16 Oct CIV ratings for electric motors need closer monitoring – Referro Systems
By Adrian van Wyk, Referro Systems Managing Director
16 October 2024: As most technical industries increasingly rely on advanced motor control technologies, understanding and selecting motors with appropriate Corona Inception Voltage (CIV) ratings is vital to prevent costly failures and downtime over time.
Corona is a luminous discharge phenomenon, which is induced by the partial ionisation of air around the conductor when the surface electric field strength exceeds a critical value. This needs to be monitored more closely to ensure the reliability and longevity of electric motors used with variable speed drive (VSD) control devices.
Electric motors connected to VSDs are susceptible to voltage transients that can exceed their insulation limits, particularly where long cable runs are involved between the VSD and motor. The high switching frequencies of modern VSDs, particularly those using Insulated Gate Bipolar Transistors (IGBT), can generate voltage spikes that exceed the insulation limits of standard motors.
These voltage transients, characterised by rapid rise times, can lead to elevated voltage gradients within the motor windings, increasing the likelihood of insulation breakdown. Understanding how VSDs create these voltage spikes is crucial for selecting a motor that can withstand such stresses.
Therefore, a CIV rating on a motor is not just a number; it represents the maximum voltage that a motor’s insulation can safely withstand. Standard motors may not adequately rated to withstand the elevated voltage conditions generated by VSDs, while inverter-duty motors are specifically designed with enhanced insulation systems to manage these challenges.
Reactor devices, or line chokes, are also often employed to reduce peak voltages transmitted from the VSD to the motor. By limiting the rise time of voltage spikes, these devices can help protect motor insulation from damage caused by excessive transients.
However, while reactors can enhance system reliability, they also introduce limitations in terms of cost and overall system efficiency. The initial investment for reactors must be balanced against potential savings from reduced motor failures and maintenance costs.
Motor terminators are another effective solution for reducing transient peak voltages at the motor terminals. These devices work by absorbing voltage spikes and reducing reflections caused by impedance mismatches in long cable runs.
However, their effectiveness is contingent upon cable length; as distances increase, the ability of motor terminators to mitigate transients diminishes. Implementing motor terminators is advisable for cable lengths not exceeding 182 metres.
The risks of insufficient CIV ratings
When the voltage overshoot constantly surpasses the motor’s CIV rating, it can trigger the corona discharge damaging insulation and leading to short circuits within the motor windings to ground and even interphase.
Generally, the CIV tends to decrease with increasing carrier frequency of the alternating voltage under the influence of which the corona takes place.
This risk is exacerbated by factors such as cable length, cable type, and impedance mismatches between the motor and cable. According to industry standards, including NEMA MG1 Part 31, IEC 60034-25 and IEC 60034-18-41; motors should be rated to handle approximately 3.1 times phase to ground and 5.9 times phase to phase to their rated voltage to accommodate these transients effectively.
Using simulations to select the right motor
Several variations of corona test apparatus and circuits are available and can be used to generate typical corona environments for specific power system components and their operating conditions. The use of simulation tools can predict how different configurations will perform under operational conditions, showcasing system voltages and identifying maximum voltage levels. These simulations provide valuable insights, enabling optimisation of system performance and ensuring the apparatus is designed to handle the stress of real-world scenarios.
By leveraging advanced simulation techniques, informed decisions can be made regarding motor choices and mitigating risk. Simulations also enable the analysis of various scenarios involving cable lengths and types, ensuring that motors that meet specific application requirements are selected. These simulations should be considered for power systems operating at 300V and greater.
This proactive approach enhances system reliability and ensures that electrical motors have adequate CIV ratings.
To run your simulation: https://rockwell.transim.com/login.aspx?ReturnUrl=%2fReflectedWave or https://rockwell.transim.com/ReflectedWave/Input.aspx
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About Referro Systems:
Referro Systems is a sales and distribution company for many of the world’s leading electrical, automation and global software and hardware brands across industrial and commercial sectors. The Level 2 BBBEE company supplies best-in-class industrial automation, electrical control and instrumentation fit-for-purpose and provides cost-effective solutions.
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