Depending on the application, a motor may take 2 seconds to start and
then run for a long time. The _efficiency_ of the start up mode is
rarely an issue.
There Were two issues, 1. that the starting torque was enough to get the
load moving and 2. that the startup was fast enough since in motors like
capacitor start, there was some serious overheating going on in the
starter coil. In part since the start up stuff was designed to use the
thinnest (cheapest) wire that we could get away with.
At least those were the issues with single phase motors. I suspect
those sane issues were also of concern for 3 phase motors.
1) If started as star, the starting line current in the winding will be 1/3
of the starting current for delta connection- The winding current will be
about 58% of the delta current in the winding. The starting torque will be
1/3 of the "normal" starting torque and this will cause slower
acceleration and, depending on the load, the motor may not start. You do
need to know the speed torque characteristic of the motor and the load to
apply this properly. If it is a fan or pump motor- there should be no
problem. If it is a load that needs high starting torque there may be a
If the motor is unloaded, there should be no problem in starting or with
overheating - The current may be below rated current for the complete
starting period. Overheating should not be a problem- even if it takes 2 to
3 times as long to come up to the speed where you switch to delta.
2)Three phase motors don't have starter coils (which are relatively high
resistance and intended for short duty. They are inherently self starting.
In either case, considering that the star/delta starter works- why change
it? Efficiency and losses are not a concern as, in terms of the total time
that the motor is operating- the start time is negligable.
Don Kelly email@example.com
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