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See https://www.circuitlab.com/circuit/3fw7yx/ammeter-remove-error/

If I remove any ammeter and replace it with a wire, simulation fails. "Undo" can repair the problem, but replacement with a new ammeter does not.

This is my attempt to better understand the unstable wire feed motor drive circuit in my SIP mig welder. This is as it is, it is not my design. (The power transistor is actually a TIP 35C).

I can't simulate the instability because I cannot simulate the motor back EMF. Help?!

Hi hadronuk, when you "remove" one of the ammeters, do you mean that you delete/cut it out of the circuit, or do you just move it to a different place on the sheet with nothing connected to it? If the latter, that would explain the issue. (Ground one end of the otherwise floating ammeter to fix.)

Are L1 + R9 your motor model?

Delete from circuit and replace with a wire. Yes, L1 & R9 are my feeble attempts a modelling a motor! Thanks for the advice on grounding temporally unused items, useful for future reference.

@hadronuk,

Which simulation fails?

DC, DC sweep, Time Domain?

What error message do you get?

What happens?

How do you know it has failed?

I have just run a series of Time Domain sims with the ammeters progressively removed in turn and that seems to run fine On Kubuntu 12.04 LTS using rekonq browser).

Running a DC Solve gives sensible results but also throws an error:

Error in DC Solve - Aborting TypeError: 'null' is not an object

Entering R3.R into the DC sweep and using the settings you have already entered also runs fine.

Have a look at:

https://www.circuitlab.com/forums/support/topic/9x8m5kf6/7-kw-brushed-motor-rectifier-circuit-need-help-with-load/#comment_3292

for some info on modelling DC motors.

Hmmm, same error as posted by @hadronuk here in Safari:

Running DC solver from original circuit: OK

Delete AM2, connect R3 to “+”: Error in DC Solve error null is not …

Yep, seems to be a bug.

Regards, Sancho

Many thanks all -- we've identified and patched this issue! The problem occurred when a Voltmeter or Ammeter element was added, simulated, removed, and another simulation attempted.

Mike, thanks very much for the quick fix. Circuitlab really is excellent; I now understand much better how my mig welder motor drive circuit doesn't work. Thanks also to Signality for the links for modelling DC motors.