB, 0 views)

JohnnieB

"And I would put the cap in parellel with the meter shunt wires I'm assuming?"
No. In parallel with the meter shunt input at the meter. (Between the Yellow and White wires.)

"I did use twisted pair shielded wires for shunt to meter connection."
Good choice of wiring.

JohnnieB

If they just reduce rather than cure, go to 1.0uF.