Vaasan Teknillinen Oppilaitos Työohje 7.20

VAASN POLYTECHNIC UNIVERISTYLab Exercise 2

IKE1002 Distributed Energy Engineering1(7)

Charasteristics of squirrel cage motor

1.PRE-EXERCISESS

1.1 Design connection diagram to paragraph 2.3

1.2Rated values for the motor are: 1,5 kW, 1420 r/min, 400 V, 3,5 A jacos=0,8 ind.

Reactive power consumed of the motor is to be compensated, when it runs at rated load. Calculate needed values for the condensators, when they are connected in triangle. (In triangle the impedance value is tripled compared to star value).

1.3The meaning and benefits, when used with the motor.

2.MEASUREMENTS

2.1 Mark down the rated values from the motor’s nameplate and take the rest from the motor data sheet.

Values from nameplate:

Values from datasheet:

Calculate the nominal torque from nameplate values.

2.2Measure the stator resistances of the cold motor.

In equivalent circuit R1 = 

Remark – In triangle measured value is divided by 3.

2.3Measurement in idle run

Motor is in free run – not connected on the load. Measure P0=f(U) and I0=f(U). Voltage is stepped 1,2...0.2 *UNdownwards with proper steps (about nine measurements). One of the measurements has to be done at rated voltage.

Power is the sum of power at each phase. Current is the average of the phase currents.

Draw a diagram in order to Define power losses Pr, P andPkoat rated voltage by drawing a diagram. See appendix 1.

2.4Efect of voltage change on the motor

Load the motor with the rated torque. Measure motor voltage, current, power from the grid and motor speed as a function of supply voltage, when U = 1,15...0,85 UN. One of the measurements has to be done with the nominal voltage.

The load torque is to be adjusted nominal during this measurement series.

Draw the following diagrams based on the measurements: Current I = f(U), Speed n = f(U), Power factorcos = f(U) andefficiency = f(U). Analyze every diagram. Which voltage gives the powerfactor given at the nameplate.

2.5 Efect of Torque on the motor

Normally it is supposed that torque is propotional to voltage’s squared value at given speed. This measurement gives more exact value to this propotion for the motor under the test.

Load the motor with the nominal voltage so, that the speed is dropped down to about 1400 r/min (for motors with polepair 2). Mark down: Voltage, Speed (rpm) and Torque.

Lower voltage down to value UN/3 and adjust the load so, that speed stays constant. Mark the values again.

Calculate the propotional value x for the potens, T  Ux. Explain why it may differ from theoretical value 2.

Remark – Use the same speed at the both measurements.

2.6Measurements under the load

Load the motor with proper steps between 0...1,25TNand measuren, I, P and T at nominal voltage, which is kept as constant. One of the measurements has to be done with the nominal voltage.

Remark – Try to keep supply voltage at nominal value during the measurement.

2.7Measurements under the load - compensated

Use capasitance values calculated at paragraph 1.2. Measure n, I, P and T at the rated voltage as a fuction of load torque 0...1,25TN.One of the measurements has to be done with the nominal torque

Based on measurements at the paragraphs 2.6 and 2.7 draw the following graphs:

-Torque T = f(n) (Torque graph at the usable area)

-Current I = f(M)

-Reactive power Q = f(T)

-cos = f(T)

-

Draw for ex. Both 2.6 and 2.7 graphs at the same figure so, that they can be easily compared – and same for other graphs.

Comment the graphs.

2.8Short circuit test

2.8.1Set the load so high, that it exceeds start moment of the motor under the test. Increase the supply voltage so, that the current is nominal. Measure Pk, andUk

Measured Pk represents current losses at rated current.

2.8.2Remark here, like always, ask permission from the supervisor, before you make any changes on the connection. And a new permission is needed before you are allowed to put power on.

Remove adjustable transformer and all meters from the circuit – the motor is supplied directly from the panel. The values are now read from the meter at the panel.

Switch the load off. Start the motor. Train how you can read the following values: Uk, Tk, Pk and Ik.

Stop the motor. Switch the load on. Start the motor and RAPIDLY read the values: Uk, Tk, Pk and Ik.

What is proption between start torque and start current relatvie to the nominal values Ms/MN and Is/IN. If the supply voltage was not the nominal reduce the measrued values accordingly.

Compare the result to the values given at the data sheet.

2.8.3Measure immediatelly, when permission is granted, resistance of one coil.

Calculate temperature of the rotor by comparing it to the measurement at the paragraph 2.2.

RU = 

3.Documenting the measurements

Provide the mearements in table form. Anwer the questions stated at paragraph 1 and 2. Analyze the graphs.

4.POST-EXCERSICES

4.1a) Define efficiency of the motor at rated torque and at rated voltage using indirect method. The needed power losses you will get from paragraphs 2.3 and 2.8.1. The supplied power from the grid at the nominal point is at 2.6.

b) Draw motor efficiency paragraph  = f(T)measured at 2.6 and 2.7. Add a)at the same picture.

Do they differ?

Remark – attach a copy of the original measurement document to the report. It makes easier to find any possible source for errors.

Appendix 1

How to define iron losses and mechanical losses.

1. Draw the graph as a function of sqared supply voltage. This makses the sqaredpowelosses appear as a line – Iron losses and current losses in resistrances.
2. At small voltage the only losses to appear are mechanical losses P - point where the graph starts

1. Current losses when idle are Pko = 3 I02R1 . Calculate at nominal point
2. Calculate iron losses PFe = P0 - Pk0– P