Thread: 3hp table saw motor and soft start

Replace the fuse to the workshop with a 20amp fuse and replace one of your GPO's in the shed with a 15amp GPO, problem solved

It's not just a soft start you need with a 3HP saw.

The 3HP or 2250W or ~10A is purely a nominal current as the motor will draw a different current depending on the load

Free running it might only draw 5A, light load might be 8A, medium load might be 10 A, medium-heavy 12A, heavy might be 14A.

My 3HP saw will pull 17A for about 20 seconds before the thermal cut out in the motor kicks in.

If you run the saw continuously even under a medium load, then depending if you have anything running continuously (like a DC) on the same circuit it might trip the breaker

Originally Posted by rod1949

Replace the fuse to the workshop with a 20amp fuse and replace one of your GPO's in the shed with a 15amp GPO, problem solved

It does not work that way unfortunately.

It is normal to run a string of single or double 10A GPO's in a circuit from a 16A or 20A breaker. However, because they are run as a strung there is a joint in each cable (active, neutral and earth) at each outlet to allow the outlet to tap into the cables. There is a maximum number of permanent outlets permitted in the string, and the breaker and cable are sized to protect the installation and premises, not the equipment connected to the outlets. For example you might have five 3.5A constant current loads connected to five outlets in in a string and not trip a 20A breaker. Change to five 4.5A constant current loads, and the breaker should trip within seconds of the fifth load being connected to protect the installation and premises, even though the individual loads are operating safely within their own design parameters.

For a 15A circuit, the same breaker and cable size could be used though normally the cable would be upsized based on a known type of load, but the requirement is that only the one outlet can be fitted to the circuit, ie, the cable is a dedicated run from a breaker to a 15A outlet, rather than a string of cables joined at each outlet. Suppose the OP follows your advise and for convenience, the selected outlet is the last one (electrically) in the string, because that is what is closest to the saw, then every time the saw starts and runs, it gets it power through maybe 7 or 8 pairs of cable joints, plus has to share power with whatever devices are connected downstream on the circuit.

Unfortunately, it is extremely unlikely that the saw would be fitted with a 3HP 'univeral' type motor that would respond to a speed controller to reduce its start surge. It is far more likely for a 3HP motor to be an induction (brushless) motor, with a start surge that can be up to 6 times its rated current for briefly while it abrubtly accelerates to running speed. In this situation, the potential cable joints at all the other outlets in a cable string can significantly limit available current and prevent the motor starting properly, leaving it at risk of damage. The motor will exhibit a staged current demand with a minimum level set by the mechanical and electrical losses associated with rotating the motor, drive belts, arbor and blade, and increasing in proportion to the load applied to the blade when cutting timber, based upon blade type and width, timber type and depth of cut.

Therefore the OP's legal options are limited to shelling out for the circuit, or replacing the saw with a lower power/lower spec unit.

Unless there is some extraordinary geography, $1500 - $2000 seems like a lot of money for installing a breaker at one end, a power point at the other , and bit of cable in between. The hardware shouldn't cost more than $50, lets be generous and say $100. A couple of hours on site , plus, plus, plus. $600 - $800 would seem more realistic. Or am I totally out of touch with the lifestyle of a rich and famous sparky.

Originally Posted by Avery

Unless there is some extraordinary geography, $1500 - $2000 seems like a lot of money for installing a breaker at one end, a power point at the other , and bit of cable in between. The hardware shouldn't cost more than $50, lets be generous and say $100. A couple of hours on site , plus, plus, plus. $600 - $800 would seem more realistic. Or am I totally out of touch with the lifestyle of a rich and famous sparky.

I agree that's a very silly price. Even in the overheated Perth marketplace I paid $2.4k for 40 m of 3 phase cable from the front of the house, a 10 breaker box in the shed with 3 x 15A, 2 x 10A and 1 lighting lines, 12 double 10A GPOs, 3 x 15A GPOs and 4 lighting outlets and switches, although I did dig the trench (about 10m long).

Thanks to everybody who replied to my post. I will see about getting some more quotes. Keep you posted.

Safari

Originally Posted by malb

It does not work that way unfortunately.

It is normal to run a string of single or double 10A GPO's in a circuit from a 16A or 20A breaker. However, because they are run as a strung there is a joint in each cable (active, neutral and earth) at each outlet to allow the outlet to tap into the cables. There is a maximum number of permanent outlets permitted in the string, and the breaker and cable are sized to protect the installation and premises, not the equipment connected to the outlets. For example you might have five 3.5A constant current loads connected to five outlets in in a string and not trip a 20A breaker. Change to five 4.5A constant current loads, and the breaker should trip within seconds of the fifth load being connected to protect the installation and premises, even though the individual loads are operating safely within their own design parameters.

For a 15A circuit, the same breaker and cable size could be used though normally the cable would be upsized based on a known type of load, but the requirement is that only the one outlet can be fitted to the circuit, ie, the cable is a dedicated run from a breaker to a 15A outlet, rather than a string of cables joined at each outlet. Suppose the OP follows your advise and for convenience, the selected outlet is the last one (electrically) in the string, because that is what is closest to the saw, then every time the saw starts and runs, it gets it power through maybe 7 or 8 pairs of cable joints, plus has to share power with whatever devices are connected downstream on the circuit.

Unfortunately, it is extremely unlikely that the saw would be fitted with a 3HP 'univeral' type motor that would respond to a speed controller to reduce its start surge. It is far more likely for a 3HP motor to be an induction (brushless) motor, with a start surge that can be up to 6 times its rated current for briefly while it abrubtly accelerates to running speed. In this situation, the potential cable joints at all the other outlets in a cable string can significantly limit available current and prevent the motor starting properly, leaving it at risk of damage. The motor will exhibit a staged current demand with a minimum level set by the mechanical and electrical losses associated with rotating the motor, drive belts, arbor and blade, and increasing in proportion to the load applied to the blade when cutting timber, based upon blade type and width, timber type and depth of cut.

Therefore the OP's legal options are limited to shelling out for the circuit, or replacing the saw with a lower power/lower spec unit.

I knew there would be someone respond with a load of gobbledegook