Engineers are often unaware of the currents induced on motor shafts by variable frequency drives (VFDs) and the havoc these currents can wreak on bearings and motors; remember to use shaft grounding effectively.
Don’t make assumptions
Don’t assume that “inverter duty” motors are designed to prevent bearing damage. Most of these motors only protect windings, not bearings.
Bearings need protection
Machines typically quit working due to a failure to protect the motor’s Achilles’ heel – the bearings. Inadequate shaft grounding increases the possibility of electrical bearing damage in VFD-driven motors; electrical discharges can scar the race wall. During each VFD cycle, currents discharge from the motor shaft to the frame via the bearings, leaving small pits in bearings and race walls. Damage eventually leads to noisy bearings, but by the time noise is noticeable, bearing failure is often imminent.
Diminish downtime with grounding rings
Downtime is often so costly that it can wipe out the energy savings obtained with a VFD. In some applications, a momentary production stoppage due to motor failure can cost more than $250,000. Microfiber ring grounding technology offers more sustainability by protecting bearings for the motor’s life; rings are maintenance free, unaffected by dirt or grease, and easily installed on any NEMA or IEC motor.
Pay attention to drive alerts
Drive failures give warning signs. The best defense is to monitor alarms and faults for abnormalities. Ensure that the drive and its accessories are operating within specified environmental limits to avoid failures. If limits must be surpassed, check with the manufacturer for proper over-sizing information.
Know your system
If the drive input will be connected to a source that has a large short-circuit current, it may be necessary to limit that current via a reactor or isolation transformer to avoid drive damage.
Consider a filter
Drives generate high-frequency line disturbances; so if electrical noise is a concern, use an input RFI filter. If the run to the motor is long – over 250 ft – it’s also a good idea to include an output reactor to avoid motor dV/dT damage.
Choose cable carefully
Electrical phenomena in a VFD system can affect the drive and motor, as well as the cable that connects them. Be sure to choose VFD cable specifically engineered for this application.
Know your cable specs
A stress control layer or “conductor shield” is critical in a VFD cable because these cables can experience high electric fields and partial discharges. VFD cables should be rated 1,000 V continuous/2,000 V peak and include a stress control layer.
Electrical noise demands a properly grounded, double-shielded cable
To mitigate EMI and RFI effects, unshielded cable is often run in grounded metal conduits or, in less expensive methods, shielded cables are run in PVC conduits or metal trays. Due to significant noise in VFD systems, double-shielded VFD cable should be used. Look for a 100% foil shield along with a braid shield.
Whether it’s stock VFDs in conveyors, fans, and cooling towers, or specialized units in presses, extruders, roll-forming machines, lathes, and routers, their proper use follows specific guidelines. Often, these requirements are outlined in drive manuals – but here we review when critical warnings and precautions are applicable, and why.
A VFD reports a low-volts fault when the drive’s dc link voltage drops below 62% of the nominal level for the high setting (480 Vac) and 50% of nominal for the low setting (400 Vac).
The +10% and -15% voltage tolerance in most manuals is the operating range recommended to allow the drive at hand to maintain premium efficiency and proper motor current. Drives can run below these tolerances, but reduced voltage can have unpredictable effects on motor current, motor temperature, and overall performance.
More specifically, if a drive’s line volts parameter is set to high and 480 Vac is being applied, the drive will generate a low-volts fault when the dc link voltage drops to 62% of nominal – as 480 Vac·0.62·√2 = 421 Vdc. The nominal dc link voltage is 480 Vac · √2 = 679 Vdc.
Precautions when switching
Although switching a VFD from line power to a backup generator is an accepted practice in many applications, there are some limitations. Most importantly, many portable backup generators have a larger voltage swing on their phase-to-phase voltage input than a drive’s recommended phase-to-phase voltage tolerances, which are generally less than 2%. To protect against voltage swings, VFDs are equipped with surge guards in the input rectifier circuit – but external protection (such as surge arrestors) may be required for severe input disturbances. Larger variances induce greater ripple on the dc bus capacitors, and this causes damage to both the capacitors and other power components over time.
Another potential problem arises when switching from line power to a standby generator. Most drives need a minimum of two minutes before power can be reapplied; disregarding this guideline can damage the charge relay circuit, or at the very least, blow the input fuses or trip the circuit breaker.
For cases in which input power exhibits moderate spikes in voltage that cause current surges, a 3% line reactor may improve the situation.
Another solution is to add a voltage monitor with a suitable time delay. These have a protection trip level that shuts the drive down in cases of under-voltage, over-voltage, loss of phase, and voltage imbalance between phases. These devices ensure that the VFD is never exposed to unbalanced input power or powered up until voltage is within tolerance and proper delay times are met.
