Chiller motor, will not perform
Chiller motor, will not perform
(OP)
We recently rebuilt a chiller motor (600hp 3600rpm)It had a die cast rotor. The motor failed from broken rotor bars.
We removed all the old die cast alum, manufactured new rotor bars and shorting rings from alum, all sizes were dulicated within 1-2%. The rotor bars are welded to the shorting rings. The motor has been installed. it starts and runs up to speed in approx 3-4 seconds, inrush current is normal, no load current is (normal 250 amps), however when the IGVs are opened to allow the freon to flow the motor current increases to 320 amps and does not compress the freon. The chiller company (Carrier) say the chiller clearances are fine and all is working well, there is plenty of freon in the system. Puzzled ?
We removed all the old die cast alum, manufactured new rotor bars and shorting rings from alum, all sizes were dulicated within 1-2%. The rotor bars are welded to the shorting rings. The motor has been installed. it starts and runs up to speed in approx 3-4 seconds, inrush current is normal, no load current is (normal 250 amps), however when the IGVs are opened to allow the freon to flow the motor current increases to 320 amps and does not compress the freon. The chiller company (Carrier) say the chiller clearances are fine and all is working well, there is plenty of freon in the system. Puzzled ?





RE: Chiller motor, will not perform
What was the reason for the original motor failure to begin with? Was compressor checked at that time? Rotor failure usually occure due to locked rotor conditions (overheating).
If the motor is turning and taking the load, something else is wrong. Plus motor does not compress the freon, the compressor does, is the compressor turning and compressing?
May be motor is not turning sufficiently due to mechanical issues?
RE: Chiller motor, will not perform
RE: Chiller motor, will not perform
THe vibration test will indicate motor speed and compressor speed and any irregular mechanical conditions. I would be most concerned with compressor impellor condition. The vibration test can indicate deficiencies in the impellor as well as irregular gas flows in the machine.
If you have a similar machine, obtain data from both for comparisons.
RE: Chiller motor, will not perform
Consider the possibility that you have significantly altered the rotor resistance and therefore the torque speed curve. Maybe when you load the motor it is exceeding breakdown torque situation. Have you measured speed under load?
RE: Chiller motor, will not perform
A few questions:
1. What was the LRA of the original motor?
2. What was the rotor speed when the IGV opened?
3. What type (turbine, screw, vane, axial, etc.)of compressor?
4. Was there any flow and what were the readings from the suction and outlet side?
Without seeing the answers, my guess is that either the motor stalled or ran significantly slower under load than the original. Unfortunately, despite that the fact that the physical dimensions were copied, your rebuild may not have used the same type of aluminum as the original diecast material, similarly for the shorting rings. This would have a definite effect on the resistivity of the material which in turn would affect the current flowing in the rotor. The result is a motor that works fine under no-load conditions but has higher than expected slip and an altered speed-torque curve under load.
It will be obvious if the motor stalls under load, but if it runs too slow (has high slip) then the type of compressor becomes important since the characteristics vary by type. Some compresors roughly follow the cube law; therefore a 15% reduction in shaft speed would reduce its output by nearly 40%. You might try modulating the freon flow if possible, but now I'm outside my area of expertise.
RE: Chiller motor, will not perform
Normal inrush current is approx 5x or 2800 amps.
Because the motor is fully sealed it is not possible to put on a tach to measure RPM, we are going to try and monitor it through vibration, but because we balanced it so well (4W/N) the motor is very smooth, you can not feel by hand any vibrations. The compressor is centrifugal, it has 2 impellers approx 32" diameter one behind the other on the end of the motor shaft. Rotors commonly fail on these machines due to the high current at start up followed by rapid heating and rapid cooling, which stresses the di-cast aluminum, to the point of fracture at the junction of the shorting ring. The cast aluminum and the extruded aluminum have very similar electrical qualities (conductivity 64% vs 62%) The extruded material being 64%, however we reduce the section by 2% to make the bars fit easier into the slot.
RE: Chiller motor, will not perform
RE: Chiller motor, will not perform
RE: Chiller motor, will not perform
RE: Chiller motor, will not perform
Measuring speed using vibration should be no problem, regardless of the balance. The lowest bearing housing vibration I have ever seen at 1x was on the order of 0.001 ips. We could still pick it up. Put the spectrum on a log scale and it should show even the slightest peak at running speed above the noise floor. Modern 12-bit or more data collector would do a better job than a dinosaur 8-bit data collector.
If the motor is maintaining speed at/above nameplate speed, then the motor is doing its job and the problem is mechanical.