Stator winding / Coil (Motor) Burning Problem

Posted by Glenn Bullion on

1. Introduction:

Stator coil burning is one of the most common motor failures in the factory. Since the invention of AC motors at the end of 19th century, systematic innovations improved the quality of metal alloys of the coil. However, by beginning of the 21th century, motor coil failure returned as one of the biggest nuisance because of rising harmonics contents in the line which is a result of introducing more vector drives and LEDs to save energy.
Coil failure on critical machines can drag down productivity to a large extent. Given meteoric rise of harmonics in the factory power feed, we advise you to take preventive precautions. MachineSense PA can alert you about impending stress in Motor coil via SMS/Email and you can monitor coil health through mobile app.

2. Common causes of coil burning:

  • Presence of VFD/DC drive nearby
  • High current imbalance in motor input line
  • High current harmonics in motor input line

Effect of VFD/DC drive nearby

Fig. 1: Generic motor installation in factory

  • Electrical point “A” denotes the high voltage incoming mains to the input of the panel. This is the main power distribution panel
  • Electrical point “B” shows the input of the VFD drive
  • Electrical point “C” describes the output of the VFD drive where motor M2is connected
  • Electrical point “D” denotes the single utlity point where motorM1 is directly connected to the supply.

Note: There can be multiple VFDs attched to the same panel andmultiple number of motors directly connected to utilty without any drive.

Fig. 2: Three phase voltage and typical current waveforms at VFD input

Fig. 3: Typical phase voltage and current waveforms at VFD output

Fig. 4: Phase current of the motor (M1) input due the presence of VFD nearby

  • Fig. 1 shows motors are connected with VFD system and Utility directly in a factory. In most cases it is the same electrical panel where VFD is taking power to run motor and also some medium size motors are attached to it. Panel is used to distribute the power.
  • Fig. 2 shows typical voltage and current input at VFD input when VFD is having diode rectifier or thyristorized rectifier at input for the conversion of AC-DC. It is the current waveform which is highly distored and can carry upto 90% harmonics of different frquencies (100Hz, 150Hz, 200Hz…so on). These harmonics generated by VFD can affect other motors running parallel to same electrical panel output.
  • Fig. 3 tells about the phase voltage and current waveforms at the VFD output. It is clear that voltage at VFD output is a SPWM output which can vary with time depending upon load demand i.e. when speed requirement is less, frequency gets low like conveyer belt. Whereas current gets filtered with motor inductance and it looks like sinewave.
  • Fig. 4 shows how the impact of high harmonics generated by VFD keeps on going to the normal motors attached to the same electricial panel. Current waveform gets distorted and more importantly peak can even go beyond fundamental peak which unneccessairily increases heating of the coil. In the long run, insulation can get damaged and inter-turn ( a portion of the winding) can be short circuited/burnt.

Effect of high current imbalance in motor input line:

Under voltage in the line increasesthe current drawn by that line when the motor is running on constant load (power mode). This additional current can create additional heating of the coil. Voltage imbalance has the same impact as that of the under voltage. If one of the winding gets partially damaged, its winding impedance will change and will try to create magnetic imbalance which in turn will create current imbalance.

(To measure Undervoltage, Load and Current imbalance you can use MachineSense PA. For motor coil burning MachineSense PA will measure voltage/current imbalance and with an intelligent algorithm will send you a SMS/Email notification if there is a trace of Motor Coil failure.)

Effect of high current harmonics in motor input line:

Current harmonics are generated by use of SMPS based power adapters such as LEDs in the factory. Improper distribution of load can also create high neutral current with high third harmonics in Europe and India where there is a neutral. Basically, whenever there is number of nonlinear load running in the system it can experience high level of harmonics.

Not only VFD/DC drives, any power converter having nonlinear rectifier (Diode based or Thyristor based) for input power conversion can generate line harmonics in the system. These harmonics arecarried through line to some other motors which are connected to the same power line directly. MachineSense PA monitors the harmonics 24x7 and if it crosses the safe threshold, will warn you via email/SMS.


Solution

  • Isolate power source from VFD/DC drive system if they are running nearby. To do so, a transformer with proper KVA rating can be installed. To estimate the KVA rating install MachineSense PA and look for the peak usage in the Real Power.
  • Replace ordinary VFD/DC drive with IGBT based digital drive which has inbuilt PFC (Power Factor Corrector) at drive input. IGBT allows bidirectional current flow and reduces harmonics significantly and thus improves power factor at input.

    You can contact (info@machinesense.com)MachineSense Power Analyzer team to consult and get free guidance to select better quality drive to solve your factory problem.
  • Use MachineSense Power Analyzer (PA) to measure the current imbalance and current harmonics which are passing through the machine. MachineSense PA will monitor the abnormalities 24x7 and send you sms/email whenever any alarming condition is detected.

    If you have already measured the harmonics level and current imbalance level, please verify with MachineSense PA as it maintains IEC standard for each quality parameter.

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