Drives + Motors: Avoiding the Hidden Pitfalls of Mismatched Systems
Drives + Motors: Avoiding the Hidden Pitfalls of Mismatched Systems
Why Motors Alone Aren’t Enough
For decades, industry has leaned on motors as the backbone of production. But in today’s energy-conscious and reliability-driven environment, motors alone don’t unlock the efficiency or control plants need. The real gains come when motors are paired with the right drive. Variable frequency drives (VFDs) act as the brain of the system, orchestrating speed, torque, and energy use. Without them, motors run at full throttle even when processes don’t demand it — wasting energy and putting unnecessary strain on equipment.
At Marathon Electric®, we’ve seen firsthand how mismatched systems create frustration on the plant floor. A good motor without the right drive can run hot, vibrate excessively, or fail earlier than expected. A powerful drive paired with the wrong motor can waste its potential, leading to nuisance trips and downtime. In short: the motor and the drive must be treated as a system, not standalone components.
When adding a VFD to a three-phase AC induction motor, first, verify the compatibility and age of the given motor. Checking if the motor is “Inverter-Rated” NEMA Part 30 or “Inverter Duty” NEMA Part 31 will aid in determining if the given motor’s insulation treatment would handle today’s VFD’s induced voltage spikes. At Marathon® we would recommend motors with an insulation process like with our MAXGUARD system. If the motor is older, say built in the 80s or 90s you may want to change out the older motor to a new one that is able to handle the VFDs voltage spikes. Next, you will need to use the given motor’s nameplate full load amps (FLA) and other key attributes for properly sizing a VFD to a given motor and having an electrician to properly connect the motor and drive together by following the drive manual and motor connection diagrams. Finally, programing the VFD with the given motor specs/parameters (Auto-Tune). By asking the OEM or End-User if they have a requirement for insulated bearings and/or shaft grounding rings to protect the motor and what the motor is attached to as a system. Bearing shaft currents are real and can damage bearings very easily and quickly. Consider protecting your investment.
The Cost of a Mismatch
Plant teams often see the symptoms before they understand the cause. A motor that “feels” hot to the touch (knowing people do this and shouldn’t, Marathon recommends using a thermal measuring device rather than hands or fingers) even at modest load. Many different winding damages and/or premature failures can be costly resulting in downtime. Bearing Damages: Bearings that fail prematurely due to bearing shaft currents causing a phenomenon known as electrical discharge machining (EDM) or fluting of a bearing. Audible whine or chatter during acceleration. Vibration that shows up only at certain speeds. Or the dreaded nuisance trip that halts production at exactly the wrong time. All of these are costly in repair and in downtime.
According to EASA (Electrical Apparatus Service Association), many of these headaches trace back to running non–inverter-duty motors on modern VFDs. Standard insulation systems, cooling fans sized only for 60 Hz, and unprotected bearings aren’t designed for the fast voltage pulses and harmonic stresses VFDs generate. NEMA MG 1 Part 31 lays out the insulation requirements specifically for inverter-fed motors to ensure survival under these conditions. Ignore them, and failures are inevitable.
Load type is another frequent mismatch. Variable torque systems like fans and pumps behave very differently from constant torque systems like conveyors and mixers. The U.S. Department of Energy highlights that small speed reductions in fans and pumps yield outsized energy reductions — a 20% cut in speed can reduce power consumption by nearly 50%. But constant torque applications demand torque even at low speeds, which requires different drive settings, cooling provisions, and sometimes different motor technologies. Overheating: since the cooling fan is attached to a totally enclosed fan cooled AC induction motor, the fans speed can affect the overall cooling the fan was designed for say at 60HZ 1800RPM but by the VFD it can drop the speed to 6HZ 180 RPM and if this “heat” affect is not taken into account up front, there is a possibility that the winding may get too hot. The VFD also contributes additional heat to a motor that must be accounted for. Hence a potential winding failure can occur.
Finally, system details are often overlooked. Long cable runs (over 100 feet) can create reflected-wave voltages that double stress on insulation. Shaft currents, if unmitigated, etch bearings in months. Mechanical resonance zones need to be skipped in drive programming, or else vibration destroys uptime. Each detail matters if the drive-motor system is expected to last. Reduced Efficiency: a poorly matched motor and VFD can lead to energy inefficiencies, including increased harmonic distortion, which can affect the overall power quality in a plant facility.
Some additional “hidden” costs can include:
High Upfront Investments
Complex Installation and Maintenance
Electrical Interference
Increased Downtime
Additional Component Costs
How to avoid costs and damages
The Inverter-Duty Difference
“Inverter-duty” is not just marketing jargon. It’s an engineering standard that means the motor is designed to live with a drive – NEMA Part 31
By ensuring your investment with a “package VFD and an invert duty motor you will have a cost-effective solution for your plant and facility. This usually consists of the following:
Marathon’s MAXGUARD – Insulation systems built for steep-front PWM pulses and reflected wave voltages.
