thanks busbar
its a squrriel cage induction motor. by resistance ,i meant putting them in line for some time and then taking out after motor picks up speed. similarly magnetic soft starters are also available which give smooth starting as compared to star delta starter. going for electronic soft start will be costly for this application as existing resstance starter was working fine but is to be replaced by new one because of aging.what exactly i wanted was whether putting mgnetic soft starter in the motor input will be any better?
I don't think that inserting resistance in series with the stator is a common approach. It would not be very effective at reducing starting current since the resistance would be approx 90 degrees out of phase with the motor impedance which is primarily inductive during starting. Furthermore resistance would generate lots of heat. Placing an inductance in series with the stator is commonly done, but will still require an evaluation for expected behavior/effects on the motor and power system.
Also if you had a wound-rotor motor with slip rings, then you could insert resistance in series with the rotor circuit for reduction of starting current, but I don't think you have that.
Primary reistance starters are a very "old school" method for reduced voltage starting of low voltage motors.
I would look into the electronic module type of soft-start. These devices provide the very basic soft start and stop functions only and are used in conjunction with a normal full-voltage magnetic starter. For 25HP at 480V 60Hz worst case pricing is probably $800 (US).
Some manufacturers are:
Schneider Electric - type LH4N
Motortronics - XLC series
Carlo Gavazzi - RES
e*shock—it is not clear what the driver of the question? What kind of problems is the existing setup giving aside from its age?
In general, reduced-voltage starting is employed for two reasons—during the starting interval, for reduction of stress on the electrical system, and/or for reduction of mechanical stresses.
I agree with electricpete and JBD on the marginal utility of resistance starting.
There are two arrangements for Y-∆ starting—[basic] open-transition and [more complex] closed-transition types. Realize that to apply Y-∆ starting, the motor terminals must be internally configured to accommodate it.
“Reduced Voltage Starting Of Low Voltage, Three-Phase Squirrel-Cage Induction Motors”
hi everybody
thanks for your suggestions, i will consider all the above points before replacing the starter. this motor doesnt have all the 6 winding ends coming , so star/delta starting is ruled out. this is also not a wound rotor type motor.
Suggestion: It appears that a voltage regulating transformer or autotransformer could get consideration. It would be somewhat more efficient than resistor. However, this would be recommended for small motors, e.g. less than 1kW.
Normally most compressor manufacturers have their units start "unloaded". This allows the motor to start without any back pressure on the compressor. An across the line starter would be fine up to 25 HP.
IT IS ALL DEPENDS UPON THE SUPPLY GRID STRENGTH. WITH ONLY THREE TERMINALS AT MOTOR END DIRECT ON LINE WILL BE THE ONLY ECONOMICAL OPTION. BUT IT IS TO BE ENSURED THAT THE COMPRESSOR MUST BE IN UNLOADED CONDITION. OTHERWISE THE STARTING CURRENT WILL LAST LONGER TIME. IT IS ALSO TO BE ENSURED THAT WHILE STARTING THE COMPRESSOR OTHER RUNNING MACHINE SHOULD NOT SUFFER BY DROP IN VOLTAGE.
ANOTHER OPTION IS TO USE ELECTROMAGNATIC SOFT STARTER WHICH IS BIT ECONOMICAL THAN ELECTRONIC SOFT STARTER
thanks for your suggestions
normally , the compressor is started on no load only but i am ruling out direct on line starting as it will cause drop in the line causing problems to the other loads. i will take into account the suggestions given. thanks again..
