electric motors.ppt

Patrick E. Montero, SRS IIPhilippine Council for Industry, Energy, and Emerging Technology Research and Development (PCIEERD)Department of Science and Technology (DOST

Outline

• Objectives and scope of the topic

• Definition of terms

• Guiding principles

• Methods of measurement

• Energy saving opportunities

Objectives

• To determine the efficiency of electric motors by loss estimation method under operating conditions in the plant where the motor is installed and running or available as spare.

• To simplify instrumentation so that the test can be conducted with portable instruments and facilities available with plant engineers and energy auditors.

• To identify energy saving opportunities in electric motors.

Scope

• The topic deals with the commonly used electric motor in the industry which is the induction motor. These motors and driven equipments account for more than 90% of energy consumption in the industrial motor driven systems.

• The methods that will be presented in this topic can be also used for efficiency testing of squirrel cage and slip ring induction motors.

Definition of TermsSymbol Description Units

E Energy kWh

P Power Watts (W) or kilowatts (kW)

t Time duration Seconds

T Temperature °C

Pfe Core losses W

Pfw Friction and windage losses W

Pk Constant losses W

Pcu-st Stator copper loss W

Pcu-rot Rotor copper loss W

PS Stray losses W

PT Total losses W

Pmech Mechanical power W

V Terminal voltage V

I Current I

Cos θ Power factor Per unit

f Frequency Hertz (Hz)

p Number of poles - -

N Speed RPM

Ns Synchronous speed RPM

s slip Per unit

R Average D.C. resistance Ω

η Efficiency %

What is an electric motor?

• A machine that converts electricity to mechanical work.

Electric Motor Principle

DC Motor Schematic Diagram

AC Motor Schematic Diagram

Types of Electric Motors

• AC Electric Motors

Asynchronous Synchronous

Inductance

BrushlessHysteresisSine Wave Reluctance Stepper

PM Wound Field

PM Wound Field

PM Hybrid Var. Reluc.

Types of Electric Motors

• DC Electric Motors

Homopolar Commutator

Permanent Magnet Wound Field

Compound Shunt Series

Universal

Electric Motor Parts

Shaft Slinger

Rotor

Frame

Rotor Laminations

Shaft

Wound StatorInsulation

Fan

Bearing

Conduit Box

Nameplate

Facts About Electric Motors

Most electric motors are designed to run at 50% to 100% of rated load. Maximum efficiency is usually near 75% of rated load.

A motor’s efficiency tends to decrease dramatically below about 50% load.


Motor Part-Load Efficiency (as a Function of % Full-Load Efficiency)

Source: U.S. Department of Energy


Motor Power Factor (as a Function of % Full-Load Amperage)

Source: U.S. Department of Energy

Guiding Principles

Planning the Audit• METHOD – 1

– When a motor is not coupled mechanically to any load, but available as spare/newly purchased. In this case, motor efficiency at full load can be estimated.

– Motor nameplate rating of full load speed and full load output are assumed to be correct.

– Measurements are done on the motor at no load conditions.

Planning the Audit

• METHOD - 2

– When a motor is installed and coupled to driven equipment, say a pump, compressor etc.

– In addition to the measurements at no load, measurements are also required to be done at the actual operation of the motor on load.

– In this method, actual speed and power input is measured at load condition and output is estimated from power input and measured losses.

Pre-Audit Requirements

1. Conditions when it is not recommended to conduct the audit:

a) If the voltage is fluctuating by more than 5%

b) If the difference among phase voltages is more than 15V.

c) Frequency is below 48.5 hertz or fluctuating.

2. Ensure that the motors to be tested are in working condition


3. Nameplate information of the motor is required for the tests. Ensure that the nameplate information is clearly visible.


4. Any Variable Frequency drive, voltage controller or soft starter installed at the motor need to be disconnected from the line during measurements.

5. While conducting no load test, ensure that the motor is completely decoupled from the load.

6. If the motor has been in operation prior to no load test, stop the motor, decouple the load and keep the motor idle condition till the motor cools to ambient temperature.

Methods of Measurement

• The measurement of following parameters is required for efficiency testing of motor:

1. Power Input (Pi)2. Current (I)3. Voltage (V)4. Frequency (f)5. Speed (N)6. Stator Resistance (Ω)7. Ambient Temperature (°C)


• Power Input (Pi) can be measured by using a calibrated energy meter or power analyzer.

Methods of MeasurementPower input (Pi) for NO LOAD TEST using two (2) wattmeter method

3-Φ Motor

W1

W2

Contactor

A

B

C

Total Power = W1 + W2


• No Load Test Equivalent Circuit

3-Φ Motor

IC

VBC

IA

VAB

W1

W2

Contactor

A

B

C

Total Power = W1 + W2


• Voltage (V) can be measured on all the three (3) phases by using a voltmeter or power analyzer.

