drive system, energy is returned to the grid:
A. when a full car is ascending
B. when an empty car is descending
C. when a full car is descending
D. when the car is stopped
For answers, go to Appendix A.
AC AND DC ELECTRIC MOTOR MAINTENANCE, VFD TROUBLESHOOTING, AND DIAGNOSTIC PROCEDURES
Almost all modern elevators, traction and hydraulic, are driven by electric motors. Until the development of the VFD in the 1960s, electric elevators invariably made use of the DC as opposed to the AC motor because the former permitted simple, smooth speed control. That was essential for stopping the car at individual floors and also for running in inspection mode.
Since the introduction and widespread use of AC motors in conjunction with VFDs, three-phase induction motors have been used in most new construction and major rebuilds, although in the latter, a case can be made for retaining a good working DC motor. Parts and entire replacement motors are readily available and the most common repair, brush replacement, is not difficult.
In this chapter, we will discuss DC and AC motor maintenance, as shown in Figure 3-1, beginning with some comments that apply to both.
Some first signs of motor trouble are failure to start, excessive or unusual noise, reduced power, odor from insulation breakdown, overheating, and tripping out. These symptoms often occur in combination, although at first only one of them may be apparent.
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