According to the International Energy Agency (IEA), approximately half of the electricity used globally is consumed in electric motors. Industrial motor systems account for about 70% of the manufacturing electricity used.
In the U.S., pump systems account for about 40% of all industrial motor systems electricity. The trend has been to design more efficient pump systems for the highest return on energy savings.
In many applications, flow and pressure are regulated using one of two ways. They can be controlled mechanically using throttle valves or by adjusting motor speed using variable speed drives. A throttling valve, which mechanically limits the flow in a system, is both mechanically and electrically inefficient.
The preferred way to adjust pump speed and flow, therefore, is typically a variable frequency drive (VFD). The VFD electrically changes the frequency to that of an AC motor. Motor speed is directly related to frequency, while motor current is directly proportional to the required torque. As the speed of centrifugal pumps is lowered, the required torque and motor current drops dramatically.
Understanding pump operation using variable frequency drives (VFDs)
In centrifugal pump operation, flow decreases in proportion to the drop in speed. A 50% reduction in speed, in other words, produces a 50% reduction in flow. The pressure or head produced, however, decreases by the square of the speed. A 50% reduction in speed results in a 25% reduction in pressure.
Because power consumption is related to work, the relative work required is greatly reduced when both the flow and pressure are reduced. The power reduction is the cube of the decreased speed. So when motor speed is reduced by 50%, the power required is (0.5)3, or 12.5% of the required motor torque and current, compared to full-speed operation. This is known as the Affinity Law.
Dramatic energy savings can be achieved by reducing pump speed. However, it remains critical to select the correct pump for the application based on needed flow and pressure. As always, the pump speed-torque curve must be evaluated. There are mechanical limitations as pump speed decreases. For example, volume may be reduced to a point where the pump is unable to meet system requirements.
Variable speed motors provide more flexibility for using a pump that meets current needs, with possible increases in the future. A VFD helps improve pump life by operating below designed pressure limits. It also provides the ability to ramp to speed, which reduces mechanical shock or water hammers, and to match power to performance.
While VFDs deserve credit for helping systems maintain peak efficiency, they also have the potential to damage the motors they drive — unless steps are taken to reduce their harmful effects.
Understanding the challenges of VFDs on motors
The challenges posed by VFDs are related to the pulse width modulation (PWM) power waveform they use to modulate the frequency and voltage supplied to a motor. That, in turn, controls motor speed.
In VFDs, a rectifier converts AC to a DC voltage, and transistors (switches) — known as the inverter section — create pulses to output a waveform that mimics a sinewave at the desired frequency. The PWM switching waveform can affect a motor’s winding insulation and bearing life.
More specifically, PWM can cause motor windings to experience voltage spikes well above their standard voltage limits. On a three-phase AC line voltage, for example, it takes about 8 milliseconds for standard sinewave power to transition from one peak to the next. PWM causes dramatic spikes in voltage, pulsing from minimum to maximum in less than 0.001 millisecond, or 1 microsecond (μs).
Because these spikes exceed the limits of standard insulation, they can result in rapid breakdown of motor insulation, compared to what might be expected if windings did not exceed the rated maximum voltage.
Wave reflection is another phenomenon that can affect peak voltages and their effect on motor windings. Wave reflection is a function of the time it takes a wave to rise and the length of the cable between the VFD and the motor. Reflective waves that bounce back to their source can become additive to incoming waves. The longer the cable, the greater the wave reflection. Reflected waveforms can build to a point of damaging the VFD and motor. Consideration of this phenomenon should be taken into account when the distance between VFD and the motor it controls exceeds the VFD manufacturer’s suggested distance. Technologies that allow for greater distances to occur in the field can be added to the system.
Motor windings and VFD damage are not the only issues affected by PWM. Premature bearing failure can also be attributed to fast-rising voltage pulses. The high switching frequency inverters used to achieve the required PWM waveform causes capacitive energy to build between the rotor and stator.
If this energy is released on a path through the motor bearings, it can cause electrical discharge machining (EDM), also known as fluting, which can cause the bearing to overheat and potentially fail prematurely.
The voltage differential between the rotor and stator that results from use of PWM waveforms on a standard inverter can also affect an electric motor’s bearings.
