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Figure A1: Laboratory Layout

THE ELECTRIC MACHINES LABORATORY

Table of Contents

A1 THE LABORATORY A3 ADDITIONAL INSTRUMENT THEORYA.1.1 Physical ArrangementA.1.2 Power Distribution SystemA.1.3 Safety ConsiderationsA.1.4 Student Laboratory Work

A2 MEASUREMENTA.2.1 SpeedA.2.2 TorqueA.2.3 Rotational Losses and PowerA.2.4 DC Voltage and CurrentA.2.5 AC Voltage and CurrentA.2.6 Elec Power and Related QuantitiesA.2.7 Resistance and ImpedanceA.2.8 Digital Instrumentation

A.3.1 Basic DC MetersA.3.1.1 PMMC MovementA.3.1.2 The DC Ammeter CircuitA.3.1.2 The DC Voltmeter CircuitA.3.1.3 Meter Insertion DisturbanceA.3.1.5 The Ohmmeter CircuitA.3.2 Basic AC MetersA.3.2.1 Moving-coilA.3.2.2 Moving-ironA.3.3 The Wattmeter

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Figure A2: Power Panel

A.1 The Laboratory

The laboratory is the real world. This laboratory contains rotating machinery and electric powersupplies that can be dangerous if misused or used carelessly, therefore careful preparation and ano-nonsense attitude are required. All equipment that is man-made is subject to the various laws ofMurphy; fortunately "machines" experiments tend to be well behaved and predictable so that youshould expect, if properly prepared, to be able to obtain useful results in each and every experiment.It is suggested that you read this section carefully and study the content where it is not known to youso that your time spent in the laboratory is both safe and efficiently used. This section describes whatis in the laboratory, what you should do in general, and some specific information on measurement,instruments, and techniques that you will be required to use.

A.1.1 Physical Arrangement

Figure A1 shows a plan view of the laboratory. Locations 1-6 on this plan, identified by numbers onthe power panels in the laboratory, contain the 2 kW nominal rating equipment. At each of theselocations there is a table, a power panel, a console, and a set of rotating machines. See figure A.4.Locations 7-9 are similar except that the equipment has a much lower rating, nominally 200 W or 0.2kW (the manufacturer's rating is 1/4 HP or 187 W) and the rotating machines are placed in theconsole instead of on the floor. Location 10 has a power panel and can contain a console fordemonstration purposes. Equipment for most regularly scheduled experiments is pre-placed on thetable or in the console. The console has space for 9 full size modules or up to 18 half size modules.Most modules, apart from the power supply modules, are half size. Equipment not required for thecurrent set of experiments is either locked in the storage cabinet at the bottom of each console orplaced in open storage racks (SA - SD, S7 - S9). Storage units SA-SD contain 2 kW rated moduleswhereas S7 - S9 contain 0.2 kW rated modules for use with the respective 0.2 kW consoles atlocations 7-9, and occasionally at other locations.

A.1.2 Power Distribution System

All power to the laboratory (except lighting and PCs) is fed from a common source with a tripablecontactor located on the east wall near power panel 10. This contactor is closed by pushing the singleON button on the front of the case, and "power on" is indicated by a red pilot lamp. The adjacentOFF button and the five "emergency stop" buttons distributed around the laboratory will cause thecontactor to trip and disconnect power. Power is distributed via ten 50 amp 3 pole breakers in anadjacent panel.

Each Power Panel (see figure A2) contains two 20amp 3 pole breakers, each feeding a five pin socket(120/208V 3 1 4W). In the centre of each panel is a15 amp single pole breaker feeding three duplexreceptacles (120 V single phase) Normally the duplexreceptacles are used for instrumentation supplies, andone (the other is a spare) of the five pin sockets is pre-connected to the power supply module in the console.

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Figure A3: Power Supply Module

Figure A4: Typical 2kW Machine Set with Console

Each Power Supply Module (see figure A3) is normally pre-mounted in the lower left corner of the console unit as illustrated infigure A4. There are two models; #8525 for the 2.0 kW units, and#8821 for the 0.2 kW units. All units are protected by a 15 amp 3pole breaker on the input and other appropriate breakers with acommon reset on the front panel. The voltage adjustment is commonto both AC and DC outputs. There is a switchable panel meter andthree orange pilot lamps monitor the three phase input. The currentratings and terminal identification are as follows:

Terminals Type Current Rating(2 kW) (0.2 kW)

1, 2, 3, N Fixed AC 15A 15A4, 5, 6, N Variable AC 15A 5A7, N Variable DC 25A 8A

8, N Fixed DC 5A 2A

The colour coding used on the five way binding posts and banana jacks is as follows. Red signifies+ 120V relative to neutral (White) and Black signifies -120V relative to neutral (White) for dc power.Red, Black, and Blue are used for the three phases of the 120/208V ac power relative to neutral(White). On some modules, where polarity cannot be predetermined, Yellow is used. The leads usedto interconnect modules are colour coded for length, Yellow for short, Red for medium, and Blue forlong.

