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Spin Testing Air Turbines

Spin Test System Drives: Air Turbine and Electric Motor Comparison

The comparison between spin system electric drives and air turbines is analogous to the choice between electric and gas-powered automobiles.  Both have advantages and disadvantages which must be considered before deciding which is appropriate for an application.  

At first glance, the electrical drive option appears to be the most attractive for Spin Pit Systems, including Dynamic Spin Rigs.  Electric drives use clean energy, some of which can potentially be saved during the braking phase of testing.  Compressed air drives require additional hardware (compressor, piping, valves, etc.) which appear to add cost and maintenance.

However, with over 30 years of experience in designing, building, and using spin test systems, Test Devices knows that the choice of drive system is critically important to any spin system.  Most Spin-Pit drive system must handle a wide range of test requirements including: over-speed (proof), burst, Low Cycle Fatigue (LCF), & High Cycle Fatigue (HCF).  Outlined below are some of the issues one must consider when choosing a spin test system (spin-pit).

Figure 1

Flexibility

Air turbines are compact drive systems which permit easy mounting and removal from the Spin Testing Chamber, requiring only a few connections or dis-connections.  The ease of switching turbines allows customers who require testing in multiple speed ranges (i. e. 10,000 rpm, 30,000 rpm, 50,000 rpm) to increase efficiency by performing 2-3 tests of different speed ranges in the same day.  All Test Devices’ turbines are easily mounted and removed using the same methods and bolt pattern.

Electrical drive systems have only one drive (controller)/motor combination, which must accommodate the system speed range required.  This restriction is not convenient for the wide range of applications requiring multiple speed ranges inherent with Spin Test Systems.

Another great advantage of spin test systems with air turbine drives is the ease with which the system can be upgraded to new turbines to handle different speed range requirements which are inevitably necessary.  Upgrading an electrical drive system is much more difficult and requires that the complete drive (controller) be replaced at a sizable investment.  Before selecting an electric drive system, the customer must decide if any future upgrades or changes will be necessary to accommodate requirement changes.

Robustness

Based on extensive experience, Test Devices has designed robust air turbines used for testing jet engine components to failure.  The turbines typically performs the test without incident and only rarely does specimen failure result in damage to the turbine, and then only relatively minor.  Test Devices has burst parts weighing many hundreds of pounds with little or no damage to the turbine.  When there is damage to the turbine, it is generally necessary to replace only damper components (seal, bushing, wear plate).  This is easily done with the turbine mounted on the spin chamber or a workbench, and the drive is ready for service in a matter of hours.

Electric motor drive systems include a gearbox unit, which is damage prone during high-energy bursts common during failure of jet engine components.  The gears inside the gearbox assembly do not tolerate the shock well resulting from a burst event, resulting in extensive damage.  Replacement of these gears is not only very expensive but also delays testing programs due to the long lead time required for delivery of replacement parts.

Operators of electric motor systems have confirmed that problems with the drive (controller), motor, or gearbox require supplier support.  The user is unable to make most repairs, which necessitates a visit from a factory representative to investigate the problem.  If the difficulty cannot be resolved on site, vital components of the drive system must be sent to the OEM’s shop for repair.  These repairs, generally unexpected, cause considerable interruption in the use of the system and essentially result in the DSR sitting idle for many weeks while the drive is repaired.

Conversely, as discussed above, in a high percentage of the cases where an air turbine is damaged from a rotor burst, the user can repair the turbine internally in a very short period of time.  Furthermore, other tests can be run with another turbine while the repair is taking place.

Reliability and Safety

Spin testing is characterized by rotating parts with high kinetic energy.  For safe and reliable operation, the drive system must be able to be slowed to 0 rpm in the shortest period of time.  Rapid braking is required during any of the following:

• emergency event,
• rotor high vibration,
• loss of electrical power, or
• rotor cracking.

The spin pit, which uses air turbines, has a redundancy feature that decelerates the rotating part to zero even when electrical power is interrupted.  Conversely, electrical drives cannot be controlled when power is lost, and rotor speed decelerates very slowly.  Only bearing friction is available to slow the test specimen, which can be a very long time for parts with large inertias.  This long coasting period can cause severe damage to the rotating parts or the system itself to say nothing of the lost equipment time.

Furthermore, many electric drive systems incorporate a drive belt, which transmits torque from the electric drive to the spindle head.  Drive belts break occasionally, resulting in the operator having no ability to slow the speed of the rotating part.  Some operators have had to wait literally days for large inertia parts to slow to 0 rpm after a belt failure.  Additionally, the spindle head is often damaged requiring extensive repairs.

Electrical Noise

Many spin-pits are used to test instrumented (strain gages, thermocouples, pressure transducers) engine components for strain surveys and low & high cycle fatigue.  To properly perform this testing successfully, signal integrity is paramount to ensure the accuracy of resultant data.  Because signal strength from these sensors is low, they are susceptible to outside interference and degradation from interference and electrical noise can be detrimental and costly to good results.

Air turbines, because they are mechanically driven devices, provide no electrical interference and do not degrade the signal output from instruments mounted to the test disc.

Electrical drives by their vary design introduce substantial electrical noise to the spin chamber environment.  Every electrical drive system, especially variable frequency drives systems, double convert the electrical signal from the power line to the electrical motor.  This induces high electrical noise in the power line and inside the test facility by electromagnetic radiation.  Even with excellent shielding and filtering, this problem still exists along with the presence of high harmonics content, which cannot be eliminated.

