Strategy to limit inrush current

[aurmer]

I have been reading up on inrush limiting, but the advice I found was conflicting.

Here is the hardware I am working with.

VIAIR 275C Compressor ----- 12V  18Amp
Protek Power supply #PUB250-12 ----- 12V 20.84Amp
(We have a Duracell Deep Cycle 12V SLA which turns on the compressor no problem.)

What component(s) are best to help reduce the peak inrush current and allow this 20A power supply to get the compressor running? Or am I going about this the wrong way? Do we actually need a supply with more power?

[Ian.M]

A DC motor's stall current can be as much as ten times its nominal max running current.   Unfortunately, unless you are using a motor controller with current limiting, or can otherwise ramp up the speed slowly, every time you start the motor, its rotor is initially stationary, so it *will* attempt to pull its stall current, which may well be over 100A

To fix this without rewiring the compressor so its pressure switch sends an ON signal to an external motor controller would be very difficult.

Linear current limiting would have to dissipate up to 216W every time the motor starts, and have negligable voltage drop when the motor's drawing 18A.   You could probably get there with a whole bunch of paralleled power MOSFET + OPAMP constant current sinks on a*BIG* heatsink.

The best external option would probably be a large high current supercap bank to supply the surge, with a time delay relay to short out a 1.2 ohm 100W resistor once its fully charged from the PSU and also operate a >100A contactor to keep the load disconnected till the cap bank is charged.  You could also probably replace the relay and contactor with really beefy PMOSFETs and a 555 timer.

[David Hess]

Current limiting during startup can also be implemented with a switching regulator which avoids the problem of high dissipation in a linear design.  Or a buck converter can have its output voltage ramped up.  A more interesting solution would be to put an open loop pulse width modulator and filter between the power supply and motor and ramp up the duty cycle; this avoids any requirements for closed loop stability.