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     Real life episode - a recently overhauled open circuit hydraulic pump (A10VO45DFLR), equipped with pressure compensator, load sensing and torque limiter control, was about to be tested on a bench. The client had requested the torque limiter to be set to 15 KW (20HP) at 1450 rpms, which was the  nominal power/speed of the electric motor driving the pump. The request was made due to the plant's electrician advice to limit the motor current consumption to 30 amperes. In fact, the exact words were - "adjust the pump to 15 KW or 30 amperes".

    The test bench used to test and adjust the pump was of a very simple kind - a common three phase electric motor (thank you very much for these, mr. Tesla), simple mechanical coupling, an oil tank, a needle restrictor and a flow-meter - very basic yet functional and reliable rig. The technician that was testing the pump took even the trouble of finding an average AC motor current table (like this one), and indeed confirmed that the current draw of a 15 kw motor (three phase, 380 volts, 50hz) rounded 30 amps. To adjust the torque limiter he proceeded exactly in the way he had adjusted many similar controls on pumps driven by electric motors before - by reading the phase current and adjusting the control so that the current doesn't pass a certain value.

    The "adjusting maneuver" was performed flawlessly in a "by the book" fashion - the current draw in one of the phases of the test rig motor was being monitored, the torque limiter was set to start cutting the displacement at 30 amps, and then to maintain the current roughly at the 30 amps level up until the pressure compensator kicked in. However, when the actual hydraulic output (pressure times flow) power was calculated, it turned out to be in the neighborhood of 7kw (10 hp) - after several tests a conclusion was made that the pump's overall efficiency was unacceptably low (50 percent). As the pump didn't have excessive case leakage, the fault was attributed to low quality spares causing "excessive mechanical drag", the unit was rendered "faulty beyond repair", furthermore it was decided that the pump needed to be replaced with a new "original" one, and the client once again had to face expensive downtime costs until the new pump arrived.

    What is wrong with this picture? If I were asked this question, I'd say that before answering it I'd have to know how big the test bench electric motor is. Why? Because of the little thing called "power factor", which is a strictly AC power related unit, and in simple turns is the ratio of the real power (the power your motor is consuming to deliver the demanded torque) to the apparent power (line current times line voltage).

    Most industrial three phase motors have power factor of around 0.85 at nominal loads, and the above mentioned table is only valid for nominal load values. When the load of an AC motor is less than nominal, the power factor (along with the efficiency) drops (schematic relation is shown here), which means that the phase current of a lightly loaded motor can not be used as a reference to determine the output torque simply because it indicates mostly the apparent power and not the real one. I don't want to go into detailed AC power theory here, but those of you who are interested in learning more about AC motors and what a power factor is can start here.

    Due to the fact that the test bench in question was built to be able to test larger pumps (the AC motor of the rig was rated to 70 HP), the motor load was well below its nominal value during the test of the "malfunctioning" pump, and therefore the phase current the mechanic was reading didn't represent the real load current.

    In this case not knowing what a AC motor power factor is caused an extensive downtime and unnecessary expenses. This example shows that knowing a thing or two about what is at "the other end of the shaft" can't hurt a pump mechanic. Understanding at least basic theory of modern industrial motors and engines operation has become an essential part of a "knowledge package" any hydraulic technician should carry behind his back.