Archives September 2013

A  Star-Delta (or Wye-Delta)  starter is used in electrical engineering to start larger three phase motors, typically over 4 KW (5 HP).

The Star-Delta starter is chosen, rather than the cheaper D.O.L (Direct On Line) for larger motors, because when a motor is started a surge current is generated for a short initial period. This surge current is typically 5 – 7  times greater than  the normal operating current of the motor, and can cause problems due to the fact that suddenly a large load is appearing on the electrical circuit of your ship or building.

If you started a large induction motor, rated at 50 Amps, then the surge current being between 5 – 7 times that figure, would mean a current appearing on your electrical system of between 250 – 350 Amps. This surge current could potentially cause a ‘blackout’ on board a ship, or at least put strain on the generator.

Even on land, large surge currents are undesirable, and many countries require Star (Wye) – Delta starters to be used to start motors rated above 5 HP or 4 KW.

Operation

The Star-Delta Starter basically consists of three contactors, a timer, and an OCR (Over Current Relay).

When the Star- Delta is first switched on, the electromagnetic coils in the ‘Line’ & ‘Star’ contactors are energised. The magnetic field produced by the coils (in conjunction with the metal cores fitted) closes the contacts of both contactors. This allows voltage to flow through the line contactor into one side of each of the three (three phase) windings.

The Star contactor is also initially closed, and this creates a ‘star’ winding configuration, by shorting the other side of each of the three windings together.

Having the motor windings connected in Star means that surge current is reduced when the motor initially starts up.

At motor startup, a timer is also started.  To be continued………………..

(c) 2013 Craig Miles, all rights reserved.

 Comparison of True Versus Reactive Power

When a  resistive load is connected, no phase shift is occurs between the voltage and current.

The Power that is the  result of the resistive load  performs work in the circuit. This is      known as “true” power, and is measured in watts.

By  comparison, an inductive load causes a 90-degree phase shift between voltage and current.

To calculate the power that results from an inductive load, multiply voltage by current. When either voltage or current is zero, the power is zero. This type of power is known as  “reactive” power. It counteracts the true power in an electrical circuit. It is measured in reactive volt-amperes.
© 2013 Craig Miles, all rights reserved

Inductive loads are loads from electrical equipment.  Electricity circulates through coiled or wound  wire. An inductor is a component which produces a magnetic field, when a current is passed through it.

Examples  include relays, motors, solenoids, & transformers.

As a general rule, if an electrical load involves movement, it is classed as being an inductive load..

Inductive loads can cause ‘blowback’ and must be protected by diodes. Diodes allow the current to flow in one direction only.

Blowback is caused by a surge in voltage. This is caused when the magnetic field in the inductor collapses.

(c) 2013 Craig Miles, all rights reserved

Resistive Loads

Resistive loads are most often are found  in circuits where electricity  produces heat, light, or both forms of energy, Resistive loads do not produce movement.

Examples of resistive loads include Kettles, Incandescent Bulbs, and Heaters.

(c) 2013 Craig Miles