A final suggestion – the most expensive solution – is to use an isolation transformer. This ensures complete isolation of grounding and noise-related input power problems that can affect the drive. Will an isolation transformer provide better protection than a line reactor? Yes. When does an application require an isolation transformer? Incorporating such a transformer is recommended when an installation is in close proximity to a substation.
Interposing a drive isolation transformer between the VFD and its power source offers several benefits. Isolation ensures that no direct electrical connection exists between source and load – but that’s true for any transformer, other than an auto-transformer. What makes the drive isolation transformer unique is the placement of grounded electrostatic (Faraday) shielding between and around primary and secondary windings. This shielding provides up to a million-fold decrease in the capacitive coupling involved in transferring common-mode voltage disturbance. Without such shielding, that capacitance allows passage of high-frequency noise and transient voltage spikes through the transformer.
Common-mode transients are those appearing between ground and neutral of the ac system. Although those two parts of the circuit are normally bonded together at one point, they cannot be presumed to be at the same potential throughout an entire power system. Common-mode transient disturbances arise from switch-mode power supplies, drive operation, arc welders, lightning, or even from normal operation of such equipment as stepper motors. Some isolation transformers can also block “normal-mode” transients appearing between line and neutral.
Consider one application in which a daily utility-company power-up of a substation capacitor bank in an industrial park causes a transient voltage spike – amplified by reflection from onsite capacitors in a nearby plant. Assume that one facility in the park has several small drives rated at 7.5 hp. Normal-mode transients can cause these drives to shut themselves off, resulting in costly process downtime. An isolation transformer can prevent such disruption.
When it’s been sitting on a shelf
A VFD can sit unused and without power for a short time without service, but if a VFD has been stored for one or more years, it must be reformed – to recondition the dc bus capacitors for service. Here, the designer must run the drive with no motor leads connected for at least eight hours before trying to run the drive under load. Why? The electrolyte inside the bus capacitors changes state when not used for a long period of time; re-powering the drive under no load brings the electrolytic charge back to its proper charged state.
Practical installation tips
Following are some dos and don’ts when installing VFDs.
Do add a line reactor when line power source is more than 10 times the kVA rating of the drive.
3% impedance line reactors should be used to reduce power line transient voltages caused by capacitor switching, line notching, dc bus over-voltage tripping and inverter over-current and over-voltage conditions. Line reactors improve the true input power factor and reduce cross-talk between drives. The input line reactor offers some protection to the drive in short-circuit conditions. If the supply transformer kVA rating is greater than 10 times the drive kVA rating, then a line reactor is recommended to minimize damage to the drive, in case the supply transformer shorts out. This line impedance depends on the drive’s short-circuit rating, and on the supply power distribution transformer. Specifically, the line impedance must be greater than or equal to the ratio of the supply source transformer’s rating to the drive’s short circuit rating.
Do use separate conduit for input power, output power, and control wiring. More specifically, when connecting the VFD’s power and control wiring, the following guidelines should be followed:
Install the input ac power wiring in its own rigid steel conduit.
Install the output motor wiring in its own rigid steel conduit.
Install the control wiring in its own rigid steel conduit. Low-voltage dc control wiring and 120 Vac control wiring should be in separate conduits. Both twisted pair and shielded wire are sufficient when wiring to the VFD’s control board. Two and three-wire connections are recommended. For many drives, the minimum wire size is 18 AWG.
Ensure that all ground connections are tight and properly grounded. The shield should be connected to ground at only one end of the cable to avoid ground loops. When connecting the shield at the VFD end, connect it to the chassis ground lug. Caution: Make sure to remove power from the VFD prior to connecting the shield to the VFD’s ground lug.
Separate control and feedback wiring from power wiring by at least 12 inches.Caveat: In installations with multiple VFDs, input power wiring for all VFDs can be in the same conduit, and the control wiring can be in the same conduit, but the output wiring for each motor must be in a separate conduit. The only exception is that if one VFD is used to operate multiple motors, the output wiring for all of the motors can be in the same conduit.
Use the drive on a grounded system. Never use a floating ground. Some manufacturers do not recommend operating with a floating input on any sub-micro or newer designed drives. If there are no disturbances on the line, the drive should run fine – but serious common-mode noise could cause nuisance tripping or worse.
Note: Certain legacy VFDs use a string of resistors between the dc bus and ground, which means common-mode noise isn’t an issue. Some integral-hp drives also use a resistor string, so using the floating ground on these is probably okay. However, a floating-point system is not recommended for newer drive technology.
