Document NNKLkEJeZyper0nLQQejZD4v8

1004 CHAPTER 39 1958 Guide winding circuit. The relay then keeps these contacts open because there is sufficient voltage induced in the starting winding, when the motor is running, to hold the relay in the open contact position. CONTROL FOR ALTERNATING CURRENT MOTORS Squirrel-Cage motors are usually linestarted where power company limitations permit. In sizes up to 5 hp at 220 volts," or 7J hp at 440 volts, polyphase motors may be started by means of manual switches having overload current elements for motor protection. In larger ratings a linestarter is usually provided with either an additional safety switch or circuit breaker for disconnecting and short circuit protection. Reduced voltage starting may be used when the current inrush on full voltage ex ceeds the limitations set by the power company. The magnitude of this inrush current is entirely independent of the load on the motor, and is the same whether the motor is lightly loaded or heavily loaded. Table 7. Compabison op Squibbel Cage and Synchronous Motob Starting Methods Type of Starteb Percent of Motob Rated Values Line Voltage Starting Torque Line Current Full Voltage Starter 100 100 100 Autotransformer 80 percent tap 65 percent tap 50 percent tap Resistor, single step 80 64 68 65 42 46 50 25 30 80 64 80 Reactor 50 percent tap 45 percent tap 37.5 percent tap Part Winding 50 45 37.5 60 25 . 20 14 50 . 50 45 37.5 . 60 Restrictions on starting current may limit the maximum current inrush or may limit the increments of current inrush which may be drawn on each step. Table 7 shows a comparison of the various methods of reduced voltage starting. The choice may depend upon the limitation to be met. Fig. 5 shows examples for maximum current limitations. Fig. 6 shows examples for increment limitations. The increment starters would not be satisfactory for meeting maximum current limitations as they exceed the maximum value before the motor reaches full speed. Both Curves B and C of Fig. 6 would be satisfactory if the increment limitation were 400 per cent but only the starter illustrated by Curve C would meet a limitation of 200 percent increments. , When increment starters are used the motor may not develop enougn torque to start until the last step. Fig. 6 shows that with the resistor step starter the motor does not start until step 3 or after 2 seconds on Curve O- Part-winding motors may start and then stall at about half speed until the second step which applies full torque. The load inertia (WKJ) may also affect the selection of reduced current starting method since the reduced torques may not be sufficient to-accel erate the load without overheating the motor or in the case of synchronous motors, may not reach sufficient speed to pull into synchronous speed. () Motors and Motor Controls 1005 ' CurveA Full Voltage Stabtbb Cuhve B Rkactob_ CO PERCENT TAP CUBVB C Autotransformer 65 Percent Tap TIME ----- SECONDS . . Fig. 5. Starters fob-Maximum Current Limitations In these cases the solution may be to use the wound-rotor induction motor or some means of reducing the load such as suction dampers on compres sors. Fig. 7 illustrates recommended control practice for squirrel-cage motors. Wound Rotor motors require control of both primary and secondary circuits. The primary* control may be the same as for squirrel-cage motors, manual or magnetic, at full voltage. Secondary* control pro vides means of varying secondary resistance for starting and speed control. The secondary controller should be specified for starting duty only, or for speed regulating duty. If the secondary controller is to be used for speed regulating duty, the percent speed reduction, the number of speed control points, and the type of load (variable or constant torque) should be speci fied. Fig. 8 illustrates recommended control practice for wound rotor motors. Synchronous motor starters should provide pull-out protection, auto matic synchronization or automatic stopping of the motor after pull-out, and insurance of complete starting sequence, as well as overload and low voltage protection. The control may be either magnetic or semi-magnetic at full or reduced voltage. Semi-magnetic starters provide automatic new control, but require hand operation for closing the fine contactors to start and transfer to full voltage. Curve A Full Voltage Starter Curve B Past Winding Stabtbb Curve C 4-Step Resisteb Increment Starter O Fig. 6. 12 5 4 5 TIME -- SECONDS Stabtebs fob Increment Limitations Re,er 40 Glossary at end of chapter. I