Bearing Currents

High Performance Voltage Measurements in a VFD Motor

Measuring accurately voltage signals in a VFD motor is a challenging application. Typical VFD motor phase voltage measurement requires an ample voltage range, high resolution and high bandwidth. In practice, the signals exhibit sharp transients and important information is contained in small variations in high frequencies. For example, small milivolt size disturbances in phase to neutral measurements in a 600Vrms signal can indicate the presence of partial discharge which is an early fault indicator. Choosing the right sensor for your measurement system becomes crucial to obtain useful measurements. In this short application note we review a solution that uses the Verivolt Entube family of sensors combined with a National Instruments PXI system to monitor a three phase VFD motor. We discuss the various challenging requirements and how the Entube family addresses them, including finding the right combination of performance and cost, depending on the purpose of the measurements.

Example measurements

We have measured a 480V three phase VFD motor using Entube Z sensors and a PXI system. For this particular application we chose a PXI system since we desired measurements with a 50MHz bandwidth. Figure 2 displays the neutral measurements obtained using the Entube Z sensor. Notice that detailed features at various scales can be captured using Verivolt Entube Z and the PXI system. The very low noise floor can be observed in the flat regions of the signal. A 1kV Entube Z was used for the measurement. Notice in the top right panel of Figure 2 the spikes in the measurement due to fast transients.

Application Description

Three-phase and neutral measurement of a VFD motorThe usual monitoring scheme for measurements of VFD motors calls for measurements of phase and neutral voltages of the motor. In some applications, such as bearing currents monitoring, it is of interest measuring axle to ground voltage as well. A typical VFD motor voltage measurement has sharp edges corresponding to responses to the operation of the switching modulation provided by the IGBTs in the VFD PWM system. During normal operation, one common measurement is to find the harmonics in the measured voltage. Typically, measuring up to the 10th order harmonic of 60Hz might be of interest. In motor health monitoring applications, it is of interest to measure carefully the sharp transitions and even higher frequency information, easily ranging from several kHz to Mhz information. In small motors the voltages are expected to be within 600V peak. In medium voltage motors, voltages can go up to 10kV. Moreover, the peak voltages may exceed these values, if measuring fast transients that occur during a fault situation. The voltage signals usually also exhibit various pre fault signatures occurring at high frequencies but at much lower voltages, from a few milivolts to a few volts. A high performance sensor has to enable the measurement of a voltage signal with wide range, small resolution and high bandwidth to capture the various events of interest.

Measurement system requirements

There are several important requirements that voltage monitoring systems will need: Range: The large variety of VFD motors requires different measurement ranges, from 400V to 1.2kV in small drives, especially if large unexpected transients are to be captured by the monitoring system. Even larger ranges are required by medium and high voltage drives.

Bandwidth

The bandwidth requirement depends on the purpose of the measurements. They may be as low as a few kHz, for measuring harmonics and slower transients, up to 20MHz for measuring fault signatures present in partial discharges or bearing currents.

The Entube Z family of sensors has bandwidth of 50MHz enabling even the most demanding VFD motor monitoring applications. Combining Entube Z with PXI systems creates the flexibility of tailoring a measurement system for the application at hand, without the sensor being a constraint in bandwidth.

Resolution

Typical measurement resolution in VFD motor monitoring applications is 0.2% of the measurement range. But high performance measurements, such as required for detection of bearing current events and partial discharges may require a resolution of a few milivolts or less, leading to demanding requirements for the sensing and data acquisition setup.

The Entube family has sensors with resolutions of 0.2% of the measurement range and smaller. Coupled with different digitizer choices in cRIO or PXI systems, it leads to flexible, high voltage measurement platform with 12, 16 and 24 bits. For example, for measurements of bearing current events, at least 16 bits or 24 bits are required. For harmonic measurements, a 12 bit or 16 bit digitizer system may suffice.

