Energy efficiency – Part 3

What type of VFD technology fits the best your demands? The answer depends mainly on your process and operation philosophy.

Process dynamics

• Is your process very dynamic and fast response is required? Then you will need a high performance drive that can track rapid changes of torque or speed reference signals (commands).
• Is your process relatively slow? This is the case for pumps and fans. Then all you need is so called general purpose drive / standard industrial drive.

Rectifier type

• Is diode front end (DFE) sufficient or should you go for active front end (AFE)?
• If regenerative braking is not needed and input power factor > 0.95 is okay, then DFE will likely be a good solution and good value for the money.
• If you need regenerative braking, want to achieve input power factor 1.0 or even use the VFD for VAR compensation, then AFE rectifier shall be specified and purchased.
• From energy efficiency point of view you need to know how often you gonna brake. f the regenerative braking is frequent then the savings become interesting. However, if the braking is very seldom, then it might not pay off.
• Keep in mind that AFE drive with very seldom braking may overall consume more energy (integral over time) than DFE drive with or without braking chopper! The reason is that AFE drive has inherently lower efficiency compared to DFE drive (active rectifier produces more losses than passive diode rectifier).
• Knowing the system data you can easily calculate the energy balance and decide if AFE is attractive or not. The integral of the additional losses in the rectifier shall be lower than the energy you can regenerate during braking. AFE has roughly 0.8-1.2% more losses than DFE (for 10’000 kW drive this means 80 – 120 kW additional losses) and the maximum regenerated power is equal the kinetic energy stored in the rotating masses (1/2*J*w^2).
• Of course, you might need AFE for other reasons regardless the energy balance.

Output type

• Some drives have output sine filter as integral part. The sine filter contributes to the total drive losses and affects the overall efficiency. On the other hand there will be no extra harmonic losses in the motor.
• Other drives do not have a sine flter, but just a dv/dt (EMC) filter or are even filter-less (be careful about the last group). They have less passive components and may look more efficient. However, if their output waveform is far away from sine wave, they will cause lot of additional harmonic losses in the motor.

Please, please, do not compare efficiency on a product level, but rather consider the whole drive train from end to end (transformer input power to motor output). It is so easy to compare two values of two products while it gets more complex when comparing two systems. This is why many users are tempted to make the benchmark on the product level. However, this might be misleading or incorrect. This is a crucial thing and we dedicate our next post specifically to the topic how to benchmark variable speed drive systems from efficiency point of view. 

Stay with us. Cheers!