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The noise of variable frequency drive fed motors

The rotating electrical machines have basically three noise sources:

  • The ventilation system
  • The rolling bearings
  • Electromagnetic excitation

Bearings in perfect conditions produce practically despicable noise, in comparison with other sources of the noise emitted by the motor.

In motors fed by sinusoidal supply, especially those with reduced pole numbers (higher speeds), the main source of noise is the ventilation system. On the other hand, in motors of higher polarities and lower operation speeds often stands out the electromagnetic noise.

However, in variable frequency drive (VFD) systems, especially at low operating speeds when ventilation is reduced, the electromagnetically excited noise can be the main source of noise whatever the motor polarity, owing to the harmonic content of the voltage.
Higher switching frequencies tend to reduce the magnetically excited noise of the motor.

Criteria regarding the noise emitted by motors on variable frequency drive applications
Results of laboratory tests (4 point measurements accomplished in semi-anechoic acoustic chamber with the variable frequency drive out of the room) realized with several motors and variable frequency drives using different switching frequencies have shown that the three phase induction motors, when fed by VFDs and operating at base speed (typically 50 or 60 Hz), present and increment on the sound pressure level of 11 dB(A) at most.

Considerations about the noise of variable frequency drive fed motors

  • NEMA MG1 Part 30 – the sound level is dependent upon the construction of the motor, the number of poles, the pulse pattern and pulse frequency, and the fundamental frequency and resulting speed of the motor. The response frequencies of the driven equipment should also be considered. Sound levels produced thus will be higher than published values when operated above rated speed. At certain frequencies mechanical resonance or magnetic noise may cause a significant increase in sound levels, while a change in frequency and/or voltage may reduce the sound level. Experience has shown that (…) an increase of up to 5 to 15 dB(A) can occur at rated frequency in the case when motors are used with PWM controls. For other frequencies the noise levels may be higher.
  • IEC 60034-17 – due to harmonics the excitation mechanism for magnetic noise becomes more complex than for operation on a sinusoidal supply. (…) In particular, resonance may occur at some points in the speed range. (…) According to experience the increase at constant flux is likely to be in the range 1 to 15 dB(A).
  • IEC 60034-25 – the variable frequency drive and its function creates three variables which directly affect emitted noise: changes in rotational speed, which influence bearings and lubrication, ventilation and any other features that are affected by temperature changes; motor power supply frequency and harmonic content which have a large effect on the magnetic noise excited in the stator core and, to a lesser extent, on the bearing noise; and torsional oscillations due to the interaction of waves of different frequencies of the magnetic field in the motor air gap. (…) The increment of noise of motors supplied from PWM controlled variable frequency drives compared with the same motor supplied from a sinusoidal supply is relatively small (a few dB(A) only) when the switching frequency is above about 3 kHz. For lower switching frequencies, the noise increase may be tremendous (up to 15 dB(A) by experience). In some circumstances, it may be necessary to create “skip bands” in the operating speed range in order

Low impedance fault

A low impedance fault is usually a bolted fault, which is a short circuit. It allows a high amount of fault current to flow, and an upstream breaker or fuse usually senses the high current and operates, ending the event. A high impedance fault, usually an arc fault, is a fault of too high of an impedance for overcurrent protection to detect and operate, so the fault exists for long period of time without tripping upstream protection. Examples of arc faults are: A high or medium voltage distribution utility wire falling to earth in a Y grounded system and arcing to earth where no breaker or fuse will clear; another example is any fault tracking through a substance such as cable insulation or even air….this could be wiring within a building wall with a fault that lasts long enough to ignite the building wall it is installed in, which happens all the time somewhere (sometimes called “arc through char”). Another high impedance fault is one within a transformer secondary coil, arcing through the coil insulation and transformer oil (oil cooled units)…the arc will boil the oil into component gases such as acetylene and hydrogen and if the arc fault lasts long enough and gets to the gases, the gases may explode…and the primary fuse protection will likely not detect this for some time. There are many other examples of high impedance faults. One way to tell a high impedance fault or arc fault is if there is a protecting breaker or fuse that did not operate for a fault…if the breaker or fuse are correctly sized and working properly and did not operate that usually indicates a high impedance fault….a short circuit usually generates high enough current to trigger breaker/fuse operations (assuming normal circuit impedance is low). Another way to look at it is any fault in a power circuit with an impedance such that less than “available” fault current flows.

