75KW dynamometer controller 1

[iceengineer]

I have a 75KW dynamometer controller which is capable of powering several dynamometers: a 40HP, 75HP, 120HP. The 75HP and 120HP dynos work perfectly varying by less than 0.5 of an rpm from the set value, and capable of holding that under varying load. The 40HP dyno however has recently had many issues. The dyno has had issues with stability, thus the tone ring sensors have been replaced (have been verified that they work). Then the commutator was arching severly, thus the 8 brushes were replaced. Currently the dyno (when powered up) instantly runs to about 600-1500RPM when the current trips an overload sensor. The dyno is designed to be completely variable from 60rpm to 4500rpm. The controller has obviously been verified that it works with the other dynos. The dyno is an older GE 40HP dyno with a 250VDC max. Any ideas on why the dyno doesnt respond properly?

 

[waross]

A failing field winding will probably cause instability. A symptom of a weak field is over-speed and over current. A change in the field strength will also affect the commutation, hence the sparking at the brushes. Check the field winding, the field supply source and the external field wiring and connections.
yours

 

[aolalde]

As I understand a Dynamometers allow measurable mechanical torque load applied to the driving motor under test. Are you testing several motors at the same time? Are these “DC motors” regulating the operating speed, instead of "dynamometers"?

 

[waross]

Hi aolalde
I took it that these were used to drive a load such as a pump to determine the horspower required at various loadings and speeds.
iceengineer, how are these dynos used or configured?
yours

 

[iceengineer]

The dynamometer is used to drive experimental research internal combustion engines. The reason I have more than one is that, it may take two months to properly set one up (fuel, electrical, sensors, etc...) The dynamometers are used to drive the engines at a specific rpm (applying power or absorbing power as required and reporting required torque) or is used to extract a specific torque load (and reporting rpm).
The word dynamometer is used because the function like a water brake dynamometer, with an added feature of being able to measure/apply negative torque loading. This is extremely useful because in some research scenarios the engines dont create enough power to run themselves, but the sensor data is useful nonetheless. For example knowing the pressure time data of a motoring (not firing) engine is extremely useful.

 

[iceengineer]

Today I spent a ridiculus amount of time on the phone with the guy who built the system. He step me through testing individual field windings (Thanks to waross's tip) This guy immediately agreed with you, unfortunately the windings are perfect. He even had me power the field windings with an arc welder as the power source. But now a new problem has arose: The dyno will only start up (still with the current problem) if it is already spinning. Its almost as if the multiple fields are fighting each other. Short in the armature? Not that I know how to test for this. It normally has a resistance of 0.1 ohm

 

[aolalde]

I agree Iceengineer those are Dynamometers alternating performance as required Motor-generator.
The brush assemble could have shifted and to check the armature condition too, make a "Brush Neutral Position Test" a shorted armature will not reach a 0 to 0.010 Volt reading on any brush position.
Check the commutator roundout ( 0.001" TIR) and look for rising commutator bars.
Check that the brush holders allow free brush movement and the brushes are not too loose or vibrating.

 

[waross]

Hello iceengineer
That sounds like a total loss of field. I would check both current through the field and voltage at the field terminals at the machine. Check all wiring. you may have a wiring problem that is inadvertently being bypassed by your testing.
For example; If some tests are performed at the machine terminals with the wiring disconncted at the machine, and some tests are performed at the panel with the wiring disconnected at the panel you will not find a fault in the wiring.
Energize the field and check the polarity of the poles with a compass if you have not already done this.
I once encountered a DC motor of about 30 or 40 Hp. It ran poorly in one direction and not at all in the other direction. There was a bad connection in an armature lead that had corroded and produced copper oxide. The copper oxide was acting as a rectifier and blocking the current in the reverse direction. Check the wiring and connections.

To check the armature;
Apply AC to the fields. Usually 120 volts is acceptable.
Measure the voltage at the brushes. It should be zero. You can check your brush position by shifting the brush holders. It's a good idea to shift the brushes to see some voltage and confirm that the test will work. Now with AC applied to the field, rotate the armature. Any faults will show up as a fluctuating voltage at the brushes.
respectfully

 

[iceengineer]

I found an intermittent problematic relay which connects the f1-f2 field power. Whereas the f3-f4 relay was functioning. Apparently this was the problem, replaced it now the thing works perfectly. Thanks, waross, for pointing me into the right direction.

 

[waross]

Glad to help
respectfully