18/10/2010
Ezine - Baker/SKF motor testing equipment used worldwide
Baker/SKF motor testing equipment is used worldwide for condition monitoring of critical electric motors
Offline equipment tests the entire insulation system of motors and will highlight any issues that may affect the motor’s future reliability.
Online test equipment looks for issues including power quality problems (harmonic distortion etc), cracked or broken rotor bars, problems with the load, including pump cavitation, vibration issues, including bearing faults, out of balance etc. The ability to monitor starting current and voltage in-rush and the torque profile, as well as monitor 40 plus channels in real time for analysis, are all standard features.
For successful condition monitoring of electric motors, test equipment ideally needs to be computer controlled, to have the ability to accurately and consistently apply the required test voltages and to be able to trend the data over time. For offline testing, winding resistance, meg-ohm, PI and DC step voltage data needs to be automatically compensated to a fixed temperature to allow accurate trending to take place. Sufficient test voltages need to be applied to ensure that the insulation systems are sufficiently stressed to detect weakening of the insulation system. The rule of thumb typically used by Industry is to apply a test voltage of twice the motor operating voltage plus 1,000 volts. So a 415 volt motor would typically be tested at 2,000 volts. A 3.3 KV motor would be tested at 7.6 KV.
The winding resistance test should be carried out using Kelvin contacts for accuracy, and for the meg-ohm, PI and DC Step Voltage Tests, the unit must be capable of measuring up to at least 50 gig-ohms.
Baker Offline testers include the surge test, which looks at the condition of the copper to copper insulation, between turns, coils and phases. A surge test works by injecting high voltage pulses into a winding, creating a potential difference between one turn and the next.
Since 80% of electrical failures in motors, originate as a turn to turn weakness, it is vital that such weakness is detected early. Surge testing is the only method of detecting such weakness.
Paschen’s law states that a minimum turn to turn potential difference of 325 volts is required, in order to draw an arc between two conductors. The required voltage will be higher depending on the air gap between the turns, and the condition of the insulation.
Since motors see high voltage spikes on start-up and shut-down, and also during operation, it is important that testing simulates these spikes, albeit with DC voltage and low energy.
The limitation of the meg-ohm test relates to the fact that all turns are raised to the same voltage.
This explains why many engineers have experienced motors failing to earth, only a matter of days or weeks after a meg-ohm test showed several gig-ohms to earth.
Automatic trending is standard, and aids the engineer in deciding when to remove a motor from service.
Baker’s online portable tester, the EXP-4000 is suitable for testing both AC and DC motors.
Simply by measuring the three phase voltages and currents, a vast amount of information relating to the power supply, the motor itself and the load will be provided.
The EXP considers the quality of the power being fed to the motor, and will highlight issues such as voltage unbalance, harmonic distortion and total distortion.
Power issues coupled with heavy loads can lead to rapid degradation of the winding insulation, due to thermal ageing. For every 10 degrees C in temperature rise, the insulation life is halved.
The rotor bar test clearly shows the rotor condition in graph format, and this is trended automatically over time.
The EXP provides details of the instantaneous torque being requested of the motor, and displays it in graph format. Variations in the load are clearly shown, and again are trended. Pump cavitation, flapping belts, rotor bar issues can all be seen within the torque ripple graph.
The EXP will provide both V/I and torque spectra data for each test taken, allowing bearing problems, air gap problems and other mechanical issues to be found. One of the benefits of the EXP unit is that all measurements are taken from the motor control cabinet, meaning that motors in restricted areas, or submersible pumps can be tested without problem.
One of the most powerful tools within the Explorer software, is the continuous monitoring software. This is invaluable for fault finding, and allows real-time data logging of in excess of 40 channels to be made. 9 data points per second can be recorded and then viewed in playback mode. The unit can be set to record for entire the duration of the test, or it can be ‘triggered’ to record when certain parameters exceed set limits. The software will display real-time data for voltage and current levels (all phases), voltage and current unbalance, power factor, total power, percentage load, input power, output power, torque, speed, to name but a few.
The latest development from Baker/SKF is the NetEP system, which is a permanently-installed automated condition monitoring system, that will monitor up to 32 motors simultaneously, 24 hours per day. The server-based system will allow remote access from anywhere in the world, making it ideal for companies with multiple sites, shipping companies etc.
Whilst based on the proven EXP-3000/4000 technology, the NetEP removes the need for route-based monitoring, and provides a constant flow of data which allows for better trending and fault diagnosis.
The desktop Surveyor software package provides data on 120+ parameters and can produce alarms on 38 parameters.
Multiple NetEP units can be connected to one system, to monitor larger numbers of motors, where required.
For further information on any Baker/SKF motor testing products, please contact Mike Herring, Sales Director of Whitelegg Machines Ltd on 01293-526230 or email
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http://www.whitelegg.com
http://www.whitelegg.com