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. Motor coil winding formula pdf methods used for reduction of starting voltage are: Star delta starting and Auto transformer starting. 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. Delta starters are probably the most common reduced voltage starters. They are used in an attempt to reduce the start current applied to the motor during start as a means of reducing the disturbances and interference on the electrical supply. Delta starter is manufactured from three contactors, a timer and a thermal overload.
The contactors are smaller than the single contactor used in a Direct on Line starter as they are controlling winding currents only. There are two contactors that are close during run, often referred to as the main contractor and the delta contactor. The third contactor is the star contactor and that only carries star current while the motor is connected in star. Three-pole thermal overcurrent release 1 No. Fuse elements or automatic cut-outs for the main circuit 3 Nos. Fuse element or automatic cut-out for the control circuit 1No. The main contactor connects the reference source voltage R, Y, B to the primary terminal of the motor U1, V1, W1.
The Star and Delta are electrically interlocked and preferably mechanically interlocked as well. The star contactor serves to initially short the secondary terminal of the motor U2, V2, W2 for the start sequence during the initial run of the motor from standstill. This provides one third of DOL current to the motor, thus reducing the high inrush current inherent with large capacity motors at startup. The control circuit consists of push button switches, auxiliary contacts and a timer. Star Main and Auxiliary contactor change its position from NO to NC.
This alternator uses 10 poles made from ferrite magnets. The acceleration torque is the torque developed by the motor minus the load torque, dOL starting is sometimes used to start small water pumps, even a small slip induces a large current in the rotor and produces significant torque. The ideal and easiest way for overload protection for a motor is an element with current, the same winding could work with 14 magnets. DC and WRIM drives are being displaced by VFD – i’ll be waiting for your response.