DRIVES & MOTORS FEATURE Top tips for VSD selection
Frank Griffith, consultant engineer at ABB, explains the ten points to consider to ensure you select the right VSD for your application
S
electing the right variable-speed drive (VSD) for your application involves
taking into account features including the load expected, controllability, features needed and safety. Here are ten assessment stages that will help you get the VSD you need:
1. IDENTIFY THE TYPE OF APPLICATION You will need to identify the type of application and likely demand of the drive – it is important to know which type of load is to be controlled before making any selections. There are essentially three different load types: • Constant torque - This occurs when fixed volumes are being handled, such as in screw compressors, extruders, or in straight line web handling
• Quadratic load - The most common load type, covers centrifugal pumps, fans and the majority of mixers
• Constant power - This is typically used when a material is being rolled up, or unrolled down, where the diameter changes while the surface speed remains constant There may be combinations of load types, so be careful to select the correct load characteristic as opting for the wrong one could easily lead to oversizing. Where the load is constant, say at half speed, a larger motor with more thermal capacity is needed. A fan, however, needs less torque when running at half speed than when running at full speed, leading to a smaller motor frame selection. A constant torque drive should have
an overload current capacity of 150% or more for one minute, while variable torque, variable frequency drives need only an overload current capacity of 120% for one minute.
2. GATHER THE APPLICATION PERFORMANCE DATA Collect the application’s performance data including load, system inertia, acceleration and deceleration rates, minimum and maximum speeds and overload requirements. As an example, it is
essential to know the true operating speed range. This is not normally too critical with a centrifugal load, but for a constant torque load it is quite possible to over-frame the motor if the real speed range is, for example, 10% to 100% (say 5-50 Hz) and the motor is specified
for 0-100% (0-50 Hz). This is an example of motor loadability.
The ABB industrial drive, ACS880 assistant control panel offers intuitive use, easy navigation, and enables visual guidance
3. GATHER THE MOTOR DATA
Make sure you know the motor’s rated torque, power,
voltage, insulation class and speed. Whether an existing motor or a new motor is being used, this data is critical to choosing the right drive.
To identify the type of application and likely demand of the drive, it is important to know which type of load is to be controlled
4. CHOOSE A DRIVE Match the features in steps 1-3 with the features from a VSD manufacturer’s catalogue. The selected VSD should meet all the motor requirements.
5. DOES THE DRIVE HAVE THE CORRECT KW/AMP RATING? The VSD must be able to supply the correct current to the motor to provide the torque required, including normal and overload conditions. Select current from the tables in the manufacturer’s catalogue. It should be remembered that selection of a VSD is based on current not kW!
6. DOES THE DRIVE HAVE THE CORRECT ENCLOSURE AND ENVIRONMENTAL RATINGS? A number of different certifications are used to rate VSDs, including Ingress Protection (IP), NEMA and UL Type
/ DESIGNSOLUTIONS
enclosures. It is important to match the needs of the application with the right enclosure, either by using a stand-alone VSD with the proper enclosure rating, or putting the VSD inside another enclosure. The VSD must also produce the required current at the application’s altitude and ambient temperature.
7. DOES THE DRIVE HAVE THE FEATURES NEEDED TO MEET THE APPLICATION DEMANDS? The VSD must have software features that match the application’s requirements. It must have sufficient hardware, such
as inputs and outputs, feedback and communications, to perform the application. If tight process control is needed, then a sensorless vector or flux vector variable frequency drive may be needed. These allow a high level of accuracy when controlling speed, torque and positioning.
8. DOES THE DRIVE HAVE THE MOTOR CONTROL PERFORMANCE TO MEET THE APPLICATION DEMAND? The VSD must be able to produce the necessary torque at the required speeds. It must also be able to control speed and torque depending on the requirements of the application.
9. DO YOU NEED A SIMPLE KEYPAD, NO KEYPAD OR FULL TEXT ASSISTED KEYPAD? The VSD you choose must interface with the user correctly. This could depend on the number and complexity of inputs you need. You may need to alter a few parameters, choose from a menu of production recipes or alter times of operations to suit usage patterns.
10. DOES THE DRIVE HAVE THE CORRECT LEVEL OF SAFETY? The VSD you choose could be responsible for safety in your system after changes in the machinery directive. Some VSDs contain built in safety functionality. Nevertheless, VSDs should only be chosen for safety functions specified by machinery designers, not selected as per the catalogue, since a safety function may be inappropriate – e.g. Safe Torque Off would not be used as the only safety function for a crane hoist. Bearing these tips in mind when
planning your next application will help you get the most appropriate drive for your needs, keeping capital costs and operating expenses to a minimum.
ABB T: 01925 741111
www.abb.co.uk/energy Enter 203
DESIGN SOLUTIONS | JUNE 2014 11
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