Motor and Drive Interaction - Part 2.pdf

Motors and Drives Part 2

Hydraulic Institute / Pump Systems Matter • Pump Systems Matter (PSM) – is a non-profit educational organization established by the Hydraulic Institute, and leading utilities and energy efficiency organizations, to educate the industry on the benefits to pump systems optimization and energy efficiency to improve bottom-line savings of end-user companies. • Hydraulic Institute The mission of the Hydraulic Institute is to be a value-adding resource to member companies, engineering consulting firms, and pump users worldwide by developing and delivering comprehensive industry standards, expanding knowledge by providing education and tools for the effective application, testing, installation, operation, maintenance, and performance optimization of pumps and pumping systems, and by serving as a forum for the exchange of industry information. For more information on the Hydraulic Institute, its member companies and its Standards Partners, visit www.Pumps.org

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Meet Your Instructor • Currently the Power Generation Business Development Manager for WEG Motors • Over 40 years’ of experience in the pump industry designing, field testing, repairing and troubleshooting mechanical seals, compressors and pumping systems • An active Hydraulic Institute member for a number of years, including Pump Systems Matter Train the Trainer expert, Certified Hydraulic Institute Pump System Assessor and top PSO instructor 3

Motor Designs • The Material and Shape of the Rotor Bars Are the Main Factors in Obtaining Various Speed / Torque Curves • NEMA Defines 4 Basic Types of Speed/Torque Characteristics for Induction Motors: • DESIGN A • DESIGN B • DESIGN C • DESIGN D • The Stator Has Little to Do With the Shape of the Motors Speed/Torque Curve • Different Rotors Can Be Used With the Same Stator to Change the Characteristic Shape

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Definitions of Motor Torques • Locked-rotor Torque of a motor is the minimum torque, which it will develop at rest for all angular positions of the rotor, with rated voltage applied at rated frequency. • Pull-up Torque of an alternating-current motor is the minimum torque developed by the motor during the period of acceleration from rest to the speed at which breakdown torque occurs. For motors, which do not have a definite breakdown torque, the pull-up torque is the minimum torque up to rated speed. (CAN BECOME CRITICAL IN STARTING APPLICATIONS) • Breakdown torque of a motor is the maximum torque which it will develop with rated voltage applied at rated frequency, without an abrupt drop in speed. • Full-load torque of a motor is the torque necessary to produce its rated horsepower at full-load speed. In pounds at a foot radius, it is equal to the horsepower multiplied by 5252 divided by the full-load speed. 5

Typical Speed - Torque Curve Motor torque is defined at four points by NEMA on a Speed-Torque curve and they are:

NEMA Design A Speed Torque Curve

Operating to the Right of Breakdown

Torque

Speed

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Comparison of NEMA Designs

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Design B Design B Standard general purpose design for fans, blowers, pumps, etc.. (60-70 % of all application)

Design B Motors:

Are the standard general-purpose design. They have low starting current, normal torque, and normal slip. Their field of application is very broad and includes fans, blowers, pumps, and machine tools.

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Design A Design A

% Full Load Torque

Same shape as Design B except A will provide higher Breakdown torque and starting torque (10-15% of all applications)

% Synchronous Speed

Design A Covers a wide variety of motors similar to Design B except that their breakdown torque and starting current are higher.

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Design C

Design C High locked rotor torque for hard to start applications, conveyors, compressors

Design C Motors have high breakaway torque, low starting current, and normal slip. The higher breakaway torque makes this motor advantageous for “hard-to-start” applications, such as plunger pumps, conveyors, and compressors.

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Design D Design D Motors have a high breakaway torque combined with high slip. Breakaway torque for 4, 6 and 8 pole motors is 275% or more of full load torque. Three slip groups are described below.

Design D

Motors have 5 – 8% and 8 – 13% slip and are recommended for punch presses, shears, and other high inertia machinery, where it is desired to make use of the energy stored in a flywheel under heavy fluctuating load conditions. They are also used for multi-motor conveyor drives where motors operate in mechanical parallel.

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Sequence of Motor Starting

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Motor Starting Considerations •

The motor torque vs. speed curve is usually provided at rated voltage. The torque and current of the motor will change with voltage. The relationship between torque and voltage is as follows:

Torque  Voltage2 •

If voltage at the motor terminals is low, the torque produced by the motor will drop by approximately the square of the voltage. Therefore, a motor with 90% rated voltage at the terminals will have about 81% of peak torque. Since the motor has lower torque, acceleration time will increase. If the torque of the motor drops such that it is not greater than the torque required to accelerate the load, the motor will not reach rated speed.

Current  Voltage •

The current of the motor drops proportional to voltage. Therefore, a motor with 90% rated voltage has current reduced approximately 10% throughout the speed range.

