VSD on a pump: when a variable speed drive pays back, and what to specify so it causes no new problems

A VSD on a pump saves energy when the pump would otherwise run at full speed against a part-closed valve or a bypass. The system's resistance must also be mostly pipe friction. A variable speed drive (VSD) is an electronic controller. The controller sets a motor's speed by changing the frequency and voltage it supplies.^[1][2]

The saving comes from the affinity laws. Natural Resources Canada's guide to variable frequency drives states that the power input to fans and pumps varies with the cube of the speed. At half speed, the power required is about one eighth of the rated maximum.^[2]

The US Department of Energy's variable speed pumping guide was written with the Hydraulic Institute and Europump. The guide says that where friction loss predominates, slowing the pump keeps it near its best efficiency point. Variable speed is then the ideal control method. The same guide notes that a control valve is normally designed to be 10% shut even at maximum flow. A fixed-speed pump therefore always pays for that valve's friction.^[1]

The guide also estimates that 75% of pump systems are oversized, many by more than 20%. A retrofit therefore often finds a pump throttled for most of its hours. Before buying a drive for an existing pump, compare the drive with other routes to the same duty. Routes on the pump supply side include a trimmed impeller or a smaller pump.^[1]

Work the payback from measured kilowatts and your own tariff, never from a percentage quoted for another site. Log the pump's input power and flow over a normal week. Estimate the input power at the lower speeds from the pump and system curves. Then multiply the kilowatts saved by the hours in each time-of-use period and that period's energy rate.^[1][3]

The national utility's large-customer tariffs price each kilowatt-hour by season and time of use. The high-demand season runs from 1 June to 31 August. The low-demand season runs from 1 September to 31 May. On Megaflex, the peak rate in the high-demand season is about six times the off-peak rate. A pump that runs mostly in peak and standard hours therefore saves more per kilowatt than one that runs at night.^[3]

The national utility's 2026/27 Megaflex rates apply to non-local authority supplies from 1 April 2026 to 31 March 2027. The National Energy Regulator of South Africa (NERSA) approved an average increase of 8.76% for the utility's direct customers. Municipal tariffs moved on 1 July 2026, with municipal bulk purchasers averaging 9.01%.^[3][5]

A demand charge falls only if the drive lowers the highest half-hour demand in the charged periods. On Megaflex, the distribution network demand charge is levied in rand per kVA on the chargeable demand. The chargeable demand is the highest average kVA in the peak and standard periods of the month. A pump slowed outside those hours changes the charge little. So does a pump whose start-up current set the peak.^[3]

The City of Cape Town's Large User Low Voltage Time of Use tariff for 2026/27 charges 742.64 c/kWh (excluding VAT) for high-demand-season peak energy. Off-peak energy costs 148.43 c/kWh. An unregulated charge of 37.06 c/kWh applies on every unit (43.11 c/kWh in 2025/26). The peak and off-peak rates in 2025/26 were 685.93 and 136.93 c/kWh. Divide the drive's installed price, VAT treated the same way, by the yearly saving to get the simple payback.^[6]

The highest 2026/27 Megaflex energy rate is the high-demand-season peak at 720.19 c/kWh. The lowest is off-peak at 120.03 c/kWh. Both apply to a supply within 300 km of Johannesburg at 500 V to below 66 kV and exclude VAT. A legacy charge of 24.14 c/kWh (22.20 in 2025/26) applies in every period on top.^[3][4]

Sources:^[3][4]

A VSD pays back poorly where the flow is constant, where static head dominates, or where the load needs constant torque. Static head is the height, or the vessel pressure, the pump must overcome at any flow. Friction head is the pipe and valve loss, and the loss rises with the square of the flow rate.^[1][2]

In a system with high static head, the US Department of Energy guide says the flow no longer falls in proportion to speed. A small turn down in speed then greatly reduces flow and pump efficiency. The guide calls using the affinity laws to calculate savings in systems with static head a common mistake. The mistake can lead to major errors.^[1]

For a positive displacement pump, the same guide states that flow is proportional to speed while pressure can be independent of it. Power and energy savings therefore do not fall as quickly when speed is reduced. Natural Resources Canada classes constant-torque loads as giving moderate savings at lower speeds. Natural Resources Canada classes constant-horsepower loads as giving none.^[1][2]

Slowing a pump has limits too. Every fixed-speed centrifugal pump has a minimum flow. The guide also warns about liquids that carry solids with a high settling rate. The velocity must stay high enough to stop solids settling and wearing the pump.^[1]

Yes: a VSD draws a distorted current. In South Africa the Distribution Network Code requires distributors and other participants to comply with NRS 048 and the NERSA Power Quality Directive. The requirement covers voltage harmonics and inter-harmonics, among other parameters (section 6.2(1), version 6.2 of January 2022).^[7][2]

The distributor studies the impact of additional loads or a major modification on quality of supply. The distributor must refuse to connect a facility if the studies show it would exceed the parameters. The parameters are those in NRS 048 and the Power Quality Directive (section 4(9) and (10)).^[7]

A customer with demand above 100 kVA must keep its power factor at no less than 0.9 lagging. The customer must not let it go leading unless the distributor agrees. The customer must also inform the distributor before installing shunt capacitors for that purpose (section 6.3).^[7]

Natural Resources Canada's guide states that harmonics cause waveform distortion. The distortion can propagate through the entire power system and make motors run hotter. The guide says a line reactor of 3% to 5% impedance generally prevents drive harmonics from interfering with sensitive equipment.^[2]

Most standard induction motors can run on a modern VSD at 400 V, but older motors, long cables and larger motors need checking first. The US Department of Energy guide says modern drive outputs cause relatively small harmonic distortion in the windings. Little derating is therefore normally needed. The guide adds that drives can be fitted to most existing motors on 400 V networks.^[1]

In practice, check these points before the drive is ordered:^[1][2]

• the winding insulation: the guide warns that older insulation systems may deteriorate faster under the drive's rapid voltage changes, which output filters prevent;^[1]
• the cable length from drive to motor, since long runs can raise the voltage at the motor terminals;^[1]
• the bearings: the guide recommends insulated non-drive-end bearings on all motors over 100 kW, because induced shaft voltages can drive currents that destroy bearings;^[1]
• cooling at the lowest speed, since a motor's own fan moves less air when it slows and a separately driven fan may be needed;^[1]
• the location: the guide says operating a drive in a potentially explosive atmosphere is not usually possible. On a hazardous-area pump, ask where the drive itself will stand.^[1]

A VSD installation needs a certificate of compliance (CoC) for the electrical work. Under South Africa's Electrical Installation Regulations, 2009, a person who undertakes electrical installation work must ensure that a valid CoC is issued for that work. Regulation 9(4) sets that duty. An addition or alteration needs a CoC for at least that work (regulation 7(4)).^[8]

The regulations define the electrical installation as the machinery that carries electricity from the point of control to each point of consumption. The definition includes the supply terminals of a hard-wired machine (regulation 1). The supply to the drive and the cable to the motor therefore fall within it.^[8]

The wiring code the CoC tests against is SANS 10142-1. Only a registered person may issue the CoC (regulation 9(1)). Ask the industrial electrician for the registration and its category before the work starts.^[8]