What it is and when you need it
A medium voltage motor usually drives the equipment the process depends on: the main pump, the compressor, the induced-draft fan, the mill. When its winding fails, the outage is measured in weeks, because a rewind or a replacement motor cannot be had overnight. Most of these failures start in the stator insulation, to ground or between turns, and they show warning signs in the measurements first.
Motor tests are electrical measurements made with the motor stopped and disconnected that show the condition of the insulation and windings without taking the machine apart. We recommend them at these moments:
- Receiving and commissioning a new or repaired motor, to set a baseline and confirm that transport, storage and installation did not damage it.
- Scheduled maintenance, to follow the trend and decide on cleaning, drying or rewinding in time.
- Before starting after a long shutdown, especially in humid sites or on motors without space heaters.
- After a trip on differential, ground fault or overcurrent, to know whether the winding was damaged before attempting another start.
In some countries these motors are called high voltage motors, because they are above 1 kV. We serve induction and synchronous motors with form-wound coils, in industrial, mining, water and generation plants. It is part of our testing and commissioning service.
What is included
The scope is tailored to the motor and the goal, and is built from these tests:
- Insulation resistance and polarization index (PI) per IEEE 43, with the test voltage that matches the winding rated voltage and correction to 40 °C. This is the test that detects moisture, contamination and ground faults.
- Winding resistance per phase with direct current, to find loose connections, cracked brazing or imbalance between phases.
- AC measurements per phase (inductance and impedance), when they add value, to reinforce the search for turn-to-turn problems.
- Comparative surge test, which stresses the turn insulation with fast pulses and compares the response of the three phases. It is the only test that evaluates the turns directly.
- Tan delta and tip-up of the stator insulation in higher-voltage motors, to see its aging.
- Offline partial discharge on the stator, when the motor is critical and its voltage justifies it.
- Supply cable test, separate from the motor, so a low cable insulation is not mistaken for a winding problem.
How we work
- Planning. We review the nameplate data, the test history, repairs and, if there was a trip, the relay event record. From that we define which tests to run and in what order.
- Preparation on site. Your staff de-energizes and locks out the motor. We verify absence of voltage, disconnect the cable at the terminal box, ground what is needed and record winding temperature and humidity, because insulation resistance depends heavily on both.
- Execution by stress level. We start with the low-energy tests, insulation resistance and winding resistance. Only if they pass do we apply the higher-voltage tests, such as the surge test or tan delta. That way we do not stress damp or contaminated insulation.
- Analysis. We correct for temperature and compare between phases, against the factory values and against the previous measurement. We read the results together: a low PI with a good surge test does not mean the same as a surge test that fails on one phase.
- Report and recommendation. We tell you whether the motor can start, whether it needs cleaning and drying, or whether a major repair should be considered.
What we need from you
- The motor nameplate data: power, voltage, current, insulation class and year of manufacture or rewind.
- The history of earlier tests, if any, and the factory or workshop report.
- The motor de-energized and locked out, with access to the terminal box, and a person from your operations to accompany the work.
- If there was a trip, the motor relay’s event record and waveforms.
- Your safety rules and work permits.
Standards
- IEEE 43: recommended practice for insulation resistance testing of rotating machines. It sets the test voltage by rated voltage, the correction to 40 °C, the minimum PI of 2.0 for class B and higher insulation systems and the minimum resistance of 100 MΩ for modern form-wound windings.
- IEC 60034-27-4: the same measurement in the IEC framework, with its recommended minimum values.
- IEEE 522 and IEC 60034-15: turn insulation testing of form-wound coils and surge voltage levels.
- IEEE 286 and IEC 60034-27-3: power factor tip-up and dissipation factor of stator insulation.
- IEC 60034-27-1 and 60034-27-2: offline and online partial discharge.
- IEEE 1415: guide for induction machine maintenance testing and failure analysis.
When a criterion depends on the manufacturer or on the trend, we say so in the report and note where the comparison value came from.
What you receive
- A report per test with values, measurement conditions, the criterion applied and the result.
- Comparison between phases, against the factory values and against earlier measurements.
- A clear conclusion: fit to start, fit with observations, or not fit and why.
- The test files, so the next measurement can be compared with this one.
Why Tiza Group
- Our team works in substations and industry, from medium to extra-high voltage, and looks at the motor together with its switchgear, its cable and its protective relay.
- If the problem is in the settings and not in the winding, we review the relay on site.
- Reports your staff understands and can use to make decisions.
Field work is done in Colombia, or elsewhere on request. Contact us to plan your motor tests.