Marathon motors has done a lot of research (published) on motor matching with load and drive. Not to say the others havn't but they have some guys over there that really are knowledgable on this stuff.
Here is some info I put together that some of you might find useful:
Potential Motor Damage from VFDs
When using VFDs for motor control, attention must be given when specifying the motor and designing the installation. The rapid switching of modern drives creates transient voltage spikes that can shoot pinholes in the insulation of the motor causing short circuits and premature failure of the motor. The damage to the motor insulation typically occurs in the first turn near the lamination stack.
In addition, the high switching frequency currents of the inverter can damage bearings. An effect called Electric Discharge Machining (EDM) occurs when the currents find a path to ground over the surface of the bearings, causing a metal transfer between the bearing and race to take place.
The damage takes place over varying periods of time, usually months, and will be first noticed by louder then normal running noise, or by vibration monitoring, when utilized. Damaged bearing(s) should be replaced immediately, before total destruction of the bearing occurs, causing further damage and loss of production.
Reflected Voltage and Heating: Due to standing waves, part of the inverter output is reflected back from the motor to the inverter. Voltage from the inverter pulse and the reflected wave combine, thereby increasing the voltage at the motor. At long distances, a 460V rated motor could see 2000V.
Waves not sinusoidal are converted to heat in the motor windings causing burnout of the motor. This was more prevalent on 6-step inverters but is still possible using PWM types.
Common Mode Current: This source of bearing current is due to the switching that occurs between phases, the neutral cannot be instantaneously cancelled out like in a pure sine wave, creating a potential difference between inverter output and ground. The stray currents created by the potential difference cause current to flow through motor cable and windings.
Shaft Earthing Current: Motors rated around 125 Hp and larger typically induce high frequency currents into the shaft simply due to asymmetrical flux distribution in the motor. The high frequency voltage pulses from the inverter, also produces current that flows through the motor leakage capacitances of the windings to ground. Together an induced voltage between shaft ends occurs. If the induced voltage is high enough to overcome the bearing oil impedance, a circulating type of high frequency bearing current occurs. When the leakage current returns to the inverter through ground, and the motor shaft is grounded through the driven equipment, part of the leakage current can flow through the driven equipment bearings and shaft due to poor stator grounding.
In smaller motors, the stray capacitance is small enough that the internal division of common mode voltage is divided, and high enough that high frequency current pulses are created.
Preventing Damage: To reduce the probability of damage to the motor, the specification must clearly indicate it shall be inverter duty rated. This will help the manufacturer to select the proper motor for the application. The motor should utilize a class H insulation, but limit it to a B temperature rise, the service factor should always be at least 1.15 and the motor should not be allowed to operate at full load.
Additionally, there are three methods of affecting the bearing currents; proper cabling and grounding system, breaking the bearing current loops, and damping the high frequency common mode current.
Cable: Only symmetrical multi-conductor cable should be utilized to help avoid bearing currents at fundamental frequencies. Either three individual phase ground conductors should be spaced symmetrically around the three conductor leads, or an earth conductor should surround all three of the phase conductors. The cable should also have a continuous shield made of copper or aluminum, and connections at both ends need to utilize 360o terminations. Cable length should be kept to an absolute minimum, less than 15 feet is preferred.
Bonding and Grounding: High frequency bonding connections should be installed at known ground reference points. The bonding should be done using flat braided copper straps, 50-100mm in width. Shaft grounding may also be optional from manufacturer, but typically not for motors smaller then 10 Hp. It is accomplished by using a brush that rides on the shaft that shorts the rotor voltage to ground.
Breaking Loop: The following methods will help ensure ground current will not flow:
? Insulate the non-drive end bearing
? Insulate the shaft from the frame
? Provide insulated coupling between the motor and driven equipment.
Dampening: Provide impedance in the common mode loop by installing dedicated filters. Load reactors add inductance to the line between the inverter and the motor. Low pass filters are a combination of inductance, resistance, and capacitance and should be used on very long runs of cable, and tuned to the natural frequency of the wire. Adding these devices is especially important where it is not physically possible to install the inverter near enough to the motor.
