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Zorluk: ZorProtective Devices

An automotive direct-current (DC) blower motor circuit with a continuous operating current of 5 A5\text{ A} experiences a brief transient inrush current of 18 A18\text{ A} for 150 ms150\text{ ms} each time the motor starts. When a technician installs a standard 6 A6\text{ A} fast-acting fuse, the fuse opens immediately upon starting the motor despite no short circuit or wiring fault being present. To resolve this nuisance tripping while still providing reliable overcurrent and thermal protection for the continuous load and wiring, which protective device should be installed?

  1. A
    A 20 A20\text{ A} fast-acting fuse connected in series with the motor feed conductor
  2. A 6 A6\text{ A} slow-blow (time-delay) fuse connected in series with the motor feed conductorCevap
  3. C
    A 6 A6\text{ A} fast-acting fuse connected in parallel across the motor power terminals
  4. D
    A metal-oxide varistor (MOV) rated for 12 V12\text{ V} connected in series with the motor feed conductor

Cevap

A 6 A6\text{ A} slow-blow (time-delay) fuse connected in series with the motor feed conductor
Inductive loads such as electric motors draw high transient inrush currents upon startup before counter-electromotive force (CEMF) develops. A slow-blow (time-delay) fuse utilizes a dual-element or thermal lag construction designed specifically to pass short-duration overcurrent surges without melting, while still opening quickly under prolonged overloads or dead short circuits. Sizing the slow-blow fuse at 6 A6\text{ A} and placing it in series provides full circuit protection matched to the 5 A5\text{ A} running current without nuisance tripping.

Adım Adım Çözüm

1
Analyze the load characteristics and failure mode
The DC motor requires 5 A5\text{ A} continuous running current but produces a temporary inductive inrush surge of 18 A18\text{ A} (3.6×3.6\times rated current) lasting 150 ms150\text{ ms}. Fast-acting fuses react instantaneously to current spikes, causing nuisance blowing during normal motor starts.
Inductive and capacitive loads naturally draw transient inrush currents that must be accommodated without blowing the protective element under non-fault conditions.
2
Determine the appropriate fuse operating characteristic and rating
A slow-blow (time-delay) fuse rated at 6 A6\text{ A} contains a dual-element or spring-loaded thermal design that absorbs brief transient current spikes without melting, yet reliably opens if current exceeds 6 A6\text{ A} continuously.
Maintaining the current rating near the continuous operating load (6 A6\text{ A} for a 5 A5\text{ A} circuit) ensures proper thermal overload protection for the wiring and motor windings.
3
Verify proper circuit topology for overcurrent protection
The protective device must be placed in series with the ungrounded supply conductor so that opening the fuse disconnects the entire current path to the load.
Series placement ensures the total circuit current passes through the protective device.

Anahtar Kavram

Selection and application of time-delay (slow-blow) fuses for inductive inrush loads
Tahmini Süre:1m 15s
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Konular

Protective Devices

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