good day and welcome to Big Bad Tech I'm your instructor Jim Pytel today's topic of discussion this primary resistor reduced voltage starters our objective is to introduce a reduced voltage starting method making use of primary resistors reduced voltage starting methods reduce inrush current and modify acceleration starting torque characteristics as you are no doubt aware the direct and instantaneous application of full voltage to a motor at a standstill produces a tremendous surge of current known as in rush you should be tired of hearing this as I am of saying it and believe me I'm tired of saying it although brief in rush can place unnecessarily high demand in the electrical distribution network and industries often pay a financial penalty for such events additionally the abrupt acceleration of a full voltage motor starter with a motor at standstill may damage the mechanical linkages like belts chains shafts gears and couplings the system uses to manipulate the applied load it is for this reason reduced voltage starting methods are employed reduced voltage starting methods include but are not limited to the electromechanical primary resistors part winding why start Delta run sometimes called star Delta starter and the solid state soft starters and motor drives all these methods serve to limit inrush current and reduce mechanical stresses to the driven load today we'll discuss one of these methods primary resistors on an introductory level after we've discussed timer relays expect to revisit reduced voltage starters and incorporating timers in later lectures as the title implies primary resistor reduced voltage starters use a bank of resistors intended to carry primary current these resistors are placed in series with the motor stator connections those of you with an understanding of basic series properties are no doubt aware current through elements in series is the same this means that when the asks or start contactor closes the current drawn by the motor must travel through the primary resistors Ohm's law perhaps you've heard of it states that there will be a voltage drop across the primary resistors proportional to the current and resistance this means of the total applied voltage the primary resistors will drop a portion than the motor the remaining portion kirchoff's voltage law quite like gravity remains in effect at all times the sum effect of the primary resistor reduced voltage starter method is that when the primary resistors are in series with the motor the motor initially receives less than the full voltage than if a direct online starter were used current being the effective voltage is therefore less an inrush demand is reduced additionally since power is proportional to the square of current the starting torque is reduced thereby limiting stress to the mechanical linkages note given starting torque is reduced this method may not be suitable for all applications may necessitate the use of more sophisticated methods like a motor drive sophisticated by the way is a euphemism for more expensive we'll discuss motor drives and their incredible advantages in later lectures importantly the primary resistors central to the primary resistor reduced voltage starting method are to be used for starting the motor only they're not intended to carry primary current for any length of time and are meant to be switched out once a motor is reached a sufficient operational speed primary resistors get hot and this dissipated heat is a loss to the system it is for this reason another contactor is used to bypass the primary resistors once this predetermined speed has been reached as illustrated the closure of the s-start contactor starts the motor with primary resistors in series with the motor the primary resistors drop a portion of the applied voltage and voltage and seen by the motor terminals is less the motor still experiences inrush current however it is substantially less than that of a full voltage starter as the motor accelerates current subsides and as such the voltage drop across the primary resistors decrease and more voltage supplied to the motor at a certain point the motor has accelerated to an acceptable speed and the R or run contactor in parallel with a primary resistor bank closes the primary resistors are in effect short circuited all current circumvents the primary resistors in favor the closed R run contact or path full voltage is applied to the motor already rotating close to nominal speed given the motor is not at a standstill inrush current occurs however it is again substantially less than if the motor was in a locked rotor condition the graph of inrush current for the primary resistor reduced voltage starter method in solid blue shows characteristics common of a closed transition reduced voltage starter learn in later lectures the specifics of other reduced voltage starters however an early discussion about open and closed transitions is pertinent at this time only the start contactor closes the primary resistors drop a portion of the applied voltage an inrush current is reduced in comparison to a regular full voltage starter in dashed lines inrush current subsides as the motor slowly accelerates at a certain point X the run contact r closes and full voltage is applied the current spikes again however not nearly what it would be for a regular full voltage starter the primary resistor reduced voltage daughter is a closed transition method the