How to save energy & costs with a Variable Frequency Drive (VFDs)?

Electrical motors drive manufacturing industries, with motor-driven equipment accounting for 64 percent of the total electricity consumed. When manufacturers search for ways to optimize their processes, energy efficiency is their paramount concern. The industry has adopted variable frequency drive (VFD) technology as their preferred solution to save energy, as explained under section 3 of "Smart Maintenance: The Evolution of Predictive Maintenance (PdM)" whitepaper.

VFDs control AC motors in applications including fans, pumps, blowers, mixers, conveyors, and other variable loads. This article explains the working of VFD and its contribution to energy and cost

What is Variable Frequency Drive (VFD)?

A Variable Frequency Drive (VFD), also known as an AC drive or inverter, is a type of motor controller that drives an AC motor by varying the frequency and voltage supplied to the motor. VFDs reduce electrical energy and allow operators to adjust the motor speed precisely to match the system-required load. In Induction AC motors, a non-magnetised (but electrically conductive) rotor rotates slightly slower than the synchronous speed of the rotating magnetic field. The rotational speed of this magnetic field is directly proportional to the frequency of the AC power and inversely proportional to the number of poles in the stator:

S = 120 f / n

Where,

S = Synchronous speed of rotating magnetic field, in revolutions per minute (RPM)

f = Frequency, in cycles per second (Hz)

n = Total number of stator poles per phase

In the above equation, the frequency directly relates to the motor’s speed. Figure 1 depicts the three parts that make up the VFD: the rectifier, the filter, and the inverter. The rectifier uses diodes to convert line AC power into DC. The filter reduces the ripple from the rectified DC power. The inverter re-converts the filtered DC power back into AC, only this time at whatever frequency and voltage levels are needed to run the motor at different speeds.

Figure1: Block diagram of a Variable frequency drive

Types of Induction motors based on different operating loads

An induction motor, depending upon the different load conditions, can be classified into the following categories:

Constant Power Load

Grinders, winding, and lathe machines require high torque at low speeds and low torque at high speeds. As the operation speed decreases, the torque increases, keeping the required horsepower constant.

Constant Torque Load

Reciprocating pumps, compressors, conveyors, and traction drives require the same amount of torque at both low and high speeds. In this condition, the torque remains constant throughout the speed range, and the horsepower increases and decreases in direct proportion to the speed. As speed changes, the load torque will remain reasonably steady, and the horsepower will change linearly with speed.

Variable torque Load

Applications like centrifugal and axial pumps, fans, and blowers require much lower torque at low speeds compared to high speeds. The torque required varies as the square of the speed, and the power required varies as the cube of the speed.

Figure2: Different load conditions of an induction motor

How do VFDs contribute to energy and cost savings?

The Laws of Affinity approximates the potential energy savings by reducing motor power to manage speed. Reducing motor speed by 25 percent decreases energy consumption by almost 60 percent, whilst a 50 percent reduction of motor speed decreases energy consumption by almost 90 percent. Therefore, reducing motor speed is often the simplest way to conserve energy for most motion control applications. VFDs can contribute to energy savings and operating cost reduction in the following ways:

Reducing Inrush Current

A high inrush current can cause nuisance by tripping protective devices or damaging the motor. It can also cause voltage drops in the supply line or even prevent the motor from starting correctly. High inrush current also leads to high torque production at startup, which can cause a sudden, severe acceleration that damages mechanical loads. High inrush current also increases the electrical demand, and high demand leads to steep demand charges. A VFD can reduce the initial startup rush current by decreasing the voltage to the motor when it begins to accelerate the loads. This lower voltage reduces the current drawn by the motor. VFD reduced current inrush, reducing the risk of premature failure and eliminating demand charges, thereby reducing the energy and maintenance costs.



Figure3: Effect of Inrush current in Motors

By taking advantage of affinity law

In the case of centrifugal pump, affinity law specifies the relation between speed and power.

Power (HP) = Flow (Q) × Head (H) …………. (1)

Where Q ∝ Speed (N)

And H ∝ N2

Therefore, HP ∝ N3 …………. (2)

It is thus the best option to adjust the supply frequency and speed to control the flow instead of traditional valve throttling. Affinity law saves a considerable amount of power.



Figure4: Affinity law for centrifugal pumps

As per Affinity Laws, lower motor speed equals less energy consumption. For example, a motor running at 80 percent full speed requires 51.2 percent of the electricity of a motor running at 100 percent speed. (Using equation no.2 80% x 80% x 80% = 0.512). A VFD controlling a pump or blower fan can yield extensive savings across its operating life.

Using Dynamic V/f mode:

A Dynamic Volts-per hertz, commonly called V/f, is designed for conditions where you should keep power losses in the motor minimum under low load conditions. In this mode, the Drive will vary the voltage to the frequency characteristic, which applies to the motor depending on load level. The voltage on the motor is thus correspondingly reduced for a given frequency at light load levels. This method reduces the magnetising current, which reduces losses in the motor.

Integrated PID controller:

New generation VFDs have an integrated PID loop, which can be enabled and set up with only a small number of parameters. You can directly enter the set point into the VFD's programming. There is no need to connect a programmable logic controller (PLC), interface it with VFD, and program it separately. The feedback signal from the sensor is sent directly to the VFD’s analog input. The set point value and sensor feedback help the VFD determine if the motor should speed up or slow down to meet the set point. It maximizes energy efficiency because the motor only runs fast enough to meet the demand of the present set point value. Simultaneously, it reduces costs as there is no need to purchase or program the PLC.

Common DC bus:

The common bus system is the most efficient way to operate the induction motors when multiple drives are in one location. A common DC bus linking multiple VFDs can cost-effectively recover regenerative power generated by a slowing motor and feed it to one requiring additional power rather than purchasing energy from the grid. If the powered system application generates more braking energy than the active motors can consume, the line regenerative systems can feed power back to the grid at a substantial cost saving.

Power factor improvement

A lightly loaded induction motor will have nearly zero VFD input current as the large reactive current does not flow from the supply side. Since only the real current component is returned to the supply side, the VFD input current remains in phase with the supply voltage under all loading conditions. Therefore, the power factor will be close to unity, resulting in lower utility charges.

Cost Savings Example

Let's say we have a fan with a 60-horsepower (hp) motor that supplies air for 15 hours a day,

300 days a year, and the energy cost is $ 0.1177 per kilowatt-hour (assumed).

The operational cost is calculated with the following formula:

Cost = Power (kW) × Running Time × Cost/kWh

So, the cost of operation for running a motor at different speeds for different durations with VFD and without VFD is shown in table 1:

Table 1: cost of operation for running a motor with VFD and without VFD

To calculate the payback period of investing in an AC Drive, we can use the following formula:

So, if our 60hp Drive costs $15000 (assumed), the payback period would be around 17 months, i.e., less than two years!

In conclusion, using a VFD with an induction motor saves significant energy, and recovery is possible using several applications and methods. Whilst there is typically a lot of focus on the VFD and motors' initial installation or retro-fit cost, you should review each application to get the maximum operating cost savings resulting from energy savings and energy recovery. In many cases, the energy savings and operating costs are higher than the installation cost.

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