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The acceleration time of the motor is dependent on many factors. The motor torque and inertia as well as the load torque vs. speed curve and inertia have to be known to determine acceleration time. The acceleration time of a motor can be calculated using the following equation. Acceleration Time = (WK2M + WK2L) x (RPM (Final)- RPM(Initial))

308 x TAVG

WK2M = Inertia of the motor in lb-ft2. WK2L = Inertia of the load in lb-ft2. RPM = Change in speed in rpm. TAVG = Average acceleration torque in lb.-ft. (Average motor torque less average load torque from minimum to maximum speed)

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Motor Starting - Inrush Currents • Locked Rotor Current typically 600-700% of full load • Current and torque aren’t linear until near full load • Beware of applying ultra high premium efficiency motors on applications that require lots of starts (or could frequently reverse) can be as much as 13x FLA due to high X/R ratio

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Upstream Protection

Motor Starting Methods

Primary Resistance

Electromechanical Starters

Primary Reactance

Variable Speed Drive

Auto transformer

Soft Starter

Star Delta

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Motor Starting Methods Variable Frequency Drive NEMA Design A and B generally acceptable for Adjustable Frequency Drives NEMA Design C and D may present problems • May produce high peak currents that could trip drive • Design C, high starting torques difficult • Design D, high peak loads cause problems

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Starting Motors with Variable Frequency Drives • AFD controls frequency, voltage and current

• The volts per hertz ratio is controlled to operate the Motor as if it were already at full speed • This allows access to maximum torque (breakdown torque) through acceleration • Available Starting Torque is high and only limited by the max current capacity of the drive and the power grid • Motor is operated on the right side of the Breakdown on the motor speed torque curve • Rotor Heating is minimal due to reduction in slip losses

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Motor Design Issues • • • • • •

Added Heating of Winding (Class F Insulation) Added Winding Insulation stresses Added chance of Bearing currents Added chance of Vibration issues Effect on Sound Levels Large Motor Concerns • How will VFD be used? • Key details needed to choose Large motors for VFD’s

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Motor Heating from VFD • The non-sinusoidal VFD waveform contains harmonics and peak voltage/current in excess of normal sine wave grid power • On LV VFD’s it is common for the motor to see an additional 10-15 degrees C temperature rise • On MV VFD’s, motors typically see only a 3-5 degree C temp. rise

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Motor Selection

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AC Motor Selection Criteria When selecting an AC motor and associated equipment for an application, the following points should be considered: Environment The environment in which the motor operates is a prime concern. Conditions such as: ambient temperature, air supply, the presence of gas, moisture or dust should all be considered when choosing a motor. (End User) Speed Range The minimum and maximum speeds for the application will determine the motor base speed. (OEM) Speed Variation The allowable amount of speed variation should be considered. Does it require constant speed at all torque values or will variations be tolerated? (OEM) Torque Requirements The starting torque and running torque should both be considered when selecting a motor. Starting torque requirements can vary from a small percentage of the full load to a value several times full-load torque. The starting torque varies because of a change in load conditions or mechanical nature of the machine. The motor torque supplied to the driven machine must be more than that required from start to full speed. The greater the excess torque, the more rapid the acceleration. (OEM) Acceleration The necessary acceleration time should be considered. Acceleration time is directly proportional to the total inertia and inversely proportional to the torque. (Motor Vendor) Duty Cycle (RMS Calculation) Selecting the proper motor depends on whether the load is steady, varies, follows a repetitive cycle of variation or has pulsating torques. The duty cycle which is defined as a fixed repetitive load pattern over a given period of time is expressed as the ratio of on-time to the cycle period. When the operating cycle is such that the motor operates at idle or a reduced load for more than 25% of the time, the duty cycle becomes a factor in selecting the proper motor. (OEM/Vendor)

Heating The temperature of an AC motor is a function of ventilation and losses in the motor. Losses such as operating self ventilated motors at reduced speeds may cause above normal temperature rises. De-rating or forced ventilation may be necessary to achieve the rated torque output at reduced speeds. (Motor Vendor)

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Load Considerations • The process of selecting an adjustable speed AC drive is one where the “load” is of primary consideration. • It is important to understand the speed and torque characteristics as well as horsepower requirements of the type of load to be considered. • The demands and economics of a particular application should be matched to the drive capabilities. After this matching process is completed, the decision regarding the type of adjustable speed drive can be made.

• When considering load characteristics, the following should be evaluated: • What type of load is associated with the application? • Does the load have a shock component? • What is the size of the load? • Are heavy inertial loads involved? • What are the motor considerations? • Over what speed range are heavy loads encountered? • Motor loads are classified into three main groups, depending on how their torque and horsepower varies with operating speed.

