High voltage ringing (reflective voltage) occurs on all VFDs. With IGBTs replacing SCRs on smaller, more inexpensive drives, ringing has become more pronounced. This capacitive-coupling effect is caused by high speed switching (commutation). Distance greatly enhances the effect. Cables on the output side of the drive act like capacitors. When the IGBTs switch or produce the PWM output, the higher frequency part of the current will find a path through that capacitance. Think of the cable as 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 shortens the life expectancy of a motor. This is especially true for motors under 10 hp.

Typically they are machine wound, have 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. Vintage and non-drive rated motors that are used with VFDs have a lifespan of unknown length. You simply cannot know what to expect from them. Long cable runs with ringing can add 10 to 15% to the drive’s current rating, causing the drive 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. Use an output line reactor on cabled distances greater than 50 feet
3. Always use a drive-rated motor.

High voltage ringing (reflective voltage) occurs on all VFDs. With IGBTs replacing SCRs on smaller, more inexpensive drives, ringing has become more pronounced. This capacitive-coupling effect is caused by high speed switching (commutation). Distance greatly enhances the effect. Cables on the output side of the drive act like capacitors. When the IGBTs switch or produce the PWM output, the higher frequency part of the current will find a path through that capacitance. Think of the cable as 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 shortens the life expectancy of a motor. This is especially true for motors under 10 hp.

Typically they are machine wound, have 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. Vintage and non-drive rated motors that are used with VFDs have a lifespan of unknown length. You simply cannot know what to expect from them. Long cable runs with ringing can add 10 to 15% to the drive’s current rating, causing the drive 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: