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In the continuity of the previous article on 5 Steps for Efficient Motor Management Design, we tackle power quality domain and raise awareness on a particular case of motors with individual power factor correction (PFC) capacitors.
Whether direct on line (DOL) or with a motor starter (auto-transformer, soft-starter, variable speed drive), a motor can be individually paired with dedicated capacitor unit to fulfill installation power factor objectives and ensure power quality.
Usually not well understood, capacitor protection is often underestimated. Several phenomena have to be considered in order to design an optimum protection and guarantee the safety of people and goods:
- capacitor internal fault and failure mode
- inrush current,
- auto-excitation with motor
- harmonics
Internal faults can occur in a capacitor. Protective devices shall be used to isolate promptly the faulty capacitor before the unit case rupture. Properly rated, High Rated Current (HRC) fuses is the most cost-effective solution for the protection of delta connected capacitor units used for individual compensation of motors. It is important to mention that fuses for motors are not suitable to protect capacitors. They are overrated to withstand motor starting current, and too slow to operate with capacitors.
Inrush current occurs when capacitor units are switched on. This inrush current can be significantly increased if other capacitors connected in parallel are already energized (back-to-back switching). In this case, inrush reactors are required to reduce the transient overcurrent to values acceptable for the capacitor (100 In) and the switching device.
Typical values for inrush reactors range from 50 to 400 µH.
Auto-excitation can occur when capacitor and motor remain permanently connected after disconnection. In this case, the motor could behave as a generator by self-excitation during the deceleration and may cause over-voltages large enough to produce capacitor failures. To prevent this phenomenon, the capacitor current must be lower than the no-load magnetizing current of the motor. A value of 90% of no-load current is recommended.
A motor’s no-load-current is typically in the range 30 to 40% of its rated value. From a practical point of view, auto-excitation is most likely to occur when the motor power factor is relatively low, under 0.8, and installation power factor objective is high, above 0.95.
Harmonics are high order currents leading to excessive heating of the capacitor. They have to be considered for permanent operation. Typical solution are detuned reactors.
In some cases, motor starters may produce harmonics during the start-up. Given the short duration, no specific protection will be necessary.
Summary of recommended solutions for capacitor bank protection in motor applications with individual power factor correction
On the figure below is shown a typical arrangement for individual compensation of motor.
Motor fuses (or other protection) are rated to withstand starting, 3-7 time the motor rated current. These fuses will not protect the capacitor unit.
- Capacitor HRC fuses are specific to protect capacitor in case of an internal fault. When determining fuse rating, inrush currents and capacitor case rupture curves must be taken into consideration. The rating should be at least 1.7 the capacitor rated current
- Inrush reactors are necessary in case of back-to-back capacitor configuration, to limit inrush current. It is the case with several motors with individual PFC or with capacitor bank on the same busbar. Single motor with no other capacitor bank in parallel can go without inrush reactors but usually, go with in case of the later additional motor.
- Correct-Sizing of capacitors is essential to avoid auto-excitation phenomenon when motor and capacitor remain connected after disconnection from the supply. It is recommended to verify that capacitor current remains below 90% of the motor no-load current. Conservative recommendation is to limit capacitor power below 30% of motor power. Otherwise, a dedicated contactor to disconnect the capacitor from the motor before the supply disconnection has to be added. But additional cost and place need to be evaluated.
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