Marathon’s BCP – Bearing protection (mitigation) with insulated bearing designs or shaft grounding rings, preventing electrical discharge machining (EDM).
Cooling provisions for wide speed ranges, so a totally enclosed fan-cooled (TEFC) motor doesn’t cook at 15 Hz or slower and the application may require a constant speed blower (TEBC or DPFV)
Consider mitigation technologies/techniques: adding filters on the VFD output (like dv/dt or sine wave filters) to reduce the harmful effects of high voltage spikes.
Follow installation guidelines: your plant facility may need to shorten the cable lengths between the VFD and motor. You may need to use shielded cables and ensure proper grounding to minimize electrical interference.
As EASA points out, skipping these safeguards shortens motor life and often wipes out the very savings a VFD was supposed to deliver. An inverter-duty motor paired with the right drive, by contrast, delivers efficiency, reliability, and longer service intervals.
Where the CM3 Fits
For plants looking to upgrade from across-the-line starters or basic soft starts, Marathon’s CM3 drive provides a practical entry point. It excels in variable torque applications like fans, blowers, and centrifugal pumps some constant torque applications may include conveyors, mixers, and granulators. When paired with inverter-duty or premium-efficiency induction motors, the CM3 enables smooth starts, adjustable speeds, and meaningful energy savings without unnecessary complexity.
Take a cooling tower fan as an example. Running it at 80% speed instead of 100% can cut power use nearly in half — savings that multiply across dozens of units. Pairing the CM3 with an XRI® Premium Efficiency motor gives facilities an immediate reduction in kilowatt-hours, with quieter operation and less wear on mechanical components.
The CM3 shines in everyday industrial duty applications where simplicity and savings matter most.
Where the CM5 Steps In
Some applications, however, demand more than smooth starts and simple turndowns. Frequent start/stop duty, heavy loads, or processes requiring tight speed control call for advanced capability. This is where the CM5 drive delivers.
The CM5 is built not only for variable torque applications but for constant torque and demanding loads: conveyors that need consistent speed under shifting loads, mixers and extruders that require torque at very low RPM, and crushers where shock loads are common. With advanced vector control and feedback capability, the CM5 keeps torque stable, reduces downtime, and enables precise process control.
The CM5 offers NEMA 4X – IP66 with and without a disconnect switch. Suitable for Wash Down Duty® applications or wet-moist areas that demand dirt and water protection.
Pair the CM5 with a Severe Duty or Crusher Duty motor, and you have a system designed to survive in the harshest industrial environments. Pair it with a BlackMAX® PM motor, and you unlock the next tier of performance: high torque at low speed, smooth operation across the range, and industry-leading part-load efficiency.
Why BlackMAX® PM + CM5 Is a Premium Pairing
Permanent magnet AC (PMAC) motors like BlackMAX® PM take the drive-motor partnership to another level. DOE and NREL research shows that PM motors maintain high efficiency even at low speeds and partial loads — operating conditions where induction motors often fall short. They also deliver torque without relying on rotor slip, which means stronger low-speed performance and the potential to downsize or eliminate gearboxes.
In material handling, that translates to conveyors that start smoothly and hold speed under load swings. In pumping and HVAC duty, it means steady control and reduced energy consumption across fluctuating demand. In every case, the combination of CM5 + BlackMAX® PM provides a quieter, cooler, more efficient system that pays dividends in uptime and energy bills.
Best Pairings in Practice
Fans & Pumps (Variable Torque): CM3 + inverter-duty induction motor for simplicity and savings.
Conveyors (Constant Torque): CM5 + inverter-duty induction, with feedback for tight speed holding.
Mixers & Extruders (High Duty): CM5 + BlackMAX® PM for superior torque at low speed and maximum reliability.
Five Do’s and Don’ts for Motor + Drive Systems
Do specify inverter-duty insulation per NEMA MG 1 Part 31.
Don’t run legacy (standard) general-purpose motors on modern VFDs.
Do match the motor to the load type (variable vs. constant torque).
Don’t assume fans and conveyors can share the same setup.
Do mitigate shaft currents and provide proper cooling at low speeds.
Following these rules ensures that the system operates efficiently and avoids premature failures
Takeaway
Energy efficiency, reliability, and uptime don’t come from motors alone — they come from systems designed to work together. Drives are the brains, motors are the muscle, and cabling and commissioning are the nervous system tying it all together.
With the CM3 and CM5 drives, Marathon Electric gives you the tools to build systems that fit your load, your duty, and your goals. Whether you’re looking for straightforward energy savings on pumps and fans, or advanced performance in conveyors and process equipment, there’s a drive-motor pairing designed to deliver.
And with over 110 years of experience engineering motors in Wisconsin, Marathon brings unmatched perspective to how drives and motors work in the real world. Pair CM3 or CM5 with inverter-duty or BlackMAX® PM motors, and you’ll avoid the pitfalls — and capture the promise — of modern motor-drive systems.
Not sure which drive/motor pairing fits your application? Ask your local sales rep or distributor!