Technical Article
Motor and Transformer Inrush Considerations
“Why is my 20A breaker tripping when my circuit is only 15A?”
This was a popular question I would often hear while troubleshooting transformers. There are many possible explanations for why this scenario would happen: perhaps there is a short, or another issue downstream of the breaker. Typically, however, the issue was transformer inrush.
What is inrush? Why do some devices, such as transformers and motors, pull more than their rated loads at start-up? This article will explore what inrush is, how to avoid nuisance tripping, and inrush considerations for solar, energy storage, and microgrid systems.
What is Inrush?
Inrush is a large transient current surge that occurs upon energization of an AC electrical device. For most loads and devices, especially resistive loads, this surge is negligible. However, electric motors, transformers, and devices with capacitors will often draw current many times above their rating when energized.
For transformers, this inrush is caused by the sudden magnetization of the core. When the primary windings of a transformer are first energized, the windings will induce a magnetic flux onto the core. This will occur regardless of whether or not the secondary windings are connected (i.e., with or without load present). During magnetization, the core will saturate as it has no residual flux . Saturation effectively removes any impedance provided by the core or windings, causing a large spike in current.
This transient spike is primarily made up of harmonics, the most common being the second-wave harmonic. The time it takes for the current waveform to return to normal, steady-state flux is dependent on the transformer’s reactive-inductance-to-resistance ratio (XL/R) which means it typically lasts a few cycles but can last longer for larger transformers.
Induction motors will also experience large inrush currents. At startup, the rotor is stationary and demagnetized. As the stator starts to induce magnetization on the rotor, a large current will flow through the rotor windings. At this point, there is no electromagnetic force (EMF) opposing the stator supply voltage, which means low opposition to current as there is very little impedance being provided. Once the rotor begins accelerating and slip decreases, the rotor will begin providing back EMF and inrush current will start to decay.
Since the cause of inrush differs for motors and transformers, so will their inrush profiles. Motors will typically see a lower inrush multiple than transformers, but because of the nature by which this inrush is generated, the inrush will typically decay more slowly (e.g., decaying over a few seconds rather than a few cycles). The waveform of a motor inrush transient is also more symmetrical than a transformer, being made up mostly of fundamental current.
What is nuisance tripping?
Let’s say you have four identical circuits with four identical transformers, overcurrent protection devices (OCPDs), and loads. What would cause one circuit to trip the OCPD while the other three are fine? The answer is often nuisance tripping.
When a transformer core is energized, the amount of current it draws upon inrush is largely dependent upon the instantaneous point in the sinusoidal wave when it reaches the core. The most extreme example would be if the sine wave is near the zero point in voltage. Because the transformer is a mostly inductive load, a phase shift between the current and voltage will mean that while voltage is near its zero point, current will be near its maximum. This can be further exacerbated if the core has any residual magnetization from a previous energization, which depends on the polarity of both the residual magnetization and the polarity of the magnetizing current.
This means that the inrush profiles, even for identical transformers, can differ significantly depending on the instantaneous point in the sine wave and residual flux in the core.
What about induction motors? Well, because there is very little difference in the inrush generated relative to the point in the sinewave.
How does inrush impact system design?
When accommodating devices with high inrush, care must be taken to properly size the OCPD or to lessen the inrush profile of the system. Furthermore, for microgrids and load management scenarios, proper sizing of the generation equipment must be taken into account.
For sizing transformer OCPDs, the general best practice is that the device be rated for 125-150% of the full-load current (FLC) of the transformer. This, remember, is important regardless of the actual load of the transformer, as the core will saturate upon energization even if no load is connected.
For motors, it depends on the OCPD type. For inverse time circuit breakers or instantaneous circuit breakers, it is typically around 250% or 800% FLC, respectively. Time-delay fuses usually fall under 175% FLC, whereas non-time delay will be around 300%. You could also implement a soft start or VFD, both of which help to prevent a large inrush of current at startup.
If multiple motors or transformers are integrated within a microgrid, additional design considerations come into play. Oversizing generation sources absorbs transient inrush while maintaining stable voltage and frequency within the microgrid. Energy resources that are fast acting, such as a battery energy storage system, can quickly respond to a microgrid inrush and maintain system health. Microgrid controllers can also be utilized to manage loads predictively.
System modeling and load studies can be performed by Mayfield Renewables to determine the suitability of the system. Are you looking for an engineering partner who can navigate consulting on product selection and value engineering opportunities? Reach out to discuss how we can support your team today.