1. AC Machine Base2. AC Machine (fixed stator)3. DC Machine Base4. DC Machine (trunnion mounted) 5. Coupling 6. Locking Bars (2) 7. Tachometer 8. Torquemeter 9. Storage10. Shelf11. Modules12. Power Supply Module

A.1.3 Safety Considerations

No unsupervised experimentation is permitted and specific approval (including circuit checking withevery significant change) is required before turning on any power. A telephone (dial 88 foremergency) and a fire extinguisher (suitable for electrical fires) are located by the exit between powerpanels 9 and 10. Note the location of the five emergency power disconnect (red) buttons in the

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laboratory. Do not exceed electrical or mechanical ratings of any equipment. This requires that youprepare carefully for each experiment. While all equipment in the laboratory has been designed forinstructional use, it is still necessary to be careful of rotating equipment and electrical connections.

A.1.4 Student Laboratory Work

The objectives and evaluation of laboratory work are described in a separate sheet which is issuedwith the experiment instruction sheets. The following procedural points should be noted beforecommencing any work in the laboratory.

Laboratory Groups - For reasons of safety and efficiency, experiments will be done by groups ofstudents (ideally 2 students per group). Selection of these groups is determined by student choiceat the beginning of the term for the duration of that term.

Schedules - Group numbers and members' names will be posted on the lab notice board along withthe experiment schedule for the entire term. Lab locations to be used will also be posted.

Instruction Sheets - Instruction sheets are normally available prior to the experiment and are to bestudied beforehand with any required preliminary work done before entering the lab. Read theinformation provided with each group of experiment instruction sheets for more specific details.

Foremen's Responsibilities - In each group a foreman will be elected or appointed for eachexperiment. This duty will rotate between the group members. The foreman is responsible forperforming the experiment, submission of a report at the end of the lab period, restoration of the labto its initial condition prior to the experiment, and reporting any equipment malfunction to theteaching assistant.

A.2 Measurement

This section deals with the use of instruments in the laboratory. Section (A.3) contains someadditional instrument theory for common DC and AC analog meters. All notes relating tomeasurement are grouped together in this section for convenience in referencing. It may be necessaryto read other appropriate sections on poly-phase circuits, for example, to fully understand some ofthese meter descriptions or connections. While the emphasis is on analog meters, brief mention ismade where appropriate to digital techniques.

We will now examine the basic methods used in the laboratory for measuring speed and mechanicalshaft torque, and the use of these measurements to determine rotational losses in a machine set. Thiswill be followed by measurement of electrical quantities.

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A.2.1 Speed

There are two methods commonly used in the laboratory. The first one uses a shaft mountedpermanent magnet generator to produce a voltage that is connected to a voltmeter calibrated in rpm.This instrument, called a tachometer, is useful to measure speed approximately over a wide range ofspeed but inherently has limited accuracy if one wishes to measure and compare small speeddifferences. See figure A4.

The second method which actually measures the difference in speed from a reference (usuallysynchronous speed) uses a strobotak flashing on a rotating shaft. At the reference speed (the flashrate) the shaft seems to be still. If the shaft actually turns, for example, 60 rpm slower, it will appearunder the light to rotate backwards at 1 revolution per second. In this mode, the flash rate is fixed.Another mode of operation (not used with this equipment) is to vary the flash rate (calibrating the dialin rpm) and look for a stationary pattern. This is often difficult to use because of repeated multiplepatterns.

A third method, much more accurate and expensive, is to produce pulses of voltage using, forexample, a rotor mounted permanent magnet passing near a stator mounted coil. The pulse repetitionrate is proportional to speed and is measured using a digital counter. This method is used in ourresearch laboratory.

A.2.2 Torque

Three methods of torque measurement will be described. The first two involve measurement ofreaction torque, whereas the third measures actual shaft torque.

