Asynchronous motors (full page)

The asynchronous (i.e. induction) motor is robust and reliable, and very widely used. 95% of motors installed around the world are asynchronous. The protection of these motors is consequently a matter of great importance in numerous applications.

Asynchronous motors are used in a wide variety of applications. Here are some examples of driven machines:
- centrifugal pumps,
- fans and blowers,
- compressors,
- crushers,
- conveyors,
- lifts and cranes,
-  …
The consequence of a motor failure due to an incorrect protection or inability of control circuit to operate can include the following:

• For persons:

  - Asphyxiation due to the blockage of motor ventilation
  - Electrocution due to insulation failure in the motor
  - Accident due to non stopping of the motor following a control circuit failure

• For the driven machine and the process:,

  - Shaft couplings, axles, driving belts, … damaged due to a stalled rotor
  - Lost production
  - Delayed manufacturing

• For the motor itself:

  - Motor windings burnt out due to stalled rotor
  - Cost of repair
  - Cost of replacement

Therefore, safety of persons and goods, as well as reliability and availability levels, are highly dependant on the selection of protective equipment.
In economic terms, the overall cost of failure must be considered. This cost is increasing with the size of the motor and with the difficulties of access and replacement. Loss of production is a further and evidently important factor.
Specific features of motor performance influence the power supply circuits required for satisfactory operation
A motor power-supply circuit presents certain constraints not normally encountered in other (common) distribution circuits. These are owing to the particular characteristics of motors directly connected to the line, such as:

• High start-up current (see Fig. N62) which is mostly reactive, and can therefore be the cause of important voltage drop
• Number and frequency of start-up operations are generally high
• The high start-up current means that motor overload protective devices must have operating characteristics which avoid tripping during the starting period.

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Fig. N62:Direct on-line starting current characteristics of an induction motor

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Motor control systems

Different kinds of motor control solution are compared in the following tables.

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	Is / In	Ts / Tn	Speed control	Torque control
Direct on line	5-10	5-10	No	No
Star – Delta	2-3	1-2	No	No
Auto-tranformer	2-3	1-2	No	No
Soft starter	3-5	1.5-2.5	No	Yes
Variable speed drive	1.5	1.5-2	Yes	Yes

	Intérêt principal	Inconvénient
Direct on line	Reduced cost Hight starting torque	Hight in-rush current
Star – Delta	Reduced in-rush current	Reduced starting torque
Auto-tranformer	Reduced in-rush current	Hight weight
Soft starter	Reduced in-rush current controlled start and stop	Reduced starting torque
Variable speed drive	Controlled speed Energy saving at reduced speed	Higher cost

Fig. N63a: Comparison of different motor control solution

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Motor protection functions

These are the arrangements implemented in order to avoid operation of motors in abnormal conditions which could result in negative events such as: overheating, premature ageing, destruction of electrical windings, damage to coupling or gear box, …

Three levels of protection scheme are commonly proposed: "Conventional", "Advanced", "High Performance", which can be adopted depending on the sophistication and power of the driven machine.

  - "Conventional" protection functions apply for every type of motor or application,
  - "Advanced" protection functions apply to more sophisticated machines requesting special attention,
  - "High performance" protection functions are justified for high power motors, high demanding applications, or motors in critical process.

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Protection	Conventional	Advanced	High Performance
Short-circuit
Thermal overload
Phase current imbalance
Phase current loss
Over-current
Ground fault
Long start
Jam
Under-current
Phase current reversal
Motor temperature (by sensors)
Rapid cycle lock-out
Load shedding
Phase voltage imbalance
Phase voltage loss
Phase voltage reversal
Under-voltage
Over-voltage
Under-power
Over-power
Under power factor
Over power factor

Fig. N64: Classification des fonctions de protection

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Here is a list of motor protection functions and the result of activation.
Short-circuit: disconnection in case of a short-circuit at the motor terminals or inside the motor windings.

Thermal overload: disconnection of motor in case of sustained operation with a torque exceeding the nominal value. Overload is detected by measurement of excessive stator current or by using PTC probes.

Phase current imbalance: disconnection of the motor in case of high current imbalance, responsible for increased power losses and overheating.

Phase current loss: disconnection of the motor if one phase current is zero, as this is revealing of cable or connection breaking.

Over-current: alarm or disconnection of the motor in case of high phase current, revealing a shaft over-torque.

Ground fault: disconnection in case of a fault between a motor terminal and ground. Even if the fault current is limited, a fast action could avoid a complete destruction of the motor.

