Why Overload Relays Do NOT Always Protect Motors

Various common situations are discussed in which overload relays do not always adequately protect today's motors. NEMA class designations will be explained along with their relevance to motor damage curves. Single phasing and voltage unbalance will also be shown to cause motor heating beyond that which the conventional overload relay is designed to protect. Viable solutions exist for these problems, and insight will be given as to how best to solve particular problems. The important thing is to clearly understand why motor burnouts occur even though the heaters have been sized correctly.

[1]  M. Shan Griffith.  A Penetrating Gaze at One Open Phase: Analyzing the Polyphase Induction Motor Dilemma , 1977, IEEE Transactions on Industry Applications.

[2]  R. M. McCoy,et al.  Thermal Relationships in an Induction Motor under Normal and Abnormal Operation , 1961, Transactions of the American Institute of Electrical Engineers. Part III: Power Apparatus and Systems.

[3]  M. M. Berndt,et al.  Derating of polyphase induction motors operated with unbalanced line voltages , 1962, Transactions of the American Institute of Electrical Engineers. Part III: Power Apparatus and Systems.

[4]  Jack L. Craggs.  Fabricated Aluminum Cage Construction in Large Induction Motors , 1976, IEEE Transactions on Industry Applications.

[5]  Michael J. Manthe,et al.  Trade-Offs inOverload Protection forLowVoltage Motors , 1982 .

[6]  J. E. Williams,et al.  Operation of 3-Phase Induction Motors on Unbalanced Voltages [includes discussion] , 1954, Transactions of the American Institute of Electrical Engineers. Part III: Power Apparatus and Systems.

[7]  F. R. Karr.  Squirrel-Cage Motor Characteristics Useful in Setting Protective Devices , 1959, Transactions of the American Institute of Electrical Engineers. Part III: Power Apparatus and Systems.

[8]  V. J. Picozzi.  Factors Influencing Starting Duty of Large Induction Motors , 1959, Transactions of the American Institute of Electrical Engineers. Part III: Power Apparatus and Systems.

[9]  Lyle L. Gleason,et al.  Protection of 3-Phase Motors Against Single-Phase Operation , 1958, Transactions of the American Institute of Electrical Engineers. Part III: Power Apparatus and Systems.

[10]  G. M. Rosenberry.  The Transient Stalled Temperature Rise of Cast-Aluminum Squirrel-Cage Rotors for Induction Motors [includes discussion] , 1955, Transactions of the American Institute of Electrical Engineers. Part III: Power Apparatus and Systems.

[11]  Richard L. Nailen.  Safe Locked Rotor Time: How Safe Is It? , 1971 .

[12]  J. F. Heidbreder.  Induction Motor Temperature Characteristics , 1958, Transactions of the American Institute of Electrical Engineers. Part III: Power Apparatus and Systems.

[13]  B. N. Gafford,et al.  Heating of Induction Motors on Unbalanced Voltages , 1959, Transactions of the American Institute of Electrical Engineers. Part III: Power Apparatus and Systems.

[14]  Kenneth L. Paape,et al.  Trade-Offs in Overload Protection for Low Voltage Motors , 1982, IEEE Transactions on Industry Applications.