Do I need a new motor to install a drive? 6

[jmtop]

I want to install a variable frequency drive on a chilled water pump for a commercial building. Do I need to buy a new motor? It's a 60HP, 3-phase, NEMA Design B motor. In general, and if possible, what requirements do I need to look for to determine if I can reuse motors when installing drives?

Thanks.

 

[buzzp]

Inverter duty motors are available. Not sure if this is any requirement, as of yet. Chances are, your motor would work with a drive but not near as long as one rated for vfds (generate higher amplitude spikes).
Also, wiring sizes need to be looked at.

There are many experts in here on this so I am sure you will get some better feedback.

 

[Skogsgurra]

Hello,

You can use any induction motor. But you will probably need to put a du/dt filter (or even a sine filter) between inverter and motor. A du/dt filter takes the spikes mentioned by buzzp away from the motor voltage. The motor's insulation will probably take a lot of beating without the filter. It is especially important if your motor cables are of any length (five or ten meters - 15 to 30 feet) is usually not a problem.

You should talk it over with the inverter manufacturer.

 

[DickDV]

Since the motor is NEMA, I'm going to assume you are in North America.

If the application is 230VAC, you can use an Insulation Class B or F or H motor pretty much without any insulations concerns as long as your motor leads to the drive are not more than 200 feet in length. You also have to give some thought to the motor thermal capacity. Since you state that the load is a pump (I will assume a centrifugal pump) on a chiller, you will be OK thermally with an ODP or TEFC motor. Slowest speed should not be less than 6 Hz.

If the application is 460VAC, if the motor nameplate states Insulation Class B, the motor MUST be replaced. If the motor nameplate states Insulation Class F or H and the motor leads are not longer than about 60 feet, you will be OK for insulation and the thermal issues are as above in the first paragraph for 230V

If the application is 460VAC and the motor leads are longer than 60 feet and less than 250 feet, and the Insulation Class is F or H, add a motor lead reactor and you will be OK. Leads longer than 250 feet, consult application engineer.

If you are dealing with a 575VAC application, an inverter rated motor for that voltage is mandatory.

Hope this gives some practical guidelines.

 

[dpc]

Just to play devil's advocate, I would probably just use your existing motor and see what happens provided it is not already heavily loaded. If and when it fails, replace it with an inverter-duty motor.

While I agree in general that a new, inverter-duty motor should have a longer life span on an adjustable frequeny drive than a standard motor, there are thousands of old standard NEMA motors that have been put on adjustable frequency drives and have run for years and years.

 

[BobPE]

dpc:

Not to be a jerk, but that is horible advice...The motor will fail becuse it is not the correctly engineered for the job. Running it to failure knowing it will fail is not good engineering. This motor is on a pump. Failure would most likely cause the pump to stop in mid flow could be a hydraulic catostrophe from the hydraulic transients produced when the impeller stops.

I know you are taking the devil's advocate role thinking about the motor and I agree, if we were just spinning motors not connected to anything, running it to failure would be a novel experiment provided we protected people from getting hurt. Discussions we have here in the forums can have the potential to make it into practice...As engineers we need to be careful just what we say...

BobPE

 

[buzzp]

I can relate to dpcs adivise as well as Bobs. Most motors could handle it for quit a while, others may fail immediately. What the poster does will depend entirely on his application. Some applications require the utmost in reliability while others are not near as critical. So if its critical, replace it, if it is not and you can deal with some down time or in shutting down a process with limited lost products/money then let her run. Of course, if the output of the drive is filtered then the need for an inverter duty motor is greatly diminished.

 

[dpc]

To BobPE,

Actually, I think a blanket recommendation to replace the motor is not very good advice either.

There are a lot of factors to consider:

How heavily loaded is the motor? Many motors in service never operate anywhere near their capacity due to engineering conservatism and safety factor piled upon safety factor.

How old is the motor, what is the insulation class, what type of bearings does it have?

What is distance from the drive to the motor? If the distance is small, there is much less stress on the motor windings.

It's always easy to advise someone to spend money that isn't yours.

Adjustable frequency drives were widely used before there even was such a thing as an inverter-duty motor. While there were problems with motor failures, an awful lot of the motors ran just fine and are probably still running. There is no certainty that a "standard" motor is going to fail prematurely just because it is running off a drive.

And guess what? Even inverter-duty motors fail.

So if "hydraulic catastrophe" is a concern, there had better be some additional safety measures in place regardless of what type of motor is used, since the power does go off from time to time and drives will probably fail much more often than either type of motor.

 

[lcleveland]

jmtop
first off all mentioned above is good stuff.
but how old is the pump and motor existing?
replacement of the whole pump assembly might be warranted.
there are no hydraulic problems from a chilled water pump failure, only cooling problems . depending on the reliability needed the best bet woould be to replae the motor with a suitable inverter duty one . failure of the motor should not be an option unless you own the pump .

 

[DickDV]

dpc, thanks for a nice clear good-judgement response.

