Continuing Education: Elevator Drives Replacing Motor-Generator Sets with Modern Static DC Drives by Jim Papez and Mark Kobiske Although such MG systems per- Learning Objectives formed satisfactorily for years, they After reading this article, you were not without drawbacks. Some should have learned about common concerns included: N the drawbacks of motor-gener- N High power consumption: A motor ator sets. and generator working together to N the value of static DC drives. generate DC power is inefficient. N considerations when replacing Additionally, an MG set creates a con- a motor-generator set with a stant and significant power drain – digital DC drive. even at idle speed when the elevator N input power considerations. is not in motion. N N the issue of power-line distor- High routine maintenance: MG sets tion. are often specified at minimum size N sizing criteria for transformers. and cost but must rotate at relatively N output power considerations. high speeds to meet DC power re- N ripple-filter recommendations quirements. As a result, frequent and and adjustments. costly maintenance is a necessity. N N car-controller considerations in Excessive generator brush wear: a modernization with a DC drive. Brush wear is a continuing problem. N commissioning a DSD 412 DC Additionally, the large amount of car- elevator drive. bon dust generated can spread throughout the machine room and N verification of motor data and cause failure of other electrical wiring. equipment. N motor field board setup. Eventually, the cost and high main- N recording motor data and en- tenance required for MG sets rendered coder setup. these impractical – even while the N grounding considerations. rugged, low-speed DC hoist motor N power-up verification and ad- kept operating as well as the day it vanced diagnostics. Value: 1 contact hour was first installed. Static DC Drives to the Rescue At the same time, power electronics This article is part of ELEVATOR WORLD’s For many years, high-performance utilizing silicon-controlled rectifier Continuing Education program. Elevator-industry personnel required to obtain continuing-education elevator systems utilized an extremely (SCR) static switches to directly con- credits can receive one hour of credit by reading rugged, open-frame, brush-type DC vert AC line power to DC were provid- the article and completing the assessment exami- motor to power the hoist. ing new solutions to some of the MG nation questions found on page 111. The controlled DC power was set problems. This advance virtually For this article and more continuing-education generated via an AC motor operating eliminated the need to provide rotating opportunities, visit www.elevatorbooks.com from line power and driving a DC gen- and high-maintenance components NATIONAL ASSOCIATION OF ELEVATOR CONTRACTORS ® erator – a classic motor-generator set. involved in the production of DC Approved by NAEC for CET CERTIFIED This control configuration, known as power. ELEVATOR TECHNICIAN EDUCATION PROGRAM a Ward Leonard system, basically Additionally, conversion efficiency regulates DC power to the hoist was greatly improved, and idle losses Approved by NAESAI for QEI motor armature by controlling the were drastically reduced, resulting in generator field. much lower power consumption. Continued June 2007 | ELEVATOR WORLD | 85 Continuing Education: Elevator Drives Continued Static components produced re- manufacturers to exit the MG busi- switch to another. The result is a har- markable savings on maintenance, ness, resulting in a severe shortage monic distortion on the power line. making MG set brush wear and carbon of replacement parts for existing MG Potentially harmful effects are generally dust contamination things of the past. set installations. eliminated by use of an input isolation Crowded machine rooms also bene- Thus, more and more MG sets are transformer with good secondary fited from the comparatively small size being replaced with all-digital DC reactance. of the static drive. drives as part of modernization pro- SCR Switching (commutation): Two Today’s Advanced, All-Digital Static grams or simply to reduce mainte- conditions must be met to provide DC Drives nance costs. proper operation: 1) the AC voltage Incorporation of modern, micro- Application Considerations must be equal to or greater than the processor-based controllers into today’s Three major considerations must be DC voltage, and 2) at least 2-4% re- all-digital elevator drives provides: met when replacing a MG set with a actance on the input is needed. Both N Minimum installation and operat- digital DC drive. Those considerations of these conditions can be met with ing cost. are: an input transformer. N High reliability. N Input power; Transformer Sizing Criteria: N N Exceptional performance to help N Output power; and Input primary voltage must match reduce passenger waiting times and N Car control interface. the line voltage. N floor-to-floor times. Input Power Considerations Output secondary voltage must