Compactness

Typical measurements systems for motors maybe used as a temporary setup or a fixed permanent setup. In both cases, large bulky sensors are intrusive and difficult to install. They may also lead to simultaneously measuring a smaller number of sensing points than desired. This is an important issue especially in medium and large voltage drives where some sensing technologies tend to be obtrusive and bulky.

The unique Entube Z sensor design allows it to be very compact. Entube Z sensors of upto 3kV are a compact 3.3in long and 0.68in in diameter. The Entube 30kV is 10in long and 1.75in in diameter. These compact dimensions can be combined with the rugged and compact cRIO to provide a portable and easy to setup voltage measurement platform. Entube Z and National Instruments PXI system provide a high performance, industrial grade measurement system addressing some of the most demanding VFD monitoring applications.

Easy integration

Most commercially available voltage sensors do not provide outputs that directly adapt to the chosen data acquisition system. For example, National Instruments systems accept +/-5V inputs and +/-10V inputs, and many voltage sensors return 120V, requiring the integration of a component that further reduces the voltage. This increases measurement noise and solution complexity. An easy to integrate sensor should seamlessly adapt to the data acquisition system in use.

Verivolt Entube Z sensors provide +/-5V outputs that can be directly measured by existing National Instruments Data Acquisition products. This removes the need for an additional signal conditioning, resulting in a compact and easy to use system.

Easy installation

In many setups, the data acquisition system may not be placed very close to the motor due to safety or space issues. In such cases, cable compensated probes are usually required to obtain accurate measurements. This compensation calibration adds an additional burden on the management of the monitoring setup.

Verivolt Entube Z unique compact design is prepared so the sensors directly fit the BNC connectors in PXI and cRIO. A four channel module can attach to four Entube Z sensors, turning them into modules capable of reading up to 3kV. This direct attachment of the sensor to the measurement module brings one important benefit: no cable compensation is needed for measurements far away from the acquisition system.

Noise isolation

Motors operate in a very electrically noisy environment due to Electromagnetic fields and other issues. It is quite easy for this noise to influence the sensors used in the typical measurement, in some cases leading to unusable or uninformative data. It is important that the voltage sensor in use provide strong immunity to electrical noise.

Entube Z sensors were designed to operate in scenarios where Electromagnetic noise is present, and its unique patent-pending design substantially reduces measurement noise leading to the best performing sensor of its class. Various customers have reported Entube to be the best sensor in their measurement setup and are using it as a reference sensor. Together with the low noise high performance National Instruments digitizers, Entubes enable a high performance voltage measurement platform for VFD motors. In the field we observed that low quality sensors have limited the performance of National Instruments platforms when monitoring VFD motors. The Verivolt Entube sensors use various patent-pending technologies to lead to a compact, low noise, high performance sensor that can be combined with flexible, high performance Data Acquisition systems from National Instruments to deliver a compact high performance voltage monitoring system for VFD motors. The system can be configured to address multiple applications spanning different bandwidths and voltage ranges, with different requirements.

Verivolt Entube and National Instruments PXI measurement system

a flexible and high performance voltage monitoring platform for VFD motorsTo build a flexible and high performance VFD motor voltage monitoring system we used a PXI and a cRIO measurement systems. For the high bandwidth, high resolution system for measuring bearing current and partial discharge type events, we used a combination of high performance Verivolt Entube Z sensors and a PXI system capable of sampling up to 50MHz. For a less stringent bandwidth requirement, but very low noise high quality measurements, we used a cRIO based system. The cRIO gave us the flexibility to implement simple local processing useful for computing harmonics or simple event detection.

The combined system addressed each challenge for voltage measurement in VFD motor systems range

The Verivolt Entube family provides sensors for ranges from 100V to 100kV, with very low measurement noise, enabling accurate measurements even when range has to be overprovisioned to capture high voltage transients. For example, a 1.2kV sensor maybe used in a 480V motor to capture large transients without compromising the resolution of the measurement due to sensor noise. Coupled with flexible data acquisition system choices from National Instruments, a high resolution, wide range system can be achieved.