VFD For 2 Motors Driving Same Equipment

Option "a" is the safest option. It may be more expensive in the beginning but probably not in the long run.

Two motors running from two VFDs on a machine where both motor shafts are mechanically coupled is also done all the time, the 2nd VFD is a load/torque follower so the load always splits 50-50%. No speed match or follower mismatch even when on a common shaft, done on dc motors since the 1960s, a sensorless vector drive pair will work fine.

Your parts count will be higher, so your reliability will suffer some, maybe not enough to be of concern.

As options b and c don't work, the only solution is a.

Option 'a'

If money is not an issue, I would go with #1...Why potentially create more problems if you don't have to....? I'm sure we're all proud of making things work in do or die situations, but we don't go around creating these situations, we go around correcting them...

a. (Recommended).

If it is possible to get two VFDs that can run in master/slave configuration (I don't know if this is an option) then b may be a possibility. Talk to VDF suppliers.

Do remember to upgrade all the electrical supply cables and protection equipment to cater for the new load.

First, I'd make sure I understand the speed/torque/power requirements of the diffuser. Are the power requirements proportional to the throughput, or something else? You stated that the existing motor has run at 1800 RPM, so why is that insufficient now? You may not need 90 KW. If you're going to be running at 1800 RPM, I'd consider buying a motor designed for 1800 instead of 1500.

Dear Mr. bigg

I thank you and all other participants who have shared their views/knowledge/experience and I am jubiliant to see/read such large number of response for my posting.

Now I wish to state as follows, to your views:

Further detail of the diffuser, it is processing prepared or shredded sugar cane to extract cane juice and produce sugar and the present working capacity is about 130 Tonne/Hr. of cane. The new proposed working/processing rate will be 200 Tonnes/Hr. of cane. The present motor is loaded to 65 to 68% and for enhanced crushing, the additional power is required.

Regarding the speed, the processing time for the cane now inside the diffuser is 55 Minutes retention and this results in loss of sugar by inversion due to the Temp. 80+ Deg.C and we want to reduce the retention time, and limit the processing time by about 42 45 minutes, hence the new proposed speed of motor is 1800 RPM. The reason being, at prolonged retention at higher temp., the inversion loss, which is significant will be under control

Further higher speed is recommended by the consultant in order to reduce the required torque on the gear box - and we physically verified that the torque demand has come down by about 20% when compared to working at 1500 RPM and the cane mat height has come down. This is as simple as that for the giiven power, for increased speed, the torque will come down. Earlier we did not try to run at 1800 RPM now it is concluded that the speed will be 1800 RPM for the motor.

Further at 1800 RPM, the linear speed is higher to that of 1500 rpm, and there fore the cane mat height or level will be less. Earlier the cane mat level was higher to the extent of 1.3 to 1.4 Metres as against the need of 1.0 Metre and this reduction in cane mat height will improve the percolation efficiency of the diffusion process , there by sugar extraction from the cane will improve.

Any other data required, I will furnish the same.

Thanks,

DHAYANANDHAN.S

For what it's worth, Option 'a' involves a higher initial outlay, but then you're not kludging something together that may prove to be a maintenance nightmare down the road, one which could cost you more in the long term. I haven't been on this forum very long but, in the short time I have, I have seen dozens of posts by members trying to fix this or that problem with their VFDs. You'd think they were Chevy Cavaliers. Bite the bullet, install the correct (1800 rpm, 90 kW) motor and VFD for the job, and sell the existing motor to someone else or find another use for it in your plant. Your business is processing sugar cane for profit, not maintaining kluged-together motors & VFD drives. Yes, it can be done and people do it, but is that really what you're in business for? No.

Option A is the best in the long run.

To run at the higher speed you want 1800 RPM.

With a) you can select a motor with 1800 RPM base speed, and match the drive to the motor. You have the best motor control and are power matched.

I have run multiple identical motors on one VFD without problems. Each motor then needs its own independent motor protection.

However, you appear to want to run the motors above their design speed by 20%. Power requirements are also going up. This can be problematic for some motors.

I can see that you have done a lot of research on this already, so I'm not sure how much my thoughts will add.

I'm thinking that 2 motors on the same shaft, either driven by one VFD or two VFDs, is not a good idea. The second motor would be new and probably not identical in characteristics to your present one. Usually, when there are two motors, there is a differential gearing that will allow each motor to operate independently, and from your description, I don't think that is possible in your case.

I think that you would be safest to upgrade to a larger VFD and motor (option a) to avoid unforeseen problems. Hopefully, there is a market for your present motor and VFD and you might be able to recoup some of the cost.

