STEP-BY-STEP MOTOR CONTROL SELECTION GUIDE
Variable speed, constant torque, fixed speed and reduced current inrush are some terms with which you may or may not be familiar. These fall into the general term "motor control." An explanation of these terms, and a step-by-step motor guide is explained to help you to determine your needs.
1. Fixed Speed Control - Starter. If an application requires the motor to operate at a constant speed with little or no stopping and starting at full voltage, you will most likely require an electromechanical motor starter. Starters offer full-torque at start-up, but no speed control. With each new generation of starter the frame size becomes smaller but the work load remains the same. This allows you to save panel space, thus reducing the size of the enclosure and saving money.
2. Reduced Current In-Rush - Soft-Start/Stop. If your application requires intermittent starting and stopping of the motor, you may need a solid-state reduced current inrush motor starter. These starters will reduce maintenance concerns related to the life of the contacts on an electromechanical motor starter. A solid-state starter has lower long term cost.
A reduced current motor starter functions by decreasing the starting current to the motor during start-up, allowing the motor to ramp up to full speed and off again. This puts less demand on the power distribution system. The drawback to these devices is when you reduce current, you reduce torque as well.
3. Variable-Speed Drives. When the application requires variable speed throughout the process, the solution is a variable-speed drive (VSD). The primary function of any VSD is to control the speed, torque, acceleration, and direction of the motor that is being controlled. Unlike constant speed systems, VSDs offer an infinite number of speeds within the motor's range.
VSDs can offer energy savings -- running motors at variable speeds can optimize a process in order to save energy, and rebates from utilities may be available.
A VSD may also increase productivity and boost production yields by helping to automate production processes. Long term savings will be realized by reducing maintenance and equipment replacement costs. To best determine if a VSD fits the application, ask yourself: Do I need a wide range of speed? Will I need precise control of the motor speed and/or torque? If the answer is yes to one or both of these, then consider a VSD.
AC vs. DC Variable Speed Drives
Historically, applications that required precise motor control over a wide range of speeds have used DC variable speed drives. DC drives offer full torque at zero speed -- this is due to independent controls over speed and torque. However, DC motors are more expensive and require a much higher degree of maintenance than AC motors. The newest technology is an AC drive that offers torque control, an important feature when energy efficiency and motor reliability is demanded. An AC drive initially may be more expensive than an equivalent DC unit, however, an AC motor will provide a faster return on investment.
AC Drive Advantages
- Uses conventional, low cost, 3-phase AC induction motors for most applications
- Preferred for tight motor mount applications
- Smaller, lighter and more commonly available than DC motors
- Suited for speeds over 2,500 r.p.m. since there are no brushes
- Used when multiple motors in a system must operate simultaneously at a common frequency and/or speed
DC Drive Advantages
- Less complex with single power conversion from AC to DC
- Normally less expensive for most horsepower ratings
- Long tradition of use as adjustable speed machines
- Cooling blowers and inlet air flanges provide cooling air for a wide speed range at constant torque
- Capable of providing starting and accelerating torques in excess of 400% of rated
- Less noise. Some AC drives produce audible motor noise which is undesirable in some applications.
Constant Torque vs. Variable Torque
When torque requirements are independent of speed, your solution is a constant torque drive. A variable torque drive is best suited for applications such as fans, pumps and blowers. Under these applications, you will need full torque at top speed and diminishing torque as speed decreases.
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