Pumping With a VFD: Application and Control

Short answer

In a pumping system, the VFD adjusts pump speed to hold pressure, flow or level constant, instead of throttling with valves. A PID loop is closed with a pressure, flow or level transmitter. You must define the minimum speed the pump and motor allow, the ramps that avoid water hammer, the operation of several pumps in parallel and the protections against dry running and blockage. Energy savings depend on the system curve.

Why a VFD is used in pumping

A centrifugal pump connected directly to the supply turns at a fixed speed and delivers the flow that results from the intersection of its curve and the system curve. When demand changes, regulation is needed, and the traditional ways (throttling with a valve, recirculating part of the flow, starting and stopping with a tank) waste energy, wear out equipment or cause oscillating pressures.

The VFD changes the pump speed and, with it, its curve. The affinity laws (see affinity laws and energy savings) show that flow is proportional to speed, head to its square and power to its cube, and that is the basis of the saving when the system runs at partial flow.

Control strategies

Objective	Measured variable	How the drive acts
Constant pressure in a network	Discharge pressure	Raises speed if pressure falls from higher consumption and lowers it when consumption decreases
Compensated constant pressure	Pressure and estimated flow	Lowers the pressure setpoint as flow decreases, since friction losses drop
Constant flow	Flow	Adjusts speed to hold flow in dosed processes
Tank level	Level	Regulates speed to hold a level or a smooth filling rate
Fixed or stepped speed	Operator command	No loop; useful when the process needs no feedback

The loop is usually closed with a PID controller, already included in most application drives, or in a PLC that sends the speed reference to the drive (see drive communication). The transmitter is 4-20 mA and is placed where the measurement represents the process, not next to the pump discharge if there is turbulence there. The differences between V/f, vector and DTC control are compared in V/f, vector and DTC control in VFDs. To set the PID gains, the Ziegler-Nichols method and its alternatives are explained in Ziegler-Nichols tuning and alternatives. When a single loop is not enough, cascade, ratio and feedforward control are explained in cascade, ratio and feedforward control.

Design points

Pump curve and system curve

The drive does not change the system, only the operating point. If the system has a large static head, the pump cannot lower its speed much without ceasing to deliver flow, and the saving is smaller. Overlay the system curve on the pump curves at several speeds and check that the expected flow range falls within the pump’s good efficiency zone.

Minimum and maximum speed

The minimum frequency is set by the speed at which the pump still overcomes the system head and by the limit the pump and motor manufacturers indicate (motor cooling, seal lubrication, minimum flow). The maximum must not exceed rated without manufacturer authorization.

Starting and stopping

A long start ramp avoids pressure and current surges. For stopping, the deceleration ramp and, if needed, coordinated closing of a check or control valve avoid water hammer. Coasting a pump to a stop on a long pipeline is a classic cause of hammer. Ramps are defined with the hydraulic design, not with factory values.

Several pumps

In stations with several pumps, two schemes can be used:

One drive per pump. All pumps run at the same speed, sharing equally. It is the most efficient and gives redundancy.
One drive with the others at fixed speed. One pump regulates (lead pump) and the others stage in and out. It is cheaper, but with worse efficiency and more abrupt transitions.

The logic for alternation, pump staging and wear rotation is in the PLC or in the drive’s multi-pump controller.

Special modes

Sleep mode. When demand is minimal, the drive stops the pump until pressure falls below a threshold, and prevents it from running uselessly at almost zero flow.
Pipe filling. In empty networks, a slow fill ramp avoids hammer.
Pump cleaning. Some drives include cycles to clear blockages in wastewater pumps.

Protections

Dry running: current falls below normal; detected by underload or by a level sensor.
Overload or jam: current rises; the drive protects the pump and motor.
High or low pressure: alarms and stops per the process.
Motor overtemperature: PTC probe or electronic thermal protection; see induction motor and its protections.
Cavitation: shows up as noise, vibration and current oscillation; prevented with good suction design and by not letting inlet pressure fall too low.

Illustrative example

Illustrative data, not from a real plant. An industrial water network holds 5 bar with three identical pumps and variable demand.

