Digital-computer program for design synthesis of large squirrel-cage induction motors

Motor stator winding temperature rise limits and test requirements are established by industry standards, customer specifications or the materials used in the insulation system. NEMA, IEC, and CSA standards describe how stator winding temperature rise is measured based on certain criteria, such as the rating or frame size of the motor. This paper details the differences in these standards and how the tested motor temperature rise varies depending on the standard being used. The value of temperature rise measured by embedded detectors placed in the stator slot is investigated and compared to the results from thermal stabilization tests where the temperature is determined from a rise by resistance calculation. Test data for Medium and Large motors is provided.

This article presents a cost-effective variable frequency drive for starting and operating large three-phase squirrel cage induction motors. The proposed drive employs the constant V/F control scheme, which regulates the rotor frequency such that it is always kept below a predetermined value. This has the effect of drastically reducing the motor shaft torque oscillations and significantly limiting the maximum value of the motor line current. In addition to its simplicity and hence cost-effectiveness, the proposed scheme is shown to be capable of (i) reducing the shaft torque oscillations to approximately 22% of that of the direct on-line start of the large motor, and (ii) limiting the motor line current to approximately 36% of that of the direct on-line start value.

Large induction motors can have a high inrush and run-up current during starting, often up to ten times the rated current. In weak supplies, this could be a problem, causing system stability issues and the voltage to dip below acceptable levels. In islanded systems, the capacity could be pulled below its maximum. There are several different starting methods possible, but they are often only suitable for smaller machines. One method not investigated is the use of parallel capacitor compensation during the starting because large induction motors are very inductive during the starting sequence, so that supplying reactive power may be more effective than supplying energy. This paper first investigates several different induction motors with increasing size and assesses their compensation requirements in terms of the reactive power requirement during starting. It is shown that they do generally become more inductive as they increase in size. It is illustrated, using simulations, that using parallel capacitors during starting can significantly reduce the starting current. The concept is tested using a small experimental motor. Series inductors are added to the motor to make it more inductive and look more like a larger machine. The experimental results confirm that the method works. It is also found that the parallel capacitors require series filters to prevent harmonic current, and these can be realized using inductors in series with the compensation capacitors. Point-on switching using solid-state relays was tested, and these can reduce the transient switch-on current over the first voltage cycle.

The on-line monitoring techniques which have normally been used to assess the condition of large HV squirrel-cage induction motors have been dedicated to detecting rotor dynamic problems and bearing wear [1]. This is usually achieved by sensing the shaft movement using eddy current displacement probes in the bearing housings. The technique has proved to be successful for detecting drive misalignment, a bent shaft, oil whirl, bearing eccentricity and wear [2]. Unfortunately, an HV induction motor can fail due to other fault mechanisms which are not simply mechanical or electrical but are often interrelated and complex [3]. For example, frequent direct-on-line (DOL) starting can cause broken rotor bars or end rings due to the large starting currents producing high mechanical and thermal stresses on the cage winding [4]. DOL starting also produces large end winding forces and core vibration during the initial transient. This can cause mechanical damage to the end winding insulation and electrical failure.

Starting an induction motor causes voltage sag in the industrial grid that may disturb the operation of grid equipment. Direct-on-line starting is the simplest and most cost-effective method for starting induction motors, but also the most problematic. Large industries often use internal power plants with synchronous generators and starting powerful motors may impact the generator operations. The synchronous generators could be operated with the automatic or manual mode of voltage control. As the operation experience proves, the generator voltage control mode has a significant impact on the transient behaviours in the industrial power grid when starting a large induction motor. This article presents a case study of the synchronous generator tripping within a true medium-voltage industrial grid during the direct-on-line starting of a large induction motor driving the feed water pump. An analysis of the generator protection logs after the tripping showed that the synchronous generator was controlled in manual mode and its protection relay settings were exceeded. The transients initiated by induction motor starts were studied for possible configurations and operating conditions of the grid using a model developed on the Matlab/Simulink Software platform. The simulations have shown that concern about starting large motors in industrial grids containing internal synchronous generators needs to be solved considering the grid configurations and the coordination of generator protection and control devices.

The present paper gives an overview of a software tool for a complete design of electrical machines which has been developed at the Institute of Energy Conversion Technology in Munich. Within the design of electrical machines, the thermal, mechanical and electromagnetic domains have to be taken into account. The idea is a multi-domain design, i.e. a combination of domains with different levels of detail during the design process. The user interface as well as the communication between the different domains was set up in a way which is simple to handle for the user. Algorithms were adapted and topics such as optimization or interfaces to finite element analysis software were integrated.

There are applications where low current inrush and or high starting torque are specified for reducing disturbance on the electrical distribution network. There are also situations where it is required to successfully start large AC motors at reduced voltage conditions when only one substation transformer of a double-ended transformer is in service at petrochemical plants. The latter condition requires the motors to draw lower inrush current while producing high starting torque. These requirements can create unfavorable design characteristics where one or both requirements can not be accomplished using standard techniques at final design stage or at final motor test. The authors have accomplished designing and installing magnetic material retaining rings on Induction motors with significant improvement in inrush currents and starting torques. Inrush currents have been reduced from 600% to 508% and starting torques increased up to 14%. Several case studies of using magnetic retaining rings on large induction motors are described in this paper for machines ranging from 6,000 up to 14,000HP and testing as low as 380% inrush for machines with magnetic rings. The application of this concept is not limited to only AC induction motors. The same process can also be applied to synchronous motors. Very limited information is available on the benefits of applying magnetic retaining rings and what the consequences are for large AC motors. The application of magnetic retaining rings is a practical approach to solving inrush and torque problems at the manufacturing or design stage. The theory, test, concepts and consequences along with electrical and mechanical considerations are described in this paper.

