There are many options to vary the speed of motors in automated machines. Depending on the application, servo-motor drive and dc drive can be used, but the star of the show is the variable frequency drive (VFD). When the cost, reliability and ease of operation and use are considered, it’s hard to beat the VFD when there is a need to vary the speed of motors controlling conveyors, belts, pumps, blowers and fans.
Sometimes constant speed just isn't enough. Many of you mechanical types are happy to change the diameter of the pulley or sprockets driving the rotating motion, but electronics is much more flexible for adjustable motor speed control. There are some basics to keep in mind when specifying, designing a control circuit and integrating a VFD.
To start, if there are requirements to control the speed of a three-phase ac motor, there are many options available. Gather your requirements and talk to your favorite industrial parts supplier or manufacturer and consider using the popular VFD.
While VFD input voltage can be as low as 115 Vac single phase, 208/230/460 Vac single- or three-phase supply is more common. This supply voltage is connected to the VFD through a short circuit, branch circuit protective device such as a fuse or circuit breaker. The drive's output and the motor it's controlling is always three-phase.
For the most part, a VFD can handle electromagnetic interference (EMI) noise often found in industrial facility's supply voltage, but it never hurts to install some type of line-side filter upstream of the VFD, or at least include spare back panel space to add a drive isolation transformer, filters or reactors in the future. Plan ahead.
On the load side, the inductive motors and the VFD controlling it can create EMI noise and adversely affect nearby sensitive equipment. Just like it's best-practice design to add surge and lighting protection at the main disconnect of most control panels, it's at least good practice to add drive load side reactors or isolation transformers to protect against harmonics, reflected waves and electrical disturbances. Carefully understand installation requirements and talk with the manufacturer, as random "noise" problems can be difficult to find—don't add to the possible problems.
Worth noting is the reflected wave disturbance's effect on the cable connecting the drive to the motor. These waves can double the voltage at the motor, so limit cable length and be sure to specify inverter-duty motors which have higher insulation ratings, especially on 460 Vac and higher-supply voltage applications. If the cable distance is much more than 100 feet, consider installing the drive closer to the motor.
Once the proper supply voltage and related hardware are connected, it's time to get down to what the VFD's purpose is—adjusting the speed of a three-phase ac inductive motor. But it does more than that. It also provides overload protection required by the National Electric Code so an overload relay on the load side is typically not required.
The drive also starts, stops and controls the direction, and acceleration and deceleration of the motor. In the past, start and stop signals were commonly discrete inputs, often in the form of relay contact closures. Other control modes include keypad control, as well as two- and three-wire control. A speed reference signal is also adjustable via a keypad or internal drive parameter. Analog speed references are also available via 4–20 mA and 0-10 V signals. A more modern approach is to use network communications for drive control and speed reference signals.
There are many communication options for drives, such as RS-232/485 Modbus and industrial Ethernet protocols such as EtherNet/IP, to perform the starting, stopping and speed-changing functions. This communication also returns a variety of drive status information, such as running, and alarm codes, such as overvoltage and overcurrent faults, among many others.
With all the access to the data and parameters that digital drive communication provides, all but the simplest PLC drive control applications should include it as opposed to simple discrete signals. Ladder diagram, add-on instructions or similar in the PLC provides significant VFD control and data capabilities compared to what were available in the past.
The ability to change speed to optimize the process or reduce the speed of variable torque loads such as pump, fan and blower applications can greatly improve efficiency. Check out the Affinity Laws. Even constant torque loads such as conveyors, compressors and mixers may benefit from motor speed changes and reduced acceleration when starting and stopping can reduce overloads, wear and forces on the equipment.
While there are many other things to consider when integrating a VFD into an automated system, configuring a drive properly is a must. While minimal drive configuration is necessary—even though there may be more than 50 parameters—some is almost always required. Carefully record the motor nameplate data. Use that data to program the drive parameters such as rated voltage, full load current and speed—at a minimum. Spend the time to review the user's manual to get it right.