Approaches that spell trouble
Do not use time-delay input fuses. If fuses are time-delay, the designer will have problems, because these are not made for protecting solid-state rectifier front-end equipment like VFDs. Time-delayed breakers allow the MOV (metal oxide varistor) to continue drawing current – to the point of causing the drive to burn up or the MOV itself to blow before the breaker ever trips.Either branch circuit protection via a circuit breaker or a disconnect switch and fuses must be provided to comply with the National Electrical Code (NEC) and all local codes. Consult Article 430, Section H, of the NEC handbook for more information. Select a circuit breaker or fuse rated at 1.5 times the input current rating for constant torque drives, and 1.25 times the input current rating for variable torque drives. The minimum rating should be 10 A, regardless of the input current rating – because a 10 A minimum accommodates in-rush during power-up. The VFD provides motor protection.Bussmann fast-acting current-limiting type fuses with low I2t values and 200,000 AIC rating (or equivalent) are recommended. Fuse types include:240/200 Vac models: KTK-R or JJN type, rated 250 Vac480/400 Vac models: KTK-R or JJS type, rated 600 Vac
590/480 Vac models: KTK-R or JJS type, rated 600 Vac
Do not add a contactor between the drive and motor: A contactor or disconnect switch between the drive and motor is definitely not recommended. Operating a motor contactor or disconnect between the VFD and the ac motor while the VFD is running can cause nuisance tripping. Such devices should only be operated when the VFD is in a stop mode. There is also the possibility of noise from the output feeding back into the control board through the low voltage power supply – and damaging the control or driver board.
If the contactor is absolutely necessary, an early-break auxiliary set of contacts on the device should be interlocked with the VFD’s external fault input or stop input. This way, if the device is opened while the VFD is running, it will stop the drive and immediately cut off VFD output power. In addition, use a minimum time-delay of 100 ms. Remember that if wired to the VFD’s stop input, the stop method must be set to coast. Finally, allow the drive to completely stop the motor before restarting.
Do not cycle the input power more than once every two minutes. In fact, drive manuals specifically warn that switching a drive off and on without waiting two to three minutes is detrimental: Applying input power more quickly causes a buildup of voltage in the input pre-charge circuit, and eventually burns it out. Why? Here, the dc bus capacitors don’t have enough time to discharge, and the input circuit needs time to stabilize. Otherwise, additional input can damage the charge relay circuit, or at the very least, blow the input fuses or circuit breaker.In other words, the pre-charge circuit allows a certain time limit for the inrush limiter to send current through to charge the dc bus capacitors. The inrush limiter resistance changes with temperature. The hotter the limiter gets, the lower the resistance value. When that pre-charge time ends, the relay cuts off and the capacitors hold the charge. When the drive is powered down, this voltage bleeds off through resistors in the discharge circuit. Power reapplied too quickly meets an inrush limiter that hasn’t had time to cool down to an acceptable resistance level, so the current will be higher, and consequently, could blow the fuses or possibly damage the pre-charge circuit.One solution here is to install an external time-delay relay or voltage monitor circuit at the drive input. Then, when the voltage drops below a set level, the voltage monitor cuts off and allows zero power back into the drive until two to three minutes have passed -ensuring that voltage has stabilized to an acceptable level.
Do not use a ground-fault circuit interrupter (GFCI) if the drive is equipped with a filter. Installation of these devices can cause nuisance tripping – from parasitic capacitance producing leakage currents between the motor power cable lines during VFD operation, connecting multiple drives to the same input source, and using RFI filters on the input side.
Motors account for at least half of the energy consumed in the U.S. Selecting the right control method for an application lets the motor run most efficiently while maximizing torque and overall performance. Efficiently run motors also use less energy and experience less downtime for greater overall savings.
For motors controlled by a variable frequency drive (VFD), the control method used in large part determines a motor’s efficiency and performance in an application. Once engineers and designers understand the advantages, disadvantages, and particular specifications for each control method, choosing the right one for any application becomes simple.
Many people in the industry think control methods are the sequencing methods that control VFDs, as in 2- and 3-wire setups. Such 2- and 3-wire setups determine whether a VFD’s input-control terminals interface with maintained contacts or momentary push buttons to start and stop the drive. The control methods this article focuses on are perhaps more accurately called motor-control methods. They determine how VFDs
There are four primary types of motor control methods for induction motors connected to VFDs: V/f (volts-per-hertz), V/f with encoder, open-loop vector, and closed-loop vector. These methods all use pulse-width modulation (PWM), a technique that varies the width of a fixed signal by modulating pulse durations to create a variable analog signal.
