VFDs and soft starters: is there really a competition?

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This post is part of our series on motor management which covers various aspects of motor integration in an electrical network and the industrial process.

End-users are frequently coming to us with the request to help them choose a better solution between variable frequency drive (VFD) or soft starter (SS).

For example: an intake pumping station in Egypt. The initial design was to use MV SS as a starting method for the intake pumps, keeping in mind that the intake pumps are a variable load, depending on the usage. In this case, the client was willing to change to VFD if we could prove the cost benefit on both the CAPEX and OPEX.

The post will discuss the benefits of using VFD over SS generally, and in the application of booster pumps specifically. It will show the advantages of using the VFDs in energy saving, cost optimization and reduction of local generation capacity.

What is a VFD?

• A Variable Frequency Drive (VFD) is a type of motor controller that drives an electric motor by varying the frequency and voltage supplied to it.

• Other names for a VFD include: variable speed drive, adjustable speed drive and adjustable frequency drive.

The frequency of the power applied to an AC motor determines the motor speed, approximated to the slip, based on the following equation:

Where: N= speed, f=frequency and p=no. of poles.

• Want to learn more about VFD? Read Variable speed drive: Back to the features

What is a soft starter?

A soft-starter (SS) is a thyristor based electronic device, used with AC electrical motors to temporarily reduce the torque in the power train, and the electric current surge of the motor during start-up by controlling the conduction time of the thyristors. They provide a gentle ramp up to full speed and are used for both start-up and stopping. Soft-starters reduce the mechanical stress on the motor and shaft, as well as the electrodynamic stresses on the attached power cables and electrical distribution network.

Some benefits of using VFDs over soft starters:

1. Energy saving:

The affinity laws (also known as the “Fan Laws” or “Pump Laws”) for pumps are used in hydraulics to express the relationship between variables involved in pump or fan performance (such as head, volumetric flow rate, shaft speed) and power. They apply to pumps, fans, and hydraulic turbines. In these rotary implements, the affinity laws apply both to centrifugal and axial flows.

For the relation between the pump power and speed:

Where: P1 = needed powerP2 = full power    N1 = needed speed    N2 = full speed.

Having said that, if at any time during operation we need to run the pump at 80% of its speed, using a VFD we will be able to reduce the speed during operation, and this will lead to a significant decrease in absorbed power as follows:

∴ P(needed) = 0.512*P(full).

So, at 80% speed, we can save up to 50% of the motor power using the VFD, which will lead to increasing motor life time and decreasing the project OPEX. 

For more on this you can head to this blog post: About Energy Efficiency of Induction Motor Applications: Two Reasons to Choose a Variable Speed Drive (VSD) 

2. Generator capacity:

For sizing a generator, we should consider two factors: loading and starting.

• Loading check:

When using the soft starter or VFD, total load will be considered, so we will need equal total generator power in both cases, regardless of the starting method.

• Starting check:

For soft starters:

Starting factor for soft starters is 3, so we will consider sizing on two steps as follows:

Starting of biggest Load (kVA) = Biggest load * starting factor = 3 * Biggest load

Total Load before starting of biggest load = Total load – biggest load.

∴Total Load with starting of biggest load = Total Load before starting of biggest load + 3 * Biggest load.

For VFDs:

Starting factor for soft starters is 1, so we will consider sizing as follows:

Starting of biggest Load (kVA) = Biggest load * starting factor = 1 * Biggest load.

Total Load before starting of biggest load = Total load – biggest load

Total Load with starting of biggest load = Total Load before starting of biggest load + 1  * Biggest load.

Obviously using VFDs will lead to lower generator capacity requirement, due to the lower starting factor. Therefore smaller & cheaper generators can be used, which will lead to CAPEX reduction.

Case study:

The intake pumping station, 4×1580 kW (1859 kVA) pumps, was initially designed using soft starters. Therefore, generation station was sized to 12 MVA.

Figure 1 Initial design with soft starters (SS)

To verify that VFDs can reduce the generation capacity, we performed the following study:

• Loading check:

When using the soft starter or VFD, total load will be 7,435 kVA so we will need total generator power per standards: 9,000 kVA. (three generators, each rated at 3 MVA), to keep loading at less than 80% for standby generators

• Starting check:

1. For soft starters:

Starting factor for soft starters is 3, so we will consider sizing as follows:

Starting of biggest Load (kVA) = Biggest load*starting factor = 1859*3 = 5576.

Total Load before starting of biggest load = Total load – biggest load = 7435 – 1859 = 5576.

Total Load with starting of biggest load = 5576 + 5576 = 11152.

∴ Total needed generator power confirmed at 12,000 kVA.

2. For VFDs:

Starting factor for VFD is 1, so we will consider sizing as follows:

Starting of biggest Load (kVA) = Biggest load*starting factor = 1859*1 = 1859.

Total Load before starting of biggest load = Total load – biggest load = 7435 – 1859 = 5576.

Total Load with starting of biggest load = 1859 + 5576 = 8365.

∴ Total needed generator power can be reduced to 9,000 kVA.

As a standard, one 3000 kVA Generators will be saved. 

Figure 2 Optimized design, generation and variable speed drive

Despite the price increase due to using VFDs instead of soft starters, decreasing the generation capacity will still lead to a 35% decrease in the total project CAPEX.