How to specify motor drive filters

This article looks at motor drive filters, and how to select the best protection policy for old and new motors.

Bearing currents, voltage reflections, current spikes, motor noise and vibration send fear through process engineers around the world.

Although 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 few decades.

In particular, the insulated gate bipolar transistor (IGBT) is bringing substantially faster switching rates and lower losses than previous technologies.

It is partly because of these advantages that these devices can also be the cause of inadvertent 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 should result in a solution free from any of the above phenomena.

Despite this most motors on the market 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.

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, output filtering can usually filter out a specific undesired effect within the output voltage.

The three different filter solutions 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 (or dU/dt filters) are commonly used to overcome voltage reflections which amplify the voltage stress seen by the motor.

If this stress exceeds the insulation capacity of the motor, the motor can suffer from a catastrophic failure.

dU/dt filters can be used to reduce the rate of change and high frequency content of the inverter output voltage, and also increase the overall cable impedance, 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 motor.

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.

Normally, the motor manufacturer will decide whether or not dU/dt filters are needed.

Also, drives manufacturers, like ABB, define typical peak voltages and dU/dt values for their drives, both with and without filtering.

Given the peak voltages and dU/dt values, the motor manufacturer should be able to state whether additional filtering is needed.

dU/dt filters are also used for long motor cables where the capacitive leakage from phase conductors to screen/armouring could trigger a spurious earth fault, or overload the conducted emissions filter in the drive.

The filter increases the rise time of the voltage pulse, resulting in a smaller value for the capacitive leakage current.

In the case of two parallel cables the overall distance will halve, and with three cables it becomes one third, etc This is a typical solution when cable lengths are relatively long.

If the total sum of cable lengths exceeds 30-300m, depending on the drive size, a sinusoidal filter is recommended.

However, the higher the motor nominal current, the less affect the capacitive leakage current will have on drive performance.

dU/dt filters are basically output chokes which do not introduce phase shifts.

Thus, a 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 affect on the process.

To use long cables without the risk of current transients a near perfect sinusoidal voltage waveform is needed.

A lowpass LC sine filter will achieve this, eliminating the high frequency components of the inverter output.

Where an output transformer is fed from a frequency convertor to step-up the voltage into the medium voltage range a sine filter reduces the high frequency components that could stress the transformer's magnetic circuit while also generating voltage reflections and transients.

The filter protects the motor connected to the secondary and also reduces transformer and motor noise levels.

Sine filtering is highly suitable for old motor retrofits because there are no additional transient voltages to affect the phase-to-phase insulation.

Large reactors in the sine filter typically cause a 7-10% voltage drop, which means that normally the voltage of the convertor 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 - field-weakening operation should be avoided.

High frequency bearing currents, which can lead to excessive wear or premature failure of a motor's bearings, can occur as the result of current flow in the common-mode circuit of an AC drive system.

A common-mode circuit arises in PWM switched three-phase power supplies.

Bearing currents can be reduced by correct grounding and cabling methods in the drive system.

Also, motor manufacturers will normally recommend the use of insulated bearings for motors over about 100kW, however these may not be adequate to negate the effects of common-mode voltages in larger (bigger than IEC 355 frame) motors.

However, whenever there is a risk of bearing currents, high frequency common mode filtering can be achieved using simple toroidal cores of ferromagnetic material.

It should be noted however, that the common-mode filter does not provide any significant dU/dt filtering, so common-mode filters and dU/dt filters may both have to be used.

In summary, the du/dt filter: limits rate of change of output voltage; eliminates voltage reflections; avoids bearing currents; but has inherent voltage drop.

The sine filter: makes inverter output voltage totally sinusoidal; suppresses high frequency components; enables use of long cables; enables use of unscreened cables without EMC radiated emissions; avoids current spikes; may be used in step up applications; reduces the motor noise level; is suited to old motor retrofits; and avoids bearing currents.

However, large reactors cause voltage drop which may require compensation.

Sine filters require sophisticated motor control software when used with step up-transformers to avoid DC saturation.

Common-mode filters: limit common-mode voltages; and attenuate bearing currents; but do not provide dU/dt filtering.

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