The 2 kW Dynamometer:Refer to figure A.4 for a typical 2 kW machine set arrangement, the bracketed numbers in this sectionrefer to this figure. The basic method uses a "dynamometer" which is a dc machine (4) whose statoris mounted in an extra set of bearings (referred to as trunnion mounted) and connected to a fixed base(3) by a spring system so that the reaction torque of the stator is indicated on a scale (8). This scaleis calibrated in Newton-meters and indicates shaft torque transmitted by the coupling (5) between theleft-hand machine (2) and the right-hand machine (4).

Figure A4 illustrates the correct arrangement when the dc machine is used as a generator and the acmachine is used as a motor. The ac machine is run in a counterclockwise (CCW) direction, thetachometer switch is set to CCW, and the torque meter reads upscale. If it is required to use the dcmachine as a motor with the ac machine as a generator, the two machines are interchanged (from thatdepicted in figure A4), the dc motor is operated in a clockwise (CW) direction and the tachometerswitch set to CW.

In both cases the machines are oriented with the connecting cables and the nameplates at the back andthe tachometer mounted at the left side. Realize that in order to mechanically "load" the motorsignificantly, the generator must itself be loaded electrically by connecting an appropriate resistiveload and providing the necessary excitation.

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The 0.2 kW Electrodynamometer:In order to mechanically load the various motors in the 0.2 kW units, an "electrodynamometer" isused. This is a special purpose machine which is fundamentally a generator with its electrical loadbuilt directly into the rotor. A variable reactor is mounted on the faceplate to enable the variation ofthe excitation level and thence the loading. The stator is trunnion mounted and rotated against aspring thus indicating the reaction torque.

True Shaft Torquemeter:A third method used in the research laboratory utilizes a special section of shaft between the twomachines containing a bridge arrangement of resistances whose value is dependent on the stress/strainin the shaft. Deflection of this portion of shaft unbalances the bridge which causes a current to flow.This current is dependent on shaft torque and is indicated on a meter. In this particular instrumentthe readout is digital through the use of an analog-digital converter. The analog output has fastdynamic response and can be used for transient measurements while the digital output is only usefulfor steady state as the "counting" period is one-third of a second.

A.2.3 Rotational Losses

The rotational loss of a machine has two components: (1) Friction and Windage which is dependenton speed and; (2) Core Loss which is dependent upon the air gap flux density. In order to measurethese losses in one of the machines (the machine under test) run the coupled machine as a motor.

With no electrical connection to the machine under test, the torque meter indicates torque due tofriction and windage for that machine only. This torque will, in general, depend on speed and shouldbe measured for the desired value (or range) of speed. In many cases the normal speed range is smalland one value at the nominal speed will suffice. To obtain the power loss, multiply the torque readingby the speed.

If this test is now repeated with the machine under test excited and run at rated speed, the torquemeter indicates the friction and windage component of torque plus the component due to core loss.This additional component will depend on the amount of excitation (or the air gap flux density) henceeither a typical value of rated excitation or a series of values corresponding to different degrees ofexcitation should be taken. To obtain the core loss in watts, subtract the friction and windage torquefrom the test value (both in Newton meters) and multiply by the speed (in radians per second).

Shaft power in watts is obtained by multiplying the torque reading in Newton meters (N.m) by thespeed in radians per second (rad/s). Realize that in the steady state this product is always themechanical power transmitted at the coupling and thus represents the true external mechanical loadif the machine under test is a motor, and the actual mechanical input power if the machine under testis a generator. The previous two paragraphs detail how two of the internal losses of the machineunder test can be evaluated.

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Figure A5: DC Meter Module

A.2.4 DC Voltage and Current

DC voltage and current are measured using a DCmeter module containing one voltmeter (20 V and 200V ranges) and two ammeters as illustrated in figureA5.

For the 2 kW module (#8513) the low current meterin the centre has a 2 A range and the high currentmeter ranges are 15 A and 30 A. The 0.2 kW metermodule (#8412) has a low current range of 0.5 A and

high current ranges of 2.5 and 5.0 A. As a general rule the low current meter is used for field currentmeasurements while the high current meter is used for stator or armature current measurement.

DC Current is measured using a pmmc (permanent magnet moving coil) meter movement with ashunt. Positive current should enter the positive terminal which is marked with the current range.For multi-range meters there are a number of different current shunts connected internally across themeter movement. (Sometimes the shunt is external enabling the user to purchase one movement andseveral shunts as an economical alternative to several complete meters. These are used in our otherlaboratories.)

