Why Use A Drive with Your Motor?

Application Factors To Consider When Selecting a Drive

Torque

Torque is the most critical application factor. Assess all torque requirements, including starting, running, accelerating and decelerating and, if required, holding torque. These values help determine what current capacity the drive must have so the motor can provide the torque required. Usually, the main constraint is starting torque. 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.)

The overriding question 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. These applications involve centrifugal pumps and fans. The torque required in these applications decreases as the motor RPM decreases. Therefore, drives for variable torque loads need little overload capacity. Constant torque applications include conveyors, positive displacement pumps, extruders, mixers or other “machinery.” These need the same torque regardless of operating speed, plus extra torque to get started. Here, you will need high overload capacity.

Smaller-horsepower drives are often built to handle either application. Typically, you only need programming change 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 a very wide speed range. Also, they can provide multiple ways to control this speed. Many drives, for example, include a wide selection of preset speeds. This selection can make set-up easier. Similarly, a range of acceleration and deceleration speed “ramps” are provided. Another helpful feature is slip compensation, which maintains constant speed with a changing load. Many drives have programmable “skip frequencies.” Fans or pumps, in particular, may have specific speeds at which vibration takes place. By programming the drive to avoid these corresponding frequencies, you can minimize vibration. Another control function common with fans is the ability for the drive to start into a load already in motion (called 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 is the basis for sizing a drive. Manufacturers list horsepower ratings as a guide to the maximum motor size under most applications. But horsepower ratings 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-through” capacity of only a second or two. After this, the drive will trigger a fault and shut down. Drives are sometimes programmed for multiple automatic restart attempts. For safety, plant personnel must be aware of this. Depending on your application, you may prefer a manual restart.

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”). These disturbances are reflected back into the connected power system. While these disturbances rarely affect the drive, they can affect other electrically sensitive components.

Environmental Factors

The enemies of electronic components are well-known. Heat, moisture, vibration and dirt are chief among them. Obviously, these factors should be mitigated. Drives are rated for operation in specific maximum and minimum ambient temperatures. If the maximum ambient is exceeded, you need to provide extra cooling. If not, 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, you may need a space heater.

Drive enclosures should be selected based on environment. NEMA 1 enclosures are ventilated and must be given room to “breathe.” NEMA 4/12 enclosures do not have ventilation slots. These enclosures are intended to keep dirt out and are also used in washdown areas. Larger heat sinks provide convection cooling. These heat sinks must not be obstructed, nor allowed to become covered with dirt or dust. Higher-horsepower drives are typically supplied within NEMA-rated enclosures. “Sub-micro” drives, in particular, often need a customer-supplied enclosure 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.

Find a drive For Your Application

Regardless of AC or DC application, drives are an essential and complimentary addition to any motor system. The ability to regulate and vary motor output creates a more reliable and safer atmosphere. To learn more about the best time to utilize a drive, reach out to one of our motor experts.