# HOW DO YOU MEASURE CURENT!!

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ChaosCraig
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HOW DO YOU MEASURE CURENT!!

I am working on a electric vehicle project. Anyway, i was just wondering if i could get some advice for a problem I am having.
We have a motor that runs a on a 72V 16KHZ PWM signal that peaks at 800 AMPS and all we are trying to do is come up with an effective way to measure the current that is drawn by the motor. ANY IDEAS?

The Laird
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Re: HOW DO YOU MEASURE CURENT!!

Hello ChaosCraig,

It is easy. Pass the current through a suitable, known value, resistor and then measure the voltage which is developed across that resistor.

The 'resistor' may simply be any suitable conductor, i.e. a short piece of very heavy copper. The resistance of this copper will be of low value (otherwise you will get a large volt drop and a very large dissipation of energy (heat). At 800 amperes you would require a resistance of only 0.0001 ohms to give a volt drop of 0.08 volts and a power dissipation / loss of 64watts.

A sensitive voltmeter will measure the voltage. 0.08volts would equate to 800 amps, 0.06volts would equate to 600 amps and so on in a linear fashion.

It is perfectly feasible to use an amplifier to raise the voltage dropped across the 'resistor' to facilitate the use of a less sensitive voltmeter.

The Laird

Mik
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Re: HOW DO YOU MEASURE CURENT!!

You could also use a Hall-effect sensor around one of the cables carrying the current. An advantage of this is that it remains galvanically isolated from the HV circuit. For a few hundred \$\$ you can get a clamp meter that can be connected and disconnected within a second.

This information may be used entirely at your own risk.

There is always a way if there is no other way!

ChaosCraig
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Re: HOW DO YOU MEASURE CURENT!!

I have tried a shunt resistor with known value, But my oscilloscope just reads garbage. I amplified it too but the response of the amplifier is not fast enough to capture the messy PWM signal. I was looking at hall effect sensors but i can not seem to find any that work well over my entire range. Are there any hall effect sensors you can suggest? Or perhaps a way to filter out the PWM noise across the shunt resistor? My voltage i read across the shunt is so low that my instruments just read garbage.

ChaosCraig
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Re: HOW DO YOU MEASURE CURENT!!

comment removed, accidental repost.

PJD
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Re: HOW DO YOU MEASURE CURENT!!

Is this a brushless DC motor?.

If it is, then as well as being high-frequency pulsed, each of the three wires to the motor also alternates from from positive voltage, to zero voltage to negative voltage as the motor turns. So, I think you need to measure AC (RMS) current for one wire, and multiply it by three.

Someone please correct me on this if I'm wrong on this...

Spaceangel
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Re: HOW DO YOU MEASURE CURENT!!

Most electric vehicles I have seen have two shunts and two ammeters, one for motor current and one for battery. One the controller there is B+,B-, and some models have M-. Being that B+ is usually common place the 1000 amp shunt in B- and the other 1000 Amp shunt in M- and run 50 mv side to both meters. This way you can see current magnification like lots of motor current on volt battery in. If you have a DCP1200 the common is backwards.
To double check meter reading use a clamp on DC ammeter. Shunts are common and come with most meter but you can order extra shunt from KTA and EVA Deltec is one great shunt and others are just as good. 50 Milli-volt is standard for shunt voltage. Cruising equipment E-Meters uses 50 mv shunts. Shunts are \$19.95 for small shunts and up to \$29.95 for larger 500 ampere shunts. So 1000 amp might be a few bucks more? Even though it might peak at 800 Amperes you will see meter get pegged every so often. Especially on take off.

KB1UKU

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