Long start (stall): disconnection in case of a starting time longer than normal (due to mechanical problem or voltage sag) in order to avoid overheating of the motor.

Jam: disconnection in order to avoid overheating and mechanical stress if motor is blocked while running because of congestion.

Undercurrent: alarm or disconnection of the motor in case a low current value is detected, revealing a no-load condition (e.g.: pump drain, cavitation, broken shaft, …)

Phase current reversal: disconnection when a wrong phase current sequence is detected

Motor temperature (by sensors): alarm or disconnection in case of high temperature detected by probes.

Rapid cycle lock-out: prevent connection and avoid overheating due to too frequent start-up.

Load shedding: disconnection of the motor when a voltage drop is detected, in order to reduce the supply load and return to normal voltage.

Phase voltage imbalance: disconnection of the motor in case of high voltage imbalance, responsible for increased power losses and overheating.

Phase voltage loss: disconnection of motor if one phase of the supply voltage is missing. This is necessary in order to avoid a single-phase running of a three-phase motor, which results in a reduced torque, increased stator current, and inability to start.

Phase voltage reversal: prevent the connection and avoid the reverse rotation of the motor in case of a wrong cabling of phases to the motor terminals, which could happen during maintenance for example.

Under-voltage: prevent the connection of the motor or disconnection of the motor, as a reduced voltage could not ensure a correct operation of the motor.

Over-voltage:prevent the connection of the motor or disconnection of the motor, as an increased voltage could not ensure a correct operation of the motor.

Under-power: alarm or disconnection of the motor in case of power lower than normal, as this situation is revealing a pump drain (risk of destruction of the pump) or broken shaft.

Over-power: alarm or disconnection of the motor in case of power higher than normal, as this situation is revealing a machine overload.

Under power factor: can be used for detection of low power with motors having a high no-load current.

Over power factor: can be used for detection of end of the starting phase.

The consequence of abnormal overheating is a reduced isolation capacity of the materials, thus leading to a significant shortening of the motor lifetime. This is illustrated on Figure N65, and justifies the importance of overload or over-temperature protection.

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Fig. N65: Reduced motor lifetime as a consequence of overheating

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Overload relays (thermal or electronic) protect motors against overloads, but they must allow the temporary overload caused by starting, and must not trip unless the starting time is abnormally long.
Depending on the application, the motor starting time can vary from a few seconds (for no-load starting, low resistive torque, etc.) to several tens of seconds (for a high resistive torque, high inertia of the driven load, etc.). It is therefore necessary to fit relays appropriate to the starting time.
To meet this requirement, IEC Standard 60947-4-1 defines several classes of overload relays, each characterized by its tripping curve (see Fig. N65a ).
The relay rating is to be chosen according to the nominal motor current and the calculated starting time.
Trip class 10 is adapted to normal duty motors.
Trip class 20 is recommended for heavy duty motors
Trip class 30 is necessary for very long motor starting.

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Fig. N65a: Tripping curves of overload relays

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Motor monitoring 

The objective of implementing measurement devices is to ensure a continuous supervision of operating conditions of motors. The collected data can be used with great benefit for improving Energy Efficiency, extending lifetime of motors, or for programming maintenance operations.

Three levels of sophistication for monitoring scheme are commonly proposed: "Conventional", "Advanced", "High Performance", which can be made accessible, depending on the sophistication and power of the driven machine.

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Measurement	Conventional	Advanced	High Performance
Currents
Average current
Phase current imbalance
Thermal capacity level
Motor temperature (by sensors)
Phase to phase voltage
Phase voltage imbalance
Active power
Reactive power
Power factor
Active energy
Reactive energy

Fig. N65b: Classification of protection functions

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Here is a list of the most useful variables to be monitored, and the benefit provided by the measurement.

Currents: they are directly responsible for the conductors heating and thus for a possible time life reduction. These are the most important variables to monitor. The current measurement also gives a direct indication on the motor load and stress applied to the driven machine.

Average current: to know the average load of the motor, whether the motor is well adapted to the driven machine or not.

Phase current imbalance: as imbalance is responsible for additional losses in the motor, phase current imbalance is an important variable to monitor.

Thermal capacity level: knowledge of the remaining overload capability and safety margin.

Motor temperature (by sensors): knowledge of the real thermal operating conditions, taking account of motor load, ambient temperature, ventilation efficiency.

