82510_R.pdf

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Manual 82510(Revision R)

Original Instructions

Magnetic Pickups and Proximity Switchesfor Electronic Controls

Reference Manual

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DEFINITIONS

This is the safety alert symbol. It is used to alert you to potential personalinjury hazards. Obey all safety messages that follow this symbol to avoidpossible injury or death.

DANGERIndicates a hazardous situation which, if not avoided, will result in deathor serious injury.

WARNINGIndicates a hazardous situation which, if not avoided, could result indeath or serious injury.

CAUTIONIndicates a hazardous situation which, if not avoided, could result inminor or moderate injury.

NOTICEIndicates a hazard that could result in property damage only (includingdamage to the control).

IMPORTANTDesignates an operating tip or maintenance suggestion.

The engine, turbine, or other type of prime mover should be equipped with anoverspeed shutdown device to protect against runaway or damage to the primemover with possible personal injury, loss of life, or property damage.

The overspeed shutdown device must be totally independent of the prime movercontrol system. An overtemperature or overpressure shutdown device may alsobe needed for safety, as appropriate.

Read this entire manual and all other publications pertaining to the work to be performed beforeinstalling, operating, or servicing this equipment. Practice all plant and safety instructions andprecautions. Failure to follow instructions can cause personal injury and/or property damage.

This publication may have been revised or updated since this copy was produced. To verify thatyou have the latest revision, be sure to check the publications page on the Woodward website:

www.woodward.com/searchpublications.aspx

The current revision and distribution restriction of all publications are shown in manual 26311.

The latest version of most publications is available on the publications page. If your publication isnot there, please contact your customer service representative to get the latest copy.

Any unauthorized modifications to or use of this equipment outside its specified mechanical,electrical, or other operating limits may cause personal injury and/or property damage, including

damage to the equipment. Any such unauthorized modifications: (i) constitute "misuse" and/or"negligence" within the meaning of the product warranty thereby excluding warranty coveragefor any resulting damage, and (ii) invalidate product certifications or listings.

To prevent damage to a control system that uses an alternator or battery-chargingdevice, make sure the charging device is turned off before disconnecting the batteryfrom the system.

To prevent damage to electronic components caused by improper handling, read

and observe the precautions in Woodward manual 82715, Guide for Handling andProtection of Electronic Controls, Printed Circuit Boards, and Modules.

Woodward reserves the right to update any portion of this publication at any time. Information provided by Woodward isbelieved to be correct and reliable. However, no responsibility is assumed by Woodward unless otherwise expresslyundertaken.

Woodward 1969All Rights Reserved

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Manual 82510 Magnetic Pickups and Proximity Switches

Woodward 1

Magnetic Pickups and Proximity Switchesfor Electronic Controls

Be prepared to make an emergency shutdown when starting theengine, turbine, or other type of prime mover, to protect against

runaway or overspeed with possible personal injury, loss of life, orproperty damage.

Magnetic Speed Pickup

The magnetic speed pickup (MPU) is used to detect the speed of the primemover. It is necessary when the prime mover drives something other than analternator, and is often used where an alternator is driven directly by the primemover and when a control signal is necessary before the alternator comes up toits proper output voltage. A speed sensor circuit, either a section on the governoramplifier chassis or a separate unit, is needed to convert the MPUs output signal

to one usable by the governor amplifier.

The magnetic pickup produces a voltage output when any magnetic materialmoves through the magnetic field at the end of the pickup. Since most enginesand turbines have flywheels or other large gears made of magnetic material(usually iron or steel), magnetic pickups can usually be installed without addingattachments to a gear or shaft. Nonmagnetic materials, such as aluminum, brass,and some stainless steels, will not excite the magnetic pickup.

The MPU makes use of a stray magnetic field and no provision for returnmagnetic circuits or paths is necessary. Any device which produces a dynamicdiscontinuity of magnetic material in the field of the pickup will produce anelectrical voltage. Although gears are the normal devices measured by an MPU,

other devices such as a vibrating surface, moving bar, crank, wheel spokes, or asteel screw head mounted on some moving surface will work equally well ifsurface speed and other factors are taken into account. The MPU may be excitedby a keyway or slot in a wheel, but there is likely to be an unwanted backgroundsignal due to varying density or eccentricity of the material. It is better to excitethe MPU from a protrusion on the surface. This places the pickup at a relativelygreat distance from the materials between excitation periods and it is less likelyto pick up stray signals.

