Check your MEN (Multiple Earth Neutral Link).
Why does this MEN (Multiple Earth Neutral Link) matter so much?
Every three phase motor requires a balance supply in both Phase Angle and Voltage and if they don’t get this balanced supply their efficiency suffers.
Secondly the Rectifiers in Variable Speed Drives all create a certain amount of Common Mode Voltage and this is minimized if a balanced supply is applied.
A rectifier by nature produces Common Mode Voltage as the Positive and Negative AC Sine waves are clipped producing the DC Bus Voltage for the Inverter.
Normally in a balanced AC supply the sum of the Voltages equals ZERO, but when the supply is unbalance this Common Mode Voltage is significantly Amplified.
Unbalance supplies are produced when the Star Point of the WYE winding of a transformer isn’t tied to earth in a low resistance, allowing a partially floating Neutral and this affects every 3 Phase Motor connected to that Supply.
Two Points need to be addressed to ensure a stable Supply:
1. Transformer Earth Resistance
Being sound connection to an earth grid (Inspection Ports),
and Good Soil Conditions. Drought can have an affect and Increase the risk of Bearing Currents.
2. Neutral has low Resistance to the Earth Connection
The MEN (Multiple Earth Neutral) Link typically located in Main Supply Boards ties the Neutral to the transformer Earth.
It is recommended to regularly Inspect your MEN
Link and Measure the Neutral to Earth Resistance
with a Micro Ohm Meter.
The Measurement should be approximately
100 Micro Ohm.
This Link Resistance if too High can reduce the Im
Magnetic Trip into the thermal area of a Circuit Breaker which is extremely Dangerous.
3Phi Reliability recommend including this task of MEN Link Removal Cleaning back to bright copper and re torquing on an Annual Basis. Then rechecking the Earth Loop Impedance test which is a mandatory test each three years in many countries (Including EU & UK).
3Phi Reliability SARL
Blog Free Advice on Electric Motor Reliability and Energy Savings
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