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AC Motor Speed Control

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AC Motor Speed Control 

 

AC induction motors are based on the principle of a rotating magnetic field produced by a set of stationary windings (called stator windings) energized by AC power of different phases.

The effect is not unlike a series of blinking “chaser” light bulbs which appear to “move” in one direction due to the blinking sequence. If sets of wire coils (windings) are energized in a like manner – each coil reaching its peak field strength at a different time from its adjacent neighbor – the effect will be a magnetic field that “appears” to move in one direction.

If these windings are oriented around the circumference of a circle, the moving magnetic field rotates about the center of the circle, as illustrated by this sequence of images (read left-to-right, top-to-bottom, as if you were reading words in a sentence):

 

Any magnetized object placed in the center of this circle will attempt to spin at the same rotational speed as the rotating magnetic field.

Synchronous AC motors use this principle, where a magnetized rotor precisely follows the magnetic field’s speed.

Any electrically conductive object placed in the center of the circle will experience induction as the magnetic field direction changes around the conductor.

This will induce electric currents within the conductive object, which in turn will react against the rotating magnetic field in such a way that the object will be “dragged along” by the field, always lagging a bit in speed.

Induction AC motors use this principle, where a non-magnetized (but electrically conductive) rotor rotates at a speed slightly less (Note) than the synchronous speed of the rotating magnetic field.

Note : The difference between the synchronous speed and the rotor’s actual speed is called the motor’s slip speed.

The rotational speed of this magnetic field is directly proportional to the frequency of the AC power, and inversely proportional to the number of poles in the stator:

Synchronous speed

Where,
S = Synchronous speed of rotating magnetic field, in revolutions per minute (RPM)
f = Frequency, in cycles per second (Hz)
n = Total number of stator poles per phase (the simplest possible AC induction motor design will have two poles)

The relationship between synchronous speed, frequency, and pole number may be understood by analogy: the speed at which the lights in a “chaser” light array appear to move is a function of the blinking frequency and the number of light bulbs per unit length.

If the number of light bulbs in such an array is doubled by placing additional bulbs between the existing bulbs (so as to maintain the same array length), the apparent speed will be cut in half: with less distance between each pair of bulbs, it takes more cycles (more “blinks”) for the sequence to travel the entire length of the array.

Likewise, an AC stator with more poles in its circumference will require more cycles of AC power for the rotating magnetic field to complete one revolution. .

A synchronous AC motor will spin at the exact same speed as the rotating magnetic field: a practical example is a 4-pole synchronous motor spinning at 1800 RPM with an applied power frequency of 60 Hz.

An induction AC motor will spin at slightly less than the speed of the magnetic field: a practical example is a 4-pole induction motor spinning at 1720 RPM with an applied power frequency of 60 Hz (i.e. 80 RPM “slip” speed).

Induction motors are simpler both in construction and operation, making them the most popular of the two types of AC electric motors in industry.

While the number of poles in the motor’s stator is a quantity fixed6 at the time of the motor’s manufacture, the frequency of power we apply may be adjusted with the proper electronic circuitry.

A high-power circuit designed to produce varying frequencies for an AC motor to run on is called a variable-frequency drive, or VFD.

Variable-frequency motor drives are incredibly useful devices, as they allow what would normally be a fixed-speed electric motor to provide useful power over a wide range of speeds.

The benefits of variable-speed operation include reduced power consumption (only spinning the motor as fast as it needs to move, and no faster), reduced vibration (less speed = reduced vibrational forces), and the ability to ramp the motor’s speed up and down for reduced wear and tear on mechanical components resulting from acceleration forces.

Another feature common to most VFDs is the ability to actively brake the load. This is when the drive causes the motor to actively apply a negative torque to the load to slow it down.

Some VFDs even provide means to recover the kinetic energy of the load during the braking process, resulting in further energy savings.

Variable-frequency AC motor drives consist of electronic components to convert the constantfrequency AC input power into variable-frequency (and variable-voltage) AC output power for the motor to run on. This usually takes place in three distinct sections.

The rectifier section uses diodes to convert line AC power into DC. The filter “smoothes” the rectified DC power so it has little ripple voltage. Lastly, the inverter section re-converts the filtered DC power back into AC, only this time at whatever levels of frequency and voltage is desired to run the motor at different speeds.

