VSD Pumps

WHY VSD ON MOTORS?

How a Hybrid VSD Can Save Money on a Ruraflex Tariff ( Without Solar )

A Hybrid Variable Speed Drive (VSD) optimises motor efficiency, reducing electricity costs through two primary mechanisms:

  1. Lowering Demand Charges by Improving the Power Factor
  2. Lowering kWh Usage by Improving Power Factor and Efficiency

CASE STUDY: 13 March 2025 Case study on 50kW motor running at full capacity for 4 months, connected to Eskom’s Ruraflex tariff.

  1.   50kW motor without VSD
  2.   50kW motor with VSD
  3.   50kW motor with VSD & Solar PV Panels
  1. Lowering Demand Charges by Improving the Power Factor

Eskom applies a demand charge of R0.52 per kWh, based on the total energy consumed.

Power factor (PF) represents how efficiently electrical power is converted into useful work.

Wasted Energy

A low power factor (PF <1.0) increases demand charges because it means more apparent power (kVA) is drawn than real power (kW).

  • Given motor specifications:
    • Power = 50 kW
    • Power Factor = 0.85
    • Apparent Power (kVA) = Real Power (kW) / Power Factor
    • Apparent Power = 50 kW / 0.85 = 58.82 kVA
  • If a Hybrid VSD corrects the power factor to ~0.98, the new apparent power would be:
    • Apparent Power = 50 kW / 0.98 = 51.02 kVA
  • Reduction in apparent power:
    • Without VSD: 58.82 kVA
    • With VSD: 51.02 kVA
    • Improvement: 7.8 kVA (or 13.2% lower demand charges)

Since demand charges are based on kWh consumption, improving the power factor reduces wasted reactive power and ultimately lowers the effective demand charge component per unit of energy used.

  1. Lowering kWh Usage by Improving Power Factor & Efficiency

A Hybrid VSD also optimizes motor speed, reducing energy wastage in the following ways:

  • Eliminates excessive motor speeds: Instead of running at full capacity (50 kW continuously), a VSD dynamically adjusts motor speed to meet actual load demand, potentially reducing power draw.
  • Reduces mechanical and electrical losses: Traditional fixed-speed motors often operate inefficiently due to mechanical stress and higher heat losses. A VSD can improve efficiency by 5-15%.

Assuming a conservative efficiency gain of 10%, the effective power consumption drops from 50 kW to 45 kW.

Cost Savings Calculation

Scenario Without VSD (Fixed Speed Motor)

  • Energy consumption (kWh per day):
    • 50kW × 24 hours = 1,200 kWh / day
  • Energy consumption over 4 months (~120 days)
    • 1,200 × 120 = 144,000 kWh
  • Cost at blended tariff (R1.4369 per kWh avg.)
    • R206,913.60

Scenario With VSD (10% Efficiency Improvement)

  • Energy consumption (kWh per day)
    • 45kW × 24 = 1,080 kWh / day
  • Energy consumption over 4 months (~120 days)
    • 1,080 × 120 = 129,600 kWh
  • Cost at blended tariff (R1.4369 per kWh avg.)
    • R186,222.14

Energy savings:

  • 144,000 − 129,600 = 14,400 kWh saved
  • R19,691.46 saved over 4 months

Total Savings Over 4 Months

  1. Demand Charge Savings (13.2%)
    • If demand-related costs make up 30% of total billing, a 13.2% reduction could yield an additional ~R8,500 savings.
  2. Energy Consumption Savings (~R19,691.46)
    • Due to improved efficiency and lower kWh consumption.

Total estimated savings: ~R28,000 over 4 months.

Conclusion

A Hybrid VSD helps reduce electricity costs on Eskom’s Ruraflex tariff (without solar) through:

 Lower demand charges by improving the power factor
 Lower kWh consumption by improving efficiency

For a 50kW motor running 24/7 over 4 months, savings can exceed R28,000. Over a full year, savings could exceed R84,000, making a Hybrid VSD a cost-effective investment.

SOLAR PV SYSTEM

Integrating a 75kWp Solar PV System with a Hybrid VSD on Eskom’s Ruraflex Tariff

Adding a 75kWp solar photovoltaic (PV) system alongside a Hybrid Variable Speed Drive (VSD) can significantly reduce electricity costs under Eskom’s Ruraflex tariff.

Here’s how:​

  1. Solar PV System Benefits
  2. Solar Energy Production

In central, South Africa, solar irradiance is favorable. A 75kWp system can be expected to produce approximately 4.5 kWh per kWp per day.

  • Daily Energy Production: 75 kWp × 4.5 kWh/kWp / day = 337.5 kWh/day​
  • Monthly Energy Production: 337.5 kWh / day × 30 days = 10,125 kWh/month​
  • Four-Month Energy Production: 10,125 kWh/month × 4 months = 40,500 kWh​
  1. Reducing kWh Consumption

The solar PV system directly offsets energy drawn from the grid during sunlight hours, leading to substantial savings.​

  1. Time-of-Use (TOU) Tariff Optimization

Eskom’s Ruraflex tariff varies by time:​

  • Peak: 07:00 – 10:00   &   18:00–20:00​
  • Standard: 06:00 – 07:00,   10:00 – 18:00,   &   20:00 – 22:00​
  • Off-Peak: 22:00 – 06:00​

Solar generation aligns with daylight hours, primarily during Standard periods, thereby reducing consumption when tariffs are higher.

2. Hybrid VSD Benefits

a. Improving Power Factor

A Hybrid VSD can enhance the motor’s power factor from 0.85 to approximately 0.98, reducing apparent power (kVA) demand.

  • Without VSD: 50 kW / 0.85 PF = 58.82 kVA
  • With VSD: 50 kW / 0.98 PF = 51.02 kVA
  • Reduction: 58.82 kVA – 51.02 kVA = 7.8 kVA (13.2% reduction)

b. Enhancing Motor Efficiency

The VSD adjusts motor speed to match load requirements, potentially improving efficiency by 10%.

  • Adjusted Power Consumption: 50 kW × 0.90 = 45 kW

3. Combined Financial Impact

a. Baseline Scenario (Without Solar PV & VSD)

    • Daily Consumption: 50 kW × 24 hours = 1,200 kWh/day
    • Four-Month Consumption: 1,200 kWh / day × 120 days = 144,000 kWh
    • Estimated Cost: 144,000 kWh × R1.4369 / kWh = R206,913.60


b. With Solar PV & VSD

      • Post-VSD Daily Consumption: 45 kW × 24 hours = 1,080 kWh / day
      • Four-Month Consumption: 1,080 kWh / day × 120 days = 129,600 kWh
      • Solar Offset: 40,500 kWh over four months
      • Net Grid Consumption: 129,600 kWh – 40,500 kWh = 89,100 kWh
      • Estimated Cost: 89,100 kWh × R1.4369 / kWh = R128,059.79
    •  

c. Total Savings

      • Energy Savings: 144,000 kWh – 89,100 kWh = 54,900 kWh
      • Cost Savings: R206,913.60 – R128,059.79 = R78,853.81
  1. Additional Considerations
  • Demand Charge Reduction: Improved power factor reduces demand charges by approximately 13.2%.​
  • Environmental Impact: Generating 40,500 kWh via solar reduces carbon emissions, supporting sustainability goals.

Conclusion

Integrating a 75kWp solar PV system with a Hybrid VSD at the specified location can lead to substantial electricity cost savings under Eskom’s Ruraflex tariff.

Over a four-month period, potential savings are approximately R78,853.81, with additional benefits from reduced demand charges and environmental advantages.

Contact us today to discuss your VSD needs!