Just a thought...
It could be that the transients associated with pulsed currents to the
motors is causing the controller or power supply to shut down. The
transients might not be large enough to cause disruption until the fourth
motor is on-line. If so, adding a fairly large electrolytic capacitor of a
few thousand uF at the power supply connection to the controller might solve
the problem. To handle the high-frequency compoents of the transients, I
would parallel the large electrolytic with a tantalum capacitor of 100 uF or
larger.
A clue that transients are likely to be causing the problem is that it
doesn't matter which of the motors is the "fourth" motor. That pretty much
rules out mis-wiring or faulty motors. You won't see the transients with a
voltmeter and the symptoms (power supply shut down) can be exactly the same
as for simple over-current protection.
I would tack solder the large capacitor pair across the power supply
connection inside the controller and see if the behavior changes. That in
itself would be a significant clue.
73, Ward N0AX
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