Inside PQ: Evaluating Motor and Transformer Inrush Currents

Inrush currents associated with motor starting and transformer energizing can cause interaction problems with other loads in a facility or on the power system. These inrush currents can cause sags that trip loads. Also, protection devices can misinterpret these as fault currents if not coordinated properly. Coupled with the tendency of other constant power devices to increase current to make up for the reduced voltage, the inrush current may cause protection devices to trip. Energizing a transformer has the additional issue of harmonics in the inrush current. These harmonics can excite system resonances and cause dynamic overvoltages. Evaluating these concerns requires measurement equipment that can capture the waveforms over the full duration of the event (can be a number of seconds). Let's look at some of the important issues and how to deal with them.

Motor Inrush Characteristics

Motors have the undesirable effect of drawing several times their full load current while starting. By flowing through system impedances, this large current will cause voltage sags. These dim lights, cause contactors to drop out, and disrupt sensitive equipment. The voltage sag due to motor starting also affects the starting itself — large enough sags will prevent a successful start.

Even small and medium horsepower motors can have inrush currents that are six to 10 times the normal steady-state current levels. High-efficiency motors can have even higher inrush currents.

Characterizing inrush currents and their effects requires a monitor that can capture the waveforms for a long duration. Most monitors lack this capability, so you may need to use a monitor other than the one you have. Figs. 1a and 1b, on page 9, illustrate an inrush of current to a motor that resulted in a significant sag for more than 1 sec.

Ohm's and Kirchoff's Laws can help you analyze what happened. Per Ohm's Law, voltage = current × impedance. Per Kirchoff's Law, the sum of voltages around a closed loop must be zero. If we assume 0.5 ohm source impedance and a 10A nominal on a 480V system, the inrush current can result in a drop of 30V to 50V. This would be a sag to 430V at the load, down from the nominal 475V. This sag occurs because the impedance of the motor initially (when the rotor is stationary) looks much like a short circuit. Once the rotor starts turning, the current decreases and eventually goes to a much lower, steady-state value. However, if a load change causes the motor to come close to stalling or being in the locked rotor condition, another sag can result for similar reasons.

Transformer Inrush Characteristics

Transformers also exhibit inrush currents upon initial energization. In this case, the high currents occur to energize the transformer core. The steady-state magnetizing current for a transformer is very low, but the momentary current when the transformer is first energized can be quite high.

The concerns are the same as for motor starting, except for one important difference — besides being a high current magnitude, the transformer energizing current is full of harmonics. Both even and odd harmonic components occur during the transformer energizing, and they can excite system resonances — resulting in dynamic overvoltages.

These dynamic voltages can cause surge suppressor overheating, capacitor fuse blowing, capacitor failures, or misoperation of electronic equipment. Again, monitoring equipment that can characterize the waveforms over the full duration of the event will allow you to see what is going on. Fig. 2a, on page 10, illustrates a typical transformer energizing current waveform and Fig. 2b, on page 10, shows the low order harmonic components in the current.

Solving Inrush Problems

The most significant problem associated with these inrush currents is the resulting voltage sag. As mentioned above, if the voltage sag is too severe, the motor won't even start (ANSI C50.41-1982 indicates that motors are required to be able to start as long as the voltage is not less than 85% of the rated voltage). Also, most utilities limit the allowable voltage variation at the point of common coupling caused by a single motor start to about 4%. The voltage variation on the distribution system is determined by the impedance of the distribution system supply in relation to the impedance of the step down transformer and secondary cabling to the motor.

If the motor starting operation results in a voltage sag that causes tripping of equipment within the facility or at other customer facilities, you can use one of the following methods to reduce the voltage sag.

Keeping large motors on a separate supply from the sensitive loads can usually prevent problems with other equipment. This means the point of common coupling will be at the distribution voltage level, where the voltage sag is less severe than at the motor terminals.
Resistance and reactance starters initially insert an impedance in series with the motor. After a time delay, the starter bypasses this impedance. Starting resistors may be bypassed in several steps; starting reactors are bypassed in a single step. This approach requires the motor to be able to develop sufficient torque with the added impedance.
Delta-wye starters connect the stator in wye for starting, then after a time delay, reconnect the windings in delta. The wye connection reduces the starting voltage to 57% of the system line-line voltage, which causes the starting torque to fall to 33% of its full start value. The reduced voltage during the initial stage of the starting reduces the inrush current — and the resulting voltage sag.
Shunt capacitor starters work by switching in a large shunt capacitor bank with the motor. This bank supplies a large portion of the motor VAR requirements during the start process, and it automatically disconnects once the motor is up to speed (usually based on overvoltage relay).
Series capacitors on distribution circuits supplying large motors will, if used, reduce the effective impedance seen by the motor during starting. This reduces the resulting voltage sag on the motor side of the series capacitor. However, the source side of the series capacitor may still experience a more severe voltage sag.

Transformer energizing concerns are usually related to the harmonics in the inrush current (see Fig. 2b). If these harmonics can excite a system resonance, the best solution might be to change the system frequency response characteristics to avoid the resonance (at least during the transformer energizing operations). This may be possible by switching one or more shunt capacitors out prior to energizing the transformer.

Motor starting and transformer energizing operations can have important effects on facility equipment and on the supplying power system. Characterizing these events requires sampling the voltage and current waveforms over a relatively long duration (seconds), which can be a problem for monitors that record only a few cycles of information for a disturbance. Once you understand the characteristics of the inrush current and the resulting voltage disturbances, you can take the appropriate remedial measures to avoid interference with other loads and other customers.

McGranaghan directs power quality projects and product development at Electrotek Concepts. You can reach him at [email protected].

Bingham directs product development at Dranetz-BMI. You can reach him at [email protected].