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Reaaceleration of Electric Motor
Transcript of Reaaceleration of Electric Motor
It is becoming increasingly common for customers to specify the requirement for reacceleration and/or automatic re-starting of electric motors in project documents.
Reacceleration is defined as the feature which allows a motor to ride-through a short voltage dip or interruption in the electrical supply (<200ms voltage dip) during which time the motor has started to slow down. These dips or short interruptions can be caused by short circuits, either in the plant, the utility system or by the loss of a generating unit or utility tie connection.
The primary purpose of reacceleration is to maintain the operation of process-critical drives; utilities such as air, water, steam and equipment lubrication are a few examples.
Whenever an AC motor is disconnected from its source, the magnetic field in the core takes time to collapse because of residual magnetism in the steel components. If the motor is reconnected before the magnetic field collapses, the motor acts as a generator that is out of phase with the line source. It is possible that the high residual motor voltage could be 100% out of phase with the supply voltage and the resultant torque transient created on reconnection of the line source can be up to 5 times the starting torque, high enough to damage the motor and likely to break the coupling between the motor and the rotating machine.
Reacceleration of Electric Motors
The duration of the power supply interruption is of critical importance. It is safe to re-accelerate the motor if the interruption lasts less than 2-6 cycles (about 100 -200 milliseconds) as neither the residual motor voltage nor the angle between the supply voltage and motor voltage has changed sufficiently and can still be considered synchronised.
If the interruption is longer than this, re-energising of the motor should be inhibited for at least 1.5 open circuit motor time constants, i.e. the time in which the magnetic field in the rotor decays to 36.8% of its initial value, which is typically more than 2 seconds.
Electrically, it is straight forward to implement reacceleration in a direct-on-line motor starter. Traditional hard-wired time-delay under voltage relays have recently been replaced by reacceleration relay modules or integrated microprocessor based motor starter relays. The latter protect against high transient shaft torques by inhibiting the re-application of the supply voltage until the motors magnetic field has decayed to a safe level.
In the case of variable speed drives, the frequency inverter units tend to have ride-through capabilities to a significantly lower voltage than the magnetically held contactor. Consideration must be given to how the drive will respond after voltage is restored, and each drive manufacturer has different schemes for protecting the drive while allowing a restart.
For example, API Standard 671 recommends that couplings should be capable of operating with a service factor of 1.75, which is significantly less than the magnitude of the torque transients caused by the residual motor voltage being 180 degrees out of phase with the supply voltage.
For large machines, it is impractical to over-design the coupling to withstand 5 times the starting torque owing to the excessive physical size and inherent inertia of the coupling. Consequently, the coupling will no longer provide the desired mechanical protection.
Reacceleration of Electric Motor
Some recent projects have included a requirement that machine/motor couplings are designed to withstand torque transients experienced when motors are reaccelerated following a power supply voltage dip.
Compliance with this requirement may lead to an unsafe situation by transmitting excessive forces from the motor to the machine which could result in significant equipment damage, loss of containment and/or injury.
High torque transients can be avoided by implementation of the correct electrical motor starter scheme. Overdesign of the coupling to compensate for high torque transients is not an acceptable solution.
The consequence of adopting customer requirements such as
‘suitable for 100% overvoltage in order to reaccelerate after a short interruption of power supply’ and/or,
‘able to withstand the mechanical forces generated by 100% overvoltage caused by motor voltage 180 degree out of phase with incoming supply voltage’
is that motors/package equipment must be designed to withstand the resulting torque transients, but in order to achieve this the coupling design will exceed normal design limits. In this situation, the torque transients (torsional stresses) will be transmitted through the motor shaft to the machine potentially exceeding its design limits and causing it to fail in an unpredictable, uncontrolled and ultimately unsafe manner.
Impact on Coupling Design