Pump Motor Drives & Output filters
How to select the best protection policy for old and new motors

Source: ABB Motors & Drives

Bearing currents, voltage reflections, current spikes, motor noise, vibration …. selection of issues that are guaranteed to send fear through many hardened pump engineer in process plants around the world. However, technical solutions do exist and especially the Insulated Gate Bipolar Transistor (IGBT) had proven to be a very efficient and reliable device. Despite this there are many motors in the marketplace - some 97% - that are not yet controlled by variable speed drives.

While each of these phenomena is usually associated with electric motors, their cause can often be traced to AC drives and the incredible developments that have taken place in power semiconductor technology within the last decade. In particular, the Insulated Gate Bipolar Transistor (IGBT), which has proved to be a reliable and high performance device, bringing substantially faster switching rates and lower losses than were ever possible with previous technologies. It is partly because of these advantages that these devices can also be the culprit for motor damage.
However, motor technology has not been slow to react to the challenge of AC drives and today, purchasing an AC drive and a motor from a reputable manufacturer will result in a smooth solution, free from any of the above phenomena. Despite this there are many motors in the marketplace - some 97% - that are not yet controlled by variable speed drives and because of their age, the windings may not be up to the fast switching - and subsequent voltage stress - presented by IGBT technology.
But it is not just the fact that the motor may not have been designed for use with an AC drive that needs to be considered. In practice, there are several other installation parameters that can affect the operation of the motor, such as variations in cable length, the use of an output transformer or an application with multiple motors.
Whatever the problem, a remedy is usually available with some form of output filtering, which can filter out a specific undesired effect within the output voltage. There are basically three different filter solutions, each of which protects against specific problems. These are: output reactors (dU/dt filters), sine filters and common mode filters.
Each method concentrates on a particular type of filtering. The dU/dt filter limits the rate of change of output voltage and the rate of change in the switching event. The sine filter makes the inverter output voltage totally sinusoidal. The common mode filter, reduces so called common mode voltages.

Output reactors (du/dt filters)
These filters are commonly used to overcome voltage reflections. A voltage reflection occurs when the impedance of the motor cable differs from that of the motor itself. When this occurs a voltage wave will reflect from the drive to the motor, and thus amplify the voltage stress seen by the motor. If this stress exceeds the insulation capacity of the motor, the motor suffers catastrophic failure in the long run.
Should you be suffering from voltage reflections then help is at hand through the use of output reactors - or dU/dt filters. Such filters can be used to decrease the rate of change and high frequency content of the inverter output voltage. The reactors increase the overall cable impedance thereby decreasing the difference between the cable and motor impedances.
The length of the cable affects the dU/dt and peak voltage value as seen by the inverter. As the cable length is a typical installation parameter, the inverter manufacturer can deliver the dU/dt value and peak voltage value as a function of cable length, depending on the cable type recommended by the manufacturer.
But how do you know if you need such a filter?
Normally, it is the motor manufacturer that will define whether or not to use dU/dt filters. Also drives manufacturers define typical peak voltages and dU/dt values for its drives, both with and without filtering. Such information can then be supplied to the motor manufacturer. Given the peak voltages and dU/dt values, the motor manufacturer should be able to state whether additional filtering is needed or not.
One further use of dU/dt filters is in cases where a long motor cable is used and the capacitive leakage from phase conductors to screen/armouring could otherwise trigger a spurious earth fault. In this instance, the filter reduces the rise time of the voltage pulse, resulting in a smaller value for the capacitive leakage current.
This is a typical solution when cable lengths are some hundred meters long. If the total sum of cable lengths exceeds 500 meters (as in the case of multimotor applications fed by a single inverter), a sinusoidal filter is recommended. However, it should be remembered that the higher the motor nominal current, the less the capacitive leakage current effects to drive performance.
du/dt filters are basically output chokes which do not disturb the inverters current measurement by introducing phase shifts. Thus, normal high performance Direct Torque Control method can be used. Because of the reactance and thus voltage drop over the reactor in the circuit the pull-out torque limit of the motor may be decreased by a few percent. This will not normally have any effect on the process.

Sine filters
A sine filter has two main benefits. Firstly, to be able to use long cables without the risk of current transients, caused by stray capacitance within the system, it is necessary to have a near perfect sinusoidal voltage waveform.
To achieve such a waveform, a low pass LC sine filter is needed in the inverter output. This will eliminate the high frequency components of the inverter output.
Secondly, a sine filter can also be used in step-up applications. Here an output transformer is fed from a frequency converter to step-up the voltage into the medium voltage range. The filter reduces the high frequency components that would otherwise saturate the transformers magnetic circuit while also generating voltage reflections at the transformer's high voltage secondary and the motor cables. The filter, therefore, protects the medium voltage motor. Inherently, motor noise levels decrease because of the sinusoidal voltage.
Sine filtering is highly suitable for old motor retrofits -also in low voltage installations because there are no additional transient voltages to make demands on the phase-to-phase insulation. Large reactors in the sine filter cause typically some 5-7% voltage drop which means that normally the voltage of the converter needs to be boosted to correctly flux the motor. Also in the field weakening range the pull-out torque drops faster than without the sine filter. Basically, field-weakening operation should be avoided.

Common mode filters
Another phenomena that can be experienced in motors is high frequency bearing currents, which can lead to excessive wear or premature failure of a motor's bearings. Such bearing currents can occur as the result of current flow in the common mode circuit of an AC drive system. Induced shaft voltages will always occur in induction motors, and will be related to the physical frame size, these voltages tend to be higher in inverter fed solutions and have a number of sources.
A common mode circuit arises in PWM switched three-phase power supplies. With such supplies, the DC voltage is converted into three phase voltages and it is impossible to make the sum of the three output voltages instantaneously equal to zero. This is contrary to a balanced and symmetrical three-phase sinusoidal power supply, where the vector sum of the three phases always equals zero. In the PWM system, this voltage is called a common mode voltage.
Any time one of the three, inverter outputs is changed, a current proportional to this voltage is forced to flow to earth via the earth capacitance of all the components of the output circuit. This common mode current may have several different frequencies due to the different natural frequencies with one resulting effect being the introduction of bearing currents.
Bearing currents can be reduced by correct grounding and cabling methods in the drive system. Also motor manufacturers may recommend the use of insulated bearings. But whenever there is a risk of bearing currents, high frequency common mode filtering can be achieved using simple toroidal cores of ferromagnetic material.
However, the common mode filter does not provide any dU/dt filtering which means that in some cases common mode filters and dU/dt filters may have to be used at the same time. <<