> Subject: [LNC2] Oildyne Hydraulic Pump Inrush > Date: November 28, 2013 4:56:30 PM EST > To: lml@lancaironline.net >=20 >=20 > 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.) >=20 > 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. --Apple-Mail=_1AF8CAF8-9DEA-4BA8-AF07-CD27A7568C43 Content-Transfer-Encoding: quoted-printable Content-Type: text/html; charset=iso-8859-1 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:

From: John Cooper <Subject: [LNC2] Oildyne = Hydraulic Pump Inrush
Date: November 28, 2013 = 4:56:30 PM EST
To:
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.

= --Apple-Mail=_1AF8CAF8-9DEA-4BA8-AF07-CD27A7568C43--