Variable Frequency Drive Harmonics

For the AC power line, the system (VFD + motor) is a non-linear load whose current include harmonics (frequency components multiples of the power line frequency). The characteristic harmonics generally produced by the rectifier are considered to be of order h = np±1 on the AC side, that is, on the power line (p is the number of pulses of the variable frequency drive and n =1,2,3).Harmonics Thus, in the case of a 6 diode (6 pulses) bridge, the most pronounced generated harmonics are the 5th and the 7th ones, whose magnitudes may vary from 10% to 40% of the fundamental component, depending on the power line impedance. In the case of rectifying bridges of 12 pulses (12 diodes), the most harmful harmonics generated are the 11th and the 13th ones. The higher the order of the harmonic, the lower can be considered its magnitude, so higher order harmonics can be filtered more easily. As the majority of VFD manufacturers, Iacdrive produces its low voltage standard variable frequency drives with 6-pulse rectifiers.

The power system harmonic distortion can be quantified by the THD (Total Harmonic Distortion), which is informed by the variable frequency drive manufacturer and is defined as:

THD = √(∑∞h=2 (Ah/A1)2)

Where
Ah are the rms values of the non-fundamental harmonic components
A1 is the rms value of the fundamental component

The waveform above is the input measured current of a 6-pulse PWM variable frequency drive connected to a low impedance power grid.

Normative considerations about the harmonics
The NEMA Application Guide for variable frequency drive systems refers to IEEE Std.519 (1992), which recommends maximum THD levels for power systems ≤ 69 kV as per the tables presented next. This standard defines final installation values, so that each case deserves a particular evaluation. Data like the power line short-circuit impedance, points of common connection (PCC) of variable frequency drive and other loads, among others, influence on the recommended values.

Voltage harmonics
Even components 3%
Odd components 3%
THDvoltage 5%

The maximum harmonic current distortion recommended by IEEE-519 is given in terms of TDD (Total Demand Distortion) and depends on the ratio (ISC / IL), where:
ISC = maximum short-current current at PCC.
IL = maximum demand load current (fundamental frequency component) at PCC.

Individual Odd Harmonics
(Even harmonics are limited to 25% of the odd harmonic limits)
Maximum harmonic current distortion in percent of IL
ISC/IL <11 11<h<17 17<h<23 23<h<35 35<h TDD
<20 4 2 1.5 0.6 0.3

Negative sequence

Negative sequence will not cause a physical rotation. This component creates a field which, though not strong enough, tries to counter the primary field, An increase in this component will cause the motor to overheat due to the opposition. a physical rotation is not likely to occur.

Negative sequence currents are produced because of the unbalanced currents in the power system. Flow of negative sequence currents in electrical machines (generators and motors) are undesirable as these currents generates high temperatures in very short time. The negative sequence component has a phase sequence opposite to that of the motor and represents the amount of unbalance in the feeder. Unbalanced currents will generate negative sequence components which in turn produces a reverse rotating filed (opposite to the synchronous rotating filed normally induces emf in to the rotor windings) in the air gap between the stator and rotor of the machines. This reverse rotating magnetic field rotates at synchronous speeds but in opposite direction to the rotor of the machine. This component does not produce useful power, however by being present it contributes to the losses and causes temperature rise. This heating effect in turn results in the loss of mechanical integrity or insulation failures in electrical machines within seconds. Therefore it is undeniable to operate the machine during unbalanced condition when negative sequence currents flows in the rotor and motor to be protected. Phase reversal will make the motor run in the opposite direction and can be very dangerous, resulting in severe damage to gear boxes and hazard to operating personnel.

Why companies don’t invest in variable frequency drive control

Investing in energy efficient variable frequency drives (VFD) seems like an obvious path to cutting a company’s operating costs, but it is one that many companies ignore. This article explores some possible reasons for this reluctance to invest in VFD.

