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Variable Speed Drive Basics

The variable speed drive (VSD) is one part of a total system that includes a motor and a load. The motor acts as a power transducer, converting electrical power to rotational mechanical power. AC induction motors, 600V and below, often are paired with a variable speed drive. variable speed driveThese AC motors fall into classes with different torque speed curves. Most variable speed drive manufacturers assume use of a Class A motor and that the torque speed curve will be almost linear at the operating point. The variable speed drive will shift the whole curve left or right to change the operating point. Note the "slip" in the figure. Every motor suffers from some slip or difference between rotor and stator fields. This is quite different than stiction, the term used with control valves for the needed stem force to overcome static friction. The load - mechanical conveyor, pump, fan, compressor or the like - has inertia, rotational friction and stiction. The process also has dynamic characteristics that may change when using a variable speed drive instead of a control valve. It takes time to accelerate the load to operating speed and this is proportional to the inertia and the motor torque.

Today variable speed drives use a technique called pulse width modulation (PWM). First AC power is rectified to DC and filtered. Next, a solid-state semiconductor called an insulated gate bipolar transistor (IGBT) creates a voltage waveform to the motor that is a series of pulses of varying widths. The result is a varying frequency AC sine wave. The switching frequency determines the shaft rotational speed. Because the power waveform from a variable speed drive isn't purely sinusoidal, it's important to only use motors specially designed to run with PWM variable speed drives - these are "inverter duty" motors, Class F winding. If a conventional motor is used, it may burn out.

A variable speed drive also has other control electronics; these may include current, voltage and speed sensors.
The variable speed drive electronics has limitations that affect control performance. One limitation is current. The inrush peak starting current of an "across the line" starter, one without a variable speed drive, is eight times the full load current. Such a current would damage a variable speed drive's rectifiers and semiconductors. Another constraint: the variable speed drive electronics is designed to prevent the motor flux from saturating the core.

The process transmitter sending its output signal to the proportional-integral-derivative (PID) controller that acts as the outer loop senses the process dynamics; its output is cascaded to the variable speed drive.

Within the variable speed drive electronics are algorithms that control the electrical motor power, frequency, voltage and current. The current and speed set the motor torque. So the variable speed drive doesn’t control just the speed, it also regulates the torque delivered to the rotor shaft. This torque produces the rotational force applied to the load (pump, etc.) that powers the process.

Properly understanding the dynamics of a variable speed drive control loop requires considering all elements of the system and how they interact.

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