VFD output transformers: Where do we need them?

VFD output transformer

Our VFD transformer series already guided you through many topics of VFD input transformers from their purpose, through multi-winding design and other design aspects, such as short circuit impedance or harmonics, up to protection or testing. However, there might be one more transformer in a variable speed drive system: VFD output transformer.

Almost every medium voltage VFD is connected to the grid through an input isolation transformer. The purpose of this transformer was explained in the very first article of this series. Let’s just mention voltage adaptation, galvanic isolation or multi-winding design as the main features. Some AFE drives allow direct connection to the grid. They might be called ‘transformerless’ or ‘direct to line’. However, as per 2021 they still remain a small group within MV VFD world. The basic set-up is therefore input transformer – VFD – motor. Nevertheless, specific drive systems have one additional system component: VFD output transformer. By ‘output’ we mean a transformer located between the inverter section of the VFD and the motor. In the next paragraphs we will explain the purpose of this VFD output transformer and specific design considerations.

* Regenerative VFDs allow bi-directional power flow. As such, the electric machine can be operated in motor or generator mode. We use the convention that motor mode is the basic mode of operation, i.e. active power flow is from the grid to the motor. Consequently, input transformer is on the grid side while output transformer is on the machine side.

Output transformer is frequently called a step-up transformer. While it mostly steps up the inverter voltage to match the motor voltage, there are few (exotic) cases where it is used as step-down transformer. We better don’t use those terms to avoid confusion.

Purpose of VFD output transformer

The main purpose of VFD output transformer is the adaptation of voltage level – same way as most other transformers work. There are predominantly economical reasons to use an output transformer.

VFD soft starters

For higher motor rated voltages it might be overall cheaper to use a VFD with lower output voltage and use an intermediate transformer to match the voltage. This is very often the case for VFD soft starters. Why exactly this category? Well, if the motor is used in inverter duty only and is not intended to work as direct on-line then the system designer shall be free to choose the motor rated voltage. And of course, he would normally select motor rated voltage that matches the inverter output voltage. However, in VFD soft starters we don’t have such degree of freedom. The motor shall be started up, synchronized and transferred to the grid. Logically, the machine and grid voltage must match. Then the choice is simple: use a VFD that provides directly the right output voltage or design a system with an output transformer.

Typical example here is an LCI soft starter. As the power of LCI scales with the voltage, the combination of low power and high output voltage leads to an uneconomical design. It is overall more cost effective to use an output transformer, especially considering the short duty cycle.

Also VSI soft starters can utilize an output transformer when it makes sense.

Test stands with multiple voltage levels

Another use case for an output transformer are certain test stands. Quite often the requirement is to make the test bench as flexible as possible and multiple nominal voltages are required. You make ask: VFD can adjust the output voltage so why is an additional transformer needed? There are several reasons:

— VFD can certainly adjust the inverter voltage, but at lower voltages the power is reduced. If the customer wants the same power at each voltage level or relatively high power at lower voltages, the output transformer is beneficial.

— Some of the required voltage levels may be higher than what the inverter can directly provide therefore same reasoning as for VFD soft starter applies.

— At lower output voltages the power quality is worse (at least for VSI drives) and generally does not fulfill the performance requirements of the test stand.

Design aspects of VFD output transformers

In this section we will mostly talk about output transformers used in VFD soft starters. That is the most frequent application of an output transformer.

VFD output transformers has several things in common with VFD input transformers. However, there are also few fundamental differences that you shall be aware of.

Similarities of VFD input and output transformers

What is common for VFD input and output transformers? Both animals are exposed to harmonics. The thermal design of transformer must consider additional losses caused by harmonics. Moreover, the designer shall eliminate any risk of local hot spots.

The transformer will also see certain level of common mode voltage unless the VFD has a grounded sine filter at its output or it is a current source inverter.

There might be a specific requirement on the short circuit impedance of the transformer.

Differences of VFD input and output transformer

One difference might be the winding connection or vector group. Input transformers may be more complex (multi-winding) while the output transformer is just a simple 2-winding transformer. For some drives, such as LCI soft starter, there is often 2-winding transformer on each side.

Main difference is, however, the operation at variable frequency. The input transformer works at a given constant voltage and frequency of the grid. Of course, there is some natural fluctuation, e.g. ±10% for voltage and ±2% for the power frequency. It needs to be considered, but is not a major obstacle.

In contrast, VFD output transformer works with voltage and frequency that are changing over a wide range. Generally, the ratio of voltage and frequency (V/f) is maintained to be almost constant, but not entirely. Why is it so and what does it mean for the transformer design?

Transformer saturation

For harmonic signals the ratio of voltage and frequency is proportional to the magnetic flux. And the amount of flux defines the required cross section of the core. VFD transformers, as most other transformers, have a ferromagnetic core. Each ferromagnetic material has certain maximum flux density where is still has a linear behavior or slowly starts to saturate. Exceeding the maximum flux density means a saturation of the magnetic circuit. Such operation is not allowed in our application (yet there are “exotic” applications where this effect is a functional feature). Saturation causes rapid increase of magnetizing current and high distortion of current waveform.

For VFD output transformer a saturation must be avoided. The transformer would lose the capability to transform the voltage according to turn ratio. Consequently, the VFD would not be able to start the motor.

Voltage boost

In order to avoid saturation of VFD output transformer a so called voltage boost factor is defined. Put it in simple terms, this factor defines the relative increase of the cross section of the magnetic core compared to a cross section required for nominal voltage and nominal frequency.

Where does the voltage boost come from? We have said that the V/f ratio is maintained almost constant. The key is in the word “almost”. At low frequencies the V/f curve is not straight anymore. There is a voltage drop in the system that needs to be compensated. Therefore, the voltage from the inverter is proportionally higher. As a result, the flux increases.

There are several factors affecting the voltage boost factor, both system parameters as well as control algorithms. We reveal them in our premium version along with some tips and tricks how to optimize the transformer design.

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References

[1] VFD transformers: Introduction, https://mb-drive-services.com/vfd-transformers-introduction/

[2] VFD soft starters, https://mb-drive-services.com/vfd-as-soft-starter/

[3] Medium voltage AC drives, https://new.abb.com/drives/medium-voltage-ac-drives

VFD output transformers: Where do we need them?

Published by admin on February 3, 2021February 3, 2021