Advice on motor protection system
Advice on motor protection system
(OP)
Hi. I am a new starter with a company which manufactures water-filled motors. There was an incident on a 6kV motor 400kW 2960rpm motor driving a centrifugal pump. It was summarised that the impeller damaged to the extent of fully worn out. As a result, the rotor shaft became unstable and caused damage to the bearings.
There was only thermal trip and starting current protection system for that motor installed by the client. As a result of the lack of protection system, the client decided to
My opinion is that the damage to the bearing can be prevented by adding a simple overload current protection. Since worn out impeller causes higher motor current above the allowable limit, the overload trip can be activated to switch off the motor to prevent another set of failure. Thermal protection does not effectively protect the motor as there is a lag before the thermal limit is reached, therefore allowing the motor to run abnormally for an extended period of time until it tripped on thermal overloading.
What is your experience on this?
There was only thermal trip and starting current protection system for that motor installed by the client. As a result of the lack of protection system, the client decided to
My opinion is that the damage to the bearing can be prevented by adding a simple overload current protection. Since worn out impeller causes higher motor current above the allowable limit, the overload trip can be activated to switch off the motor to prevent another set of failure. Thermal protection does not effectively protect the motor as there is a lag before the thermal limit is reached, therefore allowing the motor to run abnormally for an extended period of time until it tripped on thermal overloading.
What is your experience on this?





RE: Advice on motor protection system
RE: Advice on motor protection system
RE: Advice on motor protection system
Without it RTF is the default mode.
RE: Advice on motor protection system
I agree with wroggent, thermal trip and overload trip are roughly interchangeable - itended to detect mechanical overloads. Often the overload trip setpoint is based on some margin above nameplate, if it is known that the machine oeprates lower then it may be possible to lower the overload trip setpoint.
In addition to other fluid system monitoring mentioned, for large machines it may be helpful to have continuous vibration monitoring to develop alarms or trips.
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(2B)+(2B)' ?
RE: Advice on motor protection system
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(2B)+(2B)' ?
RE: Advice on motor protection system
"Will work for (the memory of) salami"
RE: Advice on motor protection system
RE: Advice on motor protection system
Overload protection and thermal protection are the same thing. Both work on a curve. A small overload has a long trip time and a large overload has a short trip time. I believe you're thinking an over current relay which has a definite time trip. You'd have to bypass it during the motor start or it will trip due to the high starting currents.
I agree with the others. Try pressure or flow or bearing temperature or vibration protection.
There are many people who think an overload or over current can protect against a failing bearing believing the motor current will rise significantly as the bearing starts to go bad. Well, I personally believe the bearing will be to the point of causing significant damage to the motor before monitoring current will tell you it's failed.
RE: Advice on motor protection system
Add me to the list of people who are skeptical about using current to detect failing bearings. Even when trouble shooting motors with failing bearings a current reading won't tell you much.
Consider that pumps seldom run at the full rated power of the motor.
Your 30 HP pump may only be producing 25 HP, 20 HP or less. Overload protection is sized for full output. Even if your bearing failure causes extra loading, it will be destroyed long before the overload protection trip point is reached.
Bill
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"Why not the best?"
Jimmy Carter
RE: Advice on motor protection system
"Will work for (the memory of) salami"
RE: Advice on motor protection system
If the impeller wear ring becomes fully worn, I would imagine that the efficiency will drop, and pump will absorbed more power. This led me to believe that current should increase, and and if current is for example 10% higher than full load current than it should immediately trip - I guess I was mistaken. Like Lionel pointed out that it will cause ".. significant damage to the motor before monitoring current will tell you it's failed".
If vibration was monitored for the motor, does it trip when it detects high amplitude velocity?
RE: Advice on motor protection system
Also, if the pump is operating outside of this intended duty, it would also tell that it is not within its normal operating mode.
RE: Advice on motor protection system
When bad bearings cause enough increase in the current to be noticeable it is too late.
A trend that I am seeing is trending a combination vibration monitoring at each bearing and temperature monitoring AT EACH bearing. The transducers are computer monitored and the software alarms on sudden changes of temperature or vibration. These were skid mounted pumps in petroleum service. I am not sure how effective bearing temperature monitoring will be on a submersible motor.
One advantage of vibration monitoring is that it will also respond to impeller damage.
Bill
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"Why not the best?"
Jimmy Carter
RE: Advice on motor protection system
RE: Advice on motor protection system
The best way to protect a submersible motor is by setting the current overloads based on the actual operating point and not the nameplate. If the name plate says FLA of 60A and the motor is running at 55A, base your overloads on the 55A. Also monitor the voltage, current, and pump flow for trends. A slowly increasing current and/or decreasing flow will indicate things are starting to get out of spec, and a time for repair can be scheduled before then entire thing gets destroyed.
The problem with the hydrodynamic bearings used in water filled motors is that if they lose their fluid wedge while in operation, it does not take long for them to be destroyed. The fluid wedge can be overcome by vibrations and such, as well as by the internal water heating up too high.
RE: Advice on motor protection system