A simplified schematic diagram for a VFD is shown here, with a rectifier section on the left (to convert AC input power into DC), a filter capacitor to “smooth” the rectified DC power, and a transistor “bridge” to switch DC into AC at whatever frequency is desired to power the motor.

Note the reverse-connected diodes across the source and drain terminals of each power transistor. These diodes serve to protect the transistors against damage from reverse voltage drop, but they also permit the motor to “back feed” power to the DC bus (acting as a generator) when the motor’s speed exceeds that of the rotating magnetic field, which may happen when the drive commands the motor to slow down. This leads to interesting possibilities, such as regenerative braking, with the addition of some more components.

The transistor control circuitry has been omitted from this diagram for the sake of simplicity:

 

As with DC motor drives (VSDs), the power transistors in an AC drive (VFD) switch on and off very rapidly with a varying duty cycle. Unlike DC drives, however, the duty cycle of an AC drive’s power transistors must vary rapidly in order to synthesize an AC waveform from the DC “bus” voltage following the rectifier.

A DC drive circuit’s PWM duty cycle controls motor power, and so it will remain at a constant value when the desired motor power is constant.

Not so for an AC motor drive circuit: its duty cycle must vary from zero to maximum and back to zero repeatedly in order to create an AC waveform for the motor to run on.

The equivalence between a rapidly-varied pulse-width modulation (PWM) waveform and a sine wave is shown in the following illustration:

This concept of rapid PWM transistor switching allows the drive to “carve” any arbitrary waveform out of the filtered DC voltage it receives from the rectifier.

Virtually any frequency may be synthesized (up to a maximum limited by the frequency of the PWM pulsing), and any voltage (up to a maximum peak established by the DC bus voltage), giving the VFD the ability to power an induction motor over a wide range of speeds.

While frequency control is the key to synchronous and induction AC motor speed control, it is generally not enough on its own.

While the speed of an AC motor is a direct function of frequency (controlling how fast the rotating magnetic field rotates around the circumference of the stator), torque is a function of stator current.

Since the stator windings are inductors by nature, their reactance varies with frequency as described by the formula XL = 2πfL. Thus, as frequency is increased, winding reactance increases right along with it. This increase in reactance would result in decreased stator current if the VFD’s output voltage remained constant.

This undesirable scenario would result in torque loss at high speeds, and excessive torque (as well as excessive stator heat!) at low speeds. For this reason, the AC voltage output by a VFD is made to vary (Note) in proportion to the applied frequency, so that the stator current will remain within good operating limits throughout the speed range of the VFD.

This correspondence is called the voltage-to-frequency ratio, abbreviated “V/F” ratio or “V/Hz” ratio.

Note : The VFD achieves variable output voltage using the same technique used to create variable output frequency: rapid pulse-width-modulation of the DC bus voltage through the output transistors. When lower output voltage is necessary, the duty cycle of the pulses are reduced throughout the cycle (i.e. transistors are turned on for shorter periods of time) to generate a lower average voltage of the synthesized sine wave.

 

To give an example of a VFD programmed with a constant V/F ratio, if the output line voltage to the motor is 480 volts RMS at full speed (60 Hz), then the output line voltage should be 240 volts RMS at half-speed (30 Hz), and 120 volts RMS at quarter-speed (15 Hz).

Variable-frequency motor drives are manufactured for industrial motor control in a wide range of sizes and horsepower capabilities.

Some VFDs are small enough to hold in your hand, while others are large enough to require a freight train for transport. The following photograph shows a pair of moderately-sized Allen-Bradley VFDs (about 100 horsepower each, standing about 4 feet high), used to control pumps at a wastewater treatment plant:

Variable-frequency AC motor drives do not require motor speed feedback the way variable-speed DC motor drives do. The reason for this is quite simple: the controlled variable in an AC drive is the frequency of power sent to the motor, and rotating-magnetic-field AC motors are frequencycontrolled machines by their very nature.

For example, a 4-pole AC induction motor powered by 60 Hz has a base speed of 1728 RPM (assuming 4% slip). If a VFD sends 30 Hz AC power to this motor, its speed will be approximately half its base-speed value, or 864 RPM.