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Load Types Constant Torque Load • This type of load is the one most frequently encountered. In this group, the torque demanded by the load is constant throughout the speed range. The load requires the same amount of torque at low speeds as at high speeds. Loads of this type are essentially friction loads. In other words, the constant torque characteristic is needed to overcome friction.

Examples of this type of load are conveyors, extruders and surface winders. Constant torque is also used when shock loads, overloads or high inertia loads are encountered.

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Load Types Constant Horsepower load • In this type of load, the horsepower demanded by the load is constant within the speed range. The load requires high torque at low speeds. From Equation L1, you can see that with the horsepower held constant, the torque will decrease as the speed increases. Put another way, the speed and torque are inversely proportional to each other.

Examples of this type of load are center-driven winders and machine tool spindles. A specific example of this application would be a lathe that requires slow speeds for rough cuts and high speeds for fine cuts where little material is removed. Usually very high starting torques are required for quick acceleration.

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Load Types Variable Torque Load

• With this type of load, the torque is directly proportional to some mathematical power of speed, usually speed squared (Speed2). Mathematically:

Horsepower is typically proportional to speed cubed (Speed3). The below figure shows the variable torque and variable horsepower demanded by the load.

Examples of loads that exhibit variable load torque characteristics are centrifugal fans, pumps and blowers. This type of load requires much lower torque at low speeds than at high speeds.

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What does this look like to the motor?

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Motor Insulation • Existence of Corona / PD can quickly degrade insulation systems • Corona is caused by Voltage TRANSIENTS • Need to ELIMINATE (not just resist) corona for full life expectancy • Coordinate Motor - VFD / Installation as a “system”

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Effect of Corona on Motor Insulation “Partial Discharge” The corona discharges in insulation systems result in voltage transients EFFECTS OF CORONA

Corona, also known as partial discharge, is a type of localized emission resulting from transient gaseous ionization in an insulation system when the voltage stress, i.e., voltage gradient, exceeds a critical value.

2,400 V PEAKS 5000 V/msec dV/dt

The ionization is usually localized over only a portion of the distance between the electrodes of the system.

Corona Inception Corona inception voltage is the lowest voltage at which continuous corona of specified pulse amplitude occurs as the applied voltage is gradually increased. Corona inception voltage decreases as the frequency of the applied voltage increases. Corona can occur in applications as low as 300V

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Circulating Current Paths in AC Motors

Because of the transient nature of the common mode voltages and common mode currents, we have to look at current paths a bit differently than we would for 50 or 60 Hz sinewaves. Specifically, we have to see the transitions as high frequency events. With high frequencies, there is a greater opportunity for capacitive current flow, so insulated parts of motors, even air gaps, need to be thought of as capacitors. While these capacitors may be absolutely negligible in magnitude for 60 Hz waveforms - being down in the nano-farad range - they are not at all negligible for PWM waveforms. We can see in this sketch, capacitors representing the non-conductors of air gaps, stator winding to stator core and stator winding to rotor. While the shaft extension is not normally a conductor, it can in fact end up participating in carrying common mode currents to the coupled equipment such as a pump, gearbox, or other machinery.

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Grounding System Insulated Bearing Housing

Ceramic Bearing

Low Impendence Grounding Strap Shaft Grounding Brush

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Incorrect Grounding

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Bearing Damage Mechanisms from Electrical Current Fluting in outer race, from prolonged operation after damage from current flow

Individual arc damage spots

With that brief background, let's look at the topic of just how current flow in a rotating bearing does its damage. The photo on the left is a section of an outer race of a ball bearing that has progressed to a point where the surface is said to be "fluted.” This fluting or washboard pattern is commonly associated with current flow, however, as we will see the existence of fluting does not require a root cause of current flow. The photo on the upper right is a high power magnification of individual arc damage in a bearing race. In fact, this is taken with a scanning electron microscope, and these pits are small enough that you would not expect to find them with the naked eye. It is possible, however, that with sensitive vibration monitoring, an "outer race defect frequency" may show up in association with these pits - though the amplitude would be expected to be quite low.

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Bearing Damage Mechanisms from Electrical Current

“Frosting” Fluting on inner race, from prolonged operation after damage from current flow

Fluting in outer race

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Motor Windings Coil Head Failure

Did not have shaft grounding or isolated bearings.

