VFD retrofit on a throttled 110 kW cooling-water pump
A 110 kW, 415 V, 4-pole, 1485 rpm cage induction motor with full-load efficiency 96% and power factor 0.86 drives a cooling-water pump through a partly closed discharge valve. At the design point the pump delivers 900 m3/h at 40 m head, of which 15 m is static (the cooling-tower elevation) and 25 m is friction. The plant now runs at 585 m3/h, i.e. 65% of design, with the valve throttled, and the measured motor input is 88 kW. The tariff is Rs 7.5/kWh and the pump runs 4,000 hours per year. A vendor offers a 110 kW, 6-pulse VFD with a claimed payback of 14 months. The maintenance engineer is sceptical: 'the motor is already only drawing 88 kW, so there is very little left to save, and the VFD itself will lose 3-4%'.
1. Calculate the motor's full-load current and estimate the input power with a VFD at 65% flow, taking the static head into account.
Read the worked answer
Full-load input power = 110/0.96 = 114.6 kW, so full-load current = 114,600/(1.732 x 415 x 0.86) = 185 A. This is the figure to use for the VFD, cable and protection sizing, not the 88 kW currently measured.
For the VFD case the valve is fully open and the pump speed sets the duty. At 65% flow the system head is the static part plus the friction part at that flow: H = 15 + 25 x 0.65^2 = 15 + 10.6 = 25.6 m. For the same pump curve, head scales as n^2 and flow as n, so n = sqrt(25.6/40) = 0.80, i.e. 80% speed, not 65% speed.
Shaft power then scales as n^3: 110 x 0.80^3 = 56.3 kW. Allowing for motor efficiency falling to about 94% and drive losses of about 2% at this load, the input is approximately 56.3/0.94/0.98 = 61 kW.
The saving is 88 - 61 = 27 kW, not the 88 - (0.65^3 x 110) = 58 kW that a naive cube-law estimate would claim. That distinction is the whole point of the static-head correction.
2. Is the vendor's 14-month payback credible? Quantify it and list the assumptions that could invalidate it.
Read the worked answer
Energy saved per year = 27 kW x 4,000 h = 108,000 kWh, worth 108,000 x 7.5 = Rs 8.1 lakh. A 110 kW 6-pulse VFD with bypass, filters, cabling and installation is typically Rs 8-12 lakh, so the payback is around 12-18 months. The vendor's order of magnitude is credible; the precise figure is not, because it depends on assumptions the vendor has not stated.
Assumptions to test:
- The affinity laws assume the system curve is unchanged (valve fully open, no bypass flow) and pump efficiency holds at 80% speed. If the pump efficiency drops from 78% to 72% at 80% speed, the saving falls by roughly 6 kW.
- The 15 m static head is 37.5% of the total, which is why the speed is 80% rather than 65%; if the tower height or the minimum back-pressure is higher than stated, the saving shrinks further.
- The 88 kW throttled reading should be verified with a calibrated meter over a full week, at the actual flow, not a spot reading.
- If the valve is throttled partly to guarantee a minimum flow through the pump, the VFD must retain a minimum-speed or bypass arrangement, which reduces the achievable saving.
- The tariff and 4,000 h/yr must be the real figures; a lower load factor stretches the payback proportionally.
3. What electrical and mechanical checks should be completed before the VFD is ordered and installed?
Read the worked answer
Motor suitability: check whether the motor is inverter-duty. A standard motor on a PWM drive sees steep voltage fronts; if the cable from drive to motor exceeds about 30 m, add a dv/dt or sine filter, and confirm the motor's insulation class and winding temperature rise. At 80% speed the shaft-mounted fan delivers only about 50% of its cooling, so confirm the motor can run continuously at 80% speed and 61 kW without exceeding class F rise; derate 5-10% if the ambient is above 40 degrees C.
Bearing currents: with a 110 kW frame, use an insulated non-drive-end bearing or a shaft grounding ring, and follow the manufacturer's earthing arrangement (separate PE conductor for the motor, shielded cable bonded at both ends).
Harmonics: a 6-pulse front end draws 25-35% 5th and 7-12% 7th harmonic. Check the existing capacitor bank's resonant order (h = sqrt(Ssc/Qc)) before adding the drive, and confirm the voltage THD at the point of common coupling stays within 5%. A 4% DC or 5% AC line reactor may be needed.
Process and control: confirm the minimum allowable speed and minimum flow, the pump's allowable run-out flow at full speed, the pressure-surge risk on fast ramps (set the acceleration and deceleration ramps to 20-30 s), and the interlocks. Finally, verify the VFD's rated output current exceeds the motor's 185 A at 45 degrees C, and confirm the bypass arrangement for drive maintenance.