The output voltage of a magnetic pickup is affected by three factors.

Voltage increases with increases of the surface speed of the monitoredmagnetic material.

Voltage decreases as the air gap between the magnetic pickup and thesurface of the gear tooth is increased.

Voltage waveform is determined by the size and shape of the gear tooth inrelation to the size and shape of the pole piece.

With any given speed and clearance conditions, a maximum power output willresult when the field is filled with a relatively infinite mass of magnetic material atone instant and a complete absence of such material the next. A reasonableapproach to these conditions exists when the cross-section of the exciting massesis equal to or greater than that of the pole piece, and the space between is equal toor greater than three times the diameter of the pole piece (see Figure 1).

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Magnetic Pickups and Proximity Switches Manual 82510

2 Woodward

Figure 1. Magnetic Pickup Dimensions

In Figure 1, the optimum dimensions of A, B, C, and F are given as they relate to D, the

diameter of the pole piece of the magnetic pickup. The optimum relationship for maximumoutput is as follows:

A equal to or greater than DB equal to or greater than CC equal to or greater than three times DF equal to or greater than D

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Manual 8

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4 Woodward

Figure 3a. MPU Models

The surface speed in meters per second (m/s) or inches per second (IPS) isrelated to revolutions per minute (rpm) by the following:

Surface Speed =rpm x x gear diameter

60

The diametral pitch for the gear is obtained by the formula:

Diametral Pitch =number of teeth + 2

gear diameter (inches)

The gear module is obtained by the formula:

Gear Module =gear diameter (mm)number of teeth + 2

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Woodward 5

Figure 3b. CSA/ATEX Models

The standard magnetic pickup may be used with a gear with a diametral pitchcoarser than 8 (gear module 3.2), but the output will not be increased. Gears witha finer diametral pitch than 8 (gear module 3.2) may be used with acorresponding decrease in output voltage. A diametral pitch of 20 (gear module1.27) is the finest that may be used with this pickup without two teeth appearingover the pole piece at the same time. When adjusting the clearance between thepickup pole piece and the gear, adjust the pickup so the output voltage is aminimum of 1.5 Vac (rms) at the lowest speed.

If you cannot measure the gap directly, it can be determined in the followingmanner.

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Manual 8

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8 Woodward

Troubleshooting Magnetic Pickups

Disconnect the pickup leads. Check between leads with an ohmmeter forresistance as listed below. Check between the pickup case and leads with anohmmeter for near infinity resistance. Start the prime mover and control speedmanually or block the throttle. Check pickup leads with a high-impedance acvoltmeter for 10 to 20 Vac (rms). Reconnect the pickup and check at the speedsensor input terminals for a minimum 1.5 Vac (rms) at the lowest controlling

speed. Check the pickup and gear clearance if voltage is low.

MPU Resistance List

Long Standard Model 250 maxExtra Long Standard Model 250 max

Short Standard Model 220 maxStandard Metric Model 250 max

Conduit Model 114 to 140 Explosion Proof Model 165 to 225

Inspect for physical damage. Replace pickup if faulty.

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Woodward 9

Proximity Switches

Large engines usually have a larger air gap between the speed sensor probe andthe monitored gear than smaller engines. This is necessary because of therelatively large runout of the monitored gear in a large engine. The monitoredgears on large engines usually also have a slower surface speed. Because ofeither, or both, of these conditions, a magnetic pickup may not performsatisfactorily on a large engine. A proximity switch (zero velocity pickup) will,however, operate well on large engines because of its abilities to operate with alarge air gap and at low surface speeds.

The output of these pickups depends solely on the position of the gear tooth (orsimilar ferrous discontinuity) and not the velocity at which the gear tooth passesthe sensing end of the pickup.