title closed transition means that at no point is a motor using this method ever de-energized even momentarily during the transition from start to run there's always a pair of hands under it assuring it along and there's not a jarring disjoint in the transition this isn't always the case for other reduced voltage starter methods consider an open transition reduced voltage starter that must necessarily de energize a spinning motor even momentarily during the transition from start to run this would be a graph of inrush current characteristic of an open transition reduced voltage starter note the transient spikes that are occurred during the open shift that do not occur in a close transition reduced voltage starter the spike is a product of the open transition between the two axis open transition starters try to minimize the transition time between axes to minimize the disturbance however it's an intrinsic characteristic of open transition methods will revisit open transition reduce voltage starters in later lectures as long as we're looking at these inrush current graphs we should discuss how the timing of the transition from start to run mode affects e inrush current note if the transition from start to run takes place quickly motor is only given a short time to accelerate inrush current during the closed transition will be more if the transition from start to run mode takes place later motor is given more time to accelerate inrush current during the closed transition will be less I must reiterate and emphasize that primary resistors aren't intended to stay in the circuit for any length of time banks of primary starting resistors get notoriously hot when the drawing inrush current must be rated to handle this level of power dissipation this power dissipated as heat is a loss to the system and contributes to in efficiencies for this reason primary resistor reduced voltage starters are losing favor to increasingly efficient motor drives inefficiencies aside primary resistor reduced voltage starters are inexpensive and they get the job done in rush current is reduced and it minimizes mechanical stresses in the driven load thus far we've examined only the primary schematic primary resistor reduced voltage starters let's take a look at some sample pilot schematics that govern the operation of primary resistor reduced voltage starters our first ladder logic example of a primary resistor reduced voltage starter features an operator initiated start followed by an operator initiated transition from start to run mode rung 1 & 2 consists of a three wire control circuit governing the start contactor only rung 3 & 4 consist of a three wire control circuit governing the run contactor only note the normally open asked to contact in rung three this prevents an operator from closing the run contactor without having first closed the start contactor when an operator presses and releases the start push button the S contactor coil is energized and its associated contacts change states the ask primary contacts closed and the primary resistors limit inrush current yes one holding contact closes as does the s2 interlock in rung three an operator witnessing the motor and applied load come up to rated speed then initiates a transition to run mode by pressing and releasing the run button when the AR contactor coil is energized the our primary contacts closed and the primary resistors are switched out of the circuit full voltage supplied to the motor the AR one holding contact closes note the e stop stop a normally closed overload contact serves to de-energize both the start and run contactor coils thus opening both primary contacts and D energizing the motor the fundamental flaw to our system as currently implemented is that it relies upon the judgment of the operator to initiate the transition from start to run operators like most of humanity have a reputation of being fickle mush heads and cannot normally be relied upon to make consistent and sound judgments based off only visual and audible clues it is for this reason the run push button has been replaced in our second iteration with a rotational speed switch that closes when the motor has reached an acceptable transition speed note since the operators subjective judgment has been removed it's really not necessary to include the s2 interlock in rung three the operator still initiates a start by pressing and releasing the start push button however only when the motor has reached the set speed of the rotational speed switch does the system automatically initiate a transition from start to run mode by bypassing the bank of primary resistors note the are one holding contact ensures full voltages still apply to the motor should it experience any lulls in speed after the transition from start to run the setting of the rotational speed switch could be reduced such that the transition occurs early or the setting of the rotational speed switch could be increased such that the transition occurs later depending upon the needs of the mechanical system the desire to limit disturbances on the electrical distribution network note that this transition could also be time-based rather than speed based will discuss primary resistor reduced voltage starters using timers and later lectures finally consider a primary resistor reduced voltage starter that uses not one but two or more banks of primary resistors to limit inrush current in a stepwise manner when the