4/19/2022

Induction Motor
Protection
BY

PROF. DR. MOUSA A. ABD-ALLAH

Categories of Motor protection

Internal
Faults
Winding Bearing
Overloads
Faults failures

External Faults

Unbalanced Reverse Phase

Undervoltage Single Phasing
Supplies Sequence

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Main causes for motor damage in Protective function needed to

industrial drives (ABB Group) detect the motor drive faults

Modern Relay Design

A relay must offer a protection have the following set of features:

1. Thermal protection 7. Winding RTD measurement/trip

2. Extended start protection 8. Negative sequence current
detection
3. Stalling protection
9. Undervoltage protection
4. Number of starts limitation
10.Loss-of-load protection
5. Short circuit protection
11.Auxiliary supply supervision
6. Earth fault protection

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Motor Life

Mechanical life Electrical life

bearings, shaft, fan and the frame Stator and rotor winding and
insulation and cable terminations in
Depends on the environment (dust, connection box. continuously monitor
moisture, chemicals, etc.), vibration and the current flowing into the motor to
lubrication detect overloading or fault conditions

Abnormal Conditions

Phase to
Stalling or Reverse phase or Single
Mechanical prolonged Unbalanced phase
Undervoltage Voltage phase to phasing
Overload starting of sequence earth
motor

Overload HRC fuse,

release, instantaneous
thermal Thermal
Thermal overcurrent
overload Negative overload
relay, Undervoltage Phase relays, For
relay, sequence relay, single
instantaneous relay reversal relay large motors
overcurrent relay phase
overcurrent differential
relays, preventer
relays protection
Miniature CB used
with built in
trip coil

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Thermal (Overload) Protection

Winding failures due to overloading (either prolonged or cyclic).

Unbalanced supply voltage, or single phasing, can lead through excessive
heating.

The generally accepted rule is that insulation life is halved for each 10°C
rise above the rated value, modified by the length of time spent at the higher
temperature.

Temperature rises for commonly used insulation classes are:

Class B: 80°C above max. ambient temp. of 40°C (i.e. max. continuous temp. of 120°C)
Class F: 100°C above maximum ambient of 40°C (i.e. max. continuous temp. of 140°C)

IEC 60085 Old IEC 60085

Thermal class Thermal class Typical materials

90 Y Unimpregnated paper, silk, cotton, vulcanized natural rubber, thermoplastics that soften above 90 C

105 A Organic materials such as cotton, silk, paper, some synthetic fibers
120 E Polyurethane, epoxy resins, polyethylene terpthalate, and other materials that have shown usable
lifetime at this temperature

130 B Inorganic materials such as mica, glass fibers, asbestos, with high-temperature binders, or others with
usable lifetime at this temperature

155 F Class 130 materials with binders stable at the higher temperature, or other materials with usable
lifetime at this temperature

180 H Silicone elastomers, and Class 130 inorganic materials with high-temperature binders, or other
materials with usable lifetime at this temperature

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Sources of heating
1. Copper losses are I² (Rs + Rr): The heating and cooling of insulation time constant is
consequently quite long.
2. Iron losses : are relatively small and are quickly dissipated into the body of the motor

Steady-state temperature rise

 Motor temp. rises exponentially, towards its
respective operative temp.
 Since a motor is not a homogeneous mass, heat is
dissipated in several stages.
 It is sufficient for a thermal overload relay to be
set to the mean time constant of the motor.

 Most modern thermal overload relays only measure current, filtering out the highest of
the three-phase current.
 The critical cases of starting/stalling and failure of a phase are taken care by other
protective functions.

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Motor current during start and stall conditions

 Magnitude and duration of Istart and permissible duration of Istalling

are major factors to be considered in the application of overload
protection.
 Istarting remains approx. constant for 80–90% of total starting time.
 Istarting depends on motor design of starting method.
 For direct-on-line starting, DOL, Istarting= 4-8 times Ifull-load
1
 For Y-∆ starter, the line current = 3
the DOL starting current.

 If a motor stall whilst running, or fail to start, due to excessive loading, the motor will draw a
current equal to its’ locked rotor current.
 Discrimination between stall condition and healthy start is based on the duration of the current
drawn not on the value.

Stalling of motors

Motor may fail to accelerate from rest for a

number of reasons:
1- loss of a supply phase
2- mechanical problems
3- low supply voltage TR Relay time
Ts Stall withstand time
4- excessive load torque

Relay operation time less than stall withstand

time: relay gives stall protection

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The majority of loads are such that:

tstarting ≤ 10 s

The allowable stall time to avoid motor damage≥ 15s

Stall protection by:

- IDMT relay
- Undervoltage protection can be used

Relay operation time greater than stall withstand

time: relay does not give stall protection

Unbalanced supply voltages

 Loss of one-phase represents the most dangerous case of unbalance.

 Therefore motors protected against short circuit by fuses must equipped with fast-operating
loss of phase protection.

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Derating due to unbalanced currents

 I –ive seq does not contribute to providing the driving torque

 it produces a small negative torque < 0.5% of the full-load-

rated torque

 A voltage unbalance in the order of 10% can be neglected.

 Hence, presence of negative sequence currents does not

appreciably affect the starting characteristics.

 The main effect of the negative sequence current is to

increase the motor losses, mainly copper loss.

Maximum continuous output vs voltage unbalance

Electrical faults in stator windings

1- Earth Faults
 Earth faults can be detected using an
instantaneous relay, usually with a setting of
approx. 20% of Ifull-load, connected in the
residual circuit of three current
transformers.
 Relay must not operate due to saturation of
one or more CTs during initial peak of Istarting
 To achieve stability under these conditions,
it is usual to increase the minimum
operating voltage of the relay by inserting a
stabilizing resistor in series with it.

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Phase–phase faults
 It is seldom to occur because of the relatively greater amount of insulation between
phase windings.

 The fault would very quickly involve earth, as the stator windings are completely
enclosed in grounded metal.

 Differential protection is sometimes provided on large (2 MW) and important motors to

protect against phase–phase faults, but if the motor is connected to an earthed system
there does not seem to be any great benefit to be gained if a fast-operating and
sensitive earth fault is already provided.

Larg motor – Two sets of CT’s for differential protection

87S Stator Differential 66 Starts per hour

49 Thermal Overload 46 Current Unbalance
49RTD RTD Biased Thermal Overload 47 Phase Reversal
49S Stator RTD 27P Undervoltage

38 Bearing RTD 59P/N Overvoltage

51R Mechanical Jam 67P/N Directional Overcurrent
50P/G Instantaneous Overcurrent 32 Directional Power
51P/G Time Overcurrent 81U Underfrequency
50BF Breaker Failure 81O Overfrequency

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Medium Size Motor

49 Thermal Overload
49RTD RTD Biased Thermal Overload
49S Stator RTD
38 Bearing RTD
51R Mechanical Jam
50P/G Instantaneous Overcurrent
51G Time Overcurrent
46 Current Unbalance
66 Starts per hour
37 Undercurrent

Small size – low voltage motor

49 Thermal Overload
49RTD RTD Biased Thermal Overload
49S Stator RTD
38 Bearing RTD
51R Mechanical Jam
50P/G Instantaneous Overcurrent
46 Current Unbalance
27P Phase Undervoltage
37 Undercurrent

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