The alternating current (AC) induction motor is often referred to as the workhorse of the industry because it offers users simple, rugged construction, easy maintenance and cost-effective pricing. As a result of these factors, more than 90 percent of motors installed worldwide are AC induction.
Despite its popularity, the AC induction motor has two limitations: It is not a constant-speed machine, and it is not inherently capable of providing variable-speed operation. Both limitations require consideration, as the quality and accuracy requirements of motor/drive applications continue to increase.
Figure 1. Cutaway of squirrel cage AC induction motor opened to show the stator and rotor construction, the shaft with bearings and the cooling fan.
Motor slip is necessary for torque generation
An AC induction motor consists of two 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 degrees apart are standard for industrial, commercial and residential use.
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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 resembles a squirrel cage; hence, the term squirrel-cage induction motor. The name "induction motor" comes from the AC "induced" into the rotor via the rotating magnetic flux produced in the stator.
The interaction of currents flowing in the rotor bars and the stators' rotating magnetic field generate torque. 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 is necessary for torque production.
Slip depends on motor parameters
The formal definition of slip is:
S = (ns , n) x 100 percent/ns , where
ns = synchronous speed
n = actual speed
At low values, slip is directly proportional to the rotor resistance, stator voltage frequency and load torque, and inversely proportional to the second power of supply voltage. The traditional way to control wound-rotor-induction-motor speed is to increase slip by adding resistance in the rotor circuit. The slip of low-hp motors is higher than that of high-hp motors because rotor-winding resistance is greater in smaller motors.
As seen in Table 1, smaller and lower-speed motors are associated with higher relative slip. However, high-slip large motors and low-slip small motors also are available.
Table 1. Motor slip of selected aluminium and cast iron NEMA motors, with synchronous speed ranging from 3600 RPM to 900 RPM.
As one can see, full-load slip varies from less than 1 percent (in high-hp motors) to more than 5 percent (in fractional-hp 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.
As shown in Figure 2, rotor speed decreases in proportion to load torque. This means that rotor slip increases in the same proportion.
A = Synchronous speed
B = Rotor speed
C = Rotor slip
D = Torque
Figure 2. The speed curve of an induction motor. Slip is the difference in rotor speed relative to that of the synchronous speed. CD = AD , BD = AB.
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 in Figure 3 show how greater rotor impedance in motor B reduces the starting current and increases the starting torquebut it causes a greater slip than in standard motor A.
Figure 3. Torque/speed and current/speed curves for a standard motor A (full lines) and a high-torque motor B (dotted lines).
Methods to reduce slip
Synchronous motors, reluctance motors or permanent-magnet motors don't slip. Synchronous motors commonly are used for very high-power and very low-power applications, but to a lesser extent in the medium-hp range, where many typical industrial applications are found. Reluctance motors also are used, but their output/weight ratio is not good and, therefore, they are less competitive than squirrel-cage induction motors.
Permanent magnet (PM) motors, which are used with electronic adjustable-speed drives, provide benefits such as 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 gearboxes. PM motors are limited to special applications, mainly because of high cost and the lack of standardization.
Figure 4. The effect of the slip compensation.
Selecting an oversized AC induction motor also reduces slip. Larger motors exhibit less slip, and it gets smaller with a partial (rather than full) motor load.
For example, refer to Table 1. The required power is 10 hp at about 1,800 rpm and 1.5 percent speed accuracy is required. We know that a 10-hp motor has a slip of 4.4 percent. Can we achieve an accuracy of 1.5 percent with a 15-hp motor?
Answer: The full-load slip of the 15-hp motor is 2.2 percent, but the load is only 10/15 = 0.67. The slip will be 67 percent of 2.2 and equals 1.47 percent, which fulfils the requirements. A disadvantage to oversizing is that larger motors consume more energy, increasing investment and operation costs.
Adjustable-speed AC drive often is the best solution
Using adjustable-speed control can solve AC induction motor limitations. The most common AC drives use pulse-width modulation (PWM). Line voltage is rectified, filtered and converted to a variable voltage and frequency. When frequency-converter ouput is connected to an AC motor, it's possible to adjust motor speed.
When an AC drive is used to adjust motor speed, motor slip is no longer a problem in many applications. A number of drive applications still exist, including printing machines, extruders, paper machines, cranes and elevators, in which high static speed accuracy, dynamic speed accuracy or both are required.
Rather than oversizing the motors to eliminate the slip-induced speed error, it may be better to use sectional drive line-ups with separate inverters for each motor. The inverters are connected to a direct current (DC)-voltage bus bar supplied by a common rectifier. This is an energy-efficient solution because the driving sections use the braking energy from decelerating sections (regeneration).
To reduce motor slip, compensation can be added to AC drives. 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 percent of the slip because rotor temperature variations cause over-compensation and unstable control.) But the compensation can achieve an accuracy as great as 80 percent, reducing slip from 2.4 percent to 0.5 percent.
Figure 5. Block diagram of Direct Torque Control, DTC
Vector and direct torque control improve speed control
The newest high-performance technologies in adjustable-speed drives field are vector control and direct torque control (DTC). Both use some type of motor model and suitable control algorithms to control torque and flux, rather than the voltage and frequency parameters used in PWM drives. The difference between traditional vector control and DTC is that the latter has no fixed switching pattern for each voltage cycle. DTC switches, instead, the inverter according to the load 40,000 times per sec. This makes DTC especially fast during instant load changes and minimizes the need for and effect of dramatic speed changes once the load or process is in operation.
What is DTC?
DTC is an optimized AC drives control principle, in which inverter switching directly controls flux and motor torque.
The input variables for DTC are motor current and DC link and voltage. 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 updates stator flux and torque every 25 microsec.
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 microsec. and they indicate whether the torque or flux must be changed.
Depending on the outputs from the two-level controllers, the switching logic optimizes inverter switch positions. This means that each single voltage pulse is determined separately at "atomic level." The inverter switch positions determine motor voltage and current, which, in turn, influence the motor torque and flux (this closed loop control eliminates the need for encoders in many applications).
The reason DTC control reacts faster than PWM control is shown in Figure 6. 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 reducing speed from A to B, which is quite slow. The higher torque with the DTC control is achieved by direct increase of torque from A to C about 10 times faster than that of PWM control.
Figure 6. Comparison between PWM modulation and DTC drive control during load impact: A to B with PWM control and A to C with DTC control.
Slip compensation with DTC is instantaneous and produces a nominal slip of 10 percent. This translates into a speed accuracy of 0.1 percent to 0.5 percent. This enables DTC drive use in many applications where a tachometer-based vector control was needed previously. For applications demanding an even higher accuracy, it's possible to add a pulse encoder to a DTC drive.
Mauri Peltola is the former marketing manager of ABB Oy, Drives in Helsinki, Finland. He can be reached at Mauri.Peltola@fi.abb.com.