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Motor / Voltage Spikes Motor Insulation Possible solutions to the above compatibility issues include: • Keep lead lengths between the VFD and the motor within the motor manufacturer’s tolerances • Use a motor designed with insulation specified for VFD use and capable of handling these peak voltages • Reduced carrier frequency • Apply external RLC filters at the VFD’s output terminals • Apply line reactors at the motor’s input terminals • Motor winding insulation damage from voltage spikes become more prevalent the higher the system voltage. NOTE - An RLC filter consist of a resistor (R), an inductor (L), and a capacitor (C).

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Checklist for VFD Pump Operation • TDH vs. Static head? • Length of power cables? • Minimum speed

– Where on pump curve? – Cooling issues? • Harmonic filters? • Shielded signal cables? • How will control be programmed? 38

Case Study

Re-circulation Loop

150 HP Pump

250 HP Pump

50 HP Pump

Header

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Case Study Three Pumps • 150 hp is the main pump. • With increased demand, the 50 hp is started.

• With even more increased demand, 150, 50 shut down, start 250 hp. • Finally, run all three together.

• All adjustments done manually. • Circulation loop (blue valve) – 6” valve into 2” restriction, back into 6” pipe – controlled with a “dump” valve.

Lots of Cavitation 40

Case Study Making the Business Case • Pump Failures every 6 months -------------- $80,000 year

• Motor Failures ----------------------------------- $45,000 year • Cost of Down Time ---------------------Minimum 12 hours, reduced production

• Labor Cost ------------------------------------------ $2,000 per failure x 2 = $4,000

Total cost less energy and down time = $129,000

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Case Study

# 5 Water Pump 150 hp pump delivered 1500gpm

Average demand 800-1000gpm

Pumping more water than required

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Case Study

• Pressure needed was not 130psi, but 100psi (230 ft. of head). • Retrofitted to use 150hp pump all the time with a VSD. • New average power is 73 kW.

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System Implications to Consider VFD Pump Systems • Select pump duty point to the right of BEP • Consider full speed motor / VFD efficiency? • Motor heat load due to drive wave form ? • Check cable lengths (Critical Cable Length Calculation) • Check if filters are needed (Inlet at drive or termination) • Design VFD for by-pass operation at 60Hz

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Conclusion • Understand Application / System Requirements • Develop Drive Specification • Test Motor Drive System (prior to shipping) • Follow Installation Instructions (based on application • Use QA/QC Check List During Installation Process • Monitor System for Proper Operation • Implement Preventive Maintenance Program

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Start

Decision Tree

Consider System Demands

Confirm system and duration curves. Establish if not available.

Flow Chart to assess the suitability of retrofitting a VSD to an existing pump system

Consider reducing system losses

NO Is duty available?

Confirm existing fixed speed pump correctly sized

YES

Consider modification or replacement equipment

NO

YES

VFD potentially useful

Retain existing installation if efficient NO Mostly friction (rotodynamic only) YES

Check overall benefits include non energy items ie: reduced maintenance cost

Calculate total annual operating cost with alternative system solutions

VFD potentially useful Is VFD suitable?

NO

NO Does pump run most of the time? YES VFD almost certainly beneficial

Are existing pump and motor suitable for proposed variable speed

NO

YES

Select drive and perform financial justification Source – Hydraulic Institute LCC Guide Book

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Key Points • Use a system approach to design and manage the pumping system operation, motor, pump, drive, equipment • Understanding the solutions available to improve performance with help you realize that there may be more than one solution to solve the problem!

• Variable Speed Drives are best applied when demand varies over time

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Closing Thoughts • If you want to improve your pumping systems, follow the plan • Pump systems are big energy users in many plants – Know how much these systems are costing you

• Lifetime energy costs can be 25 times the installed cost of a pumping system Think Life Cycle Costing • Look beyond energy savings • Systems Optimization focuses on improving the reliability of the system thus reducing total system costs

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Closing Thoughts • Need to understand the symptoms that occur to the system when it operates away from BEP • Many pumping systems are not well designed or controlled • Efficiency and reliability go hand-in-hand • Screening provides valuable information on how many systems should be further assessed • Build a Business Case, Speak the Language of Management (bottom line results)

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Take Away Provide Solutions Listen to the customers issues and concerns

- Ask qualifying questions to establish business case - Ask the right questions understand the problem(s)

- Think what is the best solution (provide options) - Solve the Problem using the best solution along with Life Cycle Cost justification (build business case)

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Where to Go to Get Help •

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Visit the these Website Resources: • www1.eere.energy.gov/industry • www.superiorenergyperformance.net • www.Pumps.org • www.WEG.net • www.energyquickstart.org Send your staff to web seminars and courses available from the DOE, PSM, Hydraulic Institute and others

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Explore local efficiency programs and utility rebates!

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Bring in a pumping system specialist to help you

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Purchase the ASME Energy Assessment for Pumping Systems Standard 51

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Thank you Questions / Comments

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