RC snubber circuits RC Snubbers The RC snubber is the simplest and lowest cost of methods employed. In its simplest form it consists of resistors and capacitors configured as shown in Fig. 3.1. The RC snubber is typically installed at the motor terminals and acts as an impedance matching network. The snubber components are carefully selected to cause the load impedance to match the characteristic impedance of the motor cables. When the motor surge impedance is equal to the line characteristic impedance, then voltage reflection does not occur and excessive voltage will not be experienced at the motor terminals.
LC filters The LC filter combines a load reactor and a capacitor network to form a low pass filter as illustrated in Figure 4.1. The basic concept is that the filter network has a resonant frequency of approximately 1 to 1.5 kHz and frequencies higher than that will be absorbed by the filter and not passed on to the motor. Of course it is important that the inverter switching frequency be set to 5khz to prevent excessive filter current and drive malfunction. In fact, it has been found that the performance of this basic LC filter network, while very good at 5 kHz, actually improves as the switching frequency is increased.
Reactor specs (from MTE) Each variable frequency drive must be equipped with an input reactor offering noless than 4.5% effective impedance at rated motor amps (the fundamentalcurrent). They must be harmonic compensated and be UL-506 and UL-508 approved. Nema 1 enclosed units must be UL Listed. The continuous current rating of the reactor must be equal to or greater than the rms input current rating of the drive. Reactors must be copper wound with a UL class H (180 C) insulation system. They must be suitable for an ambient temperature of 45 C and a have a maximum temperature rise of 115 C. Their watts loss must be less than 1% of the rated load. Box lug type terminals must be provided on all reactors rated from 2 amps thru 400 amps. Higher current reactors may be supplied with copper tab type terminals. Reactor must be MTE Corporation type “RL” series.
Motor efficiency Because harmonic currents and frequencies cause additional watts loss in both the copper windings and the iron of a motor, the actual mechanical ability of the motor is reduced. These watts are expended as heat instead of as mechanical power. When a harmonic compensated reactor is added to the VFD output, harmonics are reduced, causing motor watts loss to be reduced. The motor is able to deliver more power to the load at greater efficiency. Utility tests conducted on VFD's with and without output reactors have documented efficiency increases of as much as eight percent (at 75% load) when the harmonic compensated reactors were used. Even greater efficiency improvements are realized as the load is increased.
Motor noise Because the carrier frequency and harmonic spectrum of many Pulse Width Modulated (PWM) drives is in the human audible range, we can actually hear the higher frequencies in motors which are being operated by these drives. A five percent impedance harmonic compensated reactor will virtually eliminate the higher order harmonics (11th & up) and will substantially reduce the lower order harmonics (5th & 7th). By reducing these harmonics, the presence of higher frequencies is diminished and thus the audible noise is reduced. Depending on motor size, load, speed, and construction the audible noise can typically be reduced from 3 - 6 dB when a five percent impedance harmonic compensated reactor is installed on the output of a PWM drive. Because we humans hear logarithmically, every 3dB cuts the noise in half to our ears. This means the motor is quieter and the remaining noise will not travel as far.