- March 01, 2003, Mauri Peltola, ABB Oy
The AC induction motor often is referred to as the workhorse of the industry, but there are inherent limitations of the AC induction motor—no constant speed and no speed control.
The AC induction motor offers users simple, rugged construction, easy maintenance, and cost-effective pricing. These factors have promoted standardization and development of a manufacturing infrastructure that has led to a vast installed base of motors; more than 90% of all motors used in industry worldwide are AC induction motors.
In spite of this popularity, the AC induction motor has two basic limitations: (1) The standard motor is not a true constant-speed machine, and (2) It is not inherently capable of providing variable-speed operation.
Both of these limitations require consideration, as the quality and accuracy requirements of motor/drive applications continue to increase.
This article explains the reason for the first limitation — slip — and ways to minimize it.
The best methods to control motor speed with power electronics now available also are detailed, including technology to minimize the negative effects of slip.
Motor Slip Is Necessary for Torque Generation
An AC induction motor consists of two basic assemblies: stator and rotor. The stator structure is composed of steel laminations shaped to form poles. Copper wire coils are wound around these poles. These primary windings are connected to a voltage source to produce a rotating magnetic field. Three-phase motors with windings spaced 120 electrical deg apart are standard for industrial, commercial, and residential use.
The rotor is another assembly made of laminations over a steel shaft core. Radial slots around the laminations' periphery house rotor bars — cast-aluminum or copper conductors shorted at the ends and positioned parallel to the shaft.
Arrangement of the rotor bars looks like a squirrel cage; hence the well-known term: “squirrel-cage induction motor.”
The name “induction motor” comes from the alternating current (AC) “induced” into the rotor via the rotating magnetic flux produced in the stator.
Motor torque is developed from the interaction of currents flowing in the rotor bars and the stators' rotating magnetic field.
In actual operation, rotor speed always lags the magnetic field's speed, allowing the rotor bars to cut magnetic lines of force and produce useful torque.
This speed difference is called slip speed. Slip also increases with load, and it is necessary to produce torque.
Slip Depends on Motor Parameters
According to the formal definition, the slip(s) of an induction motor is:
s = (ns - n)/ns *100% where
ns = synchronous speed
n = actual speed
For small values of motor slip, the slip(s) is proportional to the rotor resistance, stator voltage frequency, and load torque — and is in inverse proportion to the second power of supply voltage. The traditional way to control the speed of a wound rotor induction motor is to increase the slip by adding resistance in the rotor circuit. The slip of low-horsepower motors is higher than those of high-horsepower motors because of higher rotor winding resistance in smaller motors.
Smaller motors and lower-speed motors typically have higher relative slip. However, high-slip, large motors and low-slip, small motors are available.
Full-load slip varies from less than 1% (in high-horsepower motors) to more than 5% (in fractional-horsepower motors). These variations may cause load-sharing problems when motors of different sizes are connected mechanically. At low load, the sharing is about correct, but at full load, the motor with lower slip takes a higher share of the load than the motor with higher slip.
The rotor speed decreases in proportion to the load torque. This means the rotor slip increases in the same proportion.
Relatively high rotor impedance is required for good across-the-line (full-voltage) starting performance (meaning high torque against low current), and low rotor impedance is necessary for low full-load speed slip and high operating efficiency. The curves reveal how higher rotor impedance reduces the starting current and increases the starting torque — but it causes a higher slip than in a standard motor.
Methods to Reduce Slip
The use of synchronous motors, reluctance motors, or permanent-magnet motors can solve the problem of slip because there is no measurable slip in these three types of motors. Synchronous motors are used for very high-power and very low-power applications but to a lesser extent in the medium-horsepower range, where many typical industrial applications are. Reluctance motors also are used, but their output/weight ratio is not very good, and, therefore, they are less competitive than the squirrel-cage induction motors.
A potential growth market is for permanent magnet (PM) motors — used with electronic adjustable speed drives (ASDs). The main benefits are accurate speed control without slip; high efficiency with low rotor losses; and the flexibility of choosing a very low base speed (eliminating the need for gear boxes). The use of PM motors still is limited to certain special applications, mainly because of high cost and the lack of standardization.
Selecting an oversized AC induction motor is a second way to reduce slip. Why? Larger motors typically have a smaller quantity of slip, and slip gets smaller with a partial (rather than full) motor load.
Example: The required power is 10 hp at about 1,800 rpm and 1.5% speed accuracy is required. We know that a 10-hp motor has a slip of 4.4%. Can we achieve an accuracy of 1.5% with a 15-hp motor? Answer: The full-load slip of the 15-hp motor is 2.2%, but the load is only 10/15 = 0.67. The slip will be 67% of 2.2 and equals 1.47%, which fulfills the set requirements. The disadvantage with oversizing is, with the larger motors, there's higher energy consumption, increasing investment and operation costs.
Adjustable Speed AC Drive Often Is the Best Solution
The inherent limitations of the AC induction motor — no constant speed and no speed control — can be solved through use of adjustable speed control. The most common AC drives today are based on pulse-width modulation (PWM). The constant AC line voltage with 60 or 50 cycles/sec from the supply network is rectified, filtered, and then converted to a variable voltage and variable frequency. When this output from the frequency converter is connected to an AC motor, it's possible to adjust the motor speed.
When using an AC drive for adjusting the motor speed, there are many applications in which motor slip is no longer a problem. The speed of the motor is not the primary control parameter; rather, it could be the liquid level, air pressure, gas temperature — or something else. There still are many drive applications in which high static speed accuracy and/or dynamic speed accuracy are required such as presses, extruders, paper machines, etc.
There also are many machines and conveyors on which speed control between sections driven by separate motors have to be synchronized. Instead of oversizing the motors to eliminate the speed error caused by slip, it may be better to use sectional drive line-ups with separate inverters for each single motor. The inverters are connected to a DC-voltage bus bar supplied by a common rectifier. This is a very energy-efficient solution, because the driving sections of the machinery can utilize the braking energy from decelerating sections (regeneration).
Slip compensation can be added to AC drives to reduce the effect of motor slip. A load torque signal is added to the speed controller to increase the output frequency in proportion to the load. Slip compensation cannot be 100% of the slip because of rotor temperature variations that may cause over-compensation and unstable control. But the compensation can achieve accuracies up to 80%, meaning slip can be reduced from 2.4% to about 0.5%
Vector and Direct Torque Control
The newest high-performance technologies in the field of adjustable speed drives are vector control and direct torque control. Both of these use some kind of motor model and suitable control algorithms to control the motor torque and flux, instead of the voltage and frequency parameters used in PWM drives. The difference between the traditional vector control and direct torque control is that direct torque control has no fixed switching pattern for each voltage cycle. Instead, direct torque control switches the inverter according to the load needs, calculated/adjusted 40,000 times/sec.
What Is Direct Torque Control?
Direct torque control is an optimized AC drive control principle in which inverter switching directly controls the flux and torque variable of a motor/load.
The voltage is defined from the DC-bus voltage and inverter switch positions. The voltage and current signals are inputs to an accurate motor model, which produces an exact actual value of stator flux and torque every 25 microseconds.
Two-level motor torque and flux comparators compare the actual values to the reference values produced by torque and flux reference controllers. The outputs from these two-level controllers are updated every 25 microseconds, and they indicate whether the torque or flux has to be changed or not.
Depending on the outputs from the two-level controllers, the switching logic directly determines the optimal inverter switch positions. This means every single voltage pulse is determined separately at “atomic level.” The inverter switch positions again determine the motor voltage and current, which, in turn, influence the motor torque and flux (because this control loop is closed, the need for encoders is eliminated in most applications).
The reason direct torque control reacts faster than PWM control is shown [in a table in PFFC print version (March 2003)]. The motor is running with low load at point A, and the load has a stepwise increase to high load. The higher torque with the PWM control is achieved by reduced speed from A to B. This is quite a slow procedure. The higher torque with direct torque control is achieved by direct increase of torque from A to C, and this procedure is about 10x faster than that of PWM control.
Slip compensation with direct torque control is instant, and it creates an accuracy that is typically 10% of the nominal motor slip. That means speed accuracy of 0.1% to 0.5%. This enables the use of direct torque control drives in many applications in which previously a tachometer-based vector control was needed. For applications demanding even higher accuracy, it's possible to add a pulse encoder to a direct torque control drive.
Mauri Peltola is past director of marketing for ABB Oy, Finland. He began with Stromberg, ABB's predecessor, in 1964, and held sales/marketing positions throughout his tenure. He retains emeritus status with ABB's drives and motors segment. For more information contact Ken Graber, senior PR coordinator, ABB Inc., Automation Technologies, Drives, Motors and Machines, 262/780-3873; mailto:email@example.com; abb-drives.com.
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