Tag Archives: vfd

Control Panel Layout: Top Tips with some Photos

Control panel, Systems, , ,

Amongst the control panel layout tips out there, some are practical, many are good and some are downright weird. There are is a wealth of it in the /PLC sub-reddit. Some of it is amusingly opiniated:

 

The following is my collection of top tips on control panel layouts. A few of the panel posts from Reddit are embedded below. Will add more pointers as I come across them.

 

Busbar candy
byu/Otherwise_Feed_3320 inPLC

1. Heat rises

Do the heat calculations. Enclosure vendors usually have free tools for this like this .

    • If ventilation is needed, fan at the bottom, exhaust at the top, not the other way around. Hot air rises and leaves the panel, cool air comes in the bottom.

2. Power protection components at the top. Circuit breakers, disconnects. The temperature rating on circuit breakers are usually higher than the average PLC, drive or anything that has electronics for that matter. Example here – 30A breaker from SE has an operational ambient of 158 degF/70 degC. Accessibility and safety is also better with power devices at the top. 

3. Incoming power. This really depends on the install site/location. If you have a choice, some would argue that’s it’s better for incoming power to come in from the bottom. With gravity, holes and inlets at the top of the panel have poor contingencies in the event of condensation or dirt coming in( or even water ingress- say NEMA 4/4X failure situations) . 

4. Wire labels, terminals, and wire markers

    • Avoid putting the label on the device. If the device gets replaced, the label goes with it.
    • Sometimes end users may require label on device also. Check before it gets to the FAT
    • Harmonize labelling such that it can be traced back to schematics. This will help with maintenance folks and any troubleshooting efforts.

5. Wireway

    • Vertical runs should intersect with a horizontal run such that the horizontal run stops the vertical cover from sliding down
    • Plan it out such that control wiring is separated from power wiring. If they intersect, make it perpendicular.
    • Read on to number 6.

6. Electromagnetic interference

      • Separate 480Vac and  24Vdc ( control and communications) wires. 
      • If they have to cross, it’s best done perpendicularly- ie. they cross at a 90 deg angle. Still avoid having them in proximity. Good explanation of this here
      • Additional sleeving or barriers for EMI mitigation if needed.

 

7. Spacing If the project allows for it, allow for some room between devices, PLC’s, drives, power supplies. This helps with maintenance accessibility. Also, it makes way for future expansions. More I/O if the PLC needs it, another drive …etc..

My new office 😉
byu/adi_dev inPLC

8. Ground connections

    • Spec grounding washers installed and properly torqued to bite through the paint

9. Network cabling

  • Use pre-terminated cables where possible. From a good vendor, reliability is better. 
  • From item 6 above, separate controls communications cables from the power wiring.

10. Maintenance and usability

  • Add a rack on door for reference material

 

“I´m tired boss…”
byu/andisosh inPLC

Will come back and add more as I find it…

 

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VFD’s and Long Motor Leads

VFD, ,

Recently I came across an application where multiple motors were failing at a single site. This facility utilized VFDs to control several rooftop AC induction motors. The repeated motor failures raised concerns about the possible causes, prompting a deeper investigation into the relationship between long motor leads, motor failures, and VFDs.

Long Motor Leads + VFD’s

The use of long motor leads can cause issues in VFD-driven motors. Voltage reflections and voltage magnification can occur, leading to voltage spikes that can exceed the motor winding insulation capabilities. These spikes, if left unaddressed, may result in premature motor failure due to insulation degradation.

VFD Strategies for Long Motor Leads

 

  1. Adjusting Carrier Frequency: Reducing the carrier frequency of the VFD can help minimize the impact of voltage reflections and the risk of motor failure due to long motor leads. However, this may result in increased audible noise and reduced motor efficiency, so finding an optimal balance is crucial.
  2. Motor Chokes: Installing motor chokes, also known as output reactors can help mitigate voltage reflection and reduce the risk of insulation breakdown.
  3. Motor Insulation Ratings: Using motors with higher insulation ratings, such as those designed specifically for use with VFDs, can help protect against voltage spikes and reduce the risk of motor failure.
  4. Proper Grounding: Ensuring proper grounding practices can help minimize bearing currents and their associated motor failures. While this is not directly related to motor lead length, longer lead lengths can contribute to conditions that cause bearing currents. Some examples of ‘fluting’ and bearing issues here: https://empoweringpumps.com/est-aegis-protect-motors-from-variable-frequency-drive-induced-bearing-damage/
  5. Cable Shielding: Using shielded motor cables can help reduce electromagnetic interference and protect the motor from high-frequency voltage pulses.

For reference, the formula for the reflection coefficient is below. The greater the mismatch, the greater the voltage amplification factor.

