Motor acceleration model

Status
Not open for further replies.

wpm1

Member
Location
canada
I'm trying to model a motor accelerating from 0 full load speed. The motor data for Rr, Xr, Xm, Rs, Xs, when I start the model gives a much higher current than the LRA data. I am assuming Is=V/Ztotal where Ztotal= Zs+ (Xm*(Rr/s+Xr i)/(Rr/s+(Xr+Xm)i)). Using the impedance data I expect the calculated amps at slip of 1 should match LRA.

The impedance were reported in P.U. which i was never good at. I am using the phase voltage, and the P.U. impedances.

Any advice?
 

wpm1

Member
Location
canada
Wpm1...

A difference between using Ohms and Per-Unit occurs when one uses the wrong Base VA! What value did you use?

Regards, Phil Corso

Fixed it. Yes, I was using %R and %X, but using actual voltage, I took the base reference and converted to actual R and X. Acceleration model works now, torques and currents are very close to what I was expecting. Rusty on the P.U. system, Ha Ha.
Thanks.
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
Wpm1...

FWI, another common error is using the Rule-of-Thumb (ROT), i.e., kVA equals Hp!!

Phil

1 Hp = 746W (0.746kW)

conservative efficiency = 0.9
conservative pf = 0.85

therefore; 1 Hp approx equal to 1 KVA. Granted this is very conservative, but for quick estimating, works quite well.
 

templdl

Senior Member
Location
Wisconsin
I'm trying to model a motor accelerating from 0 full load speed. The motor data for Rr, Xr, Xm, Rs, Xs, when I start the model gives a much higher current than the LRA data. I am assuming Is=V/Ztotal where Ztotal= Zs+ (Xm*(Rr/s+Xr i)/(Rr/s+(Xr+Xm)i)). Using the impedance data I expect the calculated amps at slip of 1 should match LRA.

The impedance were reported in P.U. which i was never good at. I am using the phase voltage, and the P.U. impedances.

Any advice?
Are you designing a motor? I also am assuming the the motor example that you are using is 3ph which shows speed torque curves fro start through the knee of the curve to run.'
How would be different than a common NEMA speed torque curve such as a design 'B'?
Then the is the standard CO curve as I can recall which is based upon the wr2 of the load.
 

Julius Right

Senior Member
Occupation
Electrical Engineer Power Station Physical Design Retired
Leakage reactance and resistance values of the rotor depend on slip

Leakage reactance and resistance values of the rotor depend on slip

One cannot use the same rotor resistance and reactance of steady state run in order to calculate the start current. Leakage reactance and resistance values of the rotor depend on slip . The rotor frequency is elevated at start and reduced in steady state run. That means Rr(slip=0.05)<Rr(slip=1) due to skin effect. See[ for instance]:
http://piers.org/piersproceedings/d...ycmFrZXNofDNQOF8xNDUxLnBkZnwxMDA5MTUwOTU5NTQ=
 
Status
Not open for further replies.
Top