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Why is Impedance Imbalance for Electric Motors so important.

Why is Impedance Imbalance for Electric Motors so important.

Saturday, April 4, 2020

The analogy is liking it to Running a car for long periods of time with, out of balanced wheels.

Impedance imbalance is the electrical equivalent of getting your wheels balanced, its saves energy, reduces wear & tear, and risks other damage like suspension & steering. So what is it????

Impedance is the load the electrical supply see’s when running an electric motor. And it uses a lot!!!!

47% of global electricity consumption is use in motor systems.

It is the Number One Electrical consumer in your facility by far!

Electrical Energy Consumption Globally

Take a cooling tower motor which runs continuous, it will typically uses 16x its purchase value in energy annually. Based on 9.7 euro cents per kWhr.

The motor manufacturers produce motors to a standard eff2 or eff3 which has an approximate efficiency of 90+% for small motors and 95+% for motors above 100kW.

A lot of factors can eat away at this efficiency when installed, such as motor loading, harmonics, mechanical losses in impellers, alignment, and balancing.

You may not be aware but Impedance imbalance is a big factor, a 3% imbalance leads to approximately 3% efficiency loss, but a 6% imbalance is closer to 8% loss. Most of the losses end up as heat decaying Motor Insulation, Terminations and Switch gear and if left without remedy propagate to a failure.

The average Impedance Imbalance of defects in Electric Motors of 3Phi Reliability database is 8.25%. That Energy saving Opportunity is significant when purchasing or overhauling a Motor.

Impedance Z is the total resistance of a motor circuit and is made up of Resistance R, Reactance XL, and Capacitance Xc.

The R components usually made up of cable and coil resistance but can be elevated by burnt contacts, poor crimps, corroded or poor terminations, and makes up the dominant portion of defects.

XL is the motor winding impedance, defects from new or overhauled motors affect imbalance. Xc is the capacitance component dominated by cable, insulation and contamination factors.

Conducting Motor Circuit Analysis measures Impedance will identify imbalances some be remedied, but Winding and Rotor defects in Overhauled or New Motors means your stuck with them for the life of the motor (Probably a shorten the life).

There’s a strong correlation between Impedance Imbalance and developing winding defects.

While Resistance imbalances which affect winding voltage can be remedied by proper termination many defects are permanent.

Setting a purchase & overhaul specification and conducting acceptance testing serves as a filter, stops them entering your plant and someone else is likely to get them.

The rejection rate can be significant, and even if you have to pay a premium the cost will be recouped rapidly in energy & reliability savings, often in months.

3Phi Reliability Motor Health Assessment uses the ALL Test Pro MotorGenie within minutes can assess the Health of an electric Motor and is validated under IEEE 1415:2006 standard.

The Impedance Imbalance is often seen in the Vibration Analysis Spectra as Twice Line Frequency being 100 hz or 120 hz depending on the supply frequency. This frequency is produced as the magnetic out of balance pulls one way in the positive sine wave voltage, then negative on the opposite portion of the sine wave.

In this case the MotorGenie found multiple connection defects on a centrifuge motor and the remedial work dropped the Electrical harmonics by 84% saving 3500 GBP per annum in Electrical Energy.

High 2xLF Two Times Line Frequency 100hz

Before Remedial Work

2xLF Two Times Line Frequecny nearly completely eliminated

Post Remedial Work

This vibration of 100 hz is very common and a good starting point to identifying the defects is to rank your assets by 2xLF Two Time Line Frequency 100hz (120hz US) and then conduct Motor Circuit Analysis on those circuits.

This targeted approach provides a much higher return on effort if all your Motors can not be tested in a short period of time. The energy savings alone will turn your Preventative Maintenance into a profit center.

Ensure you document the "As Found" and "As Left" conditions and it is very easy to calculate the Energy Savings. If you need help

Test Methods for determining the impact of Motor Condition on Motor Efficiency

Motor Genie All TestPro

3Phi Reliability implement proven strategies to elevate your Electrical Preventative Maintenance with Best Practice Motor Management

Mark Gurney, Gerant 3Phi Reliability Sarl

Best Practice Motor Management
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