1. Thermal Analysis of Canned Motor: A Special Type of Squirrel Cage Induction Motor by Lumped Heat Analysis
Vijay V. Mehta1 , Pankaj Pandey2 & Akshinthala Raviprasad2
1Department of Mechanical Engineering, VVP Engineering College, Rajkot, India & 2Department of Nuclear Energy, Pandit Deendayal Petroleum University, Gandhinagar, India.
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ABSTRACT
Figure 2 shows construction of squirrel cage induction motors sketch view with 11 node numbers.
For steady state temperature difference calculation of the nodes, inverse metrices of conductivity (g) and losses are multiplied. The equation [3] is given as ∆T= [g]-1*P. It is calculated by MATLAB ® [4].
Figure 1 shows the sketch of Total Enclosed Fan Cooled Motor (TEFC). TEFC induction motors are the simplest category of squirrel cage induction motors (SCIM).
Primary Heat Transport (PHT) pumps in a nuclear power plant are required to be operated with minimum maintenance during their entire life of operation. Thus usage of a gland seal pack operating at high temperature and pressure is not feasible and therefore PHT pumps use thin cans that separate the rotor and stators in order to achieve zero leakage of radioactive coolants. Squirrel cage induction motors having very thin and strong can materials enveloping both rotor and stator are called canned motors. These cans should withstand high pressure of the operating fluid. They should be made of non-magnetic materials and have minimum thickness so that eddy current losses in the cans are as minimum as possible. Limiting hot spot temperatures is the major requirement of thermal design of cage induction motors. Canned motor is an important application of cage induction motor. In a nuclear power plant the maximum hot spot temperatures of motors of Primary Heat Transfer (PHT) pumps have to be limited to be within permissible values which are acceptable to nuclear operators for all the operating conditions.
Also motors of PHT pumps have very high power densities, pump liquids at high temperatures and high pressures and are operated for most severe operational transients. Thus thermal design of cage induction motors with cans for which the motors need to be qualified from safety considerations has great importance in the nuclear industry. In the paper, initially thermal design of squirrel cage induction motor a type of totally enclosed fan cooled motor is explained with some assumptions. The thermal design of motor is done using thermal network model and finite element method. The analysis results of above two methods are compared. Later this method will be applied to canned motor for nuclear safety.
LITERATURE SURVEY
Published technical papers of cage induction motors highlight difficulties in analyzing the thermal design of motors. Some of them discuss the analysis of TEFC and Totally Enclosed Water cooled (TEWC) for which the rotor construction is of squirrel cage type.
Most of the literature dealing with thermal design of motors uses thermal networks. The TNM or flow networks are analogous to electrical networks. The most famous research paper of P.H. Mellor, D Roberts and D.R. Turner described in detail lumped method [1]. In this poster, TNM is represented with consideration of assumption.
RESULT COMPARISON WITH ANSYS WORKBENCH®
Assumption of symmetry considered for the FEA analysis. The temperature and heat generation are same in the machine around circumference which is divided in to 48 (no. of slots) cuts with 7.5 degree. Finite element analysis is done using ANSYS® software. Analysis of thermal temperature in ANSYS Workbench® is shown in Figure 4. Table shows results.
LUMPED HEAT TECHNIQUE
Components of motor may be treated as lumped, if there is no significant temperature variation in its volume.
This component may be represented as a single node in the thermal network model (TNM).
Nodes get separated by thermal resistances.
Heat transfer takes place between components (nodes).
Knowing thermal properties and geometries of components of particular motor, thermal impedances, thermal resistances and thermal capacitances of all components can be calculated.
These thermal properties and heat losses are applied into the thermal network, to calculate temperature rises of the motor components for all the operating conditions of motor.
Figure 3
Figure 3 shows simplified TNM with combined axial and radial thermal resistances connected to a related nodes. The proposed TNM for the reported machine is developed according to the Principles reported by Kessler [2].
Assumptions:
Heat flows in the radial and axial directions are independent.
A single mean temperature defines the heat flow both in the radial and axial directions.
There is no circumferential heat flow.
The heat generation is uniformly distributed.
Each cylinder is thermally symmetrical in the radial direction
Losses considered in the model correspond to no-load condition.
Figure 4
Sr No.
Component
Material
Results of TNM using MATLAB (in °C)
Results using ANSYS
(in °C) 1.
Frame
Aluminium Alloy
91.01
48.70 2.
Stator Yoke
Silicon Steel
185.63
155.79 3.
Stator Teeth
187.41 - 4.
Stator Winding
Copper
185.97
166.5 5.
Air Gap
Air
166
145.08 6.
End Winding
181.56
187.92 7.
End Cap Air
125.21 8.
Rotor Bar
Aluminium
140.56
177.21 9.
Rotor Iron
115.90
198.63
10.
End Ring
136.42
11.
Shaft
Low alloy Steel
104.94
123.67
INTRODUCTION
Figure 1
THERMAL NETWORK MODEL
REFERENCES
CALCULATIONS
Mellor P. H., Roberts D., Turner D. R., “Lumped parameter thermal model for electrical machines of TEFC design”, IEE PROCEEDINGS-B, Vol. 138, No. 5, SEPTEMBER 1991, pp
A.Kessler,“Versucheinergenaueren Vorausberechnung des Zeitlichen Erwarmungsverlaufes Elektrischer Maschinenmittels Warmequellennetzen,” Arch. Elektrotech., vol. 45, no. 1, pp. 59–76, 1960.
Okoro,O. I., “Steady and transient states thermal analysis of a 7.5-kW squirrel-cage induction machine at rated-load operation,” IEEE Trans. Energy Convers., Dec vol. 20, no. 4, pp. 730–736
Kattan, Peter., “Matlab Guide to Finite Elements”, Springer International Edition.
Transient behavior is represented by given equation below:
If =0, Steady state behavior will be
Figure 2