There is a goldmine of savings waiting to be unlocked by controlling electric motors, but the reluctance to take advantage of this is a very puzzling phenomenon. Motors consume about two thirds of all electrical energy used by industry and cost 40 times more to run than to buy, so you would think optimizing their efficiency would be a priority. The reality is that this good idea is not always turned into good practice and many businesses are missing out on one of the best opportunities to boost profits and variable frequency drive growth.

It might surprise you to learn that your average 11kW motor may cost about £500 to buy but £120,000 to run at 8,000 hours per year over a 15-year lifetime (and that isn’t even accounting for inevitable increases in energy prices). It’s worth considering the payback on any investment in motor control that will reduce this significant running cost, such as using VFDs to control speed, or implementing automated starting and stopping when the motor is not needed. Payback times can often be less than 1 year and, of course, the savings continue over the lifetime of the system, particularly as energy costs rise.

The question that often arises when I talk about this subject to people is: “If the savings are so great, why don’t more people do this?” It would appear to be something that fits into the nobrainer category, however there are three main barriers to the wider uptake of motor control with variable frequency drive, none of which should stop common sense from prevailing – but all too often they do.

The first barrier is a lack of awareness of how much energy is being consumed, and where, in a business. A surprising number of companies do not have a nominated energy manager, still less have energy management as a dedicated job function or have a board member responsible for this significant cost. Those that do measure their energy consumption often have a financial rather than technical bias, so solutions tend towards renegotiating supply contracts, rather than reducing consumption.

The second barrier stems from the economic climate and the level of uncertainty about future events and policies. Businesses are still reluctant to invest in improvement projects, despite short payback periods and the ongoing benefits. The short-term focus is on cutting costs, not on spending money, even to the detriment of future growth. This make-do-and-mend attitude is often proudly touted as a strength, but it is ultimately a false economy. Saving money by cutting capital budgets, reducing staff and cancelling training is damaging to a business and to morale, making it difficult to grow again when the opportunity arises. Saving money by reducing energy consumption makes a business more competitive, while keeping hold of key skills and resources.

The third barrier is a focus on purchase cost, rather than lifetime cost. Whenever a business invests in a machine, a production line or a ventilation system, you can be sure they will have a rigorous process for getting several quotes, usually comparing price, with the lowest bid winning. Something that is not often evaluated is the lifetime energy cost of the system. Competing suppliers will seek to reduce the capital cost of the equipment but without considering the true cost for the operator, including energy consumption. What if the cost of automation and motor control added £700 to the purchase cost? Many suppliers will consider cutting this from the specification. But what if that control saved £1,400 per year in energy? It co

Variable frequency drive saves energy on fans

Like pumps, fans consume significant electrical energy while serving several applications. In many plants, the VFDs (variable frequency drives) of fans together account for 50% to 60% of the total electricity used. Centrifugal fans are the most common but some applications also use axial fans and positive-displacement blowers. The following steps help identify optimization opportunities in systems that consume substantial energy running the fan with VFDs.

Step 1: Install variable frequency drive on partially loaded fans, where applicable. Any fan that is throttled at the inlet or outlet may offer an opportunity to save energy. Most combustion-air-supply fans for boilers and furnaces are operated at partial loads compared to their design capacities. Some boilers and furnaces also rely on an induced-draft fan near their stack; it must be dampened to maintain the balanced draft during normal operation. Installing VFDs on these fans is worthy of consideration.

Similar to centrifugal pump operation, the affinity law applies here. Because constant-speed motors consume the same amount of energy regardless of damper position, using dampers to maintain the pressure or flow is an inefficient way to control fan operation.

Step 2: Switch to inlet vane dampers. These dampers are slightly more efficient than discharge dampers. When a VFD can’t be installed to control fan operation, shifting to inlet vane control could provide marginal energy savings.

Step 3: Replace the motor on heavily throttled fans with a lower speed one, if applicable. Smaller capacity fans with high-speed motor VFDs operate between 25% and 50% of their design capacity. Installing a low-speed motor VFD could save considerable energy.

