VFD & Motor compatibility

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megloff11x

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If they don't publish it in their specifications and their technical support center in New Delhi doesn't know (Ross Perot undercounted the number of sources of giant sucking sounds), what else can you look at to determine a motor's compatibility with a VFD?

Matt
 
originally there were no motors rated for vfd's. we connected them to the original motor or installed them to a regular motor on new installs. we always recommended new motors. many of these older motors failed due to insulation failure. remember the drive's output voltage is in the 700 to 800 volt range. we used to tell the customer if it(older motor) lasted through the first three months of operation chances were that they would not have any problems with the original motor. they came out with high efficency inverter rated motors -- they had a higher rated insulation and larger fans to cool the motor at low rpm operation.
 
Charlie,

Actually the drive does not put out a higher voltage. A higher voltage is seen by the motor due to refelcted waves between the motor and drive, that build up to high levels. Also, on non-inverter duty motors, a lot of times you would have bearing failure because stray current on the shaft would go to ground through the bearings, causing severe pitting. leading to faiulre. You should always have insulated bearings on the motors.

Matt,

The cable length between the drive and motor needs to be as short as possible, and it should be a multiconductor armored cable with 3 concentric grounds, twisted within the cable.

Motors not only need to be inverter duty rated, they need to be tested specifcally with the drive. The drive manufacturer should be able to tell you this info.

Robicon drives, and marathon motor websites have a lot of good info.
 
kingpb said:
Motors not only need to be inverter duty rated, they need to be tested specifcally with the drive.
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I wouldn't really think that is necessary. Is that something from a manufacturer's "sell sheet", or from personal experience? I've never seen a manufacturer list their drives as being compatable with XX brand motor, but it sounds like you know at least one manufacturer that does. Are there many?

Even if an occasional motor-drive doesn't combination doesn't work, I think in the vast majority of cases, a standard inverter rated motor and a standard drive usually wouldn't be a problem. I would think most of the problems would be in unusual applications - weird motor or drive, or weird load, low speed, or something like that.

Steve
 
It my experience the insulation ratings of the motor winding are crucial to the longevity of service for the motor. Keep the lead lengths short, if you can't, use a 3% harmonic output filter to reduce the capacitive coupling of the motor leads. A non-inverter rated motor may last for a year or two but will eventually fail. A great test for marginal insulation on a motor that has been across the line is to put on a drive. If there is a weak spot in the insulation, The PWM output of the drive will find it.
 
Several of the newest generation VFD's have a start-up function called "motor initialize" ABB and Yaskawa both have it. The VFD will go through a series of magnetization and rotational current outputs to determine what the motor runs optimally. This allows the use of any motor regardless of manufacture, as long as the insulation is rated inverter duty.
 
Kingpb,
The cable length between the drive and motor needs to be as short as possible, and it should be a multiconductor armored cable with 3 concentric grounds, twisted within the cable.
Motors not only need to be inverter duty rated, they need to be tested specifcally with the drive. The drive manufacturer should be able to tell you this info.
Click to expand...
Is most of this info just manufacturer's CYA info? I have a couple of hundred drives installed here, with none on inverter rated motors, all with THWN in conduit and at least 100' long, no problems with winding or bearing failure. Also only a few of the drives have input or output reactors. They range in horsepower from 0.5 to 500.
Don
 
A lot of data and papers have been written on this topic. Certainly, every application/installation needs to be viewed on it's own merits.

The longer the cable, the greater the chance that the standing/reflected wave can build to voltage greater then that of the insulation of the motor.

Typical explanation without getting to detailed:

VFD cables carry power from AC drive systems to AC motors. As a result, these cables must handle not only the overall high power levels of the pulse-width modulated (PWM) signals, but also the extremely high voltage which can occur when standing waves develop on the conductors. This high voltage can cause corona discharge between the conductors of conventional cables, causing damage not only to the cabling itself, but also to the motors, bearings, drives and related equipment. In turn, this damage can cause failure of the entire drive system, resulting in costly production downtime.
 
The standing wave problem is also a function of the switching frequency of the drive output.

IMHO, unless the motor is expensive to replace (don't forget downtime) or more than 10 years old, I see no need to purchase an inverter rated motor when used with PWM output drives designed in the past 5 years.
 
Is most of this info just manufacturer's CYA info?

Is most of this info just manufacturer's CYA info?