                                   R = (Z_load – Z_source) / (Z_load + Z_source)

                                   where:

                                  R represents the reflection coefficient

                                  Z_load is the impedance of the load (in this case, the motor)

                                  Z_source is the impedance of the source (in this case, the VFD)

The formula for the voltage amplification factor:

                                V_max = 1 + |R|

      where:

                            V_max represents the maximum voltage on the line

                           |R| is the absolute value of the reflection coefficient

Another important note is in situations where there are multiple motors driven by a single VFD, the length of cable to each VFD is summed up as the total motor cable length calculation.

Motor Issues Not Caused by the VFD

There are other motor failure scenarios that get misattributed to VFD’s. These may continue occurring even with mitigation techniques above. As such, it’s important to rule these out beforehand. Some common examples:

  1. Environmental Factors: Exposure to harsh environmental conditions, such as extreme temperatures, humidity, or contaminants, can cause motor failures. These issues may be mistakenly linked to VFDs when they are actually the result of inadequate motor protection or improper maintenance practices.
  2. Mechanical Failures: Mechanical issues, such as misalignment, imbalance, or excessive vibration, can lead to motor failures. These problems may be incorrectly attributed to VFDs when they are actually caused by issues within the mechanical system, such as worn-out bearings, loose components, or improper installation.
  3. Overloading: Overloading a motor, either by exceeding its rated capacity or by running it for extended periods at high loads, can result in overheating and failure. This type of failure might be incorrectly attributed to VFDs when it is actually due to improper motor sizing or incorrect application.
  4. Electrical Issues: Motor failures caused by electrical issues, such as short circuits, phase imbalances, or power quality problems, can also be mistaken for VFD-related failures. These issues may stem from faulty wiring, improper grounding, or voltage fluctuations in the electrical supply.
  5. Inadequate Lubrication: Insufficient or improper lubrication can lead to bearing wear and premature motor failure. This type of failure may be misattributed to VFD-related bearing currents when it is actually caused by poor maintenance practices or the use of inappropriate lubricants.
  6. Design or Manufacturing Defects: Motors with inherent design or manufacturing defects may experience premature failure. These failures can be incorrectly linked to VFDs when they are actually the result of flaws in the motor’s construction or assembly.

 

Conclusion:

While long motor leads can pose challenges for VFD-driven systems, by understanding the risks, adjusting VFD setup and settings, and taking appropriate preventative measures, it is possible to mitigate the risk of motor failures. Additionally, recognizing motor failures that are commonly misattributed to VFDs can help ensure accurate diagnosis and effective solutions. By incorporating proper grounding, choosing the right motor insulation ratings, and utilizing motor chokes and cable shielding, you can enhance the performance of your VFD-controlled moto

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VFD Input Rectifier

VFD

 

The VFD input rectifier is the first of the three main stages on a VFD. The other two being the DC bus and the inverter. The rectifier stage converts AC line voltage supplied to the drive to DC.

The rectifier is usually a silicon controller rectifier ( SCR) or a diode. The difference between the two being that the SCR types can increase switching gradually thus increasing the voltage applied to charge the DC bus (second stage of the three stages of an AC drive). The diode types rely on a pre-charge circuit to perform the gradual voltage ramp up of the DC bus capacitors.

For drive rectifier stages, most commonly, there is a 6 diode or  6 SCR arrangement that makes up what is called a 6-pulse rectifier. A good illustration of this process from a TranspowerNZ video on YouTube

 

 

There are 12 pulse and 18 pulse diode arrangements that can be used to make up the rectifier stage of a drive. The main purpose of the 12 diode and 18 diode arrangements are to achieve lower harmonic distortion on the line side. The 12 pulse would be made up of 2 sets of 6 pulse rectifiers supplying the DC bus in parallel and the 18 pulse with 3 sets of 6 pulse rectifiers. Understandably, the higher order arrangements take up more space and cost more.

 

Considering that the main purpose of these arrangements is to reduce harmonic distortion, there are other options besides 12 or 18 –pulse rectifiers that can be considered for the same or better results. These options are to include a filter on the line side of the drive or by adding an active front end (AFE). MTE (maker of filter and power quality equipment) has a good paper comparing 18-pulse VFD’s with a 6-pulse VFD and their Matrix filter. The essence of it is that it costs less, takes up less space and consumes less energy (specifically for the 100hp test case).

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Choosing between variable frequency drives (VFD) and soft starts

VFD,

 

Soft starts usually vary voltage with SCR'sSoft starts usually vary voltage with SCR’s

A key difference between drives and soft starts:

Drives can continuously vary the frequency and voltage supplied to the motor. Soft starts vary only the voltage supplied to the motor, and usually only when ramping a motor up and down.

What does this mean?

Soft starts may vary the speed of the motor during startup and ramp down but this is done by reducing the motor voltage. On this note, the soft start is also called a reduced voltage starter. Drives do the same but also have the option to control motor speed by varying the voltage frequency instead of the voltage.  Motor speed is directly related to its supply voltage frequency.

