Inverter current higher than input current
(New Year’s Eve issue)
Can inverter current to the motor be higher than the rectifier input current?
That is a question that puzzles some people. Let’s have a closer look in our year-end post inverter current higher than input current.
The recent advancements in VFD technology, particularly the modern topology in combination with the latest generation of power semiconductors, enabled operation with total efficiency exceeding 100%. Consequently, the output current of the VFD is higher than the input current. Is that true? Well, only partially…
Please forgive us for being unserious for a moment. You know, it is the last day of the year.
We will shortly rectify the above statement and explain how it really is. Seriously!
Although VFDs are steadily improved with regards to minimization of losses they still have certain portion of losses associated with power conversion. As the nominal efficiency is generally very high (up to 99% or even slightly higher) there is not much room for further improvement.
It is true that the development of new power semiconductors focuses on reduced losses, among others. Also the auxiliary consumption is reduced, especially at partial load, due to the use of variable speed cooling fans (EC fans) and variable speed cooling pumps. Yet, also in the future the VFD efficiency will stay below 100%. The losses can be further minimized, but they cannot be completely eliminated.
However, it does not mean that inverter output current cannot exceed the rectifier input current. In fact, the output current can be higher than the input current – both at reduced speed/partial load as well as full speed/full load.
Before we go deeper into details we shall clarify the VFD topology:
VFD with multi-pulse diode rectifier and external transformer
In this case it is anyway difficult to compare the rectifier input current with inverter current. The inverter constitutes of one 3-phase system while the retifier is supplied from multiple 3-phase systems (12-pulse DFE with 2 x 3-phase, 24-pulse DFE with 4 x 3-phase etc).
Moreover, the rectifier input voltage is generally different than the motor rated voltage.
VFD with multi-pulse diode rectifier and integrated transformer
In case of an integrated transformer inside the VFD cubicle, the customer interface point are the transformer terminals on primary (HV) side. Therefore, there are instalations with VFD input voltage being equal to motor nominal voltage (e.g. 6.6 kV grid voltage & 6.6 kV motor voltage). Comparison of input and output current can be done. Remember that in this case it is the combined efficiency of input transformer and VFD.
VFD with active front end
This is “the most relevant case” to compare VFD input and output current. Of course, it still depends whether the rectifier input voltage and motor nominal voltage are identical or not.
The grid side power factor is typically controlled to unity. However, in some cases, an AFE rectifier can be used for additional reactive power compensation – mostly overexcited operation with capacitive power factor. Such performance affects the considerations explained in the next section.
Three reasons for inverter current higher than input current
Reason 1: Different power factor
Asynchronous motors have nominal power factor in the range 0.86 to 0.93 depending on the design, number of pole pairs, power rating etc. On the other hand, the rectifier power factor is somewhat higher. DFE drives reach 0.95-0.96 at full load (even higher at partial load)
and AFE drives typically control the power factor at the input side to unity (1.0). Therefore, the inverter current is likely to be higher than the input current when nominal rectifier input voltage and motor nominal voltage is the same.
Example A:
– 2 MW asynchronous motor with nominal voltage 6.6kV and nominal power factor 0.91, efficiency at rated load 96.9%
– AFE VFD with efficiency of 98.4% at 2 MW load
– Input power factor controlled to 1.0
–> Inverter output current 198.4 A (at 2 MW load)
–> Rectifier input current 183.5 A
In this case of AFE drive the VFD output current is approx. 8% higher than the input current.
Example B:
– 10 MW asynchronous motor, nominal voltage 6.6 kV, nominal power factor 0.92, efficiency at rated output is 97.5%
– DFE VFD with integrated transformer having combined efficiency of 97.6%, grid side power factor 0.95
– Grid voltage 6.6 kV
–> Inverter output current 975.2 A (at 10 MW load)
–> Rectifier input current 967.6 A
In this case of DFE drive wih integrated transformer the VFD input and output current is almost the same.
Reason 2: Reduced motor voltage at reduced speed
The grid voltage is almost constant (apart from certain variation around te nominal value) while the motor volage is proportional to motor frequency in most of the range (up to the field weakening point).
At low speed the grid voltage is still the same, but the machine voltage is fairly low. Therefore, the machine current is much higher than the rectifier input current.
Let’s use again our example A, but we look at a load point at 10% speed.
Remark 1: It is probably obvious that the reasons 1 and 2 can be combined and reinforced.
remark 2: The considerations are valid for motor mode. In generator mode the input current (inverter side) shall be lower than the output current (grid side). Exceptions are special cases where AFE drive is used for reactive power compensation na operates with power factor significantly lower than 1 (e.g. 0.9 overexcited/capacitive).
Reason 3: Short time transients
The dc link is an intermediate energy storage. As explained in our article about undervoltage ride trough, the stored energy is far too low to consider dc link as energy source for more than few milliseconds.
However, in short transients the dc link can provide additional capability. Note that it is relevant for extremely short time only and in case of RMS measurement of current you would most likely not even notice it.
Wrap-up of the topic
Inverter output current to the motor at rated motor speed can be higher than the rectifier input current even though the grid and motor voltages are identical. Reason for that is different power factor at each side of the VFD
(which is possible due to decoupling of grid and machine through the VFD). Asynchronous motors normally have lower power factor and consume more reactive power than the grid side. If the difference in power factor is larger
than the VFD losses then the machine side current exceeds the grid side current. In case of synchronous machine drives we will normally not see such effect as the machine power factor is controlled to 1.0.
Summary
Inverter output current can exceed the rectifier input current. In fact, it is nothing unusual, particularly in asynchronous motor drives where motor power factor is lower than VFD input power factor.
This phenomenon can therefore be explained by different amount of reactive current on each side of the VFD. It is nothing to do with perpetuum mobile. The active power flow respects the system losses, i.e. rectifier input active power is higher than inverter active power delivered to the motor (and vice versa in generator mode or during regenerative braking).
References
[1] VFD input power factor, https://mb-drive-services.com/energy-efficiency-part-6-power-factor-and-reactive-power-consumption/
[2] VFD motor start, https://mb-drive-services.com/vfd-motor-start/
[3] Undervoltage ride through, https://mb-drive-services.com/ride_through/
[4] Medium voltage AC drives, https://new.abb.com/drives/medium-voltage-ac-drives
2 Comments
Davide · January 1, 2021 at 10:45 am
Hi,
thank you for the interesting article. I have a question arising from the comparison tables grid side vs. motor side.
How can apparent power be different in the 2 cases? I would have expected the same apparent power for both cases. From the examples the apparent power is even greater at motor side. Would you please help me understand?
Thank you and happy new year!
Davide
admin · January 5, 2021 at 9:37 am
Hi Davide,
That is exactly the point of the article. Reactive power on the motor side can be higher than at the VFD input. In fact, it is almost always higher in case of asynchronous machine. For synchronous motor drives you would typically not see such effect as the machine power factor is maintained at 1.0.
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