Why Parallel VFD Pumps Draw Unequal Current

Two pumps running in parallel do not automatically draw identical current. Small differences in motor efficiency, impeller diameter, speed command, branch resistance and pump condition can produce different electrical loads. The important question is not whether the current values are perfectly equal, but whether each pump operates inside its approved motor and hydraulic envelope while contributing the expected flow.

An effective diagnosis connects current to speed, pressure and pump performance. Comparing amperes alone can lead technicians to replace a healthy motor while a partly closed valve, leaking check valve, wrong rotation or mismatched pump curve remains in the system.

First Decide Whether the Pumps Are Truly Identical

Record each pump model, impeller or stage configuration, motor manufacturer, rated power, voltage, current, frequency and speed. Confirm whether the motors are asynchronous or induction motors or permanent-magnet synchronous motors. Two packages with the same sales description may contain different motor or hydraulic revisions.

Compare the actual nameplates, not only the original schedule. A motor replacement may have changed rated current or speed. A trimmed impeller, repaired pump or different number of stages changes hydraulic load. If the equipment is not identical, equal current is not a valid acceptance target.

AUSENIST YS620 and YS820 can be matched with both induction and PMSM pump motors, but each drive needs the correct motor-control mode and data. Parameter files should be tied to the specific motor code rather than copied because the kilowatt rating matches.

Normalize the Measurements Before Comparing

Measure each pump under the same system condition. Record drive output frequency, motor current, common-header pressure, suction pressure, flow where available, valve positions and which other pumps are running. A current reading from one pump at 42 Hz cannot be compared directly with another at 50 Hz.

Compare current with each motor's own rated current, not only with the neighboring pump. A motor drawing less current may simply have a different nameplate rating or be operating at a lower hydraulic load.

Verify Frequency and Control Commands

In synchronous multi-pump operation, confirm whether drives are expected to run at the same commanded frequency. In lead/auxiliary control, one pump may modulate while another runs at a different defined speed. Unequal frequency can therefore be intentional.

Check actual output frequency, not only the common pressure setpoint. Verify minimum and maximum limits, speed references, communication status and local overrides. A drive left in manual mode or with a different frequency limit can make a mechanically identical pump carry a different load.

If pump coordination uses RS485, confirm addresses, communication settings and network health. A unit that has lost coordination may retain a fallback or previous command depending on the approved control design. Diagnose communication separately from hydraulics.

Test Each Pump Alone

Where the system can be operated safely, run each pump individually against the same header condition. Record the pressure and flow it produces at several approved speeds together with current. This separates a pump-specific problem from interaction between branches.

If one pump draws different current and produces different pressure or flow when tested alone, inspect its rotation, impeller, motor data, suction path and mechanical condition. If individual performance is similar but current diverges only in parallel, focus on branch resistance, check valves, staging and combined system curves.

Do not dead-head a pump or close a valve beyond the manufacturer's permitted test procedure. Minimum-flow and thermal limits still apply during diagnosis.

Inspect Branch Valves and Check Valves

A partly closed isolation valve or restricted branch increases loss and changes the pump's duty point. Depending on the pump curve, the affected pump may draw less current while delivering less flow. A technician who increases its speed to equalize current may hide the restriction and worsen control.

A leaking check valve on an idle or lightly loaded branch can allow reverse flow. A running pump may circulate water through another branch instead of delivering it to the header. Inspect valve direction, opening, closure and pressure behavior. In parallel operation, test that stopped pumps do not rotate backward or pass unintended flow.

Confirm Correct Rotation and Pump Condition

Wrong rotation often produces poor pressure and altered current. Verify direction using the pump manufacturer's safe method. Do not rely only on sound, because a reversed impeller can still move some water. In a multi-pump header, the other pump may mask the failed contribution.

Inspect for worn or damaged impellers, deposits, rubbing, bearing problems and coupling issues where applicable. A pump handling dirty or treated water may lose performance differently from its neighbor. Compare vibration, noise and temperature as well as current.

If a complete motor-pump package was replaced, recommission its motor data, minimum speed, pump curve and protection settings before returning it to rotation.

Examine Suction Conditions for Each Pump

Parallel pumps may share a suction header but experience different inlet conditions because of branch geometry, strainers, valves or air pockets. A restricted suction path can reduce performance and create cavitation or unstable current. Observe suction pressure and noise for each branch where measurement is available.

Running another pump increases total suction demand. A source or header adequate for one pump may become inadequate for two. If both currents and pressure become unstable when the second pump starts, assess source level, suction losses and available inlet conditions rather than treating it as a drive synchronization problem.

Review Pump Curves and the Combined Duty Point

When parallel pumps operate, they share head and their flows combine according to the pump and system curves. Manufacturing tolerance, wear and speed differences affect where each unit settles. Two pumps do not simply split the total flow equally under every condition.

Plot or compare the individual curves at the operating speeds. If pumps are different sizes, define the expected contribution of each. Avoid forcing current equality between a small jockey pump and a larger duty pump. The correct target is an intentional hydraulic split with every motor inside its rating.

If the system lacks individual flow meters, temperature, current, branch pressure and test data can provide clues, but they do not replace a direct flow measurement when accurate balancing is required.

Check Motor and Drive Parameters

Verify rated current, voltage, frequency, speed, power and motor type in each drive. Confirm that motor identification or tuning was performed under the approved conditions. A wrong motor current setting can distort protection, while wrong speed or control data can change torque behavior.

YS620 covers a documented 0.75–7.5 kW range with dual RS485 throughout. YS820 covers 0.75–22 kW; its 220 V 0.75 and 2.2 kW versions use single RS485, while documented 380 V versions use dual RS485. A mixed-series system should preserve the intended communication topology and parameter control.

Review Staging and Rotation Logic

One pump may accumulate wear if it always leads, even when timed rotation is configured incorrectly or disabled. Different operating hours can create performance differences. Check accumulated hours, lead assignment, rotation interval and fault history.

The YS620 documented architecture supports two master-capable drives and up to four auxiliary pumps. Its control functions include standby-master takeover, failed-pump bypass and timed rotation. Confirm that a pump marked available actually joins the sequence and that a faulted unit is skipped as intended.

Consider Installation and Altitude

For YS620 above 1,000 m, no altitude derating is required below 1,000 m and capacity is derated by 1% for every additional 100 m. Apply the same site altitude to each drive but verify that the motor currents and drive sizes provide adequate individual margin.

Build a Repeatable Diagnostic Record

Create a table for each pump: equipment code, motor nameplate current, parameter version, speed, measured current, suction pressure, discharge pressure, flow, valve position and operating combination. Repeat the test after correcting one identified cause.

AUSENIST customization can include pump and motor matching, induction or PMSM setup, sensor compatibility, parameter presets, communication, multi-pump logic, documentation, packaging and private-label presentation. For an OEM package, define the expected current and hydraulic ranges rather than an unrealistic requirement for identical amperes.

Balance Hydraulic Contribution, Not the Display

Unequal current is a symptom to interpret, not automatically a fault. Start with equipment identity and equal test conditions, then separate individual pump performance from parallel interaction. Rotation, valves, branch losses, suction conditions, curves and parameters usually provide more useful evidence than adjusting speed until the numbers look alike.

The system is balanced when each pump delivers its intended contribution, remains within motor and pump limits, and stages reliably. That engineering definition is more meaningful than forcing every AUSENIST display to show the same current.