Is it standard practise to fit 2 devices for overload protection, in motor control panels i.e. fuse for short circuit and electronic/thermal for overloads ?
Is it standard practise to fit 2 devices for overload protection, in motor control panels i.e. fuse for short circuit and electronic/thermal for overloads ?
the best approach is a function of motor size, and what it is doing.
The fundamental issue is one of starting and running currents. For all but the smallest motors, the is no single fuse or breaker value that is both high enough not to blow on start-up and at same time not low enough to guard against the windings being damaged if there is a stall or heavy overload condition. This may be adressed by soft starter circuits, star delta switching or nowadays the electronic speed controls, but the tradition is to have a simple fuse or breaker to protect the shop wiring that is significantly ‘oversize’ for protecting the motor, and an overload trip that is chosen to suit to the thermal time constants of the machine. Really big machines may have winding temperature monitors and all sorts of other fancy solutions as well.
At the other extreme for something small like a bathroom fan motor, no-one bothers, and there is a one-shot thermal weak link embedded in the windings instead.
Mike.
the best approach is a function of motor size, and what it is doing.
The fundamental issue is one of starting and running currents. For all but the smallest motors, the is no single fuse or breaker value that is both high enough not to blow on start-up and at same time not low enough to guard against the windings being damaged if there is a stall or heavy overload condition. This may be adressed by soft starter circuits, star delta switching or nowadays the electronic speed controls, but the tradition is to have a simple fuse or breaker to protect the shop wiring that is significantly ‘oversize’ for protecting the motor, and an overload trip that is chosen to suit to the thermal time constants of the machine. Really big machines may have winding temperature monitors and all sorts of other fancy solutions as well.
At the other extreme for something small like a bathroom fan motor, no-one bothers, and there is a one-shot thermal weak link embedded in the windings instead.
Mike.
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