System Compatibility of Adjustable Frequency Drives

GENERAL
The characteristics of Adjustable Frequency Drives, as an electrical system component, are not well understood. They have been blamed for almost every electrical problem that can not be explained. It is true that AFDs have characteristics not typical of more conventional components like motors, starters, and transformers. Fortunately, the most common AFDs of the 1990’s are very easy to understand. By far, the largest installed base of AFDs is the Pulse Width Modulated, voltage source type. The unique features of the PWM, AFD can be defined and with those definitions, successfully applied for process improvements and for energy savings.

For simplicity, four categories will be explained.

1. As a source for the ac motor (Energy Transfer component).
2. As a load for the distribution system ( Energy Consumption component).
3. As a load for the ac motor (Energy Absorption component).
4. As a source for the distribution system (Energy Storage component).

Since the function of the AFD is to control 3 phase ac motors, a successful operation depends on the ac motor experiencing a voltage source that provides the following characteristics.

1. symmetrical waveform (mirrored imaged)
2. balanced waveform (+/- 2%)
3. low impedance source (less than 1%)
4. 3 phase source (120 degree displacement)
5. controlled volts/hertz relationship through the speed range. The area under the voltage curve must be fixed to the frequency. A 480V, 60Hz source and a 240V, 30Hz source have the same volts per hertz relationship. The relationship is 8 volts per hertz.

When connecting an ac motor to an AFD, the resulting current waveform is also symmetrical. As the polarity of the terminal voltage changes, the frequency of the current waveform will change. The current produces the magnetic fields whose interaction produces the motor torque and results in the rotation of the motor (rotor). Changes in motor rotation stop when one of two factors occurs. The first, and most important, occurs when there is insufficient voltage to produce additional current that produces the torque necessary to change rotational speed. The second occurs when the rotational speed of the rotor matches the speed of the rotating magnetic field (applied frequency).

There is no significant difference between the current in the motor operated directly across the ac line or connected to the output terminals of an AFD. The real current circulating in the motor always comes from the utility or power source connected to the input terminals of the AFD. The AFD merely connects the input source to the terminals of the motor. The AFD does not produce power or store power. The AFD is simply a connecting device that allows a limited amount of power to flow from a source to a load. The motor circuit carries real and reactive current. Unlike the motor operating directly across the line, the AFD operated motor does not allow the reactive current to flow in the incoming distribution system. The reactive portion of the motor current will flow in the output (inverter) section of the AFD. Only real current flows from the incoming distribution system through the AFD and into the motor. This single characteristic of PWM, voltage source AFDs means that there will always be fewer system losses when operating an ac motor with an AFD compared to an ac motor operated directly across the line.

Energy Consumption component
As a load for the distribution system, the PWM, voltage source AFD will permit only a limited amount of energy transfer. Unlike other types of loads, like motors and transformer, the short circuit conditions that can exist with AFDs are much, much less than typically found in distribution systems. Protection from short circuit demands is normally controlled by fuses or other circuit protection devices. An important feature of the AFD is its current limiting capability.

As a connecting device, the AFD can interrupt the flow of current in microseconds. This means that no high rms values of fault current will ever exist. The connecting semiconductor switch operates as a extremely fast recloser for values of current within the AFD rating. The semiconductor switch operates as a manually resettable circuit breaker when the instantaneous value of current exceeds the trip limit of the AFD. It is rare that fuses or other protective devices would function faster than the electronic current limiting that exists in AFDs.

As a result of the current limiting feature, the short circuit current let-through is well below the typical values defined by branch circuit protection found with fuses and circuit breakers. In practice, and for simplicity, branch circuit protection for AFDs is selected as if the circuit were a standard motor circuit. This is very conservative, but less confusing than attempting to redefine local electrical codes. When input fuses are specified by the AFD supplier, those fuses have, most often, been recommended to conform to some testing criteria such as UL.

When the AFD, as a load, is compared to the ac motor, electrical parameters such as power factor and surge currents are more favorable than expected. A typical ac motor may have a power factor of 0.8. The AFD controlling that same motor would have a power factor much closer to unity. That difference in power factor means that less current will flow in the distribution system. Less current means lower system losses or greater system efficiency. The losses in the AFD are similar to the losses experienced by the voltage drop across the contacts of a motor starter. As the current increases, the voltage drop will increase. As the current decreases, the voltage drop will decrease. In some applications where power reduces as speed reduces (fans), the efficiency of the system can improve significantly.

Energy Absorption component
As a load for the ac motor, the AFD is expected to absorb some energy when the mechanical energy is converted by the motor to electrical energy. This occurs when the motor rotates faster than the applied frequency. The motor, now acting as a generator, will force energy (current) back to the AFD. Since the standard AFD will transfer power in one direction only, the energy supplied by the motor must be absorbed or that energy will cause the AFD to shut down on an over voltage event.

Three factors must be considered when the motor is operating as a generator. The first is that the motor will operate as a voltage source, supplying current to the AFD as long as the rotational speed of the motor is greater than the synchronous speed of the motor. The second factor is that no generator action can occur unless there is a magnetic field in the motor. Unlike a dc motor which has a separately excited field, the ac motor depends on the operation of the AFD to maintain the field. If the AFD sends no supply voltage to the motor, then the motor can not send energy back to the AFD. The third factor is that the AFD will rectify the ac voltage being supplied by the motor and store that voltage in the filter capacitor (dc bus) of the AFD. Since the AFD can not send that motor voltage back to the distribution system, rectification will continue until the voltage being stored in the AFD filter capacitor exceeds the over voltage limit. The amount of energy that can be stored is very small, so the limit is reached very rapidly. When the limit is reached, the AFD will shut down.

