Why VFDs Draw High Amps at Low Hz: Normal vs Problem Signs

A variable frequency drive does not simply turn the motor down like a dimmer switch. To keep the motor from stalling or losing magnetic flux at reduced speed, the VFD lowers output voltage in a fixed ratio alongside frequency, commonly called the V/Hz ratio. At 60 Hz the drive might output 460V; at 30 Hz it targets roughly half that voltage so the volts-per-hertz ratio stays constant. A VFD reading a low Hz output on its display is not, by itself, a problem: the drive is doing exactly what it is designed to do. What changes the current reading is the load the motor drives, not the frequency number alone. A motor under constant torque (a conveyor, a mixer, a compressor) needs close to the same torque at any speed, and torque is what drives current draw, so current stays near the motor's full load amps (FLA) across most of the speed range. On JOHOB-supplied VFD duplex control panels, this V/Hz behavior is standard and documented on the panel's parameter sheet, so a site electrician can confirm the drive is behaving as designed before assuming a fault.

Constant torque equipment is the most common source of a VFD that looks like it is pulling too much current at a low Hz reading. Mixers, extruders, briquette lines, and conveyor drives all need roughly the same torque to turn whether they run at 50 Hz or 15 Hz, because the resistance they are working against (the mix viscosity, the material being compressed, the load on the belt) does not change with speed. Since torque is proportional to current in an induction motor, a constant torque load will hold its current close to nameplate FLA even as the frequency display drops. A reading of 40 amps at 20 Hz on a motor rated for 42 amps FLA at 60 Hz is normal operation, not an early warning sign, provided the figure does not exceed the nameplate FLA stamped on the motor's data plate. This is also why JOHOB sizes VFD duplex control panels and electrical panels (VFD, SDE, star-delta, MCC type) around the motor's FLA and duty cycle rather than around the frequency range alone, since the panel's breaker, contactor, and overload relay all have to carry that current continuously, not just at full speed.

Centrifugal pump loads behave the opposite way, which is what makes a high current reading at low Hz worth checking on a booster, transfer, or submersible pump system. By the pump affinity laws, torque and horsepower on a centrifugal load fall with the square and cube of speed, so at 30 Hz (half of 60 Hz) a centrifugal pump should need roughly a quarter of its full-speed torque, and current should drop well below nameplate FLA, not hold near it. If a booster or transfer pump on a VFD duplex panel shows current staying flat or climbing as the frequency drops toward 20 to 30 Hz, that is not the expected affinity law curve, and it points to a mechanical or hydraulic problem upstream of the drive: a partially closed valve forcing the pump to work harder at low speed, impeller wear changing the pump curve, a bound bearing, or a check valve holding back pressure the pump has to fight at every speed. On variable torque pump applications this is the clearest single signal to flag for inspection, because the physics predicts current should fall, and a reading that does not fall is the panel telling you something downstream changed, not that the VFD itself is at fault.

A short checklist separates normal V/Hz behavior from a developing fault, and it should be the first thing an electrician checks before opening the panel. Current is almost certainly normal when the load is constant torque (mixer, extruder, conveyor, briquette line) and the reading stays at or below the nameplate FLA stamped on the motor, when the reading tracks the parameter sheet the panel was commissioned with, and when the motor runs cool to the touch at the frame and bearing housings. Current points to a developing problem when a centrifugal pump load fails to drop current as frequency drops toward 20 to 30 Hz, when the reading exceeds nameplate FLA at any frequency, when the motor overheats despite a current reading that looks acceptable, or when current is stable but rises slowly over days or weeks on a load that has not changed. The first group calls for no action beyond logging the reading against the commissioning baseline. The second group calls for a technician to check the mechanical load, the overload relay setting, and the drive's voltage boost parameter before the motor or panel is assumed to be defective, since on a VFD system the fix is often in the piping, the coupling, or the parameter sheet rather than in the drive itself.

A current reading that looks acceptable on the display can still coincide with an overheating motor, because a VFD running a standard induction motor at low Hz also slows down the motor's own shaft-mounted cooling fan. That fan is sized to cool the motor at or near full speed, so at 15 to 20 Hz it is moving a fraction of the air it would at 60 Hz, even though the motor may still be carrying close to full current on a constant torque load. The result is a motor that runs hotter at low speed for the same current than it would at full speed, which is why continuous low-speed operation on a constant torque load is the case where thermal protection matters more than the amp reading alone. Motors intended for continuous low-Hz duty typically need an independently powered cooling fan or a derated duty cycle, a detail that should be confirmed at the quoting stage, not discovered after nuisance trips start. JOHOB specifies motor and panel protection, including CE-marked electrical panel components, against the actual duty cycle and speed range a buyer describes on the RFQ, rather than against the nameplate alone, because the nameplate FLA and the safe continuous current at 20 Hz are not the same number on every constant torque application.

Protecting against the real current behavior of a VFD load, rather than against the nameplate alone, is a panel design decision, not a field fix. JOHOB supplies VFD duplex control panels and VFD, SDE, star-delta, and MCC-type electrical panels sized to the motor's actual FLA and duty cycle, with components built to ISO 9001 certified partner factory standards, TUV SUD tested, and CE marking on the electrical components, so the breaker, contactor, and overload relay are rated for continuous current at the load's real speed range, not just at 60 Hz. Pricing is quote-based since panel specification depends on motor FLA, duty cycle, and site voltage, and standard lead time runs 21 to 35 days ex-stock or 45 to 90 days for a made-to-order panel configuration, with an RFQ response within 1 business day. Buyers that are PEZA or SBMA locators, or exporters, are zero-VAT on qualifying purchases; standard VAT-registered buyers carry the 12% VAT. JOHOB serves pumping, electrical panel, and VFD buyers nationwide across NCR, Luzon, Visayas, and Mindanao, including PEZA, SBMA, and Clark zone sites, so a plant engineer anywhere in the Philippines can get a panel spec reviewed against the actual current behavior described in this guide before ordering a replacement or an upsized unit.