Motor for VFD duty Vs normal duty motor 5

[RRaghunath]

I understand from discussions in this forum motor design needs to be different from the normal design when the motor is to be used with VFD. This is mainly as the motor design needs additional attention with regard to voltage transients and the harmonics associated with VFD duty, as I understand.

How about the cost, Is it possible to say the VFD duty motor is costlier by such and such percentage than the normal duty one.

Another thing, the VFD for HT motor vs that for LT motors - Is it a good idea to have two nos. LT motors (rated 2x50% duty) than 1x100% duty HT motor i.e. will two nos. LT VFDs be cheaper than one no. HT VFD? To make it more clear - Should I ask mechanical people to consider 2x50% rated fans in place of 1x100% fan (both with VFDs, 1x100% one is with HT drive) to effect saving in cost of VFD.

Any experiences / recommendations!

Thanks in anticipation.

 

[DickDV]

rraqhunath, as always, you will run into people that are more than happy to overspecify components to protect their own comfort zone. That's where you get statements like "you always should specify an inverter-duty motor, etc etc!!"

The basic facts about motors on inverters come down to primarily two issues. First, heat. This issue is significant only when a motor with a cooling fan on its rotor shaft is required to develop high torque levels at slow speed. Centrifugal loads such as most fans and centrifugal pumps unload as they slow down by the square of the shaft speed. Motors on these types of load rarely have any thermal issues because, even at very slow speed, there is little or no load. These applications can use ODP or TEFC motors without any extra thermal considerations-in other words, plain, commodity-grade motors.

If your load requires high or full-rated torque at slow speeds, then you need to check with the motor manufacturer for slow speed thermal capacity. Up to about 100hp, most TEFC motors are rated full-torque down to 1/4 nameplate speed. Below that, you will need some auxiliary method of cooling. Within that range, however, ordinary ODP and TEFC motors are ok from a thermal standpoint.

The second major consideration is insulation integrity. VFD's output high voltage pulses to the motor which can result in premature insulation failure. Roughly, here are guidelines to follow for insulation issues.

If your application is 230 or 380volt and your motor leads are not more than 100ft, and the drive carrier frequency is 4khz or less, pretty much any old motor is ok.

If your application is 460V, motor leads are under 60ft, and drive carrier frequency is 4khz or less, any standard Insulation Class F motor is ok. Insulation Class A or B is absolutely unacceptable. If, at 460V, leads are longer or carrier frequency is higher or you just need a little extra insurance, use a Class F motor with IEEE MG1-Part 31 rating. This is an extra insulation test specifically dealing with VFD-type pulses and gives you a better level of insulation integrity. This extra endorsement is not expensive and is worth specifying on any new motor purchase, whether you need it or not.

If your application is 550V or higher, use of inverter rated motors for that specific environment is mandatory. The same is true at 460V if leads are long, carrier frequency must be higher than 4khz, ambient temperatures are at or above 40 degrees C, etc. etc.

As you push the edge on insulation issues, you get into the need for more and more specialized and expensive conditioning equipment while using the better motor grades.

Hope this has helped clarify where that edge is.

 

[dpc]

I'm not sure about the cost adder for inverter-duty motors. It may depend on the size range you are considering.

One item that I don't think DickDV mentioned is protection against shaft currents through the bearings that can cause bearing damage. This is a fairly common problem on motors fed through adjustable frequency drives. Most manufacturers provide some form of insulation in one of the bearings to eliminate the shaft currents in their "inverter-duty" motors.

I would also recommend specify some type of embedded temperature detector for any motors 100 hp and above on drives.

As for deciding between LV and MV drives, this is going to be very application specific. The cost of MV drives has come down quite a bit in the past few years. I doubt that specify two LV systems in place of one MV system is ever going to be less expensive (at least above about 500 hp).

But I agree with Dick that in many cases, an inverter-duty motor may not be a necessity.

 

[jraef]

Actually dpc, you may be wrong about the cost diferential between LV and MV drives in many instances, for the simple reason that often the smallest MV drive may be far larger than you need. For instance, although a VFD manufacturer may catalog a 500HP MV VFD, in truth they use the same components as in a 2000HP unit because they are the smallest components available at that voltage. Or if you need a 2500HP drive, you are actually buying a 5000HP. Of course in that case you really didn't have an option at LV anyway, but you get my point.

"Venditori de oleum-vipera non vigere excordis populi"

 

[aolalde]

I checked list prices of an American manufacturer offering TEFC SQ CAGE INV DUTY motors; Constant HP operation to1.5 base speed; Constant torque operation from 0 to base speed on vector duty; Optimized for operation with IGBT inverter (NEMA Design A). The prices go from 200% to 140% as compared to std motors and from 1 hp to 20 HP, 1800 rpm, 3ph, 230/460V.

My personal opinion is to reduce risk of premature failures by "guessing" and installing the motor designed to match the application.

 

[dpc]

jraef,

Yes, at some hp size, the MV drive will become more costly on a per hp basis, no question about it.

When I last looked into this, a 1000 hp MV drive and motor was less costly than a 1000 hp LV drive and motor. I'm guessing that the break-over point has dropped in the past few years.

 

[jraef]

At 1000HP I don't doubt it. 1000HP LV motors are relatively rare, and as such so are the VFDs capable of running them (not much point in designing a VFD for a motor that hardly anyone uses). But at 500HP, where both motors and VFDs are plentiful in LV, I would venture to say that 2x500HP LV motors and drives is still less than 1x1000HP MV motor and drive. I don't know enough about the motor costs, but 500HP 460V VFDs are around US$35K, so US$70K for 2. Last time I checked, 1000HP 4160 VFDs were about US$180K (neither price includes switchgear, harmonics motigation etc.). So even if the 2xLV motors cost more than the one MV motor, I doubt it is $110K more. I'd say the breaking point is probably around 700HP because that is where LV motors (and drives) become relatively rare and therefore more expensive, so it helps mitigate the MV premium.

"Venditori de oleum-vipera non vigere excordis populi"

 

[RRaghunath]

Thanks every one for the valuable posts. I have a clearer picture,
1. Motors upto 700HP, two nos. LV motors (2x350HPwith drives) are economical than one no. (1x700HP with drive).

2. Smaller range motors of <20HP, the inverter duty motors are different than the normal ones and also are costlier, better to specify them accordingly. Other ranges in case of motors with normal torque requirements (not high starting torque types), the standard duty motors with Class F insulation are good enough for inverter duty.

3. It is better to insulate one of the bearings of motor when used with inverter drive to prevent bearing damage.

Thanks every one once again (the issue has been in my mind for a long time and you all helped me understand it better).