For example, a 2,900-rpm motor drove a plant’s primary combustion air fan with the discharge side damper throttled to about 75–80%. Installing a VFD on this motor would save considerable energy, but we recommended switching to a standard 1,450-rpm motor. This was implemented immediately, as 1,450-rpm motors are readily available. With the lower-speed motor, the damper can be left at near 90% open; the fan’s power consumption dropped to less than 50% of the previous level.

Step 4: Control the speed when multiple fans operate together. Fans consume a significant amount of energy in industrial cooling and ventilation systems. Supply fans of HVAC systems are good candidates for speed control by variable frequency drives, if not already present.

Step 5: Switch off ventilation fans when requirements drop. Ventilation systems usually run a single large centrifugal fan or several axial exhaust fans. A close look at their operation may indicate these fans could be optimized depending upon the actual ventilation needs of the building they serve.

Recently, we surveyed a medium-sized industrial facility where 26 axial-type exhaust fans were installed on the roof of one building. All fans were operating continuously, even though the building had many side wall openings and not much heat generated inside. To better conserve energy, we suggested the 26 fans be divided into four groups with variable frequency drives controlled for each group. As a result, energy consumption for the fans dropped by about 50%, as only the required fan groups now are switched on.

At another industrial site, the exhaust fan of a paint booth ran continuously but paint spraying was scheduled only about 50% of the time. Modifying fan operation with variable frequency drive and delayed sequencing saved energy.

Pumps and fans are the most common energy-consuming devices

Variable Air Volume System Optimization

Variable Air Volume Systems (VAV) can be optimized to increase energy savings by maximizing the efficiency of the equipment at part-load conditions. The goal with the optimization strategy is to run each subsystem (chiller, cooling tower, Airhandler, etc) in the most efficient way possible while maintaining the current building load requirement.

VAV System Optimization

As each Variable Air Volume terminal controls the space temperature – based on flow – the “worst case” zone can easily be identified by an automation system. The supply fan speed can be reduced by resetting the static pressure (see following page). As the load drops and the fan meets a preset minimum flow, the system resets the air temperature up, so less chilled water is needed. In a variable flow chiller system, this reduces pumping energy.

If the system load continues to drop, the system will reset the chiller supply water temperature upward which will then reduce the energy requirements of the chiller. Changes in the chiller head pressure and loads can then reset the cooling tower fan speed.

The key to optimizing the system operation is communication and information sharing through the entire system equipment. With the reduced cost of variable frequency drives and Building Automation Systems, (BAS) complete system optimization can be implemented as a cost effective option.

In VAV systems where the individual VAV boxes and the AHU are on a building automation system, additional savings can be achieved by implementing static pressure reset. The static pressure sensor in a VAV system is typically located two-thirds of the way downstream in the main supply air duct for many existing systems. Static pressure is maintained by modulating the fan speed.

When the static pressure is lower than the setpoint, the fan speeds up to provide more airflow (static) to meet the VAV box needs, and vice-versa. A constant set point value is usually used regardless of the building load conditions.

Under partial-load conditions the static pressure required at the terminal VAV boxes may be far less than this constant set point. The individual boxes will assume a damper position to satisfy the space temperature requirements. For example, various VAV box dampers will be at different damper positions, (some at 70% open, 60% open, etc) very few will be at design, ie 95% -100% open.

RESET STRATEGY
Essentially, resetting supply air static pressure requires that every VAV box is sampled with the static reset set to the worst case box requirement. For example, each box is polled, every 5 minutes. If no box is more than 95% open, reduce duct static pressure set point by 5%. If one or more boxes exceed 95% open, increase static pressure set point by 5%.

With a lower static set point to maintain, fan speed reduces. The result is increased energy savings in the 3 to 8% range. See figure below. If the BAS system is already installed, implementing this strategy is relatively free.
Variable Air Volume System energy savings

Power factor of a generator connected to national grid

Q: What should be the power factor of a generator connected to national grid in order to have maximum stability? Whether it should be high or low?

Steady State Stability:
1. National grid is like a infinite bus for an average size Generator. We can observe stable operation of generator within its capability limit for all ranges of power factor for infinite time , irrespective of power factor.
2. Observe the load cycle, The generators operate in overexcitation mode (lagging pf) during the day & during night ,when transmission lines generate enough reactive the same generators operate stable in underexcitation mode (leading pf).
3. Therefore as long as there is no instance of large disturbance, we can observe stable operation of generator within its capability limit for all ranges of power factor.