This shut down will result in the AFD being disconnected from the ac motor. Since the ac motor is disconnected, the magnetic field of the ac motor will disappear and the motor can no longer operating as a generator. The motor will coast to a stop unless the AFD fault is cleared and the AFD is restarted. To prevent a shutdown of the AFD when the motor is operating as a generator (regeneration), the voltage being stored on the filter capacitor can be controlled by discharging that stored energy through a power consuming path. Typically, an electronically switched resistor is connected across the filter capacitor of the AFD (Dynamic Braking Module). When the voltage on the capacitor reaches a present value, the resistor is connected across the filter circuit. When the voltage drops below a lower present value, the resistor is disconnected from the filter circuit. This action can continue until there is no more energy to dissipate and the voltage on the filter capacitor remains below the present value.

Energy Storage component
As a source for the distribution system, the standard PWM, AFD is prevented from sending energy back to the distribution system. This is due to the 3 phase, full wave bridge rectifier on the input circuit of the AFD. In the event of a fault on the input to the AFD, no energy can flow from the AFD to the distribution system. Unlike other power converters like dc motor controllers, Current Source Inverters and arc Furnaces, the PWM, AFD can only transfer energy from the input supply to the motor.

Per IEEE standard 141-1993 (Redbook), section 4.2.5, adjustable speed drives can contribute current from the motor to a short circuit... . Based on the preceding statement, it would be nature that the following questions would be raised. What percent FLA of the motor load would adjustable speed drives allow? What are the drive’s let through values? Are these maximum permissible short circuit values indicated on the drive’s label? Is the maximum rated short circuit number and interrupting rating for the drive indicated on the drive’s label?

Other power converters can feed energy back to the distribution system, however that energy can be controlled and in most cases can only contribute a very small portion of energy to the fault condition when comparing to the energy that can be provided by transformers and line operated motors. In all cases where AC drives or DC drives are applied, the value of rms current that is transferred from the ac supply to the motor or from the motor to the ac supply will be limited to the maximum ratings of the AC drive or DC drive.

Of VSDs, only DC drives and Current Source Inverters (AC Drives) have the natural capability to feed current back to the ac line and thus contribute to shorts on the ac line. However, in the case of short circuit on the line side of the drive, it is most likely that these drives would detect that condition and terminate operation.

In terms of maximum permissible short circuit values, only the DC drive lacks the capability of electronically controlling the instantaneous value of the current. The DC drive is typically a line commutated, 3 phase rectifier which connects the ac line to the armature of the dc motor. With a 60 Hz supply, the connections from the ac line to the motor can be commutated (turned off) every 8.3 milliseconds or approximately one-half cycle. Since the dc motor is providing the current, the question of how much current will the dc motor contribute to the line short will depend on the motor’s ability to increase in armature current during that 8.3 milliseconds. In the event of a short on the ac line, the dc motor can continue to send current out of the motor back to the shorted line as long as the field of the dc motor is energized and the motor is rotating. Due to the variations that would exist, it would be difficult to predict a maximum value. In practice, the characteristics of the incoming fuses are used to define the maximum let through value. The I^2T rating of the fuse defines when the fuse should open.

In Current Source AC drives (CSIs), the current path can be controlled electronically. The inverter or output section of the CSI can interrupt the flow of current and limit the let through current. Once again, it is likely that the CSI will detect the fault condition on the ac line and terminate operation, thus limiting how much current flows back to the ac line. Like the DC motor, the ac motor used with a CSI drive must maintain it’s magnetic field if it is to send current back to the ac line. Unlike the dc motor which has a separately excited field, the ac motor depends on the CSI drive to electronically maintain its magnetic field. A major difference between the DC drive and the CSI drive is the large reactor that exists between the ac line and the ac motor in the CSI drive. This link reactor will prevent current from changing rapidly and will limit the current that the CSI drive can contribute to the ac line fault. In practice, it is normal to use the I^2T value of the line side fuses to define the maximum let through current. In actual operation, the value of current that can be contributed by a CSI drive is less than the FLA of the motor and will not be sustained for more than one half cycle of the supply frequency.

With PWM, voltage source AC drives (AFDs), no energy can be returned to the distribution system unless the AFD is a special line Regenerative type drive. In the event of a short circuit on the ac line, the input supply voltage to the drive is reduced to a single phase supply. Since the motor is buffered from the conditions on the ac line by the filter section of the drive, it continues to operate in a controlled manner. Current will not increase in the motor since the motor believes that nothing has happened to it’s voltage supply to cause any changes. If motor current does not increase, than there will be no change reflected to the incoming supply line and thus no contribution to the short circuit condition on the ac line.

Summary
Some types of adjustable speed drives can contributed to the fault current on a ac line short. PWM adjustable (frequency) speed drives do not contributed to ac line shorts. The typical PWM, AFD is designed to transfer power from the ac line to the ac motor. In doing so, improvements in power factor result which reduce the value of total current that the distribution system must handle. Energy flow is a one way process so PWM AFDs can not add energy to faults that occur on the distribution system. By optimizing the voltage applied to the motor for many applications, a general reduction in total energy consumption will occur. In many applications, modifying the operating parameters of the application can result in overall energy savings.