Electrical Notes

  Introduction: Most induction motors are started directly on line, but when very large motors are started that way, they cause a disturbance of voltage on the supply lines due to large starting current surges. To limit the starting current surge, large induction motors are started at reduced voltage and then have full supply voltage reconnected when they run up to near rotated speed. Two methods are used for reduction of starting voltage are star delta starting and auto transformer stating.   Working Principal of Star-Delta Starter: This is the reduced voltage starting method. Voltage reduction during star-delta starting is achieved by physically reconfiguring the motor windings as illustrated in the figure below. During starting the motor windings are connected in star configuration and this reduces the voltage across each winding 3. This also reduces the torque by a factor of three. After a period of time the winding are reconfigured as delta and the motor runs normally. St...

  Introduction: Different starting methods are employed for starting induction motors because Induction Motor draws more starting current during starting. To prevent damage to the windings due to the high starting current flow, we employ different types of starters. The simplest form of motor starter for the induction motor is the  D irect  O n  L ine starter. The DOL starter consist a MCCB or Circuit Breaker, Contactor and an overload relay for protection. Electromagnetic contactor which can be opened by the thermal overload relay under fault conditions. Typically, the contactor will be controlled by separate start and stop buttons, and an auxiliary contact on the contactor is used, across the start button, as a hold in contact. I.e. the contactor is electrically latched closed while the motor is operating. Principle of DOL:   To start, the contactor is closed, applying full line voltage to the motor windings. The motor will draw a very high inrush current for ...

  General Terminology (1) Service Factor: The service factor is a multiplier that indicates the amount of overload a motor can be expected to handle. If a motor with a 1.15 service factor can be expected to safely handle intermittent loads amounting to 15% beyond its nameplate horsepower. For example, many motors will have a service factor of 1.15, meaning that the motor can handle a 15% overload. The service factor amperage is the amount of current that the motor will draw under the service factor load condition. (2) Slip: Slip is used in two forms. One is the slip RPM which is the difference between the synchronous speed and the full load speed. When this slip RPM is expressed as a percentage of the synchronous speed, then it is called percent slip or just “slip”. Most standard motors run with a full load slip of 2% to 5%. (3) Synchronous Speed: This is the speed at which the magnetic field within the motor is rotating. It is also approximately the speed that the motor will run u...

  Over Load Relay & Contactor for Motor Starter: Types of Over Load Relay: Class 10:  Would Trip after 10 seconds. Class 20:  Would Trip after 20 seconds. Class 30:  Would Trip after 30 seconds. Over Load Relay should be set 115% to 130% of Motor Full Load Current. Class 10 is faster than Class 20 and Class 30 over Load Relay. Size of over Load Relay: Size Amp Capacity S00 0.1 To 0.4 0.4 To 0.6 1.6 To 6 3 To 12 S0 3 To 12 6 To 25 S2 6 To 25 13 To 50 S3 13 To 50 25 To 100 S6 50 To 200 S10 & S12 55 To 250 200 To 540 300 To 63 Contactor Coil: Coil Voltage (40 To 50 Hz) Suffix 24V T 48V W 110V To 127V A 220V To 240V B 277V H 380V To 415V L Type of Contactor for Starter: Contactor Application AC1 Non-Inductive or Slightly Inductive ,Resistive Load AC2 Slip Ring Motor AC3 Squirrel Cage Motor AC4 Rapid Start / Stop AC5a Switching of Electrical Discharge Lamp AC5b Switching of Electrical Incandescent Lamp AC6a Switching of Transformer AC6b Switching of Capacitor Bank...