Symptom
The drive is running and shows a reference of, say, 60 Hz, but the output frequency stays lower, the motor runs slowly, or the pump does not deliver its rated flow, with or without a warning on the display.
The short answer
Motor Will Not Reach Speed
A motor on a drive that will not reach speed is limited by something the drive is doing deliberately or by a load it cannot overcome. The drive readouts separate them: if the output frequency matches the reference and the motor is slow, the reference or its scaling is wrong; if the output frequency stops below the reference at a limit parameter, the maximum frequency or a speed limit is set low; if the output frequency stalls with the current at the limit, the drive is holding the ramp on current limit against a load; if the frequency is right and the speed is wrong, the slip is large from an overloaded or wrong-sized motor, or the motor data in the drive is wrong.
Key points
- Compare four numbers on the drive display: the reference, the output frequency, the output current, and the motor speed estimate.
- Reference low: the scaling from the controller or the analog input. Output frequency capped: a limit parameter.
- Output frequency stalled with current at the limit: the load is more than the drive can accelerate. Find out why.
- Frequency right, speed wrong: slip from overload, or wrong motor data in the drive.
- A minimum speed set high or a maximum set low after a parameter reset is the common configuration cause.
Possible causes and what to check
| Possible cause | What to check |
|---|---|
| Speed reference is low | The reference on the drive display is below the expected value: controller analog output scaling, a reference at 10 V that the drive reads as 5 V from a wrong input range, a network reference in the wrong units, or a potentiometer. |
| Maximum frequency or speed limit set low | The output frequency stops at a round number below the reference: the maximum frequency, an upper speed limit, or a skip band. Check the limit parameters. |
| Current limit holding the ramp | The output frequency climbs and stalls while the current sits at the limit; the drive is protecting itself. The load is high: a clogged pump, a closed valve, a high head, or a motor too small. |
| Torque limit or power limit | Similar to current limit, on drives that limit torque; the drive readout shows the limit active. |
| Motor overloaded | The frequency reaches the reference but the motor slips badly; current is high; the load exceeds the motor rating. |
| Motor parameters wrong | Nameplate data or the auto-tune wrong; the drive computes a wrong slip compensation or a wrong flux, and the speed is off even at correct frequency. |
| Volts per hertz pattern wrong | A base frequency set wrong, or a pattern chosen for the wrong load type; the motor is under-fluxed or over-fluxed and cannot make torque at speed. |
| Input voltage low | A drive on a sagging supply cannot make full output voltage at 60 Hz and the motor slips under load; the bus voltage on the display is low. |
| Ramp still in progress | A very long acceleration time; the drive has not finished. Check the acceleration parameter. |
| Single phasing on the output | A lost output phase from a cable or a contactor; the motor runs rough and slow with high current on two phases. |
| Mechanical: a pump that cannot turn at speed | Rags, a bearing, or a broken coupling letting the motor spin freely; the drive shows speed and the pump does not turn. |
Four numbers
The drive display shows the speed reference it is receiving, the output frequency it is producing, the output current, and, on vector drives, its estimate of the motor speed. Those four numbers, read together, put the fault in one of a few places before anything is measured.
| Reference | Output frequency | Current | Meaning |
|---|---|---|---|
| Low | Equals the reference | Normal | The reference is wrong: scaling, input range, network units |
| Correct | Stops below the reference at a round number | Normal | A limit parameter: maximum frequency, upper limit, skip band |
| Correct | Stalls below the reference | At the current limit | The drive is holding the ramp against a load it cannot accelerate |
| Correct | Equals the reference | High | The motor is running at frequency and slipping: overload or wrong motor data |
| Correct | Equals the reference | Low | The motor is at speed and the pump is not doing work: a coupling, a broken shaft, air |
| Correct | Still climbing slowly | Normal | The acceleration ramp is long; wait or shorten it |
| Correct | Correct | Unbalanced | A lost output phase |
Diagnostic procedure
- 1
Read the four numbers
From the drive display or the network status, with the drive in the condition that shows the problem. Match them to the table.
- 2
If the reference is wrong
Follow it back: the analog output from the controller and its scaling, the drive analog input range and scaling parameters, or the network reference and its units. A 0 to 10 V reference on an input set for 4 to 20 mA reads wrong; a network reference in tenths of a hertz read as hertz is ten times off.
- 3
If a limit is active
The drive shows which: maximum frequency, upper speed limit, a skip frequency band, or a limit from a digital input such as a jog or a preset speed. A parameter reset puts the maximum at a default that may be lower than the application needs, and a preset speed input stuck on holds the drive at that preset.
