Variable Frequency Drives in Pump Panels

What the drive does

A variable frequency drive rectifies the incoming three-phase supply to DC, filters it on a capacitor bus, and inverts it back to AC at whatever frequency and voltage the control asks for, using pulse-width modulation. Motor speed follows frequency, so the pump runs at the speed the process needs. Along the way the drive ramps the motor up and down, limits current, monitors motor thermal state, and reports current, voltage, power, torque, and faults over a network or through its terminals.

For a pump the speed control is the point. A level loop holds a wet well at a setpoint by varying pump speed instead of cycling; a pressure loop holds distribution pressure without a pressure-reducing valve; a flow loop feeds a process at the rate it needs. The other benefits, soft starting and reduced inrush, matter at stations on weak power or on a generator.

When a drive earns its cost

Minimum speed

A centrifugal pump develops head roughly with the square of speed. Below some speed it cannot produce enough head to open the check valve against the static head of the system, and it spins in the well doing nothing while the level rises. The drive minimum speed is set above that point, found from the pump curve at the static head or by test, and it is often in the range of 50 to 70 percent for a wastewater pump on a force main. Setting the minimum to the drive default of zero, or to a number that felt reasonable, is one of the most common commissioning errors in pump stations.

Running near minimum speed for long periods has its own problems: low velocity in the force main lets solids settle, and the pump operates far from its best efficiency point. A level PID loop with a minimum speed, and stop-and-restart logic for when inflow drops below what minimum speed moves, is the usual answer.

What the drive costs

Heat

A drive dissipates roughly 2 to 4 percent of its rating as heat, all of it inside the enclosure. A 50 hp drive at full load makes on the order of a kilowatt. That heat sets the enclosure size and cooling, as described on the heat calculations page, and it is why drives in outdoor enclosures in the sun fail early.

Harmonics

The rectifier draws current in pulses, distorting the supply. A single small drive rarely matters; a station with most of its load on drives can exceed the distortion limits in IEEE 519 and heat the transformer. Line reactors, DC bus chokes, and for larger installations active front ends or harmonic filters address it.

Output voltage spikes

The PWM output has fast edges that reflect at the motor terminals, especially on long cable runs, producing peaks that stress motor insulation. Inverter-duty motors, output reactors or dV/dt filters, and cable length limits from the drive manufacturer are the mitigations. Submersible pumps on long cables are the sensitive case.

Bearing currents

The common-mode voltage of the PWM output can discharge through motor bearings and pit them. Insulated bearings, shaft grounding rings, and common-mode chokes address it on larger motors.

Instrument noise

Output cables radiate. Signal cables routed with them pick it up. Shielded output cable, separation, and proper grounding of the shield at the drive prevent most of it.

A single point of failure

A failed starter is replaced from stock in an hour. A failed drive may be a week away. Bypass or a spare is a design decision.

Bypass

A bypass contactor arrangement lets the pump run across the line at full speed when the drive has failed. It costs a contactor pair, an interlock so the drive output and the bypass contactor can never close together, and a selector switch. In a duplex station with two drives, many utilities skip bypass and rely on the second pump plus a spare drive on the shelf. In a station with one pump, bypass is cheap insurance. The decision is easy at design and expensive after the panel is built.

Integration with the panel

• Control: run command and speed reference by hardwired inputs or by network. Network control gives full diagnostics; hardwired control keeps running when the network does not. Many panels use network for monitoring and hardwired for the run and the reference, or the reverse with a hardwired fallback.
• Feedback: running status, fault, current, and speed back to the controller. A drive that reports a run status from its own logic is a better proof than an auxiliary contact.
• Protection: the drive provides overload, overcurrent, phase loss, and ground fault protection for the motor. Upstream, the drive itself needs short-circuit protection that matches its listing, and the SCCR of the panel depends on it.
• Enable and safe torque off: hardwired through the HOA and any safety devices, independent of the controller.
• Parameters: backed up, printed, and kept with the panel drawings. A replacement drive is configured from that backup, not from memory.