Motor controller question
- Thread starter 1/2sparky
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- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
You are correct. Thre fuses will trip on short circuit or ground fault but the overload will trip on over current
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Technically, fuses don't "trip", they blow... 
Overloads function on a curve of time and current (squared). At the top of the curve is 600% of FLA, a.k.a. "locked rotor", and the bottom is a "pick-up" point, typically at 125% of the motor FLA. Then there are "Classes" of trip curves, Class 10, 20 and 30, which relate to the trip time at the 600% level; Class 10 trips in 10 seconds, Class 20 in 20 seconds, 30 is 30 sec.. Most NEMA motors use Class 20. So even if the current jumps instantly to 600%, it's still going to take 20 seconds to trip.
In a short circuit, ALL of the available current available in a source will attempt to flow, instantaneously. For example if you have 40kA of fault current capability still available at the starter, 40kA will attempt to flow into that fault. If you wait for an OL relay to trip, a lot of damage will take place. Fuses and Instantaneous Trips (Mag Trips) on circuit breakers are intended to react to that as fast as possible to limit that damage.
Overloads function on a curve of time and current (squared). At the top of the curve is 600% of FLA, a.k.a. "locked rotor", and the bottom is a "pick-up" point, typically at 125% of the motor FLA. Then there are "Classes" of trip curves, Class 10, 20 and 30, which relate to the trip time at the 600% level; Class 10 trips in 10 seconds, Class 20 in 20 seconds, 30 is 30 sec.. Most NEMA motors use Class 20. So even if the current jumps instantly to 600%, it's still going to take 20 seconds to trip.
In a short circuit, ALL of the available current available in a source will attempt to flow, instantaneously. For example if you have 40kA of fault current capability still available at the starter, 40kA will attempt to flow into that fault. If you wait for an OL relay to trip, a lot of damage will take place. Fuses and Instantaneous Trips (Mag Trips) on circuit breakers are intended to react to that as fast as possible to limit that damage.
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