Vph = VL

FOR DELTA CONNECTION: FOR WYE CONNECTION:

Vph = VL/√3


Example:An existing motor is identified as a 40-hp, 1800 rpm unit with an open drip-proof enclosure. The motor is 12-years old and has not been rewound.

The following measurements was obtained:

Vab = 467 V

Vbc = 473 V

Vca = 469 V

Ia = 36 A

Ib = 38 A

Ic = 37 A

PFa = 0.75 A

PFb = 0.78 A

PFc = 0.76 A

Determine the Power input.


Example (Continuation):

Solution:V = (467+473+469)/3 = 469.7 V

I = (36+38+37)/3 = 37 A

PF = (0.75+0.78+0.76)/3 = 0.763

Pi = 469.7 x 37 x 0.763 x √3 / 1000

Pi = 22.9 kW


• Current (I) can be measured on all the three (3) phases by using a clamp-on ammeter or power analyzer.

FOR DELTA CONNECTION: FOR WYE CONNECTION:

Iph = ILIph = IL/√3


• Speed (N) - slip is measured from synchronous speed (Ns) and Operating Speed (NL) as given below:

SL = Ns - NL

Ns

Ns = 120 x f

p

SLIP CALCULATION SYNCHRONOUS SPEED

SFL = Ns - NFL

Ns


• Speed can be measured by using a contact or non-contact tachometer.

Non – Contact Type

Contact Type


• Resistance TestA1

B1 C1

A1 B1 C1

A2 B2 C2

WYE Connected Winding Connection at Motor Terminal Box

Rph = 0.5 x Rave

Where Rave is the average value of line-to-line resistance to phase resistance obtained.


• Resistance Test

A1 B1 C1

B2 C2 A2

A1 B2

A2

C1 C2

B1

DELTA Connected Winding Connection at Motor Terminal Box

Rph = 1.5 x Rave

Where Rave is the average value of line-to-line resistance to phase resistance obtained is designated as Rave


• The resistance measured should be corrected to the operating/full load temperature by using the following relationship.

R2 =235 + T2

R1 235 + T1

R2 = unknown resistance at temperature T2

R1 = resistance measured at temperature T1


• Temperature for Insulation Classes - IEC

Thermal class of insulation

Reference temperature °C

A 75

B 95

F 115

H 130

*Ambient Temp reference is 25°C


• Ambient Air Temperature can be measured by using either mercury in glass thermometer, infrared thermometer or thermocouple with digital indicator

Power Losses

Rotor

Frame

Wound Stator

Fan

Bearing

Core loss ( Pfe)

Windage loss (Pfw)

Friction loss (Pfw)

Stator loss (Pcu-s)

Copper loss (Pcu-rot)

Stray loss (Ps)


• Estimation of friction and windage losses by Variable Voltage Testing

Sample Plot of Power vs. voltage2 & voltage

Source: Bureau of Energy Efficiency, India


If Variable Voltage Testing is not possible, assuming friction & windage losses according to IEC standards is also reasonably correct.

For Drip proof motors ≈ 0.8% to 1.0% of motor rated output

For TEFC motors, friction & windage losses ≈ 1 to 1.5% of motor rated ouput


• Core losses estimation can be done by subtracting the friction & windage losses from constant losses times the square of rated voltage over the square of measured voltage as shown in the equation.

Pfe = Pk – Pfw xVrated

Vmeasured

2


• Stray loss estimation - IEC


• Stray loss estimation– Stray losses are very difficult to measure with any

accuracy under field conditions or even in a laboratory.

MOTOR RATING VS. STRAY LOSSES - IEEE

Motor Rating Stray Losses 1 – 125 HP 1.8 % of rated output125 – 500 HP 1.5 % 501 – 2499 HP 1.2 % 2500 and above 0.9 % Assumed values for Stray losses – IEEE Std 112 -1996

METHOD 1 - Full Load Test

1. If the motor has been in operation prior to this test, stop the motor, decouple the load from the motor and keep the motor idle till it cools down to ambient temperature. Usually it takes about 2 hours.

η = Rated Motor Output

Rated Motor Output + Losses


2. Measure winding resistance (Rph) at cold conditions. Record the ambient temperature (Ta)

3. Apply voltage across the motor at no load and start the motor

4. Measure line voltage (Vnl), line current (Inl), and frequency (Hz). Otherwise, use the Direct power input (Pi) measurement if power meter is available.


