Output Wiring / Load Reactors - Siemens LMV5 Series Technical Instructions

Technical Instructions
Document No. LV5‐1000
In general, a line reactor is recommended if the supply capacity (kVA) of the transformer feeding the
drive is greater than or equal to 10 times the capacity (kVA) of the drive for transformers 600 kVA and
larger.
Figure 5‐1 notes:
1. Drive power is shown in HP rather than kVA. This conversion can be done assuming a power factor
of unity (1) and negligible losses due to efficiency.
2. Transformers less than 600 kVA have high enough impedance ("soft" enough) so that line reactors
are typically not necessary.
Example 1: A 25 HP drive is being fed by an 800 kVA transformer. Is a line reactor required?
Assumptions:
The power factor is unity (power factor =1)
Losses due to efficiency and wiring are negligible
1. Convert horsepower to kilowatts: 25 HP x 0.745 HP/kW = 18.63 kW
2. Convert kilowatts to kVA: kW = kVA * Pf (Pf is power factor, which is assumed to be 1 in this
example)
Thus, a 25 HP drive is 18.63 kVA.
3. Calculate the kVA ratio: 800 kVA / 18.63 kVA = 42.94
Since a ratio of 42.94 is greater than 10, and the transformer is larger than 600 kVA, a line
reactor will be necessary for this application. The same conclusion can also be arrived at by
using Figure 5‐1.
Example 2: A 10 HP drive is fed by a 400 kVA transformer. Is a line reactor required?
Using the same assumptions and calculation as example 1, the kVA ratio is 53.7, but the transformer is
smaller than 600 kVA, so a line reactor is not necessary. The same conclusion can also be arrived at by
using Figure 5‐1.
Output Wiring / Load Reactors
When the VFD / motor are running, high levels of electrical noise are produced on the wiring between
the VFD and the motor. This is due to the fact that modified sine waves produced by the drive IGBTs are
basically high frequency / high voltage DC pulses. These output wires must be enclosed in some type of
shielding (metallic conduit or metal‐shielded cable) to mitigate radiated electrical noise.
Wire length between the VFD and the motor should be kept to less than 150 feet if possible due to the
reflected wave / standing wave phenomenon and voltage overshoot phenomenon. Both of these
phenomena are rather complex, and are a function of the wire length from the VFD to the motor. The
reflected wave / standing wave phenomenon and voltage overshoot phenomenon can damage non‐
inverter duty motor windings over time due to the high peak voltages that these phenomena can
produce.
Section 5 Page 4 SCC Inc.
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