Slip of ac induction motors and how to minimize it

The AC induction motor is often referred to as the workhorse of the industry. Generation of pulse width modulation This is because it offers users simple, rugged construction, easy maintenance and cost-effective pricing. Pulse width modulation controller These factors have promoted standardization and development of a manufacturing infrastructure that has led to a vast installed base of motors; more than 90 per cent of all motors used in industry worldwide are AC induction motors.

This article explains the reason for the first limitation – slip — and ways to minimize it. Pulse width modulation dc motor control And, the best methods to control motor speed with power electronics now available are detailed, including technology to minimize the negative effects of slip.


An AC induction motor consists of two basic assemblies – stator and rotor. Variable pulse width modulation The stator structure is composed of steel laminations shaped to form poles. Pulse width modulation block diagram Copper wire coils are wound around these poles. Pulse width modulation motor These primary windings are connected to a voltage source to produce a rotating magnetic field. Pulse width modulation using 555 timer pdf Three-phase motors with windings spaced 120 electrical degrees apart are standard for industrial, commercial and residential use.

The rotor is another assembly made of laminations over a steel shaft core. Sinusoidal pulse width modulation theory Radial slots around the laminations’ periphery house rotor bars – cast-aluminium or copper conductors shorted at the ends and positioned parallel to the shaft. Pulse width modulation ppt presentation Arrangement of the rotor bars looks like a squirrel cage; hence, the well-known term, squirrel-cage induction motor. Pulse width modulation circuit diagram 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. Pulse width modulation techniques pdf 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. Single pulse width modulation technique This speed difference is called slip speed. Pulse width modulation rectifier Slip also increases with load and it is necessary for producing of torque.

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. Pulse width modulation using 555 timer circuit 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. Pulse width modulation explained The slip of low-horsepower motors is higher than those of high-horsepower motors because of higher rotor winding resistance in smaller motors.

As seen in Table 1, smaller motors and lower-speed motors typically have higher relative slip. Pulse width modulation motor control However, high-slip, large motors and low-slip, small motors also are available.

You can see that full-load slip varies from less than one percent (in high-HP motors) to more than five percent (in fractional-HP motors). What is pulse width modulation tutorial These variations may cause load-sharing problems when motors of different sizes are mechanically connected. Pulse width modulation voltage regulator 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.

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. Pulse width modulation using 555 timer The curves in Figure 3 show how higher rotor impedance in motor B reduces the starting current and increases the starting torque — but it causes a higher slip than in standard motor A.

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. Pulse width modulation using 555 timer theory 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. Pulse width modulation inverter circuit diagram 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). Pulse width modulation motor speed control 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). Pulse width modulation stepper motor The use of PM motors is still 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. Types of pulse width modulation Why? — larger motors typically have a smaller quantity of slip, and slip gets smaller with a partial (rather than full) motor load.

Example: Refer to Table 1. Pulse width modulation advantages The required power is 10 HP at about 1,800 rpm and 1.5 percent speed accuracy is required. Pulse width modulation and demodulation experiment theory We know that a 10 HP motor has a slip of 4.4 percent. Introduction to pulse width modulation 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. Pulse width modulation for power converters principles and practice ebook The slip will be 67 percent of 2.2 and equals 1.47 percent, which fulfils the set requirements. Pulse width modulation dc motor speed control The disadvantages with over sizing are: with the larger motors, there’s higher energy consumption, increasing investment and operation costs.

The inherent limitations of the AC induction motor mentioned at the beginning of this article — no constant speed and no speed control — can be solved through use of adjustable speed control (ASDs). Pulse width modulation dimmer The most common AC drives today are based on pulse-width modulation (PWM). Pulse width modulation speed control of dc motors The constant AC line voltage with 60 or 50 cycles per second from the supply network is rectified, filtered, and then converted to a variable voltage and variable frequency. Pulse width modulation control theory When this output from the frequency converter is connected to an AC motor, it is possible to adjust the motor speed.

When using an AC drive for adjusting the motor speed, there are many applications where motor slip is no problem anymore. Pulse width modulation led The speed of the motor is not the primary control parameter; rather, it could be the liquid level (as in Fig. Theory of pulse width modulation 4), air pressure, gas temperature — or something else. Pulse width modulation control circuit There are still many drive applications where high static speed accuracy and/or dynamic speed accuracy are required. Pulse width modulation power supply Such applications are printing machines, extruders, paper machines, cranes, elevators, etc.

There also are many machines and conveyors where speed control between sections driven by separate motors have to be synchronized. Pulse width modulation converter Instead of over sizing the motors to eliminate the speed error caused by slip, it can be better to use sectional drive line-ups with separate inverters for each single motor. What is pulse width modulation used for The inverters are connected to a DC-voltage bus bar supplied by a common rectifier. Pulse width modulation using 8051 microcontroller 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. Pulse width modulation circuit A load torque signal is added to the speed controller to increase the output frequency in proportion to the load. Pulse width modulation inverter Slip compensation cannot be 100 per cent of the slip because of rotor temperature variations that may cause over-compensation and unstable control. Advantages of pulse width modulation But the compensation can achieve accuracies up to 80 per cent, meaning slip can be reduced from 2.4 per cent to about 0.5 per cent.

The newest high performance technologies in the field of adjustable speed drives are vector control and direct torque control, DTC™. Pulse width modulation vhdl code 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. Single pulse width modulation pdf The difference between the traditional vector control and DTC is that DTC has no fixed switching pattern for each voltage cycle. Pulse width modulated dc dc power converters marian k kazimierczuk DTC, a technology proprietary to ABB, switches, instead, the inverter according to the load needs, calculated/adjusted 40,000 times per second. Pulse width modulation project This makes DTC especially fast during instant load changes and minimizes the need/effect of dramatic speed changes, once the load/process is in operation.

The measured input values to DTC control are motor current and DC link and voltage. How pulse width modulation works The voltage is defined from the DC-bus voltage and inverter switch positions. Pulse width modulation for power converters principles and practice 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. Pulse width modulation techniques 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. Simple pulse width modulation circuit This means that 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 why DTC control reacts faster than PWM control is shown in Fig. Pulse width modulation using op amp 7. Pulse width modulation applications The motor is running with low load at point A and the load has a stepwise increase to high load. Pulse width modulation vhdl The higher torque with the PWM control is achieved by reduced speed from A to B. Pulse width modulation lab experiment This is quite a slow procedure. Pulse width modulation raspberry pi The higher torque with the DTC control is achieved by direct increase of torque from A to C and this procedure is about ten times (10x) faster than that of PWM control.

Slip compensation with DTC is instant and it creates an accuracy, which is typically 10 per cent of the nominal motor slip. Led pulse width modulation That means speed accuracy of 0.1 to 0.5 per cent. Advantages and disadvantages of pulse width modulation This enables the use of DTC drives in many applications where previously a tachometer-based vector control was needed. Pulse width modulation amplifier For applications demanding even higher accuracy, it is possible to add a pulse encoder to a DTC drive.

For more information on ABB Drives & Power Electronics, contact: Becky Nethery, Manager, Marketing Communications, ABB Inc., Automation Technology Products Division, Drives & Power Electronics, 16250 West Glendale Drive New Berlin, WI 53151-2840, Tel: (262) 785-8363, Fax: (262) 780-5120, e-mail: becky.nethery@us.abb.com, http://www.abb-drives.com

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