Question

What is ringing and what happens to the motor when I place it more than 50 ft away from the drive with no line reactor?

Answer

High voltage ringing (reflective voltage) occurs on all VFD's. With the advent of IGBT's replacing SCR's on smaller more in-expensive drives, it has become more pronounced. It is capacitive coupling effect caused by high speed switching (commutation.) Distance greatly enhances the effect.

Cables on the output side of the drive act like capacitors. When the IGBT's switch or produce the PWM output, the higher frequency part of the current will find a path through that capacitance. Think of the cable being a capacitor that increases in size as the cable length increases.

Ringing is very detrimental to motors; it weakens the first stage rotor windings and is detrimental to the life expectancy of a motor. This is especially true for motors under 10 hp, they are machine wound typically, thinner coats of varnish, and have neither phase nor end paper (I.E. aggregate insulation properties.) That's why we always recommend a drive rated motor. Quality is everything concerning motors. Vintage and non-drive rated motors that are used with VFD's have a lifespan of unknown proportions. You simply cannot know what to expect from them.

Long cable runs with its ringing can also add 10 to 15% of the drive current rating. Therefore, the drive may start to trip out on excessive current. The capacitance will also cause a voltage drop that may cause speed performance problems on the lower end of the speed scale (increased current demand for demanded torque.)

A few general guidelines:
(1) Try to reduce distances as much as possible
(2) Distances of greater than 50 ft (cabled distance not physical distance,) use an output line reactor
(3) Always use a drive rated motor.

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FAQ Subcategory: None

This item was last updated on 2018-02-07
Document ID: 747

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