Choosing wrong inverter motor can reduce life

A WEG Electric Motors (UK) product story

Edited by the Manufacturingtalk editorial team Oct 30, 2002

AC Inverters deliver many benefits in terms of optimised motor performance. Yet, in many ways, these benefits blind users to the more negative effects of inverter use.

AC Inverters are the workhorse of industry, delivering many benefits in terms of optimised motor performance.

Yet, in many ways, these benefits blind users to the more negative effects of inverter use.

True, much has been written about the effects of premature bearing failures resulting from the use of inverters, but no one seems concerned about other effects that might affect motor performance and life span.

This is short sighted, as experience shows that the service life of an ordinary electric motor fed by an inverter depends upon its switching frequency.

Furthermore, in some cases where the voltage spikes resulting from the inverter carrier frequency are severe, motor life can be reduced by as much as 75%.

Clearly this figure has serious cost implications for users of certain types of motors, and quality implications, also, for a number of motor manufacturers whose materials and technologies are not suitable for today's demands.

What needs to be realised is that motors suitable for DOL (direct on line) starting could experience premature failure when fed by certain types of inverters.

In addition, users should also realize that the price differential between products of different quality is justified if the longevity of the higher quality product is taken into account.

Motor life span - to understand the impact of inverter operation on motor life span it is instructive to refer to two standards: IEC 34-17 and DIN VDE 530.

These state that a general application motor used with an inverter shall withstand voltage peaks (Vpeak) up to 1,000V and dV/dt (change of voltage with time) up to 500V/ms without any significant decrease in motor life.

However, in practice, the values motors are exposed to are substantially higher, reaching 5000V/ms and 1,500V.

To illustrate this point: in a 400V supply drive-motor application the voltage on the motor terminals might be as high as Vpeak = 400V x 1.1 x Oe2 x 2 = 1245V Where: 1.1 = positive supply voltage tolerance, Oe2 = voltage peak and 2 = inverter overshoot factor.

The rate at which these voltage peaks - or spikes - occur depends on the inverter carrier frequency, but typical values are 2,000 to 20,000 times per second.

The repeated stress these cause at motor terminals gradually breaks down the dielectric strength of the winding, resulting in corona and partial discharges that eventually destroy the motor.

Whereas voltage changes with time (dV/dt) affect the insulation between turns, the high voltage spikes affect the insulation between phases and phase to ground.

In these harsh conditions traditional LV motor insulation systems consisting of varnish, insulation films or tapes and impregnation, are no longer suitable.

Neither are traditional motor windings, which are usually enamelled with three to seven layers of standard class "F" varnish to provide a nominal voltage of 600V RMS.

So what is the answer? Well firstly the dielectric strength of the motor needs to be considered.

Dielectric strength depends on the type and thickness of the insulation used, the quality of the impregnation, winding geometry, temperature and humidity.

The higher the humidity, the lower is the motor dielectric strength.

In addition, the motor dielectric strength will be much lower at higher temperatures.

Because of this, users and OEMs should select inverters with reduced harmonic content, and new technology motors, such as WEG's W21 Line, with enhanced insulation and low and homogenised internal temperature.

The W21 Line is the result of unique techniques and manufacturing processes developed by WEG.

To illustrate this: all WEG motors are wound with class "H" spike resistant wire, which in common with all WEG products, was developed by WEG itself.

The wire is produced using the most modern equipment available, at WEG's own wire drawing centre.

It is of superior quality, providing 12 to 24 layers of enamel to optimise the insulation performance of motor windings.

Another area critical to dielectric strength where WEG has invested heavily is motor impregnation.

All WEG W21 Line motors are impregnated, using either VPI or CRFI, with high quality epoxy resin.

This ensures a higher percentage of retained solids in the winding, thus improving dielectric strength.

Additional benefits of using epoxy are greater mechanical rigidity, improved heat transfer and longer life.

Conclusion - the negative effects that inverters have on AC motors is a relatively new subject, which explains why only a few motor manufacturers have reacted and why many others are still building motors using materials and technology which are not suitable for today's demands.

For many, an alternative to new materials technology is to employ filters.

True, these can be used to minimise insulation stress, reduce harmonic distortion and common mode voltages.

However, there is often a compromise with voltage drop, energy losses and cost.

In addition the effectiveness of some types of filters is questionable.