Power Flex 700vsd

Surely you must have the purchase requisition for the current motor. That should give you a lead.

I would use standard/typical data for a 55kW motor as far as protection, then let the PF700 do an Autotune for the vector control. I can't comment on the applicabilitity of running it at 58Hz if it is a 50Hz designed motor, because I have no clue what your load is. But if it has been working fine for 10 years, that's a pretty good indication that it is OK. However if it has needed rebuilding 4 times in 10 years, that to me would be a red flag that the motor is being stressed. I personally would buy a new motor, larger if at all possible.

I prefer to start with a manual and everything will be more clearly understood

The VSD will be working to an internal model of what it thinks a 55kW motor should do. Its usual. Can you let the VSD know that this is a fan duty application.

A fan load is an appalling load for a motor and more so if you are going to overspeed it.

I'm surprised you don't have the details of the motor. If you are sure it is 55kW then put the typical overcurrent and runnign current details in. Ramp the fan up gently and this wil cause much less stress to the motor.

It is usual to oversize a VSD to cope with a fan load but remember a 55kW motor is only ever that.

Again as has been stated you seem to be going through a lot of motors. Get the mechanical guys to work out what size you really need to do the job.

The number of poles is used by the drive to calculate slip in the motor and an incorrect entry will simply cause a speed error, which is not very important in a fan application. An incorrect entry will not hurt the drive or the motor.

Set the drive up for a 2 pole motor at 50 Hz. Set the drive maximum current rating for a 55Kw motor as follows : 180Amps for a 200 Volt motor, 104 amps for a 380 Volt motor, 100 amps for a 415 Volt motor or 78 amps for a 500 Volt motor, whichever you have. The voltage supplied to your motor determines the maximum current setting from the drive, for the motor.

Start the motor and do an rpm check. You should get a reading close to that of the motors nameplate speed. From this you can calculate the number of poles in the motor and you can enter the correct values into the drive.

Increase the frequency to 58Hz and monitor the current used by the drive. The current value will tell you if the motor is overloaded or not. So if the drive output is at the full load current of the motor and the rpm. is well below the name plate speed, the motor is undersized. The drive will protect the motor if the drives maximum output is set for the full load current of the motor.

If the rpm is correct and the current is not at full load value, the motor size is OK and I would be looking at the mechanics of the system for cause of past failures.

Put the information on the print so you have them for future reference.

Actual data is always preferable but this will get you close.

Elroy

As stated a fan load is not a good load for a motor. The kW / HP follows a cube law in relation to the RPM. So increasing the speed of the motor will massively increase the power consumed.

Although you probably don't have to enter the pole pairs you usually have to enter the nominal speed of the motor. In the UK at 1500 rpm you can deduce that thats a 4 pole motor. So entering the speed is critical. By entering this information the VSD knows the number of pole pairs. In the unlikely event you do have to enter the number of poles don't guess it. Depending on freqeuncy 3000 rpm is 2 pole at 50Hz, 1500 rpm at 4 pole etc. Real speed is obviously slower due to slip.

If the VSD is any good you should be able to limit the current. As you start to accelerate the fan the system inertia is massive and the load on the motor very large. By limiting the current and ok drawing out the acceleration time you will stay much more within the limits of the motor capability and relieve the internal stresses.

Again with current limiting in place you can try and increase the motor to 58 rpm, 60rpm whatever. current limiting might stop you getting to the increased RPM. The faster you turn, the higher the RPM the more power you consume and as I said it follows a cube law with a fan. If you are within the current range of the motor you can raise the RPM further.

To bear in mind the fan might not like being thrown around too quickly - you might get blade failure.

Limit the motor acceleration and do current limiting at the VSD.

With the information provided, your motor and VFD are grossly undersized. For a fan load, the following apply:

flow = rated flow * (speed/rated speed)

pressure = pressure * (speed/rated speed)^2

power = power * (speed/rated speed)^3 = 55kW * (58hz/50hz)^3 = 95kW

this all depends on the mechanical load implied by the fan.

If at design speed the mechanical power input required by the fan is less than the motor can deliver you have the job sorted.

a mechanical engineer must consider the maximum power the fan can stand together with the rotating speed so bits of blades don't fall off. We have no real power required figures all we have is what a motor could deliver. Who is to say at sync speed the motor is developing 55kW. We can determine it from the current reading and as I said if this is lower than the motor is rated for we can safely speed things up. Current limiting is the answer and let the mechanics stick the blades back together.

Agreed - I made too many assumptions. My above loading remarks were based on the presumption that the motor was sized so that it was fully loaded at 50 Hz.

Since the motors do not last, the place to begin is with the mechanical loading requirements as you have stated. I suppose the reason the motors do not last might not be related to driven load. They might be failing because they are not being rewound with enough insulation to withstand an inverter drive.

We don't know that the motors were not over-sized to accomodate the higher speed or not, we don't know if this is a centrifugal fan or some other type that is not going to have the cubed power relationship, we don't know that the user has ALREADY been running it at 58Hz for the past 10 years or is just NOW planning to, we don't know anything about this other than the sketchy information provided by the OP, who has likely been scared off now.

The only things we know:

1. He does not know everything about this motor, but he does know what is already programmed into the existing drive, so that is going to include the number of poles, the original FLC, etc. So there is no need to re-invent the wheel here, he has at least a sketchy grasp on the basics of that motor, at least as it existed prior to 4 replacements. But it's a relatively safe bet that when it was replaced, they did not change the base motor speed, that would be ridiculous. So whatever he has programmed in there is going to be fairly close.
2. The new drive is perfectly capable of determining the motor model on it's own via its "Autotune" feature, so the specific details of the motor, especially for a fan application, are not that critical here. The only really critical issue is the Motor FLA, which with no nameplate data, is going to end up being a SWAG (Scientific Wild Ass Guess.) Hence my comment on using standard data from a motor of that size. That's the best you can do.

Anonymous Poster #1 if you are still out there,

Your best bet: scrap the anonymous motor because the chances are it is not correct for the application and you will likely be replacing it soon anyway because you will have no way of effectively protecting it. Buy a new one that you KNOW will work for the way the fan NEEDS to operate, i.e. 58Hz. And as suggested, get a Mechanical Engineer involved in determining the shaft power required for the fan at the flow you are looking for.

We do not know much of anything other than this is a 55 Kw motor. Follow the prevoios posts information to determine if the motor is adequate or not and do not replace a good motor unless it is inadequate.

Elroy

I would agree, I just think the rate of replacement is a fairly good indicator that this motor has always been inadequate.