Motor help

[maint2004]

We service customers who have three phase motors and I like to ask some question on that topic.

If a motor is going to be replaced for let say it burned out.

How should it be replaced, by using the nameplate or by using some type of upgrade means to a efficent motor ?

If a motor is using less than half it rated amperage is it oversized for the job it is doing and should it be sized smaller ?

When megging a motor how do I make temperture correction to get a correct reading ?

If it meggers to 100 m and the motor is in 40 deg. temp. What is the next step ?
Thank you for the help here.

 

[Besoeker]

If a motor is using less than half it rated amperage is it oversized for the job it is doing and should it be sized smaller ?

If the measured motor current is less than half its nameplate current, it is likely that it was very lightly loaded - that is at the time when the measurement or observation was made.
You'd need to know all of the operating conditions it sees, or is likely to see, before making a decision about reducing the size.
The initial rating probably wasn't an arbitrary decision.

 

[Strahan]

We service customers who have three phase motors and I like to ask some question on that topic.

If a motor is going to be replaced for let say it burned out.

How should it be replaced, by using the nameplate or by using some type of upgrade means to a efficent motor ?

If a motor is using less than half it rated amperage is it oversized for the job it is doing and should it be sized smaller ?

When megging a motor how do I make temperture correction to get a correct reading ?

If it meggers to 100 m and the motor is in 40 deg. temp. What is the next step ?
Thank you for the help here.

First question would be why did it burn out? And I ask this because I work 3rd shift wet clean up and there are other factors outside sizing that do have an affect. Next question would be how long was this motor in operation? Finnally 40 deg should not affect the megger reading , and what are you controlling this motor with a motor starter or VFD? Either way both in my experience would run this motor.

 

[charlietuna]

If this motor is designed and engineered for the task in hand(?) then replace it with the same HP -Voltage -Frame Size - etc. You can go to a high efficiency motor with matched nameplate if you want--no problem.
I meg a motor first to ground, and then between leads to confirm continuity. Motor current is a factor of the amount of work it is doing. As it does more work the current will increase proportionaly. Example is a pump. With the pump motor running and discharge valve close, the current will be very low - just the current needed to turn the rotor. As the valve is opened, the motor starts doing more work of moving water and the current increases. If this motor is designed properly, it will move the rated system water (GPM) without exceeding 80% of the motor's rated max. current. Motors can fail for many reasons - your first step is finding out why it failed...

 

[x743]

We service customers who have three phase motors and I like to ask some question on that topic.

If a motor is going to be replaced for let say it burned out.

How should it be replaced, by using the nameplate or by using some type of upgrade means to a efficent motor ?

If a motor is using less than half it rated amperage is it oversized for the job it is doing and should it be sized smaller ?

When megging a motor how do I make temperture correction to get a correct reading ?

If it meggers to 100 m and the motor is in 40 deg. temp. What is the next step ?
Thank you for the help here.

When megging a motor to ground I use a figure of 1m/1000 volts. So .480m on a 480 volt motor. anything less most of the time will cause the OCPD to trip. I've never worried about temp correction for these tests. Normally a motor will start out with a very high reading and then over time go lower and lower. If I'm getting close to 1m/1000 volts, normally I'll let the customer know they may want to have a new one on the shelf.

I was always told in apprenticeship, if a motor burns out it's the electricians fault. The is due to incorrect sizing of the overloads. Correctly sized overloads will catch anything that will cause a motor to heat up. (except for an incorrectly wired motor at the motor junction box)

If a motor is drawing half it's current, your most likely correct it is undersized. I don't know the application but if you down size the motor and do the overloads correctly, any possible mechanical issues with the machine may cause the overloads to start tripping. In doing this you also need to look at the starting load. The code allows you to increase your overloads above 115% or 125% in these cases. I've never used it, I've always went with the class 20 or class 30 overloads. Protecting a motor with a class 10 overload above 115% or 125% will shorten the life of the motor. The other two classes will do a much better job protecting and not tripping at the standard rating.

If you can sell them on the money savings going to a more efficient design motor go for it. Just realize sometimes doing this you may have to increase the OCPD due to the higher inrush.

When replacing a motor. You will normally want to make sure and get the same frame size. This covers you for mounting up. RPM and horsepower is about all that really matters. (we are all electricians so I know we will get the voltage correct)

 

[charlietuna]

I was always told in apprenticeship, if a motor burns out it's the electricians fault. The is due to incorrect sizing of the overloads. Correctly sized overloads will catch anything that will cause a motor to heat up. (except for an incorrectly wired motor at the motor junction box)

I do not agree with the above statement. There are many reasons that can cause motors to fail OTHER than electricians! What about just insulation break down, or lack of ventilation and mechanical issues related to their application ?