 

Inherently, a drive or a soft start reduces the inrush current that every motor is subject to when starting across the line. From this perspective, either a drive or a soft start will probably prolong the life of a motor -specifically if compared to an across the line starter. This is a general statement and like most general statements, there are some conditions.  Specifically for drives, this general statement usually applies to inverter duty rated motors which can withstand the continuous high frequency switching (PWM) of the drive. Otherwise, there is a risk in applying a drive to a motor. It might heat up the winding insulation and ultimately break it down.

 

The soft start topology is usually a single stage SCR based switching scheme with a bypass. The bypass takes over  for operation at full speed (diagram above). The SCR’s fire for gradually longer parts of the AC voltage cycle until the entire AC wave is passed through to the motor. At this point, operation is handed over to a bypass.

 

VFD's vary voltage and frequency with the 3 stage design.VFD’s vary voltage and frequency with the 3 stage design.

 

The drive topology has 3 stages (diagram above), with a rectifier taking in line supply and then a DC bus capacitor that stores and buffers the DC energy within the . The final stage is the inverter which is usually made up of IGBT’s at the motor supply side of the drive. The IGBT’s can continuously operate at varied gating frequencies to produce a variable frequency supply to the motor.

When choosing between drives and soft starts, some key differences to consider are:

Key differentiators

VFD Soft Start Meaning
Speed Variability Continuously variable speed throughout operation Initial ramp up of voltage/speed. Subsequently pegged to line frequency. Drives can save energy if the load does not need to run at full speed. Soft starts do not save energy in full speed operation. Then again, some applications are designed to operate in full speed operation. These loads will not benefit from a drive from this perspective.
Control features More control features: Features that take effect during operation at regulated speeds. Less control features as speed is not regulated besides during startup and ramp down.
Application Torque Constant torque supported- i.e. high torque at low speed.

Examples: Screw compressors, conveyors.

 Variable torque applications- lower torque at low speed.

Examples: Centrifugal pumps, fans.

 Some loads require a high amount of torque when starting. A VFD is applied on these applications
Main features Reduce inrush and continuously vary frequency and motor speed. Energy savings during operation. Reduce initial inrush. Energy savings during ramps, no energy savings after ramp up and ramp down.

 

 

 

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Auto tuning VFD’s

VFD, ,

 

What is auto-tuning to a VFD?

Auto tuning a VFD is a process by which a drive measures the impedance of a motor for the purpose adjusting the motor control algorithm. The measured value may be matched to known impedance for a given motor size and used in determining voltage and current relationships at different speeds. Ultimately, this allows for more effective  driving of a motor load as well as better speed regulation specifically when running without feedback ( open loop).

When not to auto-tune?

    1. Auto-tunes are generally to be performed when the motor is cold. Auto-tuning with a hot motor may result in a variance in impedance which will subsequently cause the execution of a motor control algorithm which does not accurately match the true motor impedance.
    2. When multiple motors are connected, an auto-tune will result in the reading of multiple motor impedances connected in parallel. Some auto-tune functions match impedance readings to known typical motor impedance values ( for instance a typical NEMA B motor). As such, the reading of multiple motor impedances in parallel can not be matched to a known motor impedance value or may match a different type of motor. This results in an unsuccessful auto-tune which may be signified by a higher than usual noise levels.
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Characteristics of VFD regeneration situations

VFD,

 

The motor operation characteristics during VFD regeneration, also referred to as regen are:

1. The motor flux fields as controlled by the drive are spinning in the same direction as the load that is driving it. If the shaft is being driven by the load but the inverter is not gating, no regen is captured as the stator circuit would be open.

2. Slip is negative. Note, slip is defined as:

2015-01-15 22_08_43-Slip

Synchronous speed => speed of rotation of the stator induced flux field as drive by the VFD

Motor speed => Speed at which the load is driving the rotor

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What is a Variable Frequency Drive or a Variable Speed Drive?

VFD, ,

VSD’s or VFD’s , also referred to as frequency converters or adjustable speed drives, are devices that convert fixed frequency supply voltage ( typically 50Hz or 60Hz) to a variable frequency voltage. The frequency of voltage supplied to a motor determines the speed at which that motor rotates.

VSI_Topology

By Cblambert (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

What are the benefits of a drive (VFD) over a motor starter?

    • Drives protect motors from the in rush of current when starting.
    • In applications where full speed operation is not required, drive saves energy by facilitating operation at lower speeds.
    • Drives allow for speed regulation to maintain the set point of a process ( could be a pump motor speed for pressure and flow or fan speed for temperature)
    • In applications where a high torque is required at a low speed, drives are able regulate both speed and torque at its output to allow for continuous operation a low speed. An example could be a hoist where the load is suspended ( at zero speed)in the air without the engagement of brakes.
    • Drives are able to provide current and torque limiting functionality so as to prevent motor and other equipment damage.
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