There is really no need for speed-sensing feedback in an AC drive, because the motor’s real speed will always be limited by the drive’s output frequency.

To control frequency is to control motor speed for AC synchronous and induction motors, so no tachogenerator feedback is necessary for an AC drive to “know” approximately9 how fast the motor is turning.

The non-necessity of speed feedback for AC drives eliminates a potential safety hazard common to DC drives: the possibility of a “runaway” event where the drive loses its speed feedback signal and sends full power to the motor.

As with DC motor drives, there is a lot of electrical “noise” broadcast by VFD circuits. Square-edged pulse waveforms created by the rapid on-and-off switching of the power transistors are equivalent to infinite series of high-frequency sine waves, some of which may be of high enough frequency to self-propagate through space as electromagnetic waves.

This radio-frequency interference or RFI may be quite severe given the high power levels of industrial motor drive circuits. For this reason, it is imperative that neither the motor power conductors nor the conductors feeding AC power to the drive circuit be routed anywhere near small-signal or control wiring, because the induced noise will wreak havoc with whatever systems utilize those low-level signals.

RFI noise on the AC power conductors may be mitigated by routing the AC power through filter circuits placed near the drive. The filter circuits block high-frequency noise from propagating back to the rest of the AC power distribution wiring where it may influence other electronic equipment. However, there is little that may be done about the RFI noise between the drive and the motor other than to shield the conductors in well-grounded metallic conduit.

Source: <https://https://instrumentationtools.com/ac-motor-speed-control/>

When and How Should I Select a Braking Resistor?

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When and How Should I Select a Braking Resistor?

 

When designing a motor control system, it is not always clear if a braking resistor is required and, if it is, how to proceed in selecting a braking resistor. This post is intended to simplify that process so it is clear when and how to select a braking resistor.

Why are braking resistors necessary?

Braking resistors are introduced into a motor control system in order to prevent hardware damage and/or nuisance faults in a VFD.  They are required because in certain operations, the motor controlled by the VFD is acting as a generator and power is flowing back towards the VFD, rather than towards the motor.  A motor will act as a generator whenever there is an overhauling load (e.g. maintaining a steady speed as gravitational forces try to accelerate an elevator as it moves down) or the drive is being used to decelerate the motor.  This causes the drive’s DC bus voltage to rise and will lead to over voltage faults in the drive if the generated energy is not dissipated.

There are a few basic ways to deal with the energy generated by a motor. First, the drive itself will have the capacitance to absorb some amount of this energy for a small amount of time. This is typically the case when overhauling loads are not present and a fast deceleration is not required. If there are portions of the duty cycle where the energy generated is too great for the drive alone, then a braking resistor can be introduced. The braking resistor will dissipate the excess energy by converting it to heat across a resistor element. Finally, if the regenerated energy from the motor is continuous or the duty is high, then it may be more beneficial to use a regen unit rather than a braking resistor. This will still protect the VFD from hardware damage and nuisance faults, but allows the user to capture and reuse the electrical energy rather than dissipating it as heat.

What should be considered when selecting a braking resistor?

Once it has been decided a braking resistor is needed for the application, there are two main factors when selecting the resistor: the resistance value and the power dissipation capacity of the resistor.

 

Minimum Resistance Value

VFDs that use a brake resistor will also have a “chopper circuit” or brake transistor.  When the DC bus voltage gets too high, the brake transistor shunts current from the DC bus across the brake resistor.  This brake transistor circuitry has current limitations and the VFD manufacturer will typically list a maximum current value and duty cycle.

Since V=IR, if the voltage is constant then a smaller resistance will lead to a larger current. Thus if the max voltage is known to be the KEB over voltage level of 840VDC, it is possible to calculate the minimum resistance that would keep the current value below the braking transistor’s max current rating. While the minimum resistance value doesn’t affect the operation of the resistor or its ability to dissipate power, it is crucial to ensure it works properly with the VFD.

Power Dissipation Capacity

The second consideration when selecting a braking resistor is its ability to dissipate power.  KEB braking resistors are listed with the amount of power they can safely dissipate if used continuously (PD) as well as three values for intermittent duty.  Each of the numbers in P6, P25, and P40 refer to the cumulative number of seconds the resistor is used over the course of two minutes.  For example, the KEB 10BR100-1683 resistor could safely dissipate up to 2200W for one stretch of six seconds over the course of two minutes or instead could do two cycles of three seconds each over the course of two minutes.

