Go/no-go tests are an invaluable part of every maintenance regimen. Options include insulation resistance, leakage current, polarisation index and step voltage testing, and there are few who would argue against the benefits of these core electrical tests. However, these tests will not reveal one of the most common early initiators omotor faults: inter-turn insulation breakdown. To detect such a breakdown, it is necessary to use a surge test. Unfortunately, this is the area where some confusion has been generated in relation to the ‘high’ voltages involved. When the probes of a voltage tester are connected to two points in an electrical circuit (commonly one end to a suspected live conductor and the other to a ground source), a closed circuit or loop is formed. This allows electrical current to flow from the higher potential to the lower potential. Ratio validation to confirm that the performance of the CCVT matches its nameplate values for ratio and phase accuracy requires the use of specialised test equipment. For this reason, it is not commonly carried out in the field. If validation testing is performed, however, the results should fall within the appropriate accuracy parallelograms given in standards, such as IEEE C57. 13- 2016. One such example is provided in Figure 2, wherein a CCVT is used for metering purposes. When testing between phase and earth on any circuit protected by an RCD, RCBO, and Safety Breaker, the TPT420 is designed to work below the tripping threshold of these devices to help avoid unintentional disconnection. The phase rotation indication test has been simplified, which now avoids the crossing of the test probes. The TPT420 can also perform a single-pole voltage indication test. There are other effects due to filtering of the higher frequency components of the pulse-wave as a result of attenuation by the steel and the coils own inductance. The speed of propagation of the pulse wave is, however, the key aspect that allows the surge test to detect bad turns.

In a surge test, a short-duration test current with a fast rise time – typically around 100 ns – is generated and applied to the motor coil. The test equipment captures the coil’s response, which takes the form of a decaying oscillation – or ‘ringing transient’. If the motor coil were in air, the pulse would be travelling at almost the speed of light and the voltage would be evenly distributed across the coils. But in a motor, the coil is not in air, it is wrapped around a steel core, and so the pulse travels much more slowly. The EMU, in addition to an inductive voltage transformer, contains a tuning circuit and protection against ferroresonance (Figure 1). The tuning circuit is a reactor that compensates for magnitude errors and phase shift caused by the CVD, making it possible to have the CCVT with a characteristic on the secondary side that is similar, in terms of error and phase deviation, to that of a purely inductive voltage transformer. The instrument has abright, integrated LED torch to helpyou when working in poorly lit environments and, it is safetyrated at CAT IV 1000 V with an IP64 housing, and conforms to the latest IEC/EN 61243-3 and DIN VDE 0682-401 standards. For compliance, GS38 probe tip shrouds are provided as standard, along with a protective storage and carry pouch. Inside the voltage tester, this current passes through a known resistance. The voltage drop across this internal resistance is then measured and displayed on the tester's meter or screen. This provides the user with a reading of the voltage present. In some cases, for basic testers, this voltage may simply trigger a light or a sound to indicate the presence of voltage rather than providing a precise numerical value. On-Line testing – that is, testing while the motor is running – can also be carried out on an as-needed basis with a portable on-line monitor like the Baker EXP4000.

Megger VF5 Voltage Tester Key Features

A bad turn will short and this will be shown by a jump in the ringing frequency of the coil (it becomes in effect a ‘different’ coil at this point in the test). Large frequency jumps can be clearly seen on the tester display, but the instrument’s software also uses mathematical analysis to reveal anomalies that are less easy to spot by eye.

But what overall voltage is necessary to show up these turn-to-turn faults, and will this voltage be harmful if applied to a fault-free motor? How do the test voltages relate to the dielectric strength of the windings? And it’s the same with a surge test generated by a Baker DX tester. It applies a voltage and rise-time to enable you to see the inter-coil response, but with a signal controlled in voltage, time and energy, so that the impact on the motor is similar to the spikes that the motor receives as a result of typical power-system variations during everyday operation. Works below the tipping thresholds of RCDs, RCBOs and safety breakers when testing circuits between phase and earthWhen testing motors, it’s important to keep in mind that they are complex electro-mechanical devices with a complex set of failure modes and related diagnostic options. We will look at the failures of motor electrical/insulation systems, how to manage the life of motors by adopting an appropriate test regime and the concerns that are sometimes voiced about the ‘high’ voltages used for surge testing. The Megger TPT420 Two-Pole Voltage Tester is designed to provide electricians & electrical engineers with a simple, intuitive voltage indicator compatible with both AC and DC voltages. This device can measure AC voltage between 12 to 1000 V and DC voltage between 12 to 1500 V. The TPT420 will even continue to alert the user of a present voltage once the batteries have been completely exhausted. Including a continuity function within 0 to 500 kΩ and a frequency range of 40 to 400Hz, this unit can be applied to numerous situations such as automotive, fire alarm & solar/PV systems. The basic insulation tests just described can be usefully augmented by adding PF measurements at different frequencies and a tip-up test. A PF test at 1 Hz specifically, provides a better indicator of developing issues, facilitating early detection. 1 Hz PF is particularly sensitive to moisture contamination, a commonplace CCVT failure mode. Narrowband DFR tests include power factor tests at several discrete frequencies, including 1 Hz up to 505 Hz. Finally, this ergonomic tester is equipped with a bright LED torch, GS38 probe tip shrouds, and an IP64-rated housing. The Megger TPT420 is also supplied with a carry pouch.

As mentioned in the previous article, there are many options for testing insulation, and Megger supplies an extensive range of testers for such applications, from 50 V to 15 kV insulation testers, through VLF and AC Tan Delta test sets to diagnostic Dielectric Frequency Response instruments and, under the Baker brand, specialist motor testing equipment up to 40 kV. Transformers are an integral part of the power grid. Their reliability directly impacts the reliability of the grid. The failure of this critical asset can handicap the grid and increase its volatility. Because replacing a high voltage transformer requires planning for many reasons, including long manufacturing lead times that can exceed a full year, it is widely accepted that asset management, particularly of transformers, is a beneficial contribution to the operation of the grid. Continuity function: detects continuity between 0 and 500kΩ and alerts the user by issuing an audible and visual alertIn some circumstances, the CVD capacitive reactance can resonate with the magnetizing reactance of the inductive voltage transformer and the compensating reactor cores. This unwanted effect is called ferro-resonance and can give rise to large and damaging voltages across the inductive and capacitive elements. To avoid this, a ferroresonance damping circuit is installed in parallel with one of the secondary windings. In addition to this, the Megger TPT420 LCD/LED Two-Pole Voltage Tester features a Continuity function which will alert the user to the presence of continuity between 0 and 500kΩ via a visual and audible alert. The Megger TPT420 LCD/LED Two-Pole Voltage Tester will also issue an audible alert upon detection of voltage and will continue to warn the user of dangerous test voltages even when its batteries have expired. This voltage tester has a safety rating of CAT IV 1000V. A basic ratio test can be carried out by exciting the primary side of the CCVT with a 10 kV source and measuring the secondary voltage with a digital multimeter. However, this supplies no measurement of phase deviation, which is required to validate the accuracy of the CCVT. The Megger TPT420 LCD/LED Two-Pole Voltage Tester is an AC/DC voltage indicator. It can detect and measure voltages between 12 and 1000V AC and 12 and 1500V DC. The detected voltage range is indicated on the LED scale while the numerical reading is displayed on the LCD.