There are applications where low-current inrush and/or high starting torque are specified for reducing disturbance on the electrical distribution network. There are also situations where it is required to successfully start large ac motors at reduced voltage conditions when only one substation transformer of a double-ended transformer is in service at petrochemical plants. The latter condition requires the motors to draw lower inrush current while producing high starting torque. These requirements can create unfavorable design characteristics where one or both requirements cannot be accomplished using standard techniques at the final design stage or motor test. The authors have accomplished the design and installation of magnetic material retaining rings on induction motors with significant improvement in inrush currents and starting torques. Inrush currents have been reduced from 600% to 508%, and starting torques have been increased up to 14%. Several case studies of using magnetic retaining rings on large induction motors are described in this paper for machines ranging from 6000 up to 14 000 hp and testing as low as 380% of inrush for machines with magnetic rings. The application of this concept is not limited to only ac induction motors. The same process can also be applied to synchronous motors. Very limited information is available on the benefits of applying magnetic retaining rings and what the consequences are for large ac motors. The application of magnetic retaining rings is a practical approach to solving inrush and torque problems at the manufacturing or design stage. The theory, test, concepts, and consequences along with electrical and mechanical considerations are described in this paper.

Design and operation analysis of electrical machines can be performed by using either full-size or different scale models. This paper deals with the development of electromagnetic field analysis and electrical circuit models of geometrically similar electrical machines based on similarity theory and scale factors. The proposed methodology can be exploited for the design of micro and nano scale electrical machines used in medical and other applications, as it allows the construction of up-scale models more suitable for laboratory measurements.

Capacitor bank switching on medium voltage power systems has been in practice for nearly half a century. However, capacitor bank switching causes high inrush currents and voltage transients (as high as 3.0 p.u.) in the power distribution system potentially damaging electrical equipment, impacting the system reliability and power quality. Additionally, capacitor switching transients affect the life of the capacitor itself and can cause damage or sudden failure. Pipeline substations are often located in remote areas with insufficient SCC available on the distribution system to start a large medium voltage induction motor. In many such instances, the end user often ends up using a medium voltage soft-starter, a higher rated MVA transformer in conjunction with a reactor start, or a low inrush motor with less than ideal motor efficiency. This paper introduces a transient-free capacitor switching system for large motor starting on MV power distribution systems that can be used with a traditional capacitor bank system while restricting the voltage transient to less than 1.05 p.u. during capacitor switching. This solution eliminates the requirement of a MV soft starter or an auto-transformer motor start which are cost prohibitive. As a result, in lieu of a low inrush, low efficiency induction motor; a regular induction machine with higher efficiency can be applied.

Recently, the interest in the use of wind energy has increased considerably in the world's energy markets. In the same way there is a visibly tendency in distributed generation due to market deregulation, technological advances, governmental incentives and environment impact concerns. Considering these facts, it is important to understand the different impacts of using wind generation on several technical issues. This work aims to study the behavior of induction generator based wind farms during the connection of large induction motors to the network. Moreover, the influences on voltage sag, (LVRT) capability and short circuit characteristic are discussed.

The paper describes the transient behaviour of a synchronous generator when a large induction motor is switched direct on line. The influence of the AVR (Automatic Voltage Regulator), the damper winding parameters and the pre-load of the generator are described. Comparison is made with analytical formulae for calculating voltage drops at sudden load increase. The simulations in the time-domain are performed using the flux linkages of generator and motor as state space variables. The AVR and equations of motion (for motor and generator) are described as a set of first order differential equations.

The electric power system consists of static load, lump load, and motor. A large industry requires a large induction motor to run its production. However, the state of this induction motor harms the electric power system. The starting current on the motor is so high that it can be 5 to 10 times the nominal current. On the other hand, the motor absorbs much power from the electric power system and results in a voltage drop on the bus. This paper aims to improve the voltage drop on the bus and reduce the starting current of the induction motor. The analysis in this paper uses a transient curve of changes in voltage and starting current with time. Starting an induction motor with the Direct online (DOL) method as the basis for determining the voltage drop and the starting current of the induction motor. Furthermore, using the Capacitor Bank method, Fault Current Limiting (FCL), and Static Var Compensator (SVC) improves transients of voltage drop and motor starting current. All the results of these methods are compared to find out which method is the best.

The most commonly used technique for the detection of faults in large three-phase induction motors is to measure the supply current fed into the motor and analyze the signal spectrum. This technique is well established and has been shown to be indicative of a faulty condition. However, current signature analysis is usually used by very skilled technicians using expensive equipment. A cost effective condition monitoring technique is needed for smaller motors (those smaller than 200 HP). This paper explores the possibilities of using wireless sensors inside the motor. Wireless sensors are gaining popularity in condition monitoring applications because of their relatively low cost and ease of installation. This paper proposes a system of condition monitoring of the three-phase induction motor using wireless sensor networks (WSN) to measure the temperature and the vibration signals. The sensor nodes are placed on the rotor and the stator. The data acquisition is accomplished at a base station located at a distance of 6 feet. Issues related to electromagnetic interference between the wireless devices and the magnetic fields present within the motor are investigated

Machines designed for hazardous areas are fundamentally no different from any other type of electrical machine. However, they are subject to a number of design constraints such as maximum surface temperatures and minimum air gaps to ensure that they remain safe for the particular application. With the increasing use of very large cage induction motors offshore on relatively small electrical systems, the authors describe how it is often beneficial to have the ability to start them directly online with very low starting currents. Adopting a low capital cost, a fit for purpose approach will (with careful design) result in a low starting current motor.