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Volts/Hertz, vector control, flux vector control, field-oriented control ... many names label the techniques ac drives use to operate ac motors. Each technique manipulates current and frequency differently, and uses diverse methods to manage temperature changes in motors. These differences can be crucial to your design.

Torque - the motor perspective

Before analyzing how ac drives operate ac induction motors, lets quickly review a few fundamentals of motor operation. First, a motors ability to produce torque is based on its breakdown torque capacity. For example, a NEMA Design B motor has breakdown torque between 175 and 300% of its full-load torque rating. A typical 1-hp, 1,750-rpm motor with 3 lb-ft of torque at full load, therefore, could produce 5.25 to 9.0 lb-ft of torque before the motor would stall from overload.

Torque at the shaft is related to the amount of amps required to produce it, or the drive systems ability to supply current. Typically, though, only about 80% of breakdown torque is usable torque. Thus, that 1-hp motor s usable torque would range from 140 to 240% torque, taking into account the fact that overload and excess current create additional motor heating.

Usually, the relationship between current and torque is considered linear. Thus, an ac drive with an overload capacity of 150% for one minute can produce 150% torque for that time period. But, if a motor only has a usable torque capacity of 140%, it may not track with the amplifier or drive proportionally. On the other hand, if usable torque capacity is 240%, obviously not all of the capacity will be used.

Then, theres motor temperature to consider. As motor temperature moves from cold, to normal, and on to an overheat condition, it changes a motor s ability to produce torque. Drive manufacturers can either make basic assumptions about temperature, which will be true 95% of the time, or make measurements and calculations to adapt to actual temperature for optimal torque production throughout the range.

The variable here is rotor temperature. As a rotor changes temperature, it affects the working current s ability to produce torque.

Rotor design can significantly influence how well a motor operates on a variable- frequency ac drive. For across-theline- starting, many motors use a double squirrel-cage rotor-bar design to improve starting torque operation. Unfortunately when applied to a variable frequency control, this design increases hysteresis, creating additional rotor heating, which affects the drive s ability to produce motor torque.

Starting up and going slow

There are several techniques that develop torque, especially starting torque, which in drive applications, is usually the area of greatest concern.

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Historically, ac drive manufacturers implementing various forms of V/Hz control included a feature called voltage boost. Taking into account a motors V/Hz ratio, voltage boost over-excites motor windings by increasing the voltage either momentarily or over some scaled percentage beyond the motors fixed ratio. The drawback is that it increases motor heating.

The latest techniques to enhance starting torque are the result of improvements in semiconductor technology. In low speed operation, typically from just above 0 to 3 Hz, current available to motors may exceed the 150% overload, and for short periods of time reach more than 200%. Current availability at 0 Hz is limited in V/Hz algorithms to avoid damage to drive IGBTs in the inverter section.

A key area of focus in algorithm development is regulating speed as well as producing torque from 0 to 1.5 Hz without a motor-speed feedback device. V/Hz and vector-controlled drives depend on motor stator and rotor counter electromotive force (CEMF) measurement and calculations respectively, which become impossible to perform at this low or nospeed threshold. Therefore open-loop speed operation beyond 40:1 begins to suffer in both speed regulation and torque production and becomes progressively worse as the motor speed approaches 0 rpm.

The search is on for a substitute for CEMF feedback information. Many engineers are studying ways to artificially identify the needed stator and rotor information without an encoder or resolver. Some of these techniques include injecting variable-frequency current into a motor at low speed.

The speed to move load

V/Hz ac drives can produce variable speed by maintaining a fixed ratio from zero to some commanded speed. The motor V/Hz ratio is calculated by simply taking the nameplate information for voltage and dividing it by nameplate frequency. A nameplate showing 460 Vac and 60 Hz would result in a ratio of 7.67 V/Hz.

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