The winding in the motor must be split in order for it to use a part winding start. A portion of the winding, either half or two thirds, will be connected when the motor is started. Dual voltage motors have split windings, but they can only be started on the lower voltage winding. A 230/460 volt motor, to give an example, will have the 230 winding powered initially. The rest of the winding is added in with a relay or a timer when the motor has achieved a certain voltage.
Loads that have a requirement for low starting torque, such as low inertia fans and blowers, can be started using part-winding starters.
The part winding motor will initially draw full voltage, but only 65% to 70% of full current. The motor will rise to full current when the rest of the winding is added into the system, meaning that one of the contactors will be carrying at least 65% of the current while the motor is starting up. When the motor is running at full current, each contactor will carrying half the current.
If the load torque exceeds the torque provided by the part winding starter at any speed, then the motor will be locked in at that speed until the second part of the winding is introduced. Full torque will develop at that speed. A lower full torque speed will lead to a bigger surge of current when the second part of the winding is added into the system.
Pumping is the most common application for a part winding start. They are the least expensive starter, but they require a special motor, which can be hard to find a replacement for and may be more expensive. They also require additional devices to add overcurrent protection for the motor.
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