Post on 02-Jan-2016
TESTING MOTOR AT LOW VOLTAGE AND SEVERAL
FREQUENCIES
MOTOR 20 HP, 230 VOLTS,1800RPM, TESTING
VOLTAGE 10VAC
CONDITIONS:
• The voltage source was a sine wave inverter.• The voltage was kept sinusoidal by variation of the
capacitance between 40 F to 300 F.• The current was monitored on 1 Ohm resistor in series
with the winding.• Both voltage and current were fed to the two channel
analyzer. • The watts were calculated from the collected data in Excel.• Following slide is a sample of the voltage wave form on
the terminals of the tested motor. You can notice that the distortion was very low.
Voltage at 300Hz
TEST RESULTS
• Following are several graphs showing the impact of the frequency while the voltage is kept constant at 10 Vac.
• Note that some trend-lines are shown with analytical equations (blue thin lines).
• The analytical equations can be compared to what would be expected under simplified conditions.
Current=f (frequency)I=f[f] y = 0.1889x-0.7179
R2 = 0.9965
0
0.1
0.2
0.3
0.4
0.5
0.6
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
frequency [kHz]
curr
ent
[Am
ps]
Comment:
• Though the test was also performed at 60 Hz, it is impossible to make a reasonable graph, because the current is so much higher (looks like a hockey stick).
• The current at 60 Hz was 4.77 Amps
• One would expect decreasing the current with the power of –1. Compare to the power of the trend-line: -0.7179. It means the current is dropping with frequency at much slower rate.
Watts = f [frequency]W=f[f], Total Watts
y = 0.636x-0.6083
R2 = 0.9976
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Freqency [kHz]
Wat
ts
Comments:
• In case of a linear circuit, the curve should be dropping with the power of –2. However it is not the case.
• The watts drop very slowly with the power of –0.6083.
• The effect of iron at high frequencies is really profound.
• The watts at 60Hz: 15.65 W (and 10 V ac).
COMPARISON OF THE TOTAL WATTS TO THE WATTS LOST IN THE RESISTANCE OF THE STATOR WINDING
comparison Total Watts to RI^2
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
frequency [kHz]
Wat
ts
TOTAL WATTS
RI 2
Comments:
• The watts in the copper of the winding were calculated as: P=RI^2.
• The resistance R from line to line was 0.2064 Ohm.
• For comparison, the total losses at 60 Hz were 15.649 W.
• The loss in the stator winding only was 4.7 W. (60 Hz)
• Notice how much closer those numbers are compared to higher frequencies.
• It is obvious, that those two curves have to meet at one point at zero frequency.
• For the ease of the comparison a ratio was created from those two sets of watt readings.
• The ratio r = (W-RI^2)/RI^2. W are the total watts.
Ratio r = f [f]
Ratio r=f[f]
0
10
20
30
40
50
60
70
80
90
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
frequency [kHz]
rati
o r
Comments:
• Note that the ratio for e.g. 800 Hz is about 70. It means that the losses other than those in the stator winding were 70 times higher.
• For the comparison, the ratio for 60 Hz was only 2.33.
• This curve shows clearly, how much different the low voltage is from the conditions at normal operation.
• The next slide will show the inductance, once calculated from the McKinnon-Smolleck formula and once from the correct formula (REAL).
INDUCTANCE AS A FUNCTION OF FREQUENCY
Inductance=f(f)
7
7.5
8
8.5
9
9.5
10
10.5
11
11.5
12
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
frequency [kHz]
Ind
uct
ance
[m
H]
Comments:• The inductance of the circuit is far away form being a
constant.
• The formula for calculating the inductance:
• The formula takes into consideration all losses P in the circuit.
• The inductance at 60 Hz was only 5.284 mH (10 VAC). This inductance drops further to 4.45 mH at 50 V (60 Hz).
2
2
2
*2
1000][
I
P
I
V
fmHL
ac
ac
Phase Angle = f [frequency]phase angle (phi)
0
10
20
30
40
50
60
70
80
90
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
frequency [ kHz]
ph
i [d
egre
es]
Comments:
• It is remarkable,that the phase angle is virtually constant with frequency.
• It would be interesting to perform the same test on the motor with open rotor slots.