Bearing Current PMP Voltage Waveform

33.6.6.6 Bearing Current PMP Voltage Waveform

Bearing current and shaft voltages under 50/60 Hz sine-

33.6.7.1 Output Line Reactor

wave operation has been recognized since 1924. The bear-

A reactor increases the rise time but the benefit of its ing impedance characteristics largely determine the resulting connection may be negated as follows: bearing current that will flow for a given shaft voltage [11].

The rotating machines have three basic sources of shaft • Beneficial connection if cable length is short enough for voltage. These are:

reflections to be superimposed within rise time, i.e. if rise time is increased beyond critical value of cable length.

• Electromagnetic induction from the stator winding to the • Harmful connection if cable length is too long, the reactor rotor shaft (due to small asymmetries of the magnetic may have negligible effect on peak voltage (theoretically field in the air gap that is inherent in a practical machine its presence is insignificant in this case) or ringing period design. The design limit is <1 V RMS. but it will increase the duration of each overshoot, thus • Electrostatic coupled from internal sources: such a voltage increasing the probability of partial discharge. in motors where rotor charge accumulation may occur

(belt-driven coupling, ionized air passing over rotor fan Adding a series line reactor between the motor and inverter blades).

is not as simple as illustrated above because the reactor adds • Electrostatic coupled from external sources such as PWM or adjusts other resonant modes where the reactor rings inverter. The presence of high dV/dt across the stator with lumped capacitance’s. These resonant modes are pure neutral to frame ground causes a portion of the voltage transmission line modes and can double voltage. Some line to ground due to capacitor divider action. The presence inductance helps short circuit protection. If earth current is of PWM related voltage components is undesirable and limited by other means, then the coupled reactors may be lead to a premature bearing failure.

helpful.

33 Drives Types and Specifications 907 TABLE 33.16 An overview of techniques used as a counter measure to EMI

Effect Frequency range (f )

Counter measure

At load Mains

At source

≤100 Hz

• Avoid circulating currents

• Balanced signal circuits • Avoid earth loops in signal paths • Screening (electric field only)

Mains harmonics 100 < f ≤ 2.5 kHz

• Line and/or DC link reactor on rectifiers.

• Balanced signal circuits

• Higher pulse number rectifier (e.g. 12, 18, or 24)

• Avoid earth loops in signal paths

• Low impedance supply

• Filtering

• Harmonic filters

• Filtering • Screening • Balanced signal circuits

Low-frequency 150 kHz < f ≤ 30 MHz

• Filters – one per apparatus

• Filtering

• Cable screening

• Screening

High frequency

• Internal filtering

• Reduced torque response due to time delay in the filter, This mechanism filters the PWM carrier frequency; thus the

33.6.7.2 Sine-wave Filter

sine-wave type

converter output voltages are sinusoidal. This type of filter is • Potential oscillations which have to be electronically best suited for low performance drives and/or retrofit applica-

dampened

tions (old or standard motors). Reference [13] and Table 33.17 • Potential induction motor self excitation illustrates the filtering options for high power VSDs. Employing a filter at the inverter output has some practical consequences:

33.6.7.3 PWM (dV/dt) Filter

• Cost and weight of filter This reduces the dV/dt seen by the motor to a level, which • Filter power losses, voltage drop

does not compromise the motor or EMC. It is ideal for high

A small derating of power switches due to circulating performance drives with custom-built motors. current between filter L, C, and DC link capacitor