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Servo Drive & Motor Expert | VFD AC Drive Repair

Posted on 19th Aug 2023

Efficiency of Adjustable Speed Drive

Siemens Drive

According to CM Industry Supply Automation, device called an adjustable speed drive (ASD) regulates the rotational speed of machinery that is powered by motors. The most popular form of ASDs are variable frequency drives  such as Lenze Drive, Keb Drive solid-state electronic motor controllers that effectively adapt to changing process requirements by varying the frequency and voltage of power supplied to an alternating current (AC) motor to allow it to operate over a broad speed range. External sensors measure flow, liquid levels, or pressure and send a signal to a controller, which modifies the motor's frequency and speed to suit the needs of the operation.

Different Torque Loads

Pulse-width-modulated (PWM) VFDs are used most frequently in variable torque applications with motor sizes ranging from 1 to 1,000 horsepower (hp). The fan or affinity rules say that the fluid or airflow delivered varies directly with the pump or fan rotational speed for centrifugal fans or pumps without static lift. Figure 1 illustrates how the input power requirement varies as the cube or third power of the speed ratio. Small reductions in fluid flow or equipment rotational speed result in large energy use savings. For instance, cutting the flow rate of rotating equipment by 20% can cut the input power requirements by around 50%.

Always-On Torque Loads

When a load requires a consistent amount of torque, speed is unimportant.
Input power varies linearly with speed for constant torque applications because horsepower requirements are equal to the product of necessary torque and speed. Cranes, hoists, conveyors, extruders, mixers, positive displacement pumps, reciprocating air compressors, and rotary screw air compressors are a few examples of continuous torque loads.

How to Calculate Energy Savings

You must ascertain the load duty cycle, or the proportion of time the driven equipment is in operation at each system operating point, in order to calculate the potential energy savings that can be achieved when a VFD such as Siemens Drive is used to drive a variable or constant torque load. Additionally, you need to be aware of the efficiency of the drive motor's partial load operation at a decreased speed to meet varied flow needs.

When thinking about PWM VFDs, you may find the efficiency numbers for drives with different ratings that power motors linked to either constant or variable torque loads by consulting the manufacturer's data or Table 1. Keep in mind that motor efficiency decreases with light loads and when the motor is given a nonsinusoidal waveform

VFD efficiency declines as motor load is reduced. When drives have lower horsepower ratings, the efficiency reduction is more noticeable. This decrease in effectiveness is not as harmful as it initially appears, as demonstrated in the case that follows.


Consider a VFD connected to a motor that, when run at its maximum recommended speed, needs 16.4 kilowatts (kW) to supply 20 shaft horsepower to an exhaust fan. The fan produces 50% of its rated airflow at half its rated operating speed while utilising only 1/8 of full load electricity.

With adjustable speed operation, the fan and VFD only use 2.8 kW of power, even with a reduced motor efficiency of 77.8% and drive efficiency of 86%. The amount of input power used in this scenario is decreased by 82.9%.

kW 50% = 0.746 kW/hp x (20 hp x (1/2)3 / (0.778 x 0.86) = 2.8 kW

According to CM Industry Supply Automation, Keep in mind that the efficiency of the system is equal to the product of the efficiency of the VFD, the efficiency of the motor at its load point, and the efficiency of the driven equipment (system = VFD x Motor x Equipment). With the help of the U.S. Department of Energy's MotorMaster+ 4.0 software programme, one can easily determine the efficiencies for integral horsepower NEMA Design A and B motors at full and part load. The appropriate pump or fan performance curves must be used to extrapolate efficiency values for driven equipment.