Twin Lobe Blower High Amps: Process Load, Not the Motor

The panel shows one 50 HP twin lobe blower at 62-63 A while an identical blower and motor run at 51-52 A. Replacing the motor did not change the current, so start with the process and blower load, not another motor. The first checks are local suction pressure, discharge pressure, valve position, and pipe resistance under matched operating conditions.

Read the symptoms before changing parts

The current difference is about 19-24%, based on the reported 62-63 A versus 51-52 A range. That is large enough to investigate, but current alone does not identify the restriction or mechanical fault.

Understand what raises blower current

A twin lobe blower traps and transfers a volume of gas each revolution. The shaft torque rises as the pressure difference between suction and discharge rises. A discharge restriction raises discharge pressure; an inlet restriction lowers suction pressure. Either condition can increase the pressure ratio and change the required power.

Motor current is only a proxy for shaft load. Its relationship to power also depends on supply voltage, phase balance, power factor, efficiency, connection, and speed. Never treat 30 A as a direct percentage of full mechanical load, especially at light load where magnetizing current remains significant.

Parallel blowers need special attention. Two identical machines connected to a common header can operate at different points when their branch lengths, fittings, valves, check valves, or deposits produce unequal resistance. Read pressure close to each blower connection; a common-header reading can hide a restrictive individual branch.

Wear does not automatically explain high current. Excessive internal clearance can increase backflow from discharge to inlet and reduce delivered capacity. Internal rubbing, failing bearings, or deposits add friction directly and are stronger mechanical explanations for elevated current. Use flow, temperature, vibration, sound, and clearance inspection to separate leakage from friction.

Run the diagnostic checks in order

1. Verify the electrical reading first. Record current on every available motor conductor and measure supply voltage under load. Compare the motor connection and measured current with the nameplate. A large current imbalance directs the investigation toward the supply, connections, or winding circuit; similar currents that rise together point toward shaft load.
2. Create a valid comparison. Run the suspect and reference blowers at the same speed and with the same service gas, inlet temperature, valve lineup, and header demand. If they operate in parallel, test each machine separately when the process permits. Do not compare one blower during peak header demand with another during a lighter condition.
3. Measure suction and discharge pressure locally. Take readings near both blower flanges. Calculate the pressure difference using the same pressure basis for both readings. Compare that result with the reference blower and the manufacturer performance curve for the actual speed and inlet condition.
4. Inspect the inlet path. Check the intake filter, inlet valve, silencer, screens, and ducting for contamination or collapse. A failed filter can also admit debris that fouls the blower.
5. Trace the discharge branch. Confirm that isolation valves and dampers are in their required positions. Check the nonreturn valve, silencers, flexible connections, branch piping, and common-header connection for blockage or abnormal resistance. Never run the blower against a closed discharge path.
6. Check mechanical condition. Isolate the equipment using the site energy-control procedure. Inspect bearings, lubricant condition, coupling condition, rotor freedom, deposits, and signs of rotor or end-wall contact. Use the manufacturer procedure and clearance limits rather than judging internal clearance by feel.
7. Compare condition indicators. Record discharge temperature, bearing temperature, vibration, noise, delivered flow, and current on both units. High temperature with poor flow strengthens the case for recirculation, excess pressure ratio, fouling, or internal damage.

Correct the measured cause

1. If the suction measurement identifies a restriction, clean or replace the filter and remove inlet deposits. Repair damaged ducting or a valve that does not reach its commanded position.
2. If discharge pressure is excessive, clean the restricted line and repair the obstructed valve, check valve, silencer, or branch component. For unequal parallel branches, correct the piping resistance or operating arrangement through an engineering review; do not throttle the healthy blower merely to make its current match.
3. If inspection finds fouling, clean the blower by the approved method and identify how contamination entered. Repair the inlet filtration problem before returning the machine to service.
4. If bearings or internal rubbing add torque, stop operation and repair the mechanical defect. Restore rotor and end clearances to the manufacturer limits and confirm free rotation before coupling the motor.
5. If clearances indicate wear and flow is low, compare the measured performance with the blower curve. Repair or rebuild the blower when leakage prevents it from meeting duty without abnormal temperature or load.
6. Change one condition at a time, then repeat the same current, pressure, temperature, vibration, and flow measurements. This keeps a temporary header change from looking like a successful repair.

Verify the repair under matched conditions

Return the piping and valves to their normal operating lineup. Record suction pressure, discharge pressure, current on each conductor, voltage, discharge temperature, bearing condition, vibration, speed, and delivered flow. Repeat the measurements on the reference blower at the same header demand.

• The repaired blower should operate within its motor nameplate and blower performance limits.
• Its current should move toward the 51-52 A reference range only when both machines operate at comparable pressure, speed, inlet condition, and flow.
• Local pressure readings should no longer show excess resistance in the suspect branch.
• Temperature and vibration should remain stable rather than rising during the test.
• The overload protection must retain the setting required by the motor, protection scheme, and applicable design documents. Raising the setting to tolerate 62-63 A does not remove the load.

Avoid the fixes that waste time

• Do not install another motor before proving an electrical defect. The first replacement left the current unchanged.
• Do not call the 30 A reading a true no-load test. An open discharge removes piping resistance but not motor excitation or blower losses.
• Do not clean only the discharge pipe without measuring pressure. The restriction may be at the inlet, a valve, a silencer, a check valve, or the common header.
• Do not compare amperes without matching operating conditions. Identical model numbers do not create identical pressure drops.
• Do not blame ordinary clearance wear for high current without checking flow and mechanical condition. Leakage and rubbing produce different diagnostic patterns.
• Do not keep operating through rubbing noise, increasing vibration, or rising bearing temperature. Continued operation can turn a repairable clearance or bearing problem into rotor and casing damage.

FAQ