Page 208 - Root Cause Failure Analysis

196 Root Cause Failure Analysis

Volts-per-hertz technology works well in general-purpose, moderate-speed applica-
tions. However, it is unsuitable for applications that require high dynamic response
and torque control or when the motor is running at very low speeds.

Vector Control
Vector-control technology was developed to provide the ability to accurately control
the output speed of alternating-current motors in both high-torque and low-speed
applications. Alternating-current vector controls refer to the drive’s ability to control
the vector sum of flux and torque in the controlled motor, which provides precise
speed and torque performance. These capabilities enable the drive to maintain tension
when a machine stops or to quickly return to full speed when a heavy load variation is
imposed on the driven machine.

Three basic types of vector drives commonly are used in these applications: flux-vector,
voltage-vector, and stator-flux-vector controls. All these control technologies may retain
the volts-per-hertz core logic, but add other control blocks to improve drive perfor-
mance. These additional control blocks include a current resolver that estimates the flux-
and torque-producing currents in the motor and enters a correction factor to the V/Hz
primary-control logic. Where more accurate speed control is required, a current regulator
may be used to replace the standard V/Hz current-limit block. In this configuration,
shown in Figure 16-2, the output of the current regulator is still a frequency reference.

PERFORMANCE
Inverter performance is measured by the response characteristics of the motor. In
most cases, these characteristics include torque response, impact-load response, and
acceleration control.

Torque Response
Figure 16-3 illustrates the normal torque-response characteristics of a V/Hz inverter.
Note that the ability of the drive to maintain high torque output at low speeds drops
off significantly below 3 Hz. For this reason, the operating range of a V/Hz inverter is
usually less than 20 to 1 (i.e., 20: 1).

A flux-vector control improves the drive’s dynamic response and may be able to con-
trol both the output torque and speed. Figure 164 provides a typical torque-speed
response curve of a flux-vector inverter.

Impact-Load Response
Inverter drives must compensate for variations in load. Figure 16-5 compares the
impact-load response of a standard V/Hz and a sensorless flux-vector-type inverter. In