5. Calculate the phase current (Iph)

6. Calculate stator copper loss at no load and subtract this from no load power to get constant losses as shown below.

No load stator Copper loss; Pcu-st = 3 x Iph-nl2 x Rph-nl

Constant loss; Pk = Pin – Pcu-st


7. Estimate friction & windage losses (Pfw) by using the IEC standards.

8. Estimate Core losses

Pfe = Pk – Pfw xVrated

Vmeasured


9. Calculate stator winding resistance at full load (i.e. temperature as defined in the class of insulation of IEC)

RT = Rph-a x(235 + TR)

(235 + Ta)


10.Estimate Stator copper losses at full load, assuming nameplate full load current and corrected stator resistance at full load.

Pcu-st-FL = 3 x Iph-FL2 x RT


11. Obtain stray losses as a % of input power from the IEC – data


12.Calculate full load slip (SFL) from the rated speed (NFL) and synchronous speed (NS) at the rated frequency

SFL = NS – NFL

NS


13. Calculate rotor input power from rotor output at full load

Power input to rotor, Pirot = Rotor output

(1 – SFL)

Note: Rotor output at full load is the nameplate output kW rating of the motor


14. Calculate rotor copper losses from full load slip and rotor input

Rotor copper loss, Pcu-rot = SFL x Pirot

15.Total losses at full load is sum of all the above losses

PT = Pfw + Pfe + Pcu-st-FL +Ps + Pcu-rot


16. Efficiency (η) at Full load is obtained from rated output and estimated total losses as:

η = Rated Motor Output

Rated Motor Output + Losses

METHOD 2 - Operating Load Test

1. If the motor has been in operation prior to this test for more than one hour, it can be considered to be close to steady operating conditions. In this case, while testing, operation of the motor for 10 to 15 minutes is sufficient to attain steady operation

η = Motor input power - Losses

Motor input power


2. If the motor and load were idle before the test, continuous operation of motor on load for at least 2 hours is recommended to attain steady state conditions.

3. Start the motor with load and bring it up to desired steady operating conditions.


4. Measure line voltage (VL), line current (IL), and frequency (Hz). Otherwise, use the Direct power input (PiL) measurement if power meter is available.

5. Measure operating speed of motor, NL

6. Switch off the motor. Disconnect power supply. Measure DC resistance of the stator (Rph-L) winding immediately after switching off the motor.


7. Decouple motor from the load and allow the motor to cool for at least 2 hours.

8. Repeat Steps 2 to 6 from Method 1 for no load calculation


9. Calculate Stator copper losses at operating load,

10. Calculate Stray losses11. Calculate rotor input power

Pirot = PiL - Pcu-st-L – Pk – Ps-L

Pcu-st-L = 3 x Iph-L2 x Rph-L


12. Calculate the slip (SL) from the operating speed (NL) and synchronous speed (NS) at the measured frequency

SL = NS – NL

NS


13. Calculate rotor copper losses from slip and rotor input

Rotor copper loss, Pcu-rot = SL x Pirot


14. Total losses at actual load is sum of all the above losses

PT = Pk + Pcu-st-L + Ps + Pcu-rot


15. Efficiency is estimated from estimated output and measured input

η = PiL - PT

PiL

Slip Method

Slip Method

• The slip method for estimating motor load is recommended when only operating speed measurements are available. The synchronous speed of an induction motor depends on the frequency of the power supply and on the number of poles for which the motor is wound.

Slip Method

Example: Slip Load CalculationGiven: Synchronous speed (Ns) in RPM = 1800

Nameplate full load speed (Nr) = 1750

Measured speed in RPM (Nm) = 1770

Nameplate rated horsepower (hp) = 25

Determine the actual output horsepower

Load =Ns - Nm

x 100 = 1800 - 1770

x 100 = 60%Ns - Nr 1800 - 1750

Actual output horsepower would be 60% x 25 hp = 15 hp

Energy Saving Opportunities

• For every 1% increase in motor efficiency, there is about 5 kW power savings.

• Motors should be Properly size to the load for optimum efficiency. High efficiency motors offer of 4 - 5% higher efficiency than standard motors

Energy Saving Opportunities• Use energy-efficient motors where economical. • Use synchronous motors to improve power factor. • Check alignment. • Provide proper ventilation (For every 10 oC increase

in motor operating temperature over recommended peak, the motor life is estimated to be halved)

• Check for under-voltage and over-voltage conditions. • Balance the three-phase power supply. • (An imbalanced voltage can reduce 3 - 5% in motor

input power) • Demand efficiency restoration after motor rewinding. • (If rewinding is not done properly, the efficiency can

be reduced by 5 - 8%)

Thank You

electric motors.ppt

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