[PSUBS-MAILIST] Motor Reversing Current Spike

[Alan via Personal_Submersibles]

Just passing on some information from technadyne's thruster Q&A
that I thought was interesting. The question was whether you could put
the thruster from full forward to full reverse instantaneously. 

No!! It is necessary that your system has a time based ramp from zero speed to full speed and from full speed to zero speed in both forward and reverse. If you are controlling the thrusters with hand operated joystick, the time required to move the joystick from one position to another is probably enough for the thrusters. However, if you are driving the thrusters with computer, it is essential that you program a linear ramp into the thruster control algorithm. The reason that this time based ramp is necessary is because the thrusters generate back EMF and the back EMF is much greater when the speed changes very rapidly. For example, if the time ramp from full power forward to full power reverse is 20ms, the back EMF current spikes will be 10-20 times greater than the steady state current (we have actually measured this). If this ramp is increased to 50ms, then the back EMF current spikes will be about 4-8 times the steady state current. And increase the ramp to over 100ms, and the current spikes will be about 2 times the steady state current. Make the ramp as long as you can and contact the factory if you have any questions.
Alan


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