Torque it up

Aug. 1, 2000
Drives use many techniques to obtain torque from motors. Here’s a closer look at your options.

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.

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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The most common method used to improve speed regulation in a V/Hz ac drive is to monitor the current of the dc bus or the output from the PWM inverter. Sudden changes in load result in current variations that tell the drive to adjust frequency output to the motor. Therefore, increasing load will result in more frequency commands.

Responsiveness to load change is a key factor to all of todays V/Hz ac drives, especially those classified as sensorless vector. The term sensorless vector in this case is limited to what would be called a V/Hz core.

Automatic torque boost and slip compensation techniques include the effects of load changes on slip speed. They adjust the commanded speed by increasing output frequency, and adjust the methods to equate slip and frequency to rotor current. The result is that motor torque becomes directly proportional to rotor current and slip frequency. This, in turn, quickens response, improving speed regulation to 1.0% of motor base speed. Thus, any improvements in current measurement technology and controller processing speed enhance speed regulation even when a V/Hz-core sensorless-vector ac drive encounters dramatic load changes.

Vector core flux vector ac drives, however, work differently. They regulate torque directly, using speed feedback (closed loop systems with encoder feedback) or speed estimation techniques (sensorless — open loop systems) to compare controller speed reference to motor speed. Any measurable error between speed reference and motor-rotating speed results in a current (torque) command from the drive to the motor to compensate for the load change.

Using current to correct for error enables a vector drive to respond faster as well as improve the quality of motor speed regulation. Typical specifications for a closed loop feedback flux vector or field-oriented controlled drive can be from 0.01 to 0.001% speed regulation. The difference is in the current measuring technique, along with other supporting information such as motor voltage feedback to improve a drives ability to perform quickly and precisely.

Either closed or open-loop vectorcore drives do more than regulate speed, though. Each has a speed loop and a torque loop, along with adjustable associated gain parameters. Thus, these drives can handle a wide range of applications.

But, only vector core drives have the ability to directly command and regulate torque. For instance, a dynamometer often calls for a torque reference rather than a speed reference. A vector core drive would be able to take a reference directly into the torque loop, bypassing the speed loop altogether.

Running hot and cold

Drive designs with motor temperature compensation techniques are typically classified as field-oriented control drives. Early models incorporated thermistors embedded in the motor winding. Newer drives use voltage feedback circuits to calculate changes in motor resistance through voltage drop. These drives adjust the field flux, providing more accurate operation from cold to hot. Speed regulation is more stable too. Depending on the design and setup, these systems may cost a bit more and be more complex to set up.

Another vector control technique is known as direct torque control. It uses a modulating carrier frequency to help identify motor information. It provides the same level of performance as field oriented drives, but it is not suitable for applications where audible motor noise is a concern.

The information obtained by modulating the carrier frequency improves low speed regulation and torque production. Similar to frequency injection techniques, the algorithm attempts to create a form of CEMF as the motor s mechanical speed approaches 0 rpm.

Motor data is critical

All vector control algorithms have one thing in common — they need specific data about the motor to create the mathematical model. Regardless of whether the vector control is a flux vector, field oriented control, or direct torque control drive, each technique creates and regulates motor torque by decoupling the three-phase motor current into two fundamental components, magnetizing current and torque current. And only by maintaining these two components 90o apart, can engineers optimize torque production at the motor shaft.

The following information is needed to create a motor model:
¥ Rated amps
¥ Motor magnetizing current (may be measured during tuning)
¥ Rated frequency
¥ Rated speed
¥ Number of motor poles
¥ Rated horsepower (sometimes algorithm dependent)
¥ Motor rotor time constant or equivalent circuit data (typically calculated today)

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With this information, a drive algorithm can determine the proper flux levels required for good torque production as well as speed stability. Motor slip is calculated based on the nameplate versus the calculated synchronous speed (nameplate frequency and number of poles).

Magnetizing current establishes the base flux level required to energize windings and laminations, without creating torque. It establishes the torque current component range, from 0 to 100%, through the difference between the magnetizing current level and the nameplate amp value.

Inaccuracies in these data can create a false motor model in the drive, and result in either less than optimal torque production, motor speed instability, or both.

Depending on the specific methods implemented in a given algorithm, a motor may exhibit varying performance degradation levels from improper values. Values pertaining to current and slip speed are extremely critical for successful motor operation.

V/Hz versus Vector Control

Because of its simplicity, a V/Hz algorithm is more forgiving and needs less motor information. Therefore drive manufacturers continue to invest in improving its performance, especially for V/Hz core sensorless vector algorithms.

V/Hz core algorithms are also useful in applications where one drive controls several motors. Vector core algorithms typically dont offer such a feature, unless the motors are mechanically coupled to a common shaft such as a dual shafted gearbox.

For pure speed regulation, V/Hz core sensorless vector devices handle a broad constant torque to 40:1. However, it can never match the performance of a vector core drive with torque regulator in terms of response to a command, speed regulation, or just torque response. Tunable proportional and integral gain parameters let vector core drives meet the exact needs of a given application. That s why many drives today come standard with two or three algorithms built-in.

A closer look at linearity

A vector core closed-loop flux-vector drive is given a torque reference through its analog input. But temperature affects its ability to properly produce torque. Compare the plots of the motor at 30° C (room temperature), and 80° C (operating temperature). The drive in this test does not make any adjustments to the motor flux based upon motor temperature. Instead, its algorithm assumes the motor is always at operating temperature. Note that with a forward velocity and 100% torque reference, the torque produced is 98.6% of the reference value for a motor at normal operating temperature showing great accuracy. The same test for a cold motor produces torque that is 87% of the reference value. Considering that motor temperature will rise rather quickly, the performance is more than adequate for most applications.

In the loop

The torque loop circuit of a vector controlled drive interacts with other feedback devices, such as:
• Current feedback from the inverter output to the motor for comparison and correction for error
• Rotor velocity feedback (closed loop) or estimation (sensorless) for the information needed to maintain the 90° relationship between torque and magnetizing current components for optimization
• Speed feedback and reference comparison with error correction signals to the speed and torque loops
• Field flux adaptation to changes in load and speed such as flux boost when starting a load or field weakening when operating above motor base speed. If this were a V/Hz core drive, the speed reference would pass directly to the inverter section and current feedback information would feed to a current regulator block. The current regulator would adjust allowable current limits, indirectly affecting torque. In some V/Hz drives, manufacturers incorporate current feedback from the dc bus as a cost savings. While current information reflects load changes, it never provides the accuracy obtained by drives that measure the current at the inverter stage output.

Roy M. Anderson is product line manager, High Performance Drives, Reliance Electric, Cleveland, Ohio.

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