Phase to phase voltage: too high or too low phase voltages are responsible of increased motor current for a given load. Voltage monitoring is thus indicating whether the motor is operating in normal conditions or not.

Phase voltage imbalance: as imbalance is responsible for additional losses in the motor, phase voltage imbalance is an important variable to monitor.

Active power: indication of the load level applied to the motor.

Reactive power: indication of the reactive power that could be necessary to compensate by implementation of capacitors.

Power factor: indication of load level of the motor. If Power Factor is > 1: submit your candidacy for the Physics Nobel Prize.

Active energy: possibility to relate the consumed energy to the operating time or the quantity of goods produced by driven machine.

Reactive energy: possibility to determine the necessity of implementation of capacitors in order to avoid payment of penalties to the Utility.

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Fig. N65c: Example of motor management system with "High performance" protection and monitoring functions (TeSys T Schneider Electric)

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Motor starter configurations

Different configurations of switchgear and control-gear are commonly proposed. Some examples are shown on Figure N66.

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Fig. N66: The various functions and their combinations forming a motor starter

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The different applicable standards are listed on Figure N67.

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Standard	Title
IEC 60947-1	Low-voltage switchgear and controlgear – General rules
IEC 60947-4-1	Contactors and motor-starters –Electromechanical contactors and motor-starters
IEC 60947-4-2	Contactors and motor-starters – AC semiconductor motor controllers and starters
IEC 60947-6-2	Multiple function equipment – Control and protective switching devices (or equipment) (CPS)
IEC 61800	Adjustable speed electrical power drive systems

Fig. N67: Applicable standards

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Different utilization categories have been defined for contactors in IEC 60947-4-1. The selection relative to asynchronous motor control is given in Figure N68.

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Category	Typical applications
AC-1	Non-inductive or slightly inductive loads, resistance furnaces
AC-2	Slip-ring motors: starting, switching off
AC-2	Squirrel-cage motors: starting, switching off motors during running
AC-4	Squirrel-cage motors: starting, plugging(1), inching(2)

1) By plugging is understood stopping or reversing the motor rapidly by reversing motor primary connections while the motor is running.
2) By inching (jogging) is understood energizing a motor once or repeatedly for short periods to obtain small movements of the driven mechanism

Fig. N68: Different categories of AC contactors used for asynchronous motor control

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Protection coordination

Type 1 and Type 2 coordination are defined in IEC 60947-4-1.
Total coordination is offered by some manufacturers.

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Coordination	Consequence of a short circuit	Application field
Type 1	The contactor or starter shall cause no danger to persons and installation and may not be suitable for further service without repair and replacement of parts.	General purpose application. Basic machines.
Type 1	The contactor or starter shall cause no danger to persons or installation and shall be suitable for further use. The risk of contact welding is recognized, in which case the manufacturer shall indicate the measures to be taken as regards the maintenance of the equipment.	Process with availability constraints, e.g.: continuous process, critical industrial machines.
Continuity of service(total coordination)	No damage or maladjustment is permissible. Must be able to restart immediately after fault is corrected No special precaution is required.

Fig. N69 :Level of acceptable destruction according to the condition types

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Basic protection scheme: circuit-breaker + contactor + thermal relay

Among the many possible methods of protecting a motor, the association of a circuit breaker + contactor + thermal relay (1) provides many advantages

The combination of these devices facilitates installation work, as well as operation and maintenance, by:

• The reduction of the maintenance work load: the circuit-breaker avoids the need to replace blown fuses and the necessity of maintaining a stock (of different sizes and types)
• Better continuity performance: the installation can be re-energized immediately following the elimination of a fault and after checking of the starter
• Additional complementary devices sometimes required on a motor circuit are easily accommodated
• Tripping of all three phases is assured (thereby avoiding the possibility of “single phasing”)
• Full load current switching possibility (by circuit-breaker) in the event of contactor failure, e.g. contact welding
• Interlocking
• Diverse remote indications
• Better protection for the starter in case of over-current and in particular for impedant short-circuit (2) corresponding to currents up to about 30 times In of motor (see Fig. N67)
• Possibility of adding RCD:

  - Prevention of risk of fire (sensitivity 500 mA)
  - Protection against destruction of the motor (short-circuit of laminations) by the early detection of earth fault currents (sensitivity 300
    mA to 30 A).

(1) The combination of a contactor with a thermal relay is commonly referred to as a «discontactor». (2) In the majority of cases, short circuit faults occur at the motor, so that the current is limited by the cable and the wiring of starter and are called impedant short-circuits.