When wired as a sinking type proximity switch, the output is normally zero.When a gear tooth is within the sensing range of the proximity switch, the outputof the switch goes high (nearly equal to the supply voltage). After the tooth haspassed, the output switches back to zero volts until another tooth comes intoposition. Typical wiring for a sinking type proximity switch is shown in Figure 7.

When wired as a source type proximity switch, the output is normally high(nearly equal to the supply voltage). When a gear tooth is within the sensingrange of this proximity switch, the switch output goes to zero volts. After the toothhas passed, the output switches back to nearly the supply voltage until anothertooth comes into position. Typical wiring for a source type proximity switch isshown in Figure 8.

The on-off nature of the output of a proximity switch produces a square wavewhich is compatible with nearly all Woodward speed sensors and will directlyreplace the signal from a magnetic pickup as a speed sensor input.

Proximity Switch Features

As long as the air gap between the sensor end of the proximity switch and thegear being monitored does not exceed the sensing range of a particular proximityswitch, changing the air gap will not change the output voltage. The duty cycle ofthe signal from the proximity switch varies with the air gap, but this does notchange the reference signals produced in the Woodward control.

For proper operation there cannot be more than one tooth in the sensing field ofa proximity switch. (The distance between the teeth must be greater than the sizeof the sensing field.)

Types of Proximity Switches

Two different sizes of proximity switch are available from Woodward:

1. A proximity switch with a fairly narrow sensing field, suitable for medium sizegears with a tooth size from diametral pitch 8 to 12 (gear module 3 to 2).This switch can be mounted flush with the mounting plate with an air gap upto 5.00 mm (0.197 inch).

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Manual 8

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12 Woodward

Using the Output of a Woodward MPU to Drive aRemote Tachometer or Other Device

Attach to Control PostsShould a remote device be driven by a magnetic pickup, it is important that theconnection be made on the governor posts which accept the input from the MPU.Polarity must be maintained as indicated on these posts or it is possible to short

the system to ground and cause damage to the electronic circuits and render thegovernor inoperable. Woodward electronic controls are not directly grounded, butin many cases the negative () post or common is tied to ground through thebattery powered supply. Should an additional device also be grounded orpowered by the same grounded battery system, a short circuit is possible.

The MPU device provided by Woodward is not grounded, and polarity is notestablished until connections are made to the governor. Almost all remotedevices will have a power source in addition to two leads for input. One of theseinput connections will be circuit common () and this should be determined priorto connection with the governor.

EPG and 2500 Governors

Since these units are indirectly tied to ground, care must be taken that polarity ofthe remote device and of the governor is not reversed.

2301 GovernorThe 2301 governor is isolated from ground, and care should be taken that anydevice added to the system is also isolated from earth ground.

ShieldingGovernor circuits have been carefully shielded, and circuits to remote devicesmust also be shielded from interference, since this could prevent adequategovernor control. Inadequate shielding could cause unstable control.

Minimum SignalMost Woodward controls require a minimum speed-signal strength of 1.5 Vacfrom the magnetic pickup during cranking. The addition of remote devices to theoutput of the magnetic pickup could prevent the reception of an adequate signalto allow engine start-up.

Make sure all polarity has been established and the system is notshorted to ground before trying to start the engine after adding anauxiliary device to the MPU output. Attempting to start the systemwith reversed polarity between the auxiliary device and the controlcould destroy either or both devices. Should the control be damageddue to the reversed polarity, dangerous overspeed is possible,damaging equipment, causing personal injury or even death.

Woodward advises dedicating the entire output of a governormagnetic pickup to the exclusive use of driving the control system.

This information is provided for those installations where the outputof the magnetic pickup is also used to drive other devices.

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We appreciate your comments about the content of our publications.

Send comments to: [email protected]

Please reference publication 82510R.

PO Box 1519, Fort Collins CO 80522-1519, USA1000 East Drake Road, Fort Collins CO 80525, USA

Phone +1 (970) 482-5811 Fax +1 (970) 498-3058

Email and Websitewww.woodward.com

Woodward has company-owned plants, subsidiaries, and branches,as well as authorized distributors and other authorized service and sales facilities throughout the world.

Complete address / phone / fax / email information for all locations is available on our website.

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82510_R.pdf