Three-phase motors driven by standard sinewave power have a balanced charge. The PWM waveform, however, is not a true sinewave. Rather, VFDs use pulsing DC voltage to create an imitation sinewave, which has the effect of building up a differential charge between the rotor and stator. Known as common mode voltage (CMV), the differential charge that builds up between the rotor and stator must be mitigated through shaft grounding and correct system installation to avoid damaging the bearing.
Protecting VFD-enabled motorsAware of these challenges, many motor manufacturers have created product lines specifically designed to operate with a VFD. These motors include specially designed winding and bearing protections that mitigate some of the effects a PWM waveform can have on a motor.
The shift to VFDs has led to the addition of inverter wire as a standard feature in integral horsepower random wound motors. Electric motor design and manufacturing techniques have also undergone changes to accommodate VFD applications, as well as to eliminate shaft currents.
To protect against voltage spikes that exceed their rated voltage, inverter duty motors are designed for use with VFDs by typically including inverter duty insulation systems rated to handle the voltage spikes that occur. Insulation design standards are regulated by the National Electrical Manufacturers Association (NEMA), which specifies the maximum peak voltage each type of insulation must withstand, as well as the minimum rise time for power waveforms.
According to NEMA MG1 Part 31, the windings on VFD-enabled motors with a 600 volts or less voltage rating must protect, at minimum, against a voltage spike of 3.1 times the rated voltage and a rise time greater than or equal to 0.1 μs. Motors with a voltage rating greater than 600 volts must protect, at minimum, against a voltage spike of 2.04 times the motor’s rated voltage with a rise time greater than or equal to 1 μs.
To prevent CMV, bearings on these motors require protection that provides a low-resistance path to ground to mitigate any charge buildup. This typically requires mounting a shaft grounding device — either externally or internally, per local codes for the motor.
For additional protection, 100-plus horsepower motors isolate one of the bearings from the motor shaft to interrupt circulating currents that would otherwise add extra energy to the motor shaft. This is typically accomplished by coating the bearing mount with an insulating material, thereby isolating the shaft from the bearing. If the motor will be located in a hazardous environment, the manufacturer may insulate both bearings, but more often, will insulate the coupling instead.
Getting to the root of the problem
While motor protections like these are helpful, they do not address the root causes of PWM waveform damage. The fact is, manufacturers often add these protections because they cannot predict where, how or when these components will be installed.
When reviewing the system as a whole, follow the manufacturer’s guidelines to provide correctly sized wire, filters and conduit. Refer to the manual to understand the proper grounding requirements of the motor and VFD. The following guidelines can help achieve a successful design and installation (always refer to the manufacturer’s manual for any additional information):
VFD installation: The power cables that connect the VFD to the motor should be shielded and rated for use with a VFD. The manufacturer should be able to recommend cables that are appropriate for the system’s voltage and current limits. Most distributors of VFDs offer a product that meets these requirements.
Grounding: VFDs produce high frequency noise. To reduce the damaging effects of that noise, motors must be grounded back to the drive. That requires a braided type of grounding wire that grounds the motor to the drive. At minimum, that wire should be of the same size as a single power lead and be located in the power lead conduit. To obtain the best mitigating effects, installers should consider using multiple grounding wires.
Conduit selection: Power leads and grounding wire should be housed in metal conduit, which should be connected to both the motor and drive without isolating either. PVC, plastic or other insulating materials are not recommended for connecting metal conduit to a drive or motor terminal box. If these materials are used, the metal conduit carrying the power leads and ground cable must be properly connected to the grounding circuit.
The VFD manufacturer may be able to suggest other filtering options to further reduce risk.
For existing systems, it is harder to get to the root causes of PWM. Still, its damaging effects can often be reduced with system add-ons, such as:
- Load reactors: These inductive devices slow quick rise times and eliminate high peak voltages.
- Inductive absorbers: Some damaging bearing currents can be reduced using these inductive devices.
- dV/dt filters: These filters limit the peak voltage to a motor.
- Sinusoidal filters: Similar to dV/dt filters, these filters include tuning electronics that match carrier frequency for best output waveform.
Capitalizing on the benefits of adding a VFD to a pump system is possible when the entire system is reviewed for the effects of PWM power before the project is started. To achieve the greatest protections, it is necessary to design and install these components as an integrated system. Understanding both the issues and operational requirements of a VFD system will lead to years of successful operation.
Patrick Hogg is the application engineering manager of General Industry and Integral Horsepower Pumping at Nidec Motor Corporation.