As PWM AC drives have continued to increase in popularity, drives manufacturers have spent considerable research and development effort to build in programmable acceleration and deceleration ramps, a variety of speed presets, diagnostic abilities, and other software features. Operator interfaces have also been improved with some drives incorporating “plain-
English” readouts to aid set-up and operation. Plus, an array of input and output connections, plug-in programming modules, and off-line programming tools allow multiple drive set-ups to be installed and maintained in a fraction of the time spent previously. All these features have simplified drive applications. However, several basic points must be considered:
Torque: This is the most critical application factor. All torque requirements must be assessed, including starting, running, accelerating and decelerating and, if required, holding torque. These values will help determine what current capacity the drive must have in order for the motor to provide the torque required. Usually, the main constraint is starting torque, which relates to the drive’s current overload capacity. (Many drives also provide a starting torque boost by increasing voltage at lower frequencies.)
Perhaps the overriding question, however, is whether the application is variable torque or constant torque. Most variable torque applications fall into one of two categories – air moving or liquid moving – and involvecentrifugal pumps and fans. The torque required in these applications decreases as the motor RPM decreases. Therefore, drives for variable torque loads require little overload capacity. Constant torque applications, including conveyors, positive displacement pumps, extruders, mixers or other “machinery” require the same torque regardless of operating speed, plus extra torque to get started. Here, high overload capacity is required.
Smaller-horsepower drives are often built to handle either application.
Typically, only a programming change is required to optimize efficiency (variable volts-to-hertz ratio for variable torque loads, constant volts-to hertz ratio for constant torque loads). Larger horsepower drives are usually built specifically for either variable or constant torque applications.
Speed: As mentioned, AC drives provide an extremely wide speed range.
In addition, they can provide multiple means to control this speed. Many drives, for example, include a wide selection of preset speeds, which can make set-up easier. Similarly, a range of acceleration and deceleration speed “ramps” are provided. Slip compensation, which maintains constant speed with a changing load, is another feature that can be helpful. In addition, many drives have programmable “skip frequencies.” Particularly with fans or pumps, there may be specific speeds at which vibration takes place. By programming the drive to avoid these corresponding frequencies, the vibration can be minimized. Another control function, common with fans, is the ability for the drive to start into a load already in motion often called a rolling start or spinning start. If required, be sure your drive allows this or you will face overcurrent tripping.
Current: The current a motor requires to provide needed torque (see previous discussion of torque) is the basis for sizing a drive. Horsepower ratings, while listed by drives manufacturers as a guide to the maximum motor size under most applications, are less precise. Especially for demanding constant torque applications, the appropriate drive may, in fact, be “oversized” relative to the motor. As a rule, general-purpose constant torque drives have an overload current capacity of approximately 150% for one minute, based on nominal output. If an application exceeds these limits, a larger drive should be specified.
Power Supply: Drives tolerate line-voltage fluctuations of 10-15% before tripping and are sensitive to power interruptions. Some drives have “ride-though” capacity of only a second or two before a fault is triggered, shutting down the drive. Drives are sometimes programmed for multiple automatic restart attempts. For safety, plant personnel must be aware of this. Manual restart may be preferred.
Most drives require three-phase input. Smaller drives may be available for single-phase input. In either case, the motor itself must be three-phase.
Drives, like any power conversion device, create certain power disturbances(called “noise” or “harmonic distortion”) that are reflected back into the power system to which they are connected. These disturbances rarely affect the drive itself but can affect other electrically sensitive components.
Control Complexity: Even small, low-cost AC drives are now being produced with impressive features, including an array of programmable functions and extensive input and output capability for integration with other components and control systems. Additional features may be offered as options. Vector drives, as indicated previously, are one example of enhanced control capability for specialized applications.
In addition, nearly all drives provide some measure of fault logging and diagnostic capability. Some are extensive, and the easiest to use display the information in words and phrases rather than simply numerical codes.
Environmental Factors: The enemies of electronic components are wellknown.
Heat, moisture, vibration and dirt are chief among them and obviously should be mitigated. Drives are rated for operation in specific maximum and minimum ambient temperatures. If the maximum ambient is exceeded, extra cooling must be provided, or the drive may have to be oversized. High altitudes, where thinner air limits cooling effectiveness,call for special consideration. Ambient temperatures too low can allow condensation. In these cases, or where humidity is generally high, a space heater may be needed.
Drive enclosures should be selected based on environment. NEMA 1 enclosures are ventilated and must be given room to “breath.” NEMA 4/12 enclosures, having no ventilation slots, are intended to keep dirt out and are also used in washdown areas. Larger heat sinks provide convection cooling and must not be obstructed, nor allowed to become covered with dirt or dust. Higher-horsepower drives are typically supplied withinNEMA-rated enclosures. “Sub-micro” drives, in particular, often require a customer-supplied enclosure in order to meet NEMA and National Electrical Code standards. The enclosures of some “micro” drives, especially those cased in plastic, may also not be NEMA-rated.
