For engineers and facility managers

Motor and VSD savings for pumps and fans

Pumps and fans that run at full speed and throttle their flow waste energy against a valve or damper. A variable-speed drive slows the machine instead, and the affinity laws say power falls much faster than flow.

Inputs

Pre-filled with an illustrative case. Results update as you type. Jump to results

Motor and operation

Motor rating

Shaft load at full flow

Share of rated power at design flow.

Running hours

h/yr

Average energy cost

R/kWh

Duty profile

Flow as % of design flow, and the share of running hours at that flow. Time shares must add up to 100%.

Flow 1 (%)

Time at flow 1 (%)

Flow 2 (%)

Time at flow 2 (%)

Flow 3 (%)

Time at flow 3 (%)

Flow 4 (%)

Time at flow 4 (%)

System

Flow control today

Sets how today's power falls with flow (see the method).

Power at zero flow today

Throttle/damper ≈ 60% (assumed); bypass 100%.

Static head

0 for fans and closed loops; the lift share of total head for pumps.

Drive

Motor efficiency

VSD efficiency

Installed VSD cost

R/kW

Results

A VSD saves about 107,294 kWh a year (28%), worth R150k; payback 1.2 years.

Energy saved: 107,294kWh; 28% of today's use
Annual saving: R150k; At R1.40/kWh
VSD cost: R188k; 75.0 kW drive
Simple payback: 1.2years; Before maintenance and incentives

NoteToday's power at reduced flow is an assumption (60% at zero flow, rising in a straight line). Measured kW at two flows would firm it up.

Power against flow

Electrical input as a share of today's full-flow power. The gap between the lines at your duty points is the saving. Hover for values.

Today
With VSD

Duty points

kW is electrical input. Scroll sideways on a phone.

Point	Flow	Hours	Today kW	VSD kW	Today kWh	VSD kWh
D1	100%	1,500	67.1	69.2	100,658	103,771
D2	90%	2,100	64.4	50.4	135,284	105,909
D3	75%	1,800	60.4	29.2	108,711	52,534
D4	60%	600	56.4	14.9	33,821	8,966
Total	–	6,000	–	–	378,474	271,180

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What is this?

Save this result as one measure in a project kept in this browser, then combine it with results from other tools for payback, NPV, IRR and a cost curve.

Saving, year 1: R150,212/yrAssumed
Energy saved: 107,294 kWh/yrAssumed
Capex: R187,500Assumed

Every input above is still the illustrative example, so the result is tagged assumed. Enter your own numbers to make it yours.

Name in the project

LifeAssumed

years

Drives typically last 10 to 15 years. Default 10.

Running cost changeAssumed

R/yr

Fans, filters and capacitors in the drive. Default: 1% of capex a year.

Method

Affinity laws (same machine, speed ratio n): Q ∝ n, H ∝ n², P ∝ n³; Exact along the machine's own curve; the system curve decides where it operates.
Power with a VSD at flow fraction q: P_vsd(q) ÷ P_full = q × (h_s + (1 − h_s) × q²) ÷ η_vsd; System curve H = h_s + (1 − h_s) q². With no static head (h_s = 0) this is q³.
Power today at flow fraction q: P_today(q) ÷ P_full = b₀ + (1 − b₀) × q; A straight-line approximation: b₀ ≈ 0.6 assumed for a throttle valve or outlet damper, 1.0 for bypass.
Full-flow electrical input: P_full = rated kW × load fraction ÷ η_motor
Energy and saving: kWh = Σ hours × time share_i × P(q_i), saving = kWh_today − kWh_vsd
Payback: payback = rated kW × R/kW ÷ (saving kWh × R/kWh)

Assumptions

Machine and motor efficiencies are held constant across the speed range. In practice both fall at low load, which trims the saving a little.
The baseline power curve for throttling or dampers is a straight-line assumption; real curves depend on the machine. Measured kW at two flows is better.
Flow requirements are met at every duty point; there is no minimum-speed limit in the calculation.
The drive's own losses (default 3%) apply at every speed, so at full flow the drive costs a little energy.
Installed VSD cost per kW is illustrative and falls with size; harmonic filters, bypass contactors and cabling can add to it.

Sources

US DOE and Hydraulic Institute: Improving Pumping System Performance, a sourcebook for industryAffinity laws, system curves with static head and adjustable-speed drives.
US DOE and AMCA: Improving Fan System Performance, a sourcebook for industryFan laws and the comparison of dampers, inlet vanes and variable-speed control.
US DOE Better Plants: fan system measuresVariable-speed control instead of dampers or inlet guide vanes.

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