Option b) is possible, the fear is due to some reason the frequency difference is experienced – it will create problem.

Many dam gates operate with ABB VFDs. All VFDs are synchronised automatically and operate together. Hence better you contact VFD supplier whether it is possible to synchronise, so that you can have two motors and two 45KW VFDs. If this works one advantage is parallel redundancy. If one VFD or Motor fails also you can work.

'If this works one advantage is parallel redundancy. If one VFD or Motor fails also you can work.'

Not if, as the OP says, they are going to double the size of the diffuser.

All the options offered up, have their pro's and con's. As with option b), most drives these days will allow communications between two drives, operating in a Closed Loop Vector format, to synchronise/speed match them.

Best bet is option A. Most projects I see require a deadline for completion and usually you don't have a lot of time to experiment. I also recommend talking to your distributor and their drives specialist - they have a stake in the game as well, and will work hard to gain your business. And when in doubt, get a second opinion from another distributor. Think also of the simplest solution and how to maintain it.

Have you noticed that if present motor (A) is on the left, and its rotation sense is CW, then if motor (B) is placed on the right side facing motor (A), and these use the same VFD, there is no provision to have the rotation on motor (B) going CCW?

I guarantee that one VFD will spin one motor CW, and the other motor CW, unless you know to reverse the phase connections. IF the motors (A) and (B) are not electrically mirror images of one another, how do you expect one drive to power them equivalently?

Sounds like you need an (A) train (the existing one), and a (B) train made identically to the (A) train, and combine the outputs of both during high end production.

Dear Mr.James Stewart, Thank you for your response.

1. You have referred "---------- then if motor (B) is placed on the right side facing motor (A), and these use the same VFD, there is no provision to have the rotation on motor (B) going CCW? "

My Answer: The other motor phase sequence to be connected in such a way, if the EXISTING motor is run CCW the new motor also will run CCW when seen from the existing motor non-drive end. Or else the new motr will develope Torque opposit to the existing Motor.

2. You have referred "------- IF the motors (A) and (B) are not electrically mirror images of one another, how do you expect one drive to power them equivalently?"

My answer: The motors will be physically mirror image installation but the electrical connetion of the new motor will have connections in such away, it will have opposite direction while viewing from the Non-Drive end of the old motor.

3. You have referred "-----------Sounds like you need an (A) train (the existing one), and a (B) train made identically to the (A) train, and combine the outputs of both during high end production."

My answer.: Yes, it is just to increase the capacity/production.

THANKS,

DHAYANANDHAN.S

OK, so then the (A) motor will be CCW viewed from the outboard end, then the (B) motor should be CW as viewed from the outboard end. OK, I get that one must trade phases as required.

In the case where you apply the second motor, and if it is not precisely 100% the same output power at the same rpm as Motor (A), how does that play into production? Will one motor try to overload and heat up, or will load sharing be effective? Really?

Two completely separate production trains is a better answer (IMHO), since you then have equivalent to present system with 100% redundancy. Once you have 100% of the new requirement with two systems, go modular once again, and you will have 150% capacity, with 66% redundancy.

Two motors, one old, one new, can't be a good idea on one shaft, I don't care if it's one VFD or two.

As you will have a new VFD (Option 1) you might consider the following. You don’t give nominal supply, so let’s assume 400V (just for a figure), 50Hz. You could select your motor 400V Δ, 690V Y, or 400V Y, 230V Δ. Choose 400V Y, 230V Δ and connect in Δ. Set up the inverter for 230V at 50Hz. Motor is then happy at rated kW and Hz. But if Hz and speed are then increased the volts can also increase, giving higher power (which wouldn’t work if the motor were 400V D, as VFD output limited to 400V, giving constant power > 50Hz). So you can get up to √3 x the power at 50*√3 Hz and 400V. For your kit, at 1800rpm, 60Hz, power increase is 1.2 x, so you could use a motor of rated power 90/1.2 = 75kW.

you still have to size everything to do the highest amount of work. You can re-use the cabling between VFD & motor, tho...

Most efficient is to use a motor rated at the required speed and volts.

Running a 400 v drive at 230v to allow a 20% overspeed is a waste of hardware. Drives are amps rated, not KVA. The amp rating of the drive would have to be 1.74x higher.

For the KVA required get a motor wound for 60 Hz (1800 rpm) at rated volts - this gives you best utilization of the drive hardware.

I know ABB, and probably your other motor manufacturers, can supply a matched motor. There is nothing sacred for 50HZ or 60HZ motors. I have even seen motors wound for 23HZ / 480volts. 1800RPM/ 60hz / 460v motors and drives are standard North American design.