| Hi John,
A good rule of thumb on dc motors is 4 to 1 so your measurement seems good. Your readings are also consistent with the 35 amp breaker which I understand is typical for the pump.
Inductive loads like the motor are easiest to turn on for relays since the contacts are made before the current reaches the locked value. Incandescent lamps are the worst case turn on load..... 10 to 1 is the rule of thumb (no kidding) for the inrush into a cold lamp. With only the inductance of the connecting wires the relay really sees an inrush while it is making contact in lamp circuits.
If a normally sized relay were to break with the pump rotor locked it would be highly stressed, maybe even fail. Most designers rely on the breaker to trip to protect the wiring, motor, and relay and ignore the unlikely possibility of the relay being turned off during the short time it takes the breaker to trip with a locked motor. The solenoid Lancair recommends is capable of surviving turning off the starter for awhile so won't be damaged in any case. Nothing like belt and suspenders.
Jack Morgan
On Nov 29, 2013, at 6:00 AM, Lancair Mailing List wrote: Subject: [LNC2] Oildyne Hydraulic Pump Inrush
Date: November 28, 2013 4:56:30 PM EST
Today I inserted a 75A / 50mV shunt in the power lead to my hydraulic pump in order to measure the current draw. I connected my Fluke 87-V digital voltmeter on the 600mV range across the sense terminals of the shunt . With the pump running in the "gear down" direction and the equalization valve open, the motor was drawing a steady 29 Amps. I then shut off the pump, closed the equalization valve, and with the Fluke in the "250 uS peak" mode, measured the inrush when the pump started up again briefly. Would you believe 135 Amps! (Disclaimer: I purchased the shunt on eBay and can't vouch for its calibration.)
Next time the plane is up on jacks, I'll re-do this test in both gear up and gear down directions. Also, I intend to temporarily remove the ZNRs across the hydraulic relay contacts and measure the voltage spikes with no protection.
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