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primary resistance starters 3
- Thread starter tomhill56
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1
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The simple answer would be
1) Appropriate Resistance is introduced in series with each winding to start.
2) After a suitable time delay the resistance are bypssed and the motor reaches full speed.
3) Alternatively the series resitance is gradually reduced (like a reheostat) and finally the full voltage is across the winding.
This way the primary winding current is limited till the full speed is reached and back emf is prodcued.
marvin
1) Appropriate Resistance is introduced in series with each winding to start.
2) After a suitable time delay the resistance are bypssed and the motor reaches full speed.
3) Alternatively the series resitance is gradually reduced (like a reheostat) and finally the full voltage is across the winding.
This way the primary winding current is limited till the full speed is reached and back emf is prodcued.
marvin
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- #3
Skogsgurra
Electrical
There is a simpler version - the so called KUSA starter (Kurzschluss Sanft Anlauf, which means Short circuit Soft Start). It works with just one resistor connected in series with one of the windings. If the motor is wye connected, one can imagine the star point being moved out of center of the voltage star. That reduces all three winding voltages which, in turn, reduces starting torque. The torque can be adjusted with the resistor and a contactor shorts the resistor out when motor has got up in speed.
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- #4
Hello tomhill56
NYedison is correct in his description, but I woulld like to add the following:
1. the voltage across the motor terminals is able to be calculated by ohms law and is a function of the value of the resistance and the motor impedance.
2. As the motor accelerates, it's impedance changes. The intirial power factor is very low and the effective resistance is also low. As the motor acelerates, the impedance gradually increases and the power factor improves.
3. The changing impedance of the motor during aceleration, results in an increasing voltage on the terminals of the motor, giving an increasing torque relative to an constant voltage starter such as an auto transformer starter.
4. The power disipated in the primary resistors during start is very high and is of particular problems for generator starting.
skogsgurra. The single phase control method also introduces a significant twist in the current vectors and a resultant increase in negative sequence currents. Not to be used on higher inertia or frequent starting loads.
Best regards
Mark Empson
NYedison is correct in his description, but I woulld like to add the following:
1. the voltage across the motor terminals is able to be calculated by ohms law and is a function of the value of the resistance and the motor impedance.
2. As the motor accelerates, it's impedance changes. The intirial power factor is very low and the effective resistance is also low. As the motor acelerates, the impedance gradually increases and the power factor improves.
3. The changing impedance of the motor during aceleration, results in an increasing voltage on the terminals of the motor, giving an increasing torque relative to an constant voltage starter such as an auto transformer starter.
4. The power disipated in the primary resistors during start is very high and is of particular problems for generator starting.
skogsgurra. The single phase control method also introduces a significant twist in the current vectors and a resultant increase in negative sequence currents. Not to be used on higher inertia or frequent starting loads.
Best regards
Mark Empson
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