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Per IEEE standard 141-1993 (Redbook), section 4.2.5, "adjustable speed drives can contribute current from the motors to a short circuit..."
This statement has led to confusion regarding adjustable speed drives and short circuit currents. The statement is true for some adjustable speed drives such as dc drives and Current Source Inverter (CSI) drives, but incorrect for PWM adjustable frequency drives.
Examples of the questions that have been asked are:
- Do PWM drives contribute backfeed current during a short circuit on the line side of the drive? If so, what %FLA of the motor load do they allow?
- What are the drives' short circuit let through values? Are these maximum permissible short circuit values stamped next to the drive?
- Is the maximum rated short circuit number, stamped inside the drive, a value for the interrupting rating of the drive?
These are valid questions based on the statement made in IEEE standard 141-1993. However, understanding how the PWM type adjustable frequency drive differs from the older technology used in dc drives and CSI drives will quickly eliminate any concerns regarding the PWM drive as a source of current during a short circuit condition of the incoming line.
In controlling ac motors, the PWM type ac drive buffers the ac line from the characteristics of the motor. In transferring energy from the ac line to the motor, the standard PWM drive, including vector controlled PWM drives, allows power to flow in only one direction. Power is transferred to the motor, but not from the motor to the ac line. If the drive is a line regenerative type PWM drive (not normally used in standard motor control), then current can flow from the motor to the ac line.
During operation of an ac motor, energy is stored in the motor. When a short circuit condition exists on the ac line, the motor will temporarily act like a generator and try to transfer energy back to the source. Without a PWM drive, the energy can be transferred back to the ac line. However, with a PWM drive between the motor and the ac line, the energy from the motor is circulated in the output section of the PWM drive. The energy stored in the motor is converted to a dc source which is stored in the internal filter capacitor of the PWM drive. When too much energy circulates in the output section of the PWM drive, an internal overvoltage condition can be created. Without any means to dissipate the energy, the PWM drive senses the overvoltage condition and stops operation. The diagnostic function within the PWM drive indicates the overvoltage condition as an Overvoltage Fault and typically displays that information to guide the user in understanding why the drive shut down. In a line regenerative PWM drive, the energy stored as a dc source is transferred back to the ac line in a controlled manner, limiting an excessive current values.
With standard PWM drives, no energy is transferred back to the ac line, since the overvoltage condition forces the input rectifier section of the drive into an off or non conducting state. The converter or rectifier section of a PWM drive only allows power to flow towards the motor. With dc drives and Current Source drives, the converter section allows energy to flow from the motor back to the ac line. Essentially only a voltage source type drive such as the PWM drive ensures that energy flow is from the ac line to the motor and not from the motor to the ac line. In a regenerative PWM drives, the converter section is modified to allow controlled current to be transferred back to the ac line. Under a short circuit condition on the ac line, the amount of regenerated current is limited to the rating of the drive.
When a short circuit condition occurs on the output of the drive, the short circuit let through current is limited by the electronic current limiting function of the drive. Typically, the maximum allowable short circuit current is about 200% of the rating of the drive. The electronic current limit function of the PWM drive interrupts excessive currents in microseconds. The rms heating caused by the current into a drive is slightly more than the continuous rating of the drive. Typically, an rms value of 150% or the rating of the drive would be experienced. This value is far less than would typically be experienced if the motor were operating directly across the line.
In practice, the impact of standard PWM drives during a line short circuit condition can be ignored. For dc drives, Current Source Drives and line regenerative PWM drives, the maximum contributing current should be only slightly more than the overload rating of the drive. DC drives and Current Source Drives contain current limit functions which will interrupt excessive currents in milliseconds. In all cases, motors operated on adjustable speed drives, will contribute less than would be experienced with the motor connected directly on line. With the standard PWM drive, there is no contribution to short circuit current when a short circuit condition exists on the incoming ac line.
It is important the remember that standards exists as guidelines to aid in the design and operation of electrical systems. These standards often describe conditions in general terms resulting in a conservative presentation of technical information. Each installation is unique. The type of equipment changes rapidly and its use within any electrical system will depend upon that installation and the technology used within the equipment. Reviewing the installation with the equipment supplier can result in fewer problems and lower installation and maintenance costs.
