What is a VSD or variable speed pump

What is a VSD on a water pump?

VSD stands for variable speed drive. You will also see VFD, variable frequency drive, or just inverter. Same thing: power electronics between your supply and the pump motor.

The speed of an AC induction motor is set by the frequency of the supply feeding it, and New Zealand mains is 230 volt single phase at 50 hertz. So a plain fixed speed pump motor has one running speed. A drive builds its own AC output at whatever frequency it chooses. Lower frequency, slower motor.

How does a variable speed pump hold constant pressure?

A pressure sensor on the outlet, a setpoint in the controller, compared continuously.

• Open a tap, pressure falls, the drive ramps the motor up until the sensor reads the setpoint again, then holds there.
• Open a second tap, more flow leaves the system, the drive winds up further. Same pressure, more flow.
• Close everything, pressure rises, the drive winds down and stops.

A fixed speed pump on a pressure switch does none of that. It starts when pressure falls to the cut in setting, runs flat out, and stops at the cut out setting. Pressure is always climbing or dropping, and that swing is what you feel when someone flushes a toilet mid shower.

Most variable speed pumps still carry a small pressure vessel, built in or plumbed alongside, so a dripping tap is not starting the pump every few seconds.

What does it feel like at the tap?

The limit is still the pump. A VSD holds pressure only until the motor hits full speed. Past that, pressure falls away as it would on any pump. Size the pump for the flow you want.

Do variable speed pumps use less power?

Per litre pumped, usually yes. Under the pump affinity laws the power a centrifugal pump absorbs varies roughly with the cube of its speed, so a pump loafing at part speed to feed one tap draws well under what it draws flat out.

Do not take the cube literally, though. That relationship assumes the head falls away as the pump slows, and a variable speed pump is doing the opposite: it slows down while holding the pressure up. Add the static lift from the tank or bore and the real saving is a good deal smaller than the arithmetic suggests.

The honest part: a house pump runs a few minutes a day, so a percentage off a small number is still a small number. The drive also draws a small standby current while it waits.

Are variable speed pumps quieter?

Usually. Most of the time it runs well below full speed, and a motor at part speed is quieter than the same motor flat out. You also lose the hard start, the hard stop and the click of the pressure switch, which is often the noise people complain about. Not silence. Drive electronics can add a faint whine, and nothing fixes a pump bolted to timber framing beside a bedroom wall.

Will a VSD run on solar, an inverter or a generator?

A fixed speed motor started direct on line draws several times its normal running current for the first moments while it comes up to speed. On mains you never notice. On an off grid inverter or a small generator, that surge trips the supply, or forces you into a much larger inverter than the running wattage suggests.

A VSD starts the motor from zero and ramps it, so peak current on start sits much closer to running current. That is soft start, and off grid it is often the whole reason to fit variable speed.

Do variable speed pumps last longer?

Starting is the hardest thing a pump motor does: full inrush current, full torque, and a mechanical seal going from stopped to full speed in an instant. A variable speed pump ramps up, holds, ramps down, and makes a full start far less often on the same house. Gentler pressure changes are easier on pipework and fittings too.

The trade off is electronics on a machine that used to be a motor and a switch. A failed pressure switch is a cheap part any pump person carries. A failed drive board is a much bigger bill. Heat and moisture kill drives, so keep the unit shaded, ventilated and dry.

Siting it, shading it and keeping the vents clear is owner work. What is behind the cover is not. Wiring, connecting or fault finding a drive is prescribed electrical work in New Zealand and belongs to a registered electrician. If a drive is misbehaving, isolate it at the switchboard and ring us or your sparky rather than opening it up.

What does a variable speed pump cost?

Straight up: considerably more than a fixed speed pump of similar duty. Prices move, so take them off the product pages. The money buys held pressure, quieter running, soft start and fewer cycles. In a two bathroom house you notice all four. On a trough pump they are worth little.

When is a fixed speed pump plus a controller the better buy?

More often than the marketing suggests. A controller like the Davey Torrium2, the Grundfos PM PLUS or the simpler Grundfos PM Start starts a fixed speed pump when you open a tap, stops it when you close one, and shuts it down if it runs dry. No held pressure, but simple and cheap to fix.

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