Three Phase Starters
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State the formula for reactance
Why do some consumers notice dimming of lighting circuits when large motors are controlled by a Direct On Line motor starter?
Squirrel cage motors draw a very high starting current compared with starting torque. This high starting current can cause voltage drop in the distribution which can affect other electrical equiptment
State 5 ways of reducing the high current starting a motor DOL
- -A star/Delta starter
- -An Autotransformer
- -Primary Resistance Starter
- -Secondary Resistance Starter
- -Electronic Starter
In a DOL motor starter which ''device'' provides the no-volt protection?
The contactor supplying the motor
What would happen in the contacts of a motor were welded closed due to a fault after the motor had started?
The overload protection would not be able to operate and the over current stress would cause mechanical damage to the motor and eventually burnout causing hazards and physical damage
Explain what happens when the overloads operate on a motor
The supply would be disconnected from the motor and the trip currents would be isolated from the motor
Explain what happens when the pushable stop button in operated
The circuit supply would be disengaged from the contactor circuits and supply to the motor would drop out.
With respect to other voltage starters what is the advantage of using an Autotransformer starter?
It has various tappings by which an appropriate starting voltage can be chosen to provide a starting torque and current to suit a specific application. They are also efficient as no energy is wasted, as it is in other type starters
Under what circumstances can two autotransformers be used for starting a 3 phase induction motor?
This method can be used when the required starting torque is greater than can be obtained by the star-delta starting
Why does the primary resistance starter have a lower efficiency than other starters?
Because energy is dissipated as heat in the starting resistors
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