PWM is applied to VFDs by using the fixed DC voltage from the VFD’s DC bus capacitors. A set of insulated gate bipolar transistors (IGBTs) on the output side rapidly open and close to generate pulses. Varying the output pulses’ width in the output-voltage waveform can build a simulated AC sine wave. Even though the drive’s output-voltage waveform consists of square waves due to DC pulsing, the current waveform will be sinusoidal because the motor is inductive. All motor-control methods rely on a PWM voltage waveform to control the motor. The difference between control methods lies in how they calculate the motor’s voltage needs at any given moment.
AC motors are commonly controlled using pulse width modulation. In that process, the carrier frequency (shown in red) is the rate at which the VFD’s output transistors are gated or tuned on. The carrier frequency can usually be from 2 to 15 kHz. The frequency reference (in blue) is the speed signal being sent to the motor, usually from 0 to 60 Hz. When overlaying the two wave forms, engineers can use the intersection points between these two curves to modulate the output DC pulses (in black) to provide the desired speed control.
Volts-per-hertz, commonly called V/f, is the simplest motor control method. It is often used due to its “plug-n-play” simplicity and how little motor data the drive needs. It does not require an encoder and tuning the VFD to the motor is not required (but recommended). This means lower costs and less wiring. V/f control is often used when there is a demand for operation, which could exceed 1,000 Hz, so it is often employed in machine tool and spindle applications.
Different V/f patterns let VFDs control several different applications while maintaining optimal performance for each. The constant torque pattern is a straight line, which results in a constant V/f ratio that provides constant motor torque throughout the speed range. The variable-torque pattern has lower voltages at lower speeds to prevent motor saturation.
V/f is the only control method that lets several motors run from a single VFD. In such cases, all motors start and stop at the same time, and follow the same speed reference.
V/f has some limitations. For example, with V/f, there is no guarantee the motor-shaft is rotating. Additionally, the motor’s starting torque is limited to 150% of its output at 3 Hz. The limited starting torque is more than enough for most variable torque applications. In fact, just about every variable torque fan and pump app in the field uses V/f control.
The V/f method’s relative simplicity is partly due to its “looser” specifications. Speed regulation is typically 2% to 3 % of maximum frequency. Speed response is rated at 3 Hz. Speed response is defined as how well the VFD responds to a change in reference frequency. An increase in speed response results in quicker motor responses when the reference frequency changes.
Control methods also have speed-control ranges (expressed as ratios). V/f’s speed control range is 1:40. Multiplying this ratio by the maximum frequency determines the VFD’s minimum running speed at which it can control the motor. For example, with a 60-Hz maximum frequency and 1:40 speed control range, a drive using V/f control can control a motor down to 1.5 Hz.
A V/f pattern defines a ratio of voltage-to-frequency for the motor to follow and a VFD can have only one V/f pattern programmed at a time. The V/f patter, or curve, determines what voltage is output to the motor based on a given speed reference (frequency).
Operators or technicians can select preset V/f patterns in the VFD’s programming with a single parameter. Preset patterns are optimized for specific applications. Users can go one step farther, programming a custom V/f pattern or profile to tune the VFD to a specific application and motor being used.
Applications such as fans and pumps are variable torque loads. A variable-torque V/f pattern prevents faults and increases performance and efficiency. This pattern reduces the magnetized current at low frequencies by lowering the motor voltage at lower frequencies.
Similarly, constant-torque applications, such as conveyors, extruders, and hoists, should use constant-torque V/f patterns. Constant-torque applications need full magnetizing current at all speeds. So a straight slope is constructed and followed throughout the entire speed range. In general, the VFD will output a voltage based on whatever speed the motor is set to while referencing the V/f pattern.
V/f with encoder
If an application needs more precise speed regulation, along with the ability to run at a higher reference frequency, an encoder can be added to V/f control. The encoder feedback tightens speed regulation down to 0.03% of the maximum frequency. Output voltage is still determined by the selected V/f pattern programmed into the VFD by a technician. This allows for high-speed control without high dynamic responses because voltage and frequency are predetermined.
This control method is not common because it entails the costs of an encoder and feedback card, and its advantages over standard V/f control are minimal. Starting torque, speed response, and speed control range are all identical to the V/f control. In addition, higher operating frequencies are limited by the how many pulse-per-revolutions the encoder generates.