Electrical drive noise is a critical issue for blade characterization tests when investigating high frequency modes with very low signal levels.  Through the path where signals are transmitted, including slip rings mounted on the drive head or gear box, noise accumulates and negatively affects the signal to noise ratio to a level that is not acceptable for this type of testing.

The following is an excerpt from a Vishay Measurements Group (supplier of strain gages) paper on electrical noise interference with strain gages which speaks directly to this issue.

“Virtually every electrical device which generates, consumes, or transmits power is a potential source for causing noise in strain gage circuits.  And, in general, the higher the voltage or current level, and the closer the strain gage circuit to the electrical device, the greater will be the induced noise.”

Therefore, a customer can have more confidence in the data taken from an instrumented test run with an air turbine than with an electrical drive.

LCF Testing Efficiency

A spin chamber drive intended for LCF service has to accelerate and brake the test rotor inertia in the shortest possible time to maximize operating cycles per day.  Cycle rate is productivity.  Longer cycle times delay engine development programs, and thereby cost the user many times more than any savings that might accrue from operation of electric motor drives.

Air turbines are the best drive choice for LCF testing because their high operating speed and inherent torque characteristics give short cycle times and are relatively simple mechanically.

Air turbines give high drive torque at low speed, with lower torque at high operating speed.  Brake torque is high at all speeds and is an important reason for the cycle time advantage of air turbines.  The high brake torque is also very critical when the speed of a rotating part must be reduced to 0 rpm very rapidly due to high vibration, a developing rotor crack, or loss of electrical power.  The graph below shows the drive and brake torque of a typical Test Devices air turbine operated at 90 psi nozzle pressure (for model 706-90 shown).

The graph plots both torque and horsepower as a function of speed.  Note that torque is linearly related to speed, with both drive and brake torque equal at zero speed.  Notice also that drive horsepower peaks when the torque is exactly half the at-rest torque.  The speed at this point is called “design speed” and is the point where the velocity of air leaving the drive rotor is most nearly zero.

Figure 2

Torque is produced by the relative velocity between the rotor and the air exiting the nozzle ring.  In the drive direction, that relative velocity decreases as the rotor speed increases.  In the brake direction, the opposite is true and the relative velocity increases as speed increases.

Calculation of cycle time for an inertial load equating the first differential of speed with the speed function and separating variables, the differential time can be integrated to show that acceleration time from rest is:

Acceleration time = ω*I/* (ln(T_f/T_o)/(T_f-T_o)) (for linear variable torque)

Where:
T_o = Torque at zero speed
T_f = Torque at Peak Cycle Speed
ω = Cycle Speed (radians per second), and
I = Inertia (lb-in-sec²)

Because LCF cycles don't start from zero speed, and because acceleration is not linear, one must calculate acceleration time from zero speed to peak speed, calculate acceleration time from zero speed to the cycle minimum speed, and subtract the two times.  The difference is the acceleration time from minimum to maximum speed.  The braking time is calculated the same way, but the torque is different, so the calculation is repeated using the brake torque values.

Table 1

Electric motor drive torque is limited by the maximum available air gap flux density, and therefore electric motors produce constant torque.  Power is the product of torque and speed, thus, electric motors give half power at half speed, 1/10 power at 1/10 speed, etc.

Using the calculation shown below, a 75Hp, 40000 RPM constant torque drive would give a much longer cycle time:

Constant Torque Acceleration:

Acceleration time (for constant torque) = (ω_f-ω₀)*I/T
Cycle time = 2 x Acceleration time.

Net cycle time = 61 seconds

Where:
T = Torque (117 in-lb at 75 Hp)
I = Inertia (1.03 lb-in-sec²)
S₀ = Peak Cycle Speed (rpm)
ω₀ = Peak Cycle Speed (radians per second) = S₀*2 p/60
S_f = Peak Cycle Speed (rpm)
ω_f = Peak Cycle Speed (radians per second) = S_f *2 p/60

Cycle time for a constant torque drive (electric motor) is much longer than for an air turbine of the same nominal power.  For example, consider the 706-90 drive turbine from the calculations above.  It produces 90.3 horsepower at design speed, 76 Hp at 37000 rpm.  With an inertia of 1.03 lb-in sec2, the cycle rate would be 20.33 seconds. Because of its favorable torque characteristics, an air turbine produces about 3 times as many test cycles as an electric motor with the same power rating (20.33 seconds vs. 61 seconds from above).  With an air turbine drive system the Spin Test System will operate at optimum performance for the desired speed range by selecting and installing the proper turbine.  Electrical drives do not allow the customer this flexibility, because the system is fixed by the initial design.

Summary

The issues discussed above clearly indicate that an air turbine is a better choice over an electrical drive and is the best drive mechanism for spin pit systems.  The flexibility of the air turbine drive is particularly important to users who would like to use one spin pit for many different speed range and testing profiles.

The speed of TDI turbines can also be precisely controlled which is critical for any HCF testing program.  TDI turbines coupled with the speed control system can control rotor speed with an accuracy of +/- 0.01% of the full speed scale.  Also the turbines can be maintained and repaired on-site by customer personnel.

A critical requirement of any Spin Test System is the capability of the braking system to decelerate the test piece at any moment during operation.  Spin Chambers (Spin Pits) exist because of the huge energies imparted to the test specimen during testing.  To experience an event where that much energy is left uncontrolled is undesirable any unsafe.  The air turbine drive system is capable of slowing the test rotor speed to 0 rpm even when electrical power is interrupted.  A Spin System without this drive system feature is analogous to driving a car without a braking system.

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