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.
Fig. N62:Direct on-line starting current characteristics of an induction motor
Motor control systems
Different kinds of motor control solution are compared in the following tables.
| 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
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.
| 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
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.
Fig. N65: Reduced motor lifetime as a consequence of overheating
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.
Fig. N65a: Tripping curves of overload relays
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.
| 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 |