Open-loop vector control
Torque limits are broken down into four quadrants depending on motor direction (forward or reverse) and whether the motor is motoring or regenerating. For example, a bottle capper would require torque limits set up for Quadrant 1. Alternatively, an unwinding application would need forward motor rotation to feed the line but a negative torque limit due to regeneration caused by the line being pulled to create tension. So the torque limit would be set in Quadrant 4.
Open-loop vector (OLV) control is used for greater and more dynamic motor control. It independently controls motor speed and torque, much like DC motors are controlled.
When running OLV, motors can produce 200% of their rated torque at 0.3 Hz. The higher starting torque at lower speeds opens the door for a variety of applications. This control method also allows for four-quadrant torque limits.
Torque limits primarily restrict motor torque to prevent damage to equipment, machinery, or products. They are broken into four different quadrants, depending on motor direction (forward or reverse), and whether the motor is motoring or regenerating. The limits can be set independently for each quadrant, or users can program an overall torque limit into the VFD.
A motoring condition is when the motor’s speed and torque are both in the same direction. For example, forward speed and forward torque would motor a conveyor in the forward direction. Regeneration is when the motor is being overhauled by the load. On an AC motor, when the rotor rotates faster than the magnetic field in the stator, it acts as a generator. This causes regenerated energy to flow back into the VFD.
For example, a bottle capper could use a torque limit in Quadrant 1 (forward rotation and positive toque) to prevent over-torqueing the bottle caps. It moves forward and uses positive torque to put the cap on the bottles. An application involving an elevator with a counterweight heavier than the empty car would have limits in Quadrant 2 (reverse motor rotation and positive torque). If an empty car is called to a higher floor, the torque opposes the direction of the speed to maintain control over the counterweight and the elevator’s speed and position as it moves against gravity.
A machine drill backing a screw out of a block of wood (reverse motor direction and negative torque) could use limits in Quadrant 3. And an unwinding application could use Quadrant 4 limits (forward motor rotation and negative torque). The motor would spin forward motor rotation to feed the line, but it would also need a negative torque limit due to regeneration caused by the line being pulled to create tension.
The current feedback loop in these VFDs lets users set torque limits and run in all four quadrants. As motor current increases, so does motor torque. Output voltage going to the motor can be increased if the application needs more torque or decreased when reaching a torque limit. This makes open-loop control dynamic, unlike V/f control.
In addition to torque limits, open-loop control has a quicker speed response of 10 Hz, letting it handle more dynamic responses to impact loads. For example, a rock crusher’s load constantly changes, depending on the size and quantity of rock being processed.
Instead of a fixed V/f pattern, OLV control uses a vector algorithm to find the best output voltage to run the motor. Vector control accomplishes this by using current feedback from the motor. Current feedback is measured via current transformers inside of the VFD. Constant current readings and rapid calculations performed in the VFD determine present torque demand and flux. Basic vector math breaks down the motor’s magnetizing current and torque-producing current into vectors. OLV control depends heavily on the motor dynamics, so some type of motor auto-tuning must be performed to ensure the VFD has as much motor data as possible.
With the help of reliable motor data/parameters, the VFD can calculate the magnetizing current (Id) and the torque-producing current (Iq) as vectors. For maximum efficiency and torque, the VFD must keep these two vectors separated by 90°. That 90°is significant because sin (90) = 1, and the value 1 represents maximum motor torque.
Overall OLV control results in tighter control. Speed regulation is +/- 0.2% of maximum frequency, and the speed-control range jumps to 1:200, allowing for low-speed operation without sacrificing torque.
Closed-loop vector control
Closed-loop vector control uses a vector algorithm to determine output voltage, much like the open-loop control. The key difference is that closed-loop vector uses an encoder. Encoder feedback, paired with the vector control, means 200% of the motor’s rated torque is available at 0 rpm. This is a selling point for apps required to hold a load without moving, such as elevators, cranes, and hoists.
Vector control maximizes torque-per-amp by keeping torque-producing current (Iq) and magnetizing current (Id) at 90°. I1 represents total motor current (Iq + Id). If Ө > 90°, then sin Ө > 1; if Ө 1; but if Ө = 90°, then sin Ө = 1 and torque is at its maximum. VFDs try to keep Ө at 90°to mimic a DC motor. In a DC motor, the brushes are mechanically positioned 90°from the commutator to constantly produce maximum torque.
Encoder feedback allows for speed responses over 50 Hz and speed control ranges of 1:1500, the highest of all the control methods. Closed-loop control can also run a motor in torque-control mode. Torque control lets the VFD control motor torque rather than motor speed. This is needed in any application where torque is more important than speed. Winders, rewinders, capping, and web applications are good examples of where torque control is used.