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What needs to be considered before buying a variable speed drive

Variable speed drives have come a long way in the past 20 years, from being a dumb component to intelligent process controller. Following are the main points to consider:

IP ratings: is it going to be mounted inside a box? Or out in the field? What is the ambient conditions like temperature, dust, Fumes and gases etc.

Control: how is the variable speed drive controlled? Hard wired start stop (3 wire or 2 wire) and analog signals (4-20 ma or 0-10 V) OR via serial communications. What serial communications are required? PID control, Safety Requirements etc.

Load: What are the load torque requirements variable torque, Constant Torque or Constant power? What are the speed requirements? Does the variable speed drive needs to give high breakaway torque? Does the load demand high torque at low speeds. If it is used on critical applications is accuracy required and holding torque requirements at zero speed?

Motor and Cables: What kind of motor to control Normal TEFC or a PM or special pump motors requiring low du/dt? Cable distance to the motor?

HARMONICS and EMC: depending on the site conditions the variable speed drive may be required to comply with the harmonic and EMC standards.

These are only a few of the points to consider. What will be of greatest help for the variable speed drive suppliers is application details as a starting point.

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