With a lead pump on a drive and two at fixed speed, at low demand the lead pump runs at 60% of its speed and the second stages in when the lead reaches 100%.
With one drive per pump, demand is shared. Two pumps at 70% can consume less than one at 100% plus one at 40%, because of the cubic relation of power to speed. This only holds if each pump stays in its efficient zone.
The real result is confirmed with the pump curves, not only with the cube rule.

Commissioning a pump station with a VFD

An orderly startup sequence reduces hydraulic and electrical risks:

Before energizing. Check the wiring, the motor rotation direction with the pump uncoupled or the system prepared, the motor data in the drive, and that the suction and discharge valves are in the intended position.
First run at low speed. Verify rotation direction, noise, vibration and suction pressure. Prime the pump if the design requires.
Ramp test. Start and stop the pump with the design ramps and watch pressure and pipeline knocks.
PID loop tuning. With the system running, adjust the gains until the response is stable with no oscillation or excessive delay. A poorly tuned loop makes pressure oscillate and the pump speed up and down needlessly.
Protection test. Simulate dry running, high and low pressure, and loss of the transmitter signal; verify the drive reacts as defined.
Multi-pump test. Verify staging in, out and alternation under rising and falling demand.
Baseline record. Note speed, current, pressure and flow at several operating points to compare in the future and to calculate the saving.

With these data, wear is detected later: a pump that needs more speed for the same pressure indicates impeller wear or a network leak, and it is useful information for predictive maintenance.

Common mistakes

Placing the pressure transmitter at a turbulent point.
Not defining the minimum speed, and letting the PID take the pump to speeds that deliver no flow.
PID with factory gains, which cause oscillation or sluggishness.
Short ramps on long pipelines.
Leaving the throttling valve partly closed after installing the drive.
No dry-run protection.
Ignoring the effect of harmonics in stations with several drives.
Operating at a point far from the pump’s best efficiency.

Maintenance and long-term operation

A station with a VFD ages differently from a direct-on-line one. Seals and bearings work over a wider speed range, pressure transmitters become a critical component (if they drift, the loop holds the wrong pressure) and panel filters need periodic cleaning. A reasonable plan includes periodic transmitter calibration, checking current at known speed and pressure, inspecting the drive fans, and reviewing the fault logs. If a trend is detected, such as more speed for the same pressure, investigate pump wear or leaks before the problem becomes a shutdown.

Checklist

Pump curve and system curve with static head.
Expected flow profile over the day and the year.
Minimum and maximum speed accepted by the manufacturer.
Pressure, flow or level transmitter at a representative point.
Start and stop ramps consistent with the hydraulic design.
Multi-pump logic, alternation and sleep.
Protections for dry running, overload, pressure and temperature.
Test with real load and energy record before and after.
Harmonics review if several drives share the same network.

How we help

Good pumping with a VFD is decided by studying the system curve and the power quality. In Engineering design and studies we do the selection, the control scheme and the network analysis, and during commissioning, in Colombia or elsewhere on request, we verify operation with the real process. Write to us through the contact page.

References

Wikipedia, “Affinity laws” and “Variable-frequency drive” (pump relations and speed control).
Pumping application manuals from drive and pump manufacturers.
Pump curves from each equipment manufacturer.

Frequently asked questions

What signal is used to control a pump with a VFD?

The one that represents the process objective: discharge pressure to hold a network at constant pressure, flow for a dosed process, or tank level. A 4-20 mA transmitter delivers the measurement to the drive or the PLC, which closes the PID loop against a setpoint.

Can I lower a pump's speed to nearly zero?

No. Below a minimum speed the pump does not overcome the system head and delivers no flow, the motor cools poorly and the seals can suffer. The pump manufacturer states the allowable range; the drive is set with a matching minimum frequency.

How is water hammer avoided with a VFD?

With start and stop ramps long enough for the flow to change gradually, and with a controlled stop instead of a coast. On long pipelines the minimum ramp time is calculated. The drive helps, but does not correct a poor hydraulic design.

What does the drive do if the pump runs out of water?

A dry pump draws less current than normal, and many drives detect that low load and stop with an underload or dry-run fault. A level sensor or a low-pressure switch is also used. Detection must be planned in the design and set for the operating point.