Operating Motors at Variable Speeds
Electric motors are used every day. Ones that easily come to mind are in refrigerators and freezers for pumping freon and heaters blowing warm air around. Not as apparent are those in CD/DVD players and computer hard disk drives. Cars technically use gas engines not discussed here. Household motors are relatively small and easy to start and stop protected by circuit breakers and fuses.
Larger motors are often necessary in commercial facilities. Industrial complexes generate electricity, process food, procure and dispense fuel. Motors can be substantial and harder to control.
Conventional Motor Starters
More current is needed to get a motor running than keep it up to speed particularly when under load. Starting currents can be six times a motor’s rated FLA (full load amperage) for a short time. Sparks created when a switch makes contact between wires or conductors providing electricity with ones feeding big motors can be significant and burn them up.
Electric motor starters use solenoids (electro-magnets) which when energized close contactors quickly. These magnets can be controlled by voltages and currents smaller than what motors require. A 480-volt, three phase motor with 65 FLA can be controlled by a 120-volt, single phase 15-amp circuit. Control circuits with lower ratings are safer to handle and allow pressure and other limit switches with smaller contacts and wires to start and stop large motors.
Electric starters rate in size under NEMA 1 to over 12 depending on horse power of motors. Bigger starters not only take more space but are more expensive. They use time delayed thermal overload devises allowing large starting currents, but trip if persisting over full load amperage.
Larger motors are rated 60 cycles per second, either single or three phase, as provided by utility companies in North America. Because frequency of electrical current going into and out of standard motor starters does not change, motors operate at constant revolutions per minute. They cannot run faster than designed, and if caused to go slower thermal overloads will stop them before damaged by heat. Size and speed of machines operated can be altered with different size pulleys and belts or gears.
Modern electronics devices can control apparatuses more precisely by varying speed of motors.
Advantages Controlling Motor Speed
Ramping a motor gradually to speed results in less wear and tear on it and equipment operated. Loaded conveyor belts incur less damage when rollers they’re pulled by don’t slip when quickly started. Same for fan belts. If a conveyor belt transferring potatoes to a storage bin is loaded by hand instead of a hopper with a couple of potatoes on the belt every few feet dropping at a very fast rate, it makes sense to slow the whole process down.
Electronic devices controlling motor drives no longer only switch on or off but monitor activity. They output 1 to 5 volts or 4 to 20 milli-amps as pressure increases and decreases or temperature goes up and down. Values don’t start at zero since no signal indicates monitors aren’t working correctly or a broken wire to central processing units which sound that alarm. These wires should be shielded and grounded at one end to drain off surrounding electrical interference. They may be smaller than #16 gauge but that can cause problems ensuring proper termination and conductivity.
Benefits to calibrated monitors are speeding a pump up on low pressure and slowing it down when higher instead of continually starting and stopping the pump motor or making a fan on a heating or cooling devise go faster or slower as temperature varies making it more constant in rooms.
Other advantages controlling speed has to do with motors themselves. Power factors being decimals significantly less than one are not desirable when supplying power to incandescent lights or motors. This occurs when sine wave frequency of current starts later lagging that of voltage. Slowing a motor down by decreasing frequency lessens this problem resulting in amperage more constant with horse power. Electricity a fan or pump motor uses is proportional to the mathematical cube of speed. Slowing them down by twenty percent uses fifty percent less energy. Ramping motors up to speed reduces high starting currents.
Variable Frequency Drives
Variable frequency drives (VFDs) regulate revolutions-per minute (speed) and rotational force (torque) of motors designed for alternating current by changing frequency and voltage to them. Drives are also often referred to as AC, variable speed (VSDs), adjustable frequency (AFDs), adjustable speed (ASDs), variable frequency converters (VFCs), inverter drives and micro-drives.
Some use voltage source conversion (VSI), current source conversion (CSI), load-commutated inversion (LCI) or pulse-width modulation (PWM) inverters. Depending on the application one type might be more desirable with higher efficiency, reduced cost and increasing power factors or lessen clogging and pulsating rotation of motor shafts. They all rectify alternating current into direct current to an inverter converting electricity back to ac again but with different frequencies and voltages depending on 4-20 milli-amp inputs affecting speed and torque. Computers in VFDs programmed by operators interpret inputs and amperage to achieve desired results.
Some variable frequency drives are designed to accept direct current from a power source and convert it to alternating current. Great for running ac motors by batteries charged from solar panels or in electric driven vehicles. Other VFDs are designed or programmed to convert single-phase alternating current if only available at locations by utility companies into three-phase for those motors and vise-versa.
Harmonic Distortion
Harmonics are often favorable in music. When a guitar string is pressed to the fret in the middle a higher note is achieved called a second harmonic. If pressed third way up or down different frequency vibrations occur called 3rd order harmonics. The combination of different notes may make pleasant sounds.
Harmonics aren’t useful in electric circuits. Current in three phase circuits are meant to be balanced canceling each other out back to the power source thereby alleviating a fourth neutral wire. Generators produce three phase sine waves by armatures 120 degrees apart wound with wires revolving in magnetic fields sixty times a second. Intent is for VFDs to make perfect sine waves but changing frequency to vary motor speed is unaffordable with semi-conductor devices available now. Three phase motors do not have a neutral wire.