- 4
If the current limit is holding
The drive cannot deliver the torque to accelerate further. Check the load: lock out and turn the pump; look at the discharge valve and the check valve; compare the head with the pump curve. Then check the motor sizing against the pump duty point. Then, if the load is healthy, the ramp and the current limit setting against the motor rating.
- 5
If the motor slips at frequency
High current with correct frequency is an overloaded motor or wrong motor data. Confirm the nameplate data in the drive, run the auto-tune with the motor uncoupled if the manual requires it, and compare the current with the nameplate full-load current. A pump that needs more than the motor rating at speed is a sizing problem.
- 6
Check the volts per hertz
The base frequency and voltage, and the pattern, linear or squared for a pump. A base frequency set to 50 Hz on a 60 Hz motor over-fluxes it above 50 and limits the speed; a squared pattern on a positive displacement pump under-fluxes it.
- 7
Check the supply
The DC bus voltage on the display against 1.35 times the line voltage; a low supply limits the output voltage and the motor slips at full speed under load.
- 8
Check the output phases
Current on each output phase from the drive display or a clamp meter; a missing or low phase is a cable, a terminal, or a contactor.
- 9
Fix and confirm
Run the pump to full speed under load and confirm the flow matches the curve.
Common mistakes
- Raising the maximum frequency above 60 Hz to get more flow from a pump. The pump power rises with the cube of speed and the motor is overloaded in minutes.
- Raising the current limit to get through a stall. The motor and the drive are then unprotected against the load that caused it.
- Assuming the controller is sending the right reference because the HMI shows 100 percent. Read the reference on the drive.
- Not re-entering the motor data after a drive replacement or a parameter reset. The drive runs, and runs wrong.
- Confusing frequency with speed. A drive at 60 Hz on a motor with 5 percent slip under overload is a motor at 1,710 rpm, not 1,800.
Frequently asked questions
- The drive shows 60 Hz and the pump only delivers 70 percent of its flow. What is wrong?
- If the current is high, the motor is slipping under an overload and the pump is at a lower speed than the frequency suggests, or the pump itself is worn or partly clogged and delivers less at speed. If the current is normal or low, the pump is at speed and the flow shortfall is hydraulic: a worn impeller, a partly closed valve, a high head. Compare with the pump curve at the measured head.
- After replacing the drive the pump runs slower than before. Why?
- The new drive is at default parameters: a maximum frequency, a motor data set, a volts per hertz pattern, or a reference scaling that differs from the old one. Load the backed-up parameter set, or re-enter every parameter from the record, and confirm the four numbers.
- Can I run the motor above 60 Hz to compensate?
- Only with the pump and motor manufacturer approval and a check of the motor power at the higher speed. Above base frequency the motor voltage cannot rise further, so torque falls, while the pump power demand rises with the cube of speed. Most pump applications never exceed base frequency.
- The drive says the motor is at speed but the pump is not turning. What is left?
- The coupling, the shaft, or the key between the motor and the pump. The motor is spinning free at low current. Lock out and inspect.
Related topics
- Drive Faults on OvercurrentAn overcurrent trip on a variable frequency drive: what the drive measured and when, the difference between a trip at start, during acceleration, at steady speed, and at deceleration, the motor and cable faults behind each, the parameter settings that cause nuisance trips, and the tests that separate them.
- Drive Will Not Start in AUTOThe drive runs from the keypad or in HAND but not from the controller. The command source, the enable and safe torque off inputs, the speed reference, and the network control word, checked in the order that finds the problem fastest.
- Pump Runs But No FlowThe motor turns and nothing moves. How to tell a clogged impeller from an air-bound pump, a closed valve, a stuck check, a broken coupling, or reverse rotation, using motor current and discharge pressure before pulling anything.
- Variable Frequency Drives in Pump PanelsWhat a drive adds to a pump panel and what it costs: speed control, soft start, reduced inrush, and diagnostics against heat, harmonics, cable and motor stress, bypass, and the minimum speed below which a pump does no work.
- VFD Pump Control in WastewaterThe control modes for a variable speed wastewater pump: level control, flow pacing, fixed speed with soft start, and drawdown modes, plus the wastewater-specific rules: minimum speed, ragging at low speed, force main velocity, and the periodic full-speed run.
- Drive Faults on OvervoltageA DC bus overvoltage trip on a drive: where the bus voltage comes from, why deceleration and a driving load raise it, the utility-side causes from a high supply and capacitor switching, the deceleration ramp and overvoltage controller settings, braking resistors, and how to tell a regeneration trip from a supply transient.
Direct contact
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