What’s Protecting Your AC Motor Controller?

While many of today’s industrial plants are using soft-starters and drives to reduce their operating costs, there is considerable confusion about how to properly protect these devices. This has left many electronic motor controllers vulnerable to catastrophic failures that can propagate to surrounding equipment, jeopardize the safety of personnel and bring the manufacturing environment to a standstill.

“There is currently no standard approach to providing proper protection to motor controllers. In the past, all drives and soft-starters incorporated internal high-speed fusing, but as these devices have gotten smaller and less expensive many manufacturers of medium- to low-power (200HP and below) drives and soft-starters have eliminated internal fusing to satisfy cost and space constraints. The Drive manufacturers consider the lower power systems to be sacrificial. These smaller power drives are only “Type” tested with branch circuit protection to prevent explosion. Ultimately, protection is left to end users who are often unaware of this fact. Even though the manufacturer may recommend a high-speed fuse, this critical information is often overlooked because end users are simply focused on the NEC and CEC code requirements.”

Clearing the Confusion
For these applications, the CEC/NEC requires that end users establish standard branch circuit protection using a UL/CSA listed/certified 248 fuse or UL 489 Listed/CSA certified circuit breaker. This provides a level of protection that is intended to protect the conductors from damage or fire. If high-speed fuse protection is not present — inside or outside the device — neither a standard fuse nor circuit breaker can adequately protect the electronic motor controller/Starter.

To prevent unnecessary damage, end users must confirm that their component includes high-speed fusing at the time of installation. If internal fusing is not present, they must follow the recommendation included in the manufacturer’s specification sheet. But given the lack of a standard approach, not all end users recognize the need for a high-speed fuse protection — and those that do often find it difficult to coordinate the high speed fuse properly to the Electronic motor controller.

In many cases, high-speed fuses are incorrectly sized according to the motor start-up parameters, which will result in greatly oversized fuses or very slow reacting circuit breakers that will provide marginal protection in the event of a short circuit. In other cases, protection is based upon the max cable feed ampacity to the controller, but this cannot protect the controller itself. In the end, many of these potentially cost-cutting devices are needlessly scrapped, adding additional costs in lost productivity and component replacement.

Fig 1 describes the start-up characteristics of a 7.5hp drive as it relates to melting times of a standard Time Delay Class J. Note, the currents required to melt the fuse are quite higher than the actual requirements for operating the Drive.  As shown in graph, the current required to clear the fuse is far greater than the actual current needed to meet drives operating requirements.

Hard Facts About Soft-Starters
The primarily switching device in Soft-Starters, are SCR or GTO, making them susceptible to over-current current induced failures. If a short circuit occurs, the increase in current will exceed the energy capabilities of the SCR/GTO. Although most soft-starters will shut down in a half cycle, the high amounts of energy generated within that half cycle (approx 8ms) can destroy the soft-starter’s sensitive internal components. The end user may not be able to detect any visible damage, but the failure could propagate to feeder wires and the weakest components within the circuit, causing a cascade of unwanted downtime and repairs.

To properly protect soft-starters, end users should first determine if their component includes internal high-speed protection. If it doesn’t, they must select a high-speed fuse according to the operating parameters of the electronic motor controller. This means taking into account the application’s maximum inrush current and duration, the motor’s full-load current (FLA) and the duty cycle (number of starts and stops) of the controller.

“If one is required, most manufacturers will specify a high-speed fuse in the soft-starter’s documentation. It’s just that many times end users aren’t looking for it — they simply establish branch circuit protection according to the Code and put their systems online.”

Roadmap to Proper Drive Protection
The vast majority of today’s drives utilize IGBT technology as the base switching device. IGBTs have a lower energy capacity than that of SCR / GTO and are particularly susceptible to over voltage-induced failures. Due to the explosive consequences of IGBT case ruptures, most drives rated 200HP or above include internal high-speed fuse protection along with electronic controlled protection.  But that’s not always the case with medium- to low power drives (most include electronic protection only), which again leaves establishing  critical protection to the end user.

It’s extremely important that end users make certain their drives are protected by high-speed fusing. Although most drives have internal electronics to shut down the system in the event of a fault, their effectiveness is degraded over time by transient voltages. If high-speed fuse protection is not present and the internal electronics do not operate effectively, the IGBT could have a shoot-through (fault) causing case rupture of IGBT and expelling gases and particles that could propagate failure to surrounding equipment and rack up significant costs in downtime, repairs and component replacement.

The high-speed fuse should be selected according to the performance characteristics of the drive and the motor’s start-up requirements: load requirements and horsepower rating. But again, this doesn’t always happen due to the inconsistency of protection philosophies and techniques.”

To simplify matters, an innovative new solution — the High-Speed Class J (HSJ) fuse has been introduced to the marketplace recently. Designed specifically for the operating characteristics of solid-state motor controllers, the HSJ incorporates the high-speed performance and low I2t of semiconductor fuses with the overload capacity and physical dimensions of a standard Class J fuse. CSA certified and UL Listed to the 248-8 Standard, the HSJ can be used to establish both the required CEC/NEC branch circuit protection and the high-speed protection necessary to protect the drive or soft-starter in one package.

As figure 2 shows, the melting characteristics of the new High Speed Class J are much closer to the current requirements of the Drive. The new HSJ will provide exceptional protection in the event of a short-circuit (High Speed clearing) and still provide the necessary current carrying capability to allow the drive to operate normally without the possibility of nuisance operation.

The greatest advantage that the new HSJ brings is its ability to limit the let thru energy (I2t) to a fraction of the energy a circuit breaker will allow to pass (see Fig 3). Fig 3 represents the let-thru current during a short circuit. All devices were subjected to the identical fault, As the graph clearly shows, circuit breakers do very little to limit the amount of thermal energy ( I2t) associated to the fault condition which can cause extensive damage to electronic motor controllers (Drives & Soft-Starters)

The HSJ will make motor controller protection considerably easier for end users, “With a single fuse, they can meet the CEC/NEC requirements and protect their drives and soft-starters from unnecessary damage. And since the HSJ is the same size as a standard Class J fuse, it can be used with existing blocks and holders.”

To really reap the benefits of AC motor controllers, make sure your soft-starters and drives are protected with a properly coordinated high-speed fuse. Without it, your reduced operating costs can be undermined by unnecessary downtime and needlessly scrapped components.

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