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The only ammeters I can find only measure up to 10 or 20 amps.
I have seen those clamp on ones but will those allow me to measure the amps of a motor? Do they have separate probes so you dont have to use the clamp part?
For high AC current measurements I've generally seen three methods common in industural use:
CT transformer:
This is a 'donut' looking transformer designed to have the current to be measured wire to be run through the center of the donut. This acts like a one turn primary winding. The large number of turns of the donut allows a scaling of the output measurement. 5 amps AC is the 'standard' full scale measurement value of the secondary and the number of turns determines the current ratio of the donut transformer say 100:5, 200:5, 600:5, etc. This allows a standard 5 amp current meter movement to be used which then only requires it's printed scale to be changed for each range.
Current shunt:
This is in effect a very low ohm resistor usually made from strips of brass or copper and custom calibrated to develop a specific full scale millivolt AC voltage for the AC current flowing through it. It usually has four connections, two very large high current connections and two smaller connections for the millivolt output connections. A standard one I have is rated as 50amp = 50 millivolts output.
Portable clamp on AC probes:
These are variation on the CT transformer method but having a 'split core' one can slip if over an existing wire without having to disconnect the wire to be measured. These can either be designed have a self contained meter movement or display or they can be be built to plug into a standard DVM meter for display. These are not usually used for perminate installation but used as a peice of test equipment for troubleshooting problems.
For high current DC measurement the high current shunt method applies, however normal CT transformers or clamp on CT transformers will not as they won't pass DC current. There are clamp on hall effect probes that can measure DC current but I've not seen these for very high current values, but they may exist.
When using a shunt to measure current your meter only needs to be able to measure voltage, as the VOLTAGE drop measured across the shunt is proportional to the CURRENT flowing through the shunt. Searching on E-bay under current shunt should snag many listings such as this:
Here's a really cheap, easy, and accurate method - that gives zero loss to the circuit!. It was in an old Practical Electronics, back when the LM3914/5 came out!.
You've got two wires connecting from the battery to the load, motor or whatever, this wire, thick as it is, will still drop a voltage across it depending on the current through it (it's essentially a shunt). Use an opamp to amplify the voltage dropped across it, this gives a voltage output directly related to the current through the wire. Calibrate it by known loads, or by using a current meter and adjusting the opamp gain to read the same.
BTW, the original article was for a car ammeter, and it monitored the voltage across the battery earth strap - giving both charge and discharge readings, including the starter motor current!.
To employ Nigel's suggestion may not even require an op amp if your DMM has capability to measure the voltage drop that exists in your test setup. What I would do though is to make sure you measure the voltage drop in places that would not be influenced by junctions where contact resistance might not be constant. Also be sure to account for the voltage drops - in the wires supplying the current, across a shunt, etc.
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