Motor protection - Fuse

Motor protection Fuse

 

“Quick-acting” fuses

Nontime-delay fuses provide excellent short circuit protection. However, brief overloads, such as motor starting currents, may cause problems for this kind of fuse. Therefore, nontime-delay fuses are best used in circuits, which are not subject to large transient currents. Normally, nontime-delay fuses hold some
500% of their rated current for one-fourth of a second.

 After this time, the current-carrying element melts,
and opens the fuse. Thus, in motor circuits, where the
starting current often exceeds 500% of the fuse’s rated
current, nontime-delay fuses are not recommended.

“Time-lag” fuses

This kind of fuse provides both overload and shortcircuit
protection. Typically, they allow up to 5 times
the rated current for up to 10 seconds and for shorter
periods even higher currents. Usually, this is sufficient
to allow a motor to start without opening the fuse.
On the other hand, if an overload condition occurs
and persists for a longer period of time, the fuse will
eventually open.




Principle of a tripping curve for a fuse. The graph shows the relation between the actual current and the full-load current.

Fuse clearing time
 
The fuse clearing time is the response time it takes the fuse to open. Fuses have an inverse time characteristic, meaning that the greater the overcurrent, the shorter the clearing time. Generally speaking, pump motors have a very short run-up time; below 1 second. So, blown fuses during start-up are normally not an issue for pumps if the fuses match the motor’s full-load currentand is a »time-lag«
fuse.
The illustration on your right-hand side shows the principle of a tripping curve for a fuse. The x-axis shows the relation between the actual current and the full-load current: If the motor consumes the full-load current or less, the fuse does not trip. But at 10 times
the full-load current, the fuse will trip in a very short time (0.01 s). The Y-axis shows the clearing time.
During start-up, an induction motor consumes a large amount of current. In some rare cases, this may lead to a cut-out via relays or fuses. Different methods of starting the motor exist in order to reduce the locked rotor current.

 


Tripping curve for a »quick-acting« and a »time-lag« fuse.
 The “time-lag” fuse is the best choice for motors because of the high starting current