asks or start contactor only is closed both primary resistor banks a and B serve to limit inrush current after predetermined period of time or after the motor has reached predetermined speed the are a primary contact are closes and bypasses the a bank primary resistors leaving only primary resistor bank B to limit in rush finally after another longer period of time or after the motor has reached a greater speed the R be primary contactor also closes and bypasses the B bank of primary resistors full voltage is applied to the motor the stage approach therefore serves to limit inrush current and mechanical stresses to the apply load B a graduated approach note more stages would increase the number of gradual steps however directly increase the cost weight size and complexity of this system here's an example of the pilot ladder logic diagram governing the behavior of this two-stage primary resistor reduced voltage starter using two rotational speed switches rotational speed switch a has a setting less than rotational speed switch B I leave it as an exercise to the viewer to determine how this staged primary resistor reduced voltage starter operates using this ladder logic diagram you should observe that when an operator initiates a start both resistor banks a and B lemma inrush current at the speed setting of rotational speed switch a resistor Bank a should be switched out at the speed setting rotational speed switch be both resistor banks should be switched out in full voltage would be applied to the motor here's a graph of line current for a regular full voltage starter for a lightly loaded motor the vertical scale is set to 5 amps per division in the horizontal scale is set to 0.2 seconds per division notice the huge current draw peeking out around 12 ish amps upon being instantaneously and directly connected to full voltage additionally note the motor accelerates briskly and reaches a stable current draw maybe three and a half to 4 --is-- divisions meaning it took the motor less than a second to accelerate the applied load this wouldn't be the method of choice for a fragile electrical distribution network or applied loads necessitating controlled acceleration in contrast consider the graph of line current for a 5 ohm primary resistor reduced voltage starter used on the same lightly loaded motor the vertical and horizontal scale are sent to the same respective 5 amps per division than 0.2 seconds per division notice the motor now takes five and a half to six ish divisions around 1.2 seconds to accelerate the applied load additionally note that current draw Peaks out at only nine amps the primary resistor reduced voltage starter therefore serves to limit inrush current demand the mechanical stresses to the applied load whose both curves superimposed on one another so you can compare the two methods the full voltage starter is in blue it is characterized by a substantial inrush in a brief acceleration period the 5 ohm primary resistor reduced voltage starter is in yellow it is characterized by lessen rush and longer acceleration if we increase the resistance of the primary resistor reduced voltage starter we can expect in rush to decrease and the acceleration period to increase here's the graph of line current for a 10 ohm primary resistor reduced voltage starter used on the same they loaded motor the vertical and horizontal scale are set to the same respect of five amps per division and point two seconds per division notice the motor now takes eight to eight and half ish divisions or just under two seconds to accelerate the applied load additionally note that current draw Peaks out at only seven amps the primary resistor reduced voltage starter with increased resistance therefore serves to further limit inrush current demand and mechanical stresses to the applied load here's all three curves superimposed on one another so you can compare the methods it's kind of busy but you should get the picture the full voltage starters in blue it is characterized by substantial in rush and a brief acceleration period 5 ohm primary resistor reduced voltage starter is in light yellow it is characterized by lessen rush and longer acceleration the 10 ohm primary resistor reduced voltage starter is in darker yellow it is characterized by even less in rush and even longer acceleration alright this about wraps up our introduction of primary resistor reduced voltage starters again be prepared to revisit reduced voltage starters using timers once we've gotten a basic introduction to the timer relay in conclusion the selection a brief look at primary resistor reduced voltage starters we learn reduced voltage starters I used to reduce the inrush current and modify acceleration starting torque characteristics the primary resistor reduced voltage starter method uses resistors in series of the motor intended to carry primary current when started primary resistors drop a portion of the applied voltage and once the motor reaches a predetermined speed or accelerates for a predetermined time are switched out remember to review these concepts as often as you need to really drive at home imagine how well lab will go if you know what you're doing thank you very much for your attention and interest and we'll see you again there in the next lecture of our series remember to tell your lazy lab partner about this resource be sure to check out the Big Bad tech channel for additional resources and updates