Quote:"RPM and horsepower is about all that really matters. (we are all electricians so I know we will get the voltage correct)"

Again, i disagree, RPM and horsepower matter, but what about the service rating concerning ventilation relating to the motor's application and the enclosure rating ? Yes, we are all electricians, but many motor failures we come across are caused by electricians connecting 230 volt motors on 208 volt systems.

 

[petersonra]

If this motor is designed and engineered for the task in hand(?) then replace it with the same HP -Voltage -Frame Size - etc. You can go to a high efficiency motor with matched nameplate if you want--no problem.
I meg a motor first to ground, and then between leads to confirm continuity. Motor current is a factor of the amount of work it is doing. As it does more work the current will increase proportionaly. Example is a pump. With the pump motor running and discharge valve close, the current will be very low - just the current needed to turn the rotor. As the valve is opened, the motor starts doing more work of moving water and the current increases. If this motor is designed properly, it will move the rated system water (GPM) without exceeding 80% of the motor's rated max. current. Motors can fail for many reasons - your first step is finding out why it failed...

Actually that is not quite true. The power used does increase roughly proportional to the work being done, but the PF for lightly loaded motors tends to be quite low, causing more current than one might expect for the actual load.

Motors fail for a lot of reasons and being over sized is not typically one of the reasons. I would not be reducing the motor size without thinking it through. Its quite possible that a motor might only need full power a few hours a year. Thats not all that unusual of an application.

Some reasons motors fail:
- just plain failed for some reason
- overloaded
- under/over voltage
- single phasing
- motor not cooling properly (dust is sometimes a culprit)

It could even be some combination of the above.

You might want to take a look at the heater selection. Its not unheard of for someone to put the wrong size units in, either unintentionally or deliberately. Sometimes people do things like cranking up the setting on the overloads to keep the plant running after a trip and never fix the reason it tripped.

 

[Besoeker]

but the PF for lightly loaded motors tends to be quite low, causing more current than one might expect for the actual load.

Yes.
In my experience a three-phase cage motor takes around 30% of FLC on no load.

 

[PowerQualityDoctor]

Load of motor should be made by active power, as the current remains high due to low power factor on partial load. As Besoker said, 30% means almost no load on the motor.

After measuring the load during the motor work cycle you can decide how to control it.

Does it need to change speed? If yes - it probably has variable speed drive. Maybe the motor is not designed for speed control (requires higher class of insulation)?

If it doesn't need speed control - consider using motor efficiency controller such as SinuMEC by PowerSines.

 

[jmellc]

I don't do much motor work and have no deep knowledge of motors. When I have a motor problem, I call a motor shop that's been here for decades. I follow their advice. I take the motor in to them if possible, make good copy of nameplate info if not. If they tell me an improved model is available, I talk to the customer and see what he wants to do. Otherwise, I replace it with standard model. I use this shop to replace roof fan motors in places where replacing the whole fan assembly would mean extensive roof cutting or roof is too steep to safely work. They have always found something compatible, except for a strange motor on my inlaws' roof fan. Had strange little mounting studs that lock into the fan brackets. They could not find that and the motor was much too small to rewind. My father in law has somehow found more items than anyone I know that are one of a kind with no parts out there anywhere.

 

[zog]

When megging a motor how do I make temperture correction to get a correct reading ?

.

Use this table

 

[weressl]

I was always told in apprenticeship, if a motor burns out it's the electricians fault. The is due to incorrect sizing of the overloads. Correctly sized overloads will catch anything that will cause a motor to heat up. (except for an incorrectly wired motor at the motor junction box)

I do not agree with the above statement. There are many reasons that can cause motors to fail OTHER than electricians! What about just insulation break down, or lack of ventilation and mechanical issues related to their application ?

Quote:"RPM and horsepower is about all that really matters. (we are all electricians so I know we will get the voltage correct)"

Again, i disagree, RPM and horsepower matter, but what about the service rating concerning ventilation relating to the motor's application and the enclosure rating ? Yes, we are all electricians, but many motor failures we come across are caused by electricians connecting 230 volt motors on 208 volt systems.

Torque and load inertia is chopped liver?!

 

[LarryFine]

Torque and load inertia is chopped liver?!

Mmmmmm! Chopped liver!