Now that it is known what resistance values will safely work with the VFD and the power dissipation capabilities of the various resistors, it must be considered how much energy will be generated back towards the drive that will need to be dissipated.  This will ensure the selected braking resistor has enough capacity to safely dissipate the energy generated from the motor.  The first way to do this is through calculation.  It is possible to calculate the power generated from the motor if the mass moment of inertia of the motor and load, motor torque, speed change, and time of the deceleration are all known.  More information on performing these calculations can be found in the braking resistor manual.  However, in real world applications it can be difficult to know and/or calculate the mass moments of inertia, particularly of the load.  Because of that, it is commonly necessary to determine the proper power size of the braking resistor through a testing method.

The general rule is that the larger the load and the faster the deceleration, the more power that will need to be dissipated.  However, by utilizing the scope function in the Combivis 6 software, it is possible to record the drive’s DC bus voltage throughout the operation to get a more accurate picture of the braking resistor usage.  With the scope it is possible to monitor if a resistor with greater power dissipation is required or if instead the resistor is sufficiently sized.  In the latter scenario, it may be possible to adjust the operation to improve performance, such as making the deceleration faster.

Braking Resistor Installation

The final consideration when selecting a braking resistor is to ensure that it is installed properly.  If a braking resistor is not installed according to UL standards, the circuit can fail in a manner that is a fire hazard.  More information on safe connection of a braking resistor can be found here.

In addition to our traditional resistors, KEB is increasingly selling intrinsically safe brake resistors which fail much like a fuse protecting the system in the case of short circuit failure.

The installation environment is also important.  Hazardous locations and installations with fibers (textile, sawdust) that are flammable will need special consideration.

Source: <https://www.manufacturingtomorrow.com/article/2020/08/when-and-how-should-i-select-a-braking-resistor/15756>

What is a Line Choke or Reactor in a VFD?

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What is a Line Choke or Reactor in a VFD?

 

Line choke is an inductance connected to the VFD input or output circuit. It is a component that forms a magnetic field as current flows through it and when the current increases, it limits the increment in current by producing a voltage or emf across it that opposes it.

This protects the VFD from unwanted spikes, transients, and harmonics. The reactor is used on either the input side or output side of the VFD.

When a reactor is used on the input side, it is called a line reactor and when it is used on the output side, it is called a load reactor.

Refer to the above image. The inductance is shown on the input side and the output side is nothing but the line choke. It is normally used for VFD’s which have a rating of more than 1 KVA. Let us understand its basic working.

It is not always necessary that a current wave be sinusoidal. When it is not sinusoidal, it contains harmonics. Harmonics are a large current distortion.

If you see the basic equation of an inductor, it is – V = L (di/dt)

Here, V is the voltage, L is the inductance of the choke or reactor, and (di/dt) is the rate of change of current.

As the current change rate increases, the voltage too increases in proportional to the inductance.

But, this induced voltage has the opposite polarity to that of the applied voltage. This automatically reduces the rate of current change and limits the current applied.

Line Choke

One more theory can be understood here for the output side of VFD. As a high switching frequency is produced at the output of VFD, sharp changes in voltage outputs are reflected sometimes.

This high spikes in voltage can damage the motor circuit or even heat the VFD itself. This can be reduced by using a line choke.

The line choke reduces the peak of the voltage waveform and increases the changing rise time of the signal; to stabilize the final output waveform.

The stabilized waveform will indirectly provide limited current and voltage output to the motor.

Load Reactor

A load reactor is used when the distance between the motor and VFD is very large; mostly 100 feet or more.

Due to long-distance, voltage spikes are amplified more and this can affect the circuit in a large way. So, the line choke will limit this current to a safe limit.

This inductance plays a very important role in protecting the thyristors of the VFD from damage or overheating. Mostly, this device is used when several drives are connected in parallel closely; or the line power supply is imbalanced or largely variable, etc.

In this way, we understood the basic concept of the line choke.

Source: <https://instrumentationtools.com/what-is-a-line-choke-or-reactor-in-a-vfd/>