Transient Stability:
1. Depends upon the initial condition of the generator operation (see on Power vs Sin-delta plot)
2 The level of power thrown-off causing the disturbance & Equal area criterion of the energy balance & Inertia.
3 During transient/disturbance, the stability is ensured better if the angle delta (rotor angle or power angle) is small, meaning the amount of store energy in the rotating system is high. Theoretically this means delta angle =0 to have robust stability, but it is practically impossible to have power generation at that value.
4 In order to have maximum stability & power generation simultaneously , the value of rotor angle has to be non zero , on positive side. (negative means motor operation).
To Conclude : It means over-excited mode.(lagging pf ). Many colleges in discussion chain above have written near about 0.9 – 0.94 lagging . They are correct.

Frequency Inverter Direct Digital Control

Modulating Supply & Return Fans are used as a means of providing proper variable air volume (VAV) control as well as building pressurization. Many such VAV systems are still largely pneumatic with static to the downstream boxes being maintained by inlet guide vanes. To provide increased energy savings and energy comfort, these systems can be easily converted to frequency inverter fan control of the supply and return fans and Direct Digital Control (DDC) to coordinate any increased energy saving strategies. Figure 1 shows such a system.

Frequency Inverter Direct Digital Control

To increase energy savings, the DDC controller can be programmed to reduce the flow from the return & supply fans for short periods of time. Coordinated with the building pressurization system, any temporary loss of space temperature may be avoided.

In Figure 1, the supply fan is controlled by the duct static pressure sensor, via the DDC, while the outside air and mixed air dampers are optimized to provide economizer control.. The return fan is modulated to stabilize building pressure at a slight positive. For simple supply and exhaust systems the building pressure and static pressure sensors may be connected directly to the frequency inverter with an internal PID controller.

Typical Energy Savings are realized from converting pneumatic (or electromechanical) control to DDC control with frequency inverter in the following ways:

  • Locking inlet guide valves mechanically open to allow the frequency inverter to fully modulate the fans.
  • Free cooling by accurately modulating the economizer dampers and sequencing the mechanical equipment.
  • Controlling static and resetting the static pressure during short periods of time.
  • Accurate building pressurization.
  • Implementing other energy saving measures which include supply air reset, and night purge routines.

CONTROL CONSIDERATIONS

  • Placement of the indoor static pressure sensor is important as it should provide a stable signal. Entrances, dock, and other areas where large , sudden static pressure changes may occur should be avoided.
  • The outside reference static tip should be shielded from wind and rain.
  • When the exhaust fan is frequency inverter controlled, consider a 2-position air damper to prevent the outside air from entering the building (infiltration) when the exhaust fan is off or a very low speeds.
  • For simple VAV systems, consider using frequency inverters with built in PID controls such as the Iacdrive frequency inverters.. This minimizes hardware and installation costs. Static sensors provide a 0-10vdc control signal directly to the frequency inverter.
  • Duct mounted static pressure sensor should be mounted 2/3 of the distance of the distribution system.

Current transformer selection

When you want to select current transformer with appropriate rated power for your power system, you need to consider that value of rated power of selected current transformer should be higher from sum of values of load and Joules’ losses which are a consequence of flow current through conductors which connect current transformer with relay.

So, if you have a long distance between current transformer and relay, then you need to consider one of two following manners for solving this problem:
1. replacing existing current transformer with current transformer with higher power,
2. replacing existing conductors with conductors with lower cross-section.

This solution is a consequence of necessity for reducing of Joules’ losses which are a consequence of flow current through conductors which connect current transformer with relay. If you have conductors whose value of rated current is 5A, you will have Joules’ losses P=R*I^2=R*5^2=25*R. Otherwise, if you have conductors whose value of rated current is 1A, you will have Joules’ losses P=R*I^2=R*1^2=R.
On this way, Joules’ losses in your selected conductors will be reduced 25 times and selected current transformer will be unloaded by reducing additional load.