N All-digital control to ensure re- Power Feed: Since it converts AC equal or exceed DC armature volt- peatability over time and consistent to DC directly, the DC drive will draw age. If there is a potential for the line operation of all elevators in a group. approximately 30% more amps from voltage to drop, then a 10-20% in- Digital DC drives have virtually the line during car acceleration. crease in secondary voltage is rec- eliminated problems experienced Also, the requirement for peak accel- ommended as a safety factor. N 5% voltage taps are recom- with the earlier analog versions. eration amps is immediate, rather mended to allow for unforeseen field Analog DC drive problems included than built up gradually as with the conditions, such as line voltage drop difficult adjustments caused by nu- MG set. The power line must be able or the need to more closely match ar- merous potentiometer settings that to support these conditions without mature voltage for power factor or were sensitive, often interactive and excessive voltage drop affecting performance. could not be replicated successfully equipment on the same feeder. N Continuous (rated) secondary on another car; component drift with Input Voltage: MG sets convert line amps should equal or exceed 1.05 X age and environmental changes that volts to armature volts via rotating 0.82 X full-load DC armature amps. affected car performance; and high components. Static DC drives do the N Continuous (rated) KVA rating is part count with many more electrical AC to DC conversion directly via secondary amps X secondary volts X connections that negatively im- three-phase rectification. If the DC 1.732. pacted reliability. voltage required by a hoist motor is N The transformer must not saturate Today’s DC elevator drives are de- significantly lower than the line when acceleration amps are substituted signed and field-qualified specifically input voltage, the drive will lower for full-load amps in the above formula. for elevator applications. They have the input power factor. A low power N The secondary reactance should the high lift capacity demanded for factor means a higher KVA require- be at least 2-4%. minimum floor-to-floor times and can ment that needs to be supported by Output Power Considerations withstand the rigorous duty cycle as- the power distribution equipment These are somewhat more empir- sociated with high-speed elevators. and wiring. Older buildings may not ical, as they depend on both motor In addition, modern software incor- have this capability, and thus, an input and hoistway conditions. Thus, some porates unique control algorithms to transformer is recommended to make degree of application experience must provide optimum performance and a the voltage input to the drive compara- be taken into consideration. broad spectrum of input/output capa- ble to motor armature voltage. As will DC Motor Armature: Elevator mo- bilities for car control interface. be seen, there are other good reasons tors almost always have armatures DC Drives as MG Set Replacements for using this transformer. designed to minimize torque ripple Since the introduction of DC static Power Line Distortion: Static switch- and provide for a smooth ride. This is drives, MG sets have essentially disap- ing draws square-wave current from also the best motor for operation peared from use in new installations. the line and creates a notch when from a static DC drive. In the rare This, in turn, has caused some current is commutated from one older hoist motor that may not have 86 | WWW.ELEVATOR WORLD | June 2007 this armature design, torque ripple armature inductance and resistance. conjunction with the recommended may be accentuated by the static DC N Adjustable capacitance and damp- ripple filter will achieve more than drive. If that results in a performance ing resistance should be included with adequately quiet operation. Many issue, such as car vibration, then the basic design criteria of roll-off times – almost always for geared greater care in adjustment is needed. starting at approximately 300 Hz and machines – the addition of inductance An increase in the size of the DC rip- a tuned peak of approximately 100 Hz alone will achieve the desired results. ple filter may also be necessary to further (Q of 1.2-1.5). If it does, then the capacitors are per- reduce drive-generated voltage ripple. N The filter should be rated for con- forming no useful function and may DC Drive Output Voltage Ripple: tinuous operation at full-load arma- be disconnected from the filter. Doing Three-phase rectification produces a ture current and not saturate at accel- so now removes an unnecessary com- 360-Hz ripple on DC voltage. This eration current. Inductance needs to ponent from the power circuit, which ripple can excite the hoist motor and be constant from very low armature in turn, improves reliability and re- produce acoustical noise in the motor current through maximum accelera- duces future maintenance of the filter and mounting structure.