Don,

I sincerely believe that you have asked the pertinent question. Drives come with various programmable functions, such as motor characteristics. Even the latest vector drives are set up with those before running the learn sequence. Strange, to think that I was responsible for evaluating motors and drives for production, and shipped up to 500 5-20 HP units monthly, without experiencing this 'blown motor' phenomenon. The people I dealt with had been in the field from SCR days, and when a 10HP drive used a walk-in closet to house components; they didn't seem to think there was any magic beyond programming the units to what motor was being used.

A single Fuji motor for inverter duty only was tested, which had good quiet operation at our 252 Hz standard but was high priced. To quote something I read once, induction motors have been used for 100 years and some of those are still running, and the induction motor is one of the most reliable and durable devices mankind uses.
 
Kingpb,
I understand that VFDs can and do cause problems, but I haven't experienced it. I have read the papers and manufacturer's information and they both make it sound like these problems will occur every time if you don't do it their way. Most of them say that if your leads between the VFD and the motor are over 50' long you should expect problems. I can't even get out of the MCC with less than 100' of lead. The average length would be about 200' and I have a couple over 400'. Maybe we are just operating under the theory that is expounded in your signature line as the installation cost go up significantly if you do everything that is recommended.
Don
 
don_resqcapt19 said:
Kingpb,
... Maybe we are just operating under the theory that is expounded in your signature line as the installation cost go up significantly if you do everything that is recommended.
Don
Click to expand...

How true, how true! In some ways don't we all operate that way..........:D
 
i agree with don!! we have installed over 200 drives and i would guess 60 per cent with non vfd rated motors. i can only remember three or four motor failures directly after converting to drives. of course the motor manufacturer recommends "their" motors be used with "their" drives!!

remember, on many of these installs not only did the drives save energy costs, but also big time costs of maintainance. consider a belt driven cooling tower fan --- twice a year the belts would fail and every three years figure a gear box. thats alot of labor-materials and down time!!! consider a large press which originally had brush controlled motors for speed control --- the motor's location and access under the press prevented easy maintainance and most of the time the motor would wear out the brushes and cause major commutator damage and winding damage. these motors cost from $12k to $24k to replace. and the downtime is very costly.

we always recommended new motors but some installs this would put the job costs out of budget. and i know the output voltage of many drives are in the neighborhood of 780 volts--- i've measured them. and we normally used standard wiring practices and had no problems.....
 
Inverter Duty rate motors

Inverter Duty rate motors

Inverter duty rated motors are basically oversized motors. If you are using a 50Hp inverter, the motor MUST be for inverter duty. Typically this motor will actually be a 75Hp motor with a 50Hp nameplate. This is to accomodate the additional heat build-up due to running at slower speeds which reduces the effect of the cooling fan. Also the harmonics produced by inverters create excess heat and eddy currents in the motor. A 3% line reactor in between the inverter and the motor will help reduce harmonics. If the incoming line to the inverter is dipping, another 3% line reactor upstream of the inverter will help this.
Kevlar
 
kevlarster,
Inverter duty rated motors are basically oversized motors.
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Are you sure? My understanding that the biggest difference is that the winding insulation has a higher voltage range.
Don
 
Gotta go with Don, inverter duty motors have beefed up insulation to cover the voltage increase typically seen.

If the motors were simply oversized, how would that solve the issue of voltage. A larger motor is still rated for the same voltage class, and voltage class dictates insulation. That would also mean a lot of short circuit analysis would be messed up, and the oversized motors on starting would be drawing a lot more current then expected.
 
It is the winding insualtion rating that creates the inverter duty rated motor. Something like 1500 volts vs 1100 volts for non-inverter rated motors.

Here is a basic scenario I use to non-electric people explaining motor HP. A 1 HP motor if connected to a 100 hp load, will draw enough current to rotate the load. The problem is, the cooling and current carrying capability of the 1 hp motor is not sufficient to handle the amps required to rotate the 100 hp load, so it fails. The 100 hp motor if rotating a 1 hp load will only draw enough amps to rotate the load and itself. Now there is wasted money on copper and steel. Match the motor HP to the load. If external cooling is needed because of motor speed or environement, get a separate fan motor.
 
One reason some people talk about an inverter rated motor as being larger has to do with the physical size required for increased voltage insulation and the radius of the winding turns. The high voltage VFD spike wants to go in a straight line (just like a ligntning surge) and so it will often "blow out" the windings as they loop out and turn back into the motor. Smaller motors often lead to smaller turning radii.
 
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