Motor for VFD duty Vs normal duty motor
Motor for VFD duty Vs normal duty motor
(OP)
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.
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.





RE: Motor for VFD duty Vs normal duty motor
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.
RE: Motor for VFD duty Vs normal duty motor
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.
RE: Motor for VFD duty Vs normal duty motor
"Venditori de oleum-vipera non vigere excordis populi"
RE: Motor for VFD duty Vs normal duty motor
My personal opinion is to reduce risk of premature failures by "guessing" and installing the motor designed to match the application.
RE: Motor for VFD duty Vs normal duty motor
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.
RE: Motor for VFD duty Vs normal duty motor
"Venditori de oleum-vipera non vigere excordis populi"
RE: Motor for VFD duty Vs normal duty motor
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).