Medium voltage drives can be found in applications like hoists, cranes, and down-hill conveyors. And like all technical equipment, these drives are not free from defects or challenges. So, they require proper care and maintenance for consistent performance. In this article, we review the medium voltage drive technology, failure modes that lead to repair, and rectifier and IGBT module testing.
What is a Medium Voltage Drive?
Voltage drives or voltage-frequency drives are essential parts of various industrial equipment, systems, and machinery. They control the speed and torque of rotating elements by adjusting the input voltage and frequency. Moreover, there are different types of voltage drives available to cover numerous applications. One way of classifying voltage drives is on the basis of voltage rating. Medium voltage drives are those that operate between 2.3 kV and 13.8 kV. Generally, these drives can handle loads ranging from 500 to 3500 hp and are popular in industries like oil & gas, chemical processing, treatment plants, mining, etc. Below is a schematic of a typical medium voltage drive.
The functions of some of the components in the schematic are as follows:
- Rectifier: It converts incoming AC to DC.
- DC-Link Capacitor: Smoothens the DC output from the rectifier by utilizing capacitors and inductors.
- Output Inverter: Converts DC back into AC with controllable voltage and frequency.
- Control Unit: The microprocessors or Digital Signal Processors (DSPs) manage the drive’s operation.
Possible Failures that Lead to Repair in a Medium Voltage Drive
Power Component Failure
A medium voltage drive contains power components like semiconductors, capacitors, and diodes, which can fail, thus requiring repair. This failure could be due to a range of issues such as overheating, physical damage, or aging. Semiconductors, mainly IGBTs, can fail due to overcurrent as this causes excessive heat that can melt the semiconductor material. In addition, capacitors can fail due to dielectric breakdown, corrosion of the electrodes, or loss of capacitance.
Control Unit Failure
The control unit dictates the motor’s rotation at a specific speed using a microprocessor hardware and software network. However, software bugs and malfunctioning of corresponding hardware can lead to the control unit’s failure. Another area of control failure is from programmable logic controllers (PLCs), which establish communication between the control unit components. Failures in PLCs can also lead to misalignment, or the operator can lose control over the voltage drive.
Electrical Insulation Breakdown
The insulation of electrical wiring in voltage drives, like cables, motor windings, and circuit wires, can fail. Usually, this is due to excessive heat, mechanical stress, vibration, and environmental factors, which can break the electrical insulation.
Failure of Mechanical Components
The mechanical hardware in a voltage drive system might fail to perform its tasks because of wear, excessive stress, misalignment, and other causes. Generally, components at risk of mechanical failure are bearings, cooling fans, couplings, mounts & housing, etc.
Rectifier Testing in a Medium Voltage Drive
There are two common measurement techniques to assess whether the rectifier is working with the intended specifications of the voltage drive. One is Output Voltage Measurement while the other is Waveform Analysis of DC Output.
Output Voltage Measurement of Rectifier
This test ensures that the rectifier converts the incoming AC to the correct DC output according to the drive’s specifications. The basic steps in the test procedure are as follows:
- Shut down the voltage drive before connecting the testing equipment.
- Then, connect the multimeter’s probes to the rectifier’s output terminals securely.
- Power up the voltage drive and observe the DC voltage reading on the multimeter once it is stabilized.
- Analyze the reading voltage against the voltage specified in the voltage drive manual or document. If the voltage matches, the rectifier is working well.
Waveform Analysis of DC Output
This test provides information regarding how pure the converted DC is, which means the flatness of the current wave. Carrying out this test is necessary because after conversion, the DC might still contain some AC. Thus, adversely affecting the performance of the medium voltage drive, which may lead to damage and require repair. In this test, the device that captures the current waveform is called an oscilloscope.
Here are the connection and testing steps for waveform analysis:
- Connect the oscilloscope to the output of the rectifier.
- Then, operate the medium voltage drive and capture the waveform of the DC output
- Check whether the output shows minimal AC ripple and noise or not.
- The waveform should be flat and steady. Look for any instability or intermittency that might suggest rectifier problems.
IGBT Module Testing in a Medium Voltage Drive
The IGBT Module can also be tested using a multimeter. First, turn your multimeter to the diode measurement setting. Then, it is ready to test the IGBT transistor’s forward drop and reverse bias. Disconnect the copper bus bars and other components linked to the IGBT’s electrodes C1, E2, and C2E1. This isolation ensures accurate measurements without interference from other circuit elements. Finally, connect the multimeter with IGBT electrodes (terminals) and check the reading.
The connection between the multimeter and IGBT Module for testing should be as follows:
- Red probe on the electrode C1
- Black probe on the C2E1 electrode
- If the IGBT is in perfect condition, it will show “0L.”
- Repeat the measurement for reverse bias (C2E1 to C1) and forward drop (C2E1 to E2) and check the reading. They also need to show “0L”.
