Starting Method Full Voltage Starting (Number of Starts): Squirrel cage induction motors are designed to accelerate a NEMA inertia along a NEMA load curve with rated voltage applied to the motor terminals. With the motor initially at ambient temperature, it may be started twice in succession coasting to rest between starts, or once the motor has reached the rated load operating temperature it may be stopped, coasting to rest, and restarted immediately, providing the WK2 inertia of the load, the load torque during acceleration, the applied voltage and the method of starting are those for which the motor was designed. Note: When more frequent starting is necessary contact the Large AC Sales Team for pricing, frame size and availability. 23 ABB Motors and Mechanical Inc.
Voltage Dip (also known as Reduced Voltage Starting): Squirrel cage induction motors for pumps and fans are capable of accelerating the specified load inertia in WK2 with the specified load starting conditions when the motor terminal voltage does not drop below 90 percent of the nominal voltage. This applies only to ratings utilizing normal inrush current. Exception: When the load WK2 is not more that 1/2 of the NEMA normal values and where the load torque varying as the square of the speed does not exceed 60 percent of motor rated torque during the acceleration period, no price addition is required for terminal voltage down to 70 percent of rated. Notes: 1. Where motor is to accelerate a load WK2 greater than NEMA normal also make price addition for high WK2 2. Do not use this option where non-standard starting torques are required. 3. Motors designed for low voltage starting conditions may have higher than 650% inrush current at full voltage and lower than listed efficiency and power factor. 4. Customer load speed torque curve required prior to order acceptance. 24 ABB Motors and Mechanical Inc.
Electronic Soft Starting: Electronic Soft-starting is used to either limit the inrush current or to limit the torque in order to reduce the strain on the mechanical system. There are two basic kinds of electronic soft-starters: voltage ramp and current limit. In either type of device, the user pre-sets an initial voltage and ramp time or current limit that will reduce the torque available from the motor. The following data must be supplied to insure proper application of the motor: 1. Will voltage ramp or current limit be used? 2. What is the current limit and time (if applicable)? 3. What is the initial voltage and ramp time (if applicable)? 4. Provide driven equipment load speed vs. torque curve. 5. Provide Load inertia (reflected at the motor shaft). 25 ABB Motors and Mechanical Inc.
Wye Start-Delta Run: The motor normally operates while running as a delta connected stator. At start, the stator is connected as a wye winding and draws only 1/3 locked rotor current of the delta connected winding. This is because the phase voltage and current in the wye connection is 1/3 times the phase voltage and current in the delta connection. The starting torque is decreased to approximately 1/3 of its value for the delta or running connection, since the change from delta to wye decreases the phase voltage to 1/3 of its original value and the torque is proportional to the square of the phase voltage. After the motor has reached a certain predetermined speed, or locked current value, a timer in the control panel actuates the starter contacts and switches the winding connection from wye to delta. The motor now develops full design running torque to produce rated horsepower. The control timer is usually set for 5-8 seconds, so the value of locked rotor current drawn by the delta connected winding at switch over is not much greater than the initial locked current on the wye at startup. This method is often termed Star-Delta starting. Note: This feature is only practical for low voltage (typically 380 or 460V). 26 ABB Motors and Mechanical Inc.
Reactor starting: Despite the simplicity and reliability of the polyphase induction motor, high inrush current limited its usefulness in the early years of weak power system capability. This led to the use of series resistance during starting, to reduce that current. As motor size increased, such resistors required high power dissipation capability, and the autotransformer, or "starting compensator," became popular. An alternative was replacement of series resistance by series reactance. Although the solid-state electronic starter is now popular for motors under 375 kw, especially below 600 volts, the motor starting reactor is still used for many large machines and at voltages up to 11,000. Although some starting reactors are of the air-core type, most contain iron cores because such cores provide higher reactance in a smaller space. One reactor is usually connected in each phase, bypassed by a contactor once the motor reaches full speed. Alternatively, reactors are inserted in the neutral side of a wyeconnected stator winding. This permits a lower level of reactor insulation. A less-common scheme includes a single reactor in only one phase, resulting in phase voltage unbalance that reduces total kva supplied to the accelerating motor. 27 ABB Motors and Mechanical Inc.
Reactor starting (Cont.): Reactor starting always reduces motor accelerating torque. Voltage reduction in the reactor is not fixed, as with an autotransformer, but varies with the current. Since this current is determined by total circuit impedance, motor voltage during acceleration depends upon the relative reactor and motor impedances. Complete circuit analysis, and choice of reactor characteristics, involves the vectorial relationships among the circuit voltages and the current. Reactor opposition to current flow, unlike series resistance, is not necessarily a constant for all values of current. That's because of magnetic saturation in the reactor core. In some starters the reactors are "saturable"; that is, a separate control winding on the same core allows an external control voltage to vary the saturation and therefore the reactor impedance. Control voltage may be fed back directly or indirectly from the counter EMF of the accelerating motor. 28 ABB Motors and Mechanical Inc.