Isn't it amazing how many solid opinions can be generated without a reasonably full set of info.

Good grief, if this is a 230V application with motor leads under 15 feet and a carrier frequency of 2khz, any old motor will work, even an ODP Insulation Class B. And work for a good long time too, if you please.

An inverter duty motor for every application!!!!! What!!! Are we all motor salesmen here, or what!!!!

 

[RRaghunath]

The issues to be watched with introduction of VFD in the existing motor power circuit, if I understand right from the expert posts above, are voltage spikes and the additional heating due to harmonics in supply.

In such a case, I guess motor with longer leads should be better (considering that the cable capacitance can reduce du/dt). But the posts give an impression that shorter the leads the better it is. Am I missing some thing?

 

[DickDV]

rraghunath, please go to any major drive mfgr website and download their white paper on motor lead harmonics.

You will have a clearer insight into motor lead lengths after reading one or two of those.

You could also go thru Google searching for "inverter reflected wave" to find similar material.

But the summary of all of it will be that longer lead lengths, smaller motor hp's, and higher carrier frequencies all combine to make trouble on the drive output side.

 

[jraef]

Generally, I agree with dpc's approach to using the existing motor (as per DickDV's guidelines) and see if it gives you trouble as opposed to changing it out prophylactically. In the majority of industrial applications I work on I make that recommendation because they already own the motor, the cost of changing it out often means additional downtime, and they usually have spare motors on hand and an established proceedure for changing them quickly if they fail later.

But I must concede BobPE's point about the consequences of failure being at least considered. In many applications, for instance this one where a commercial building chilled water pump may be running continuously, the result of a motor failure may have far reaching effects. Damage to other equipment may be a direct loss, but reduced habitability of the building space may mean additional lost revenue for the building occupants, offsetting the cost of just replacing that motor up front. I once did a project for a large chain of department stores who had empirical data on the somewhat dramatic drop in sales volume as the building air temperature increased even slightly. For them, the chilled water system was considered critical to their revenue stream and most costs involved in maintaining reliability of it were justifiable. Casinos are savy to this as well. Sweaty (or cold) gamblers go somewhere else.

"Venditori de oleum-vipera non vigere excordis populi"

 

[SteveKW]

Sounds like a "bean counter" will have to have the final say on this discussion!

 

[BobPE]

dpc:

I can see your point, and I find a lot of people that agree with your opinion. I fix the problems left behind all the time. When fixing the problems, I am in the Monday morning quarterback mode, so it is easy to make the people that made the decision look bad. You have to remember, this isn't you fixing a problem you have, it is someone looking for advice.

I agree with the first part of DickDV's advice, don't give overly technical advice when you don't have all the information. Since we don't have all the information, a conservative opinion, would seem to be what engineers should give.

I am not a motor salesperson...LOL...I just seem to run into EE problems more than I care to say. The root of the problem, in my limited opinion, always seems to be that the driver is the main focus while the system the driver is serving is secondary...In my world, the driver (usually the cheapest part of the system) is just a small component to a much larger system that really should be the primary focus....

Funny story...My first involving variable speed and electric motor was when I was called to a job where an EE was working on a large verticle turbine pump. It was an older installation that had a series transformers arranged to control the speed by reduced voltage at pump start up. The EE replaced the transformers with an across the line starter, since reduced voltage starting wasn't necessary since the power lines in the community were improved recently, and the chances of brown out were slim. I was called to figure out why the pipe and appurtenances in the station and out in the system failed when they started the pump. The motor worked great and the community didn't brown out, but now they were without water...lol

The point to the story, look beyond what you want to focus on to see the big picture....Inverter duty motors and higher classes of insulation were invented (by us engineers, if you please) for a reason. Because of failures to systems. Now I agree, not every application may need such a motor, but when I get hired by insurance agencies and they ask what caused the failure, and I say lack of engineering consideration in equipment selection...what do you do then?

BobPE

 

[buzzp]

While all opinions in here are valid, the only person that can answer your question is you.
Some posters are more conservative than others in their suggestions. Some are liberal. All could be valid solutions given your application and resources available.
You now know the motor can fail prematurely due to running it on a VFD. How soon, no one knows. It may run for another 20 years. It may run 2 hours. The age (and therefore) the design of the motor will determine this. Older motors are generally more rugged and can handle more electrical and mechanical issues than newer motors. Of course, your motor has been in service for a while so maybe it has weakened. You just don't know.
You just need to ask yourself, what if it fails? How long will it take to replace? What processes will be affected? Whats the cost of unscheduled downtime? If this can be replaced quickly with minimal dollars lost and minimal people affected by lack of water, then maybe just put it in. However, while your engineering the replacement you better also select a inverter duty motor. Maybe you have spares already that have been around a while so you may choose to use these first. Who knows. A common sense approach will give you your solution. VFDs have been around longer than inverter duty motors. It is my opinion that newer motors can not handle the switching transients as well as the old brutes. Hence, newer motors tend to fail quicker when operated on a vfd than an older design in decent shape.