DC Voltage is measured using a pmmc movement and a built in series resistor to produce a currentproportional to the voltage.

A.2.5 AC Voltage and Current

AC Voltage is measured using module #8426 containing three identical units. Each voltmeter hasboth a 250 V and a 100 V range. These units use exactly the same pmmc movement as for DCvoltage measurement with diodes in series with the series resistor.

AC Current is measured using a three meter module. For 2 kW units the part number and meterranges are: #8514 with 5, 10, and 30 A ranges (the center meter has additionally a 1 A range). Thecorresponding module for 0.2 kW is: #8425 with 0.5, 2.5, and 8.0 A ranges (with an additional 25A range in the centre unit). These meters use a moving iron (mi) meter movement. See section A.3.2for internal details. A built in current transformer is used to obtain the multiple ranges.

A.2.6 Electrical Power and Related Quantities

The following quantities can be measured using the same general class of instrument:

Name Symbol UnitReal or Active Power P watt (W, kW, MW)Imaginary or Reactive Power Q var (VAR, kVAR, MVAR)Power Factor PF, cos -0 per unit (no unit)

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Figure A6: Single Phase Wattmeter Module

Figure A7: Three Phase Wattmeter Module

Since the basic wattmeter is fundamental to all the above power related measurements it will bediscussed first.

Single Phase Wattmeter Figure A.6 illustrates module #8431 used with 0.2 kW rated equipment.On the left is the front panel layout with a simplified internal connection diagram on the right. Thecurrent coil of the meter movement is connected between terminals 1 and 3. The potential coil isconnected in series with a resistor between terminal 1 and terminals 2 and 4 which are themselvesconnected together. The meter movement is of the electrodynamometer type which is described insection A.3.

Three Phase Wattmeter Figure A.7 illustrates module #8515 used with 2 kW rated equipment.This is a so-called two element wattmeter that contains two of the previous (single phase) wattmetercoil sets mounted on a single shaft. The meter reading is the sum of the two meter elements. Theconnections are such that this sum is equal to the three phase power under balanced conditions fora 4 wire system, and balanced or unbalanced conditions for a 3 wire system.

Three Phase Varmeter and Power Factor Measurement The modules for 2 kW rated equipment(#8516 and #8523 respectively) are extensions of the previous use of the electrodynamometermovement. The varmeter uses two current coils in a basically two element meter and manufacturesa 90 degree phase shift by appropriate connections to the potential coils. The power factor meteruses only one current coil and appropriate connections for potential. For both instruments, thereading depends on assumed voltage and current phase relationships and therefore is only correct forbalanced, 60 hertz, sinusoidal conditions.

Rdc

Vdc

I dc

Z

Vac

I ac

and � cos 1Pac

Vac I ac

then use R ac �j X Z S�

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Power Flow Direction In an ac system, voltage and current alternate at the supply frequency. Itis important to realize that the direction of real and reactive power flows is quite specific.Consequently wattmeters and varmeters must be connected correctly in the circuit and the readinginterpreted correctly. For the Lab-Volt instruments the convention is that power flows from left toright. For the wattmeter, the terminals on the left side of the modules are connected towards thesource of power and the terminals on the right side are connected towards the load so that the meterreads upscale. For varmeters, which have a zero scale, this same convention implies that when themeter deflects to the left (counterclockwise) lagging vars are being delivered to the load. Note thatthis convention is consistent with the power factor meter where the left side of the scale is markedas lagging pf.

A.2.7 Resistance and Impedance

DC Resistance of a power device is determined by passing rated dc current through the device andmeasuring the dc voltage drop with no machine motion (if any) present. The ratio of dc voltage todc current is the dc resistance between the points identified by the voltmeter connection.

AC Impedance of a power device is determined by measuring simultaneously the ac voltage, current,and real power relevant to the device, and then calculating Z and � from these measurements. Ingeneral, R � R .ac dc

A.2.8 Digital Instrumentation

There are two models of digital instrument currently in use:

Digital Multimeter A Fluke #8010A digital multimeter which is primarily useful for dc and rms acvoltage measurement and resistance estimation.

Power Analyzer A Valhalla #2101 wide band ac-dc digital wattmeter with readout of power andeither voltage or current. Its connection is similar to the single phase wattmeter discussed earlier(except that the current coil is in the lower line of the drawing).

File EML.wpd R.T.H. Alden October 23, 1997