Rectifying alternating current into dc using capacitors and diodes has been around for a while but today computers can replicate sinusoidal waves based on differential calculus. Converting ac to dc by VFDs don’t require computer assistance but changing current back to ac again does. Information from computed sine waves are transmitted on high frequency carrier waves to full-wave bridge inverters made with diodes or insulated gate bipolar transistors. IGBTs are much larger than conventional transistors. Transistors are based on outdated vacuum tubes where trivial voltages from radio waves are amplified repeatedly until capable of driving audible speakers and TV screens. This made transistor radios smaller and powered by batteries.
Sine waves are simulated by space vector pulse-width modulation. IGBTs operate on a different principle called sinusoidal pulse-width modulation. Different frequencies of sine waves are converted into switching algorithms by computers which do not change maximum output voltage but allow transistors to produce varied square waves of equal amplitude. Minute increments of time called integers between pulses and their durations determine output voltage and current frequency. Carrier frequencies from computed sine waves should be greater than ten times (2,000 to 16,000 hz) power output frequencies. Square waves leaving inverters are pronounced and cumbersome. Six pulse rectification and inversion result in approximately 25% total harmonic distortion where 18 pulse creates around 5% THD with less clipping. Everything to do with electrical circuits and devices means more is better until size and cost are considered.
Harmonic Filters
Made with the same parts, motors are basically generators in reverse. If armatures rotating in magnetic fields produce electricity, then motors can convert it back to mechanical energy. Adverse effects running motors with distorted sine waves from VFDs are apparent. It is required by NEMA standards these motors be designed definite-purpose inverter fed duty and withstand high surge voltages, can run at low speeds without overheating, and endure 200% torque overload for one minute. It is desirable to dampen or mitigate harmonic power distortion before getting to inverter duty motors. Long wires from VFDs amplify distortion due to impedance. Shielded, flexible VFD cables are offered if longer lengths are necessary. Transformers being inductors themselves dampen irregularities to motors. Output filters run the risk of damaging drives. High carrier frequencies in drives create sparking in motor bearings resulting in their deterioration.
Not as critical to operation is harmonic distortion back-fed on transmission lines supplying VFDs. Line harmonics result when converting ac to dc with voltage and current distortion. Chopping or clipping sine waves leave voltage spikes. Considered negligible with small motors, large VFDs create distortion with other motors, lighting and electronic equipment connected to the line before them.
Utility companies require filters before feeding substantial inverter drives. Not a bad idea if the line also supplies your residence. Active filters inject opposite harmonics using diodes or IGBTs requiring computer control. More common are passive filters with inductors and capacitors dampening and draining distortion to ground and providing power factor corrections.
VFDs sometimes have an efficiency loss up to five percent producing some heat. Passive harmonic filters utilize resistors, inductors and capacitors considerably bigger than in electronic circuits. It is their purpose to burn off or drain unwanted power distortions. Size and more heat dissipation requires them in separate enclosures away from drives.
Dynamic Breaking
Motors are similar to generators. If motors run faster than power intends electricity is back fed into the supply. Because conventional motor starters open contactors when stopping motors, back-fed electricity has nowhere to go causing motors to stop faster. VFDs reduce speed or stop motors by decreasing current but back-fed energy remains in the system.
Some CSI and LCI type drives are Regenerative VFDs which recover breaking energy meant to return to the power source. If motors are sped-up and slowed down often there may be some economic advantage to recovering this energy provided a place to store it such as capacitors or batteries. Utility companies don’t want distorted power on their line.
Commonly used are dynamic breaks, and like filters drain to ground or burn off undesirable energy. Breaks get much hotter making it even more important putting them in separate enclosures.
Misconceptions
Measurement of horsepower of engines and motors was originally derived by actual horses. A weighted wagon an average size horse can pull a given distance and time became one horsepower. If weight doubled or half as much time needed to pull it, two horses are required. Two horses tethered to pull the wagon with original weight and speed means each horse works half as hard resulting in one horsepower. One linear mechanical horsepower is 33,000 foot-pounds per minute (550 ft-lbf/second) requiring 746 electrical watts (volts x amps).
Motors rotate, and linear movement converts to rotational torque equal to horsepower times 5252 divided by rpms. Motor designed for specified revolutions-per-minute equipped with larger pullies on their shafts pull loads at less speeds but higher torque. Lesser horsepower might now be needed than what the motor is rated.
One misapprehension is if a twenty-horsepower motor is only needed to run a pump or conveyor belt, then a forty-horsepower motor at half rpm can be operated by a 20 hp VFD. It takes more energy to ramp a large motor to speed and if the load exceeds 20 hp for some reason smaller VFDs might be damaged. Better to use a 40 hp VFD at half capacity to run a 40 hp motor with lesser loads.
Another misconception is utility transformers providing power can be half the size if a motor runs at half the speed. The same as the above applies. KVA ratings of single-phase transformers used to operate three-phase motors should be increased by the 1.73 conversion factor. A 20 hp, three phase motor fed single phase to harmonic filters and dynamic brakes means these are rated 20 hp when verified by manufacturers.
Article 430, Part X gives more information and installation requirements by the 2020 National Electric Code for Adjustable-Speed Drive Systems.
Future Uses
Many electronic inventions improve with time. As VFD technology becomes better it will be used more often in the future.
Eberling@www.thndrsns.com
