This post is part II of our In and Out Trilogy.
Read part I here
Read part III here

So many times as engineers, technicians, and electricians we are faced with the financial side of the business to cut costs, encouraging the design to be on the edge. Some call it cost-efficient, some deal with strict budgets, and others just don’t want to deal with the critics and doubt about a long-lasting installation of crucial equipment that must perform.

First, Some History

Motors are an area where mechanical and electrical failures seem to be more acceptable, except for the fact that they are the shaft that actually produces the work. It seems to be an item that may come up constantly in the maintenance and repair budget every year.

Manufacturing plants, water supply companies, and operations with kilns are typically areas that may experience the reflected wave phenomenon as a reoccurring experience and have accepted this as a normal failure to motors and a budgeted cost.

Over the years, since the 1990s after the 4th generation of drive transistors (the IGBT), the birth of the IGBT to the present has caused failures in the first turn of the motor after the conduit box connection. The IGBT had significant rapid rise times on the first switch of the PWM waveform between positive and negative on each phase of the output.

Motor Problems Become Money Problems

The transistors overshoot the output voltages on the first switch because the transistor is not saturated. These spikes along with long lead lengths in the motor leads can cause doubling, and close to tripling, the voltage at the motor.

The wire structure is capacitive in nature and the motor is inductive in nature. This voltage overshoot will reflect off the first turn of the motor. The motor repair shop says the drive is damaging the motor. VFD cable adds impedance to the motor leads. Another investment is an MG1-rated motor which has a 1600V rating to resist corona (pitting) on the first turns of the motor. All these pieces cost extra money – they were not ordered to save money

The VFD cable and MG1 motors are bid to several sales groups and the cheapest solution is purchased and shows up in your trade shop. A year passes and you have gone through a motor or two. You replace them - the finance department starts asking questions. It is your responsibility to find the answer to a problem that should not have occurred if the proper equipment had been shipped in the first place.

The equipment has already been installed – the THHN wire (the worst you can use) is installed and the motor has a weak voltage rating (possibly a year-year-old motor found in the stock crib). What do you do now?

NOTE:  If the filter is not installed yet, please consider the 2KV inverter duty-rated output cable recommended by the manufacturer.

Bring in the dv/dt Filter

Place a dv/dt on the output near the drive. They are good for approximately 250 – 1000 feet. This will address the reflected wave and attenuate the reflected wave and relieve the stress on the first windings on the motor.

Sometimes the laws of physics trump the financial side of the science project. Maybe business majors should take a physics class (not here to save the world).

Speaking of science, let’s look at the dV-E series dv/dt filter: