Inverter Motor Help

The Plc Kid

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The Plc Kid
Join Date
Feb 2009
Location
Macon, Georgia
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3,233
I have a vfd application thatis 30 hp 1765 rpm 4 pole on 460 volts

The drive is a powerflex 700s phase 2 up to 400hz

The motor we currently have is inverter duty but 60 hz and has external blower and encoder feedback with a 1.0 service factor.

The OEM of this machine is telling us that if we use a lower rpm motor that we can have more torque on our lower speeds and then over speed the motor if we get a 200 or 400 hz motor to run our material that requires higher speeds and less torque.

Does this sound right.

Also what is the best brand or motor i should be looking at for inverter duty vector controlled 400 hz c face mount 30 hp?
 
Are you really thinking of running the motor at 400Hz? That's over 11,000 RPM. Most motors I have seen that actually provide bearing RPM ratings top out about 5k rpm.

I think you should do the math to figure out the highest top speed required of the motor, and then make sure you get a motor that can handle the speed safely.

Also realize that you will lose torque above the base RPM, so that may factor into the equation when you go well above 60Hz.

Paul
 
Okie is right on that one, you should never run over 80 hz at least not according to any VFD specialist I've ever talked to. Also if torgue is a concern you may want to look into a flux control or senserless vector VFD the torgue is more stable at different speeds Flux control being the more stable of the two
 
As far as I know the inverter rated duty is designed to run at 200% of rated speed,say maximum of 120Hz , if you are looking at high speed then AC Servomotor is the option.
 
jayc12155 said:
As far as I know the inverter rated duty is designed to run at 200% of rated speed,say maximum of 120Hz , if you are looking at high speed then AC Servomotor is the option.
Click to expand...

The rated maximum speed of ANY motor is stamped on the nameplate. If you require a specific speed range, it's best to consult with the manufacturer before purchasing.
 
The Plc Kid said:
I have a vfd application that is 30 hp 1765 rpm 4 pole on 460 volts

The drive is a powerflex 700s phase 2 up to 400hz

The motor we currently have is inverter duty but 60 hz and has external blower and encoder feedback with a 1.0 service factor.

The OEM of this machine is telling us that if we use a lower rpm motor that we can have more torque on our lower speeds and then over speed the motor if we get a 200 or 400 hz motor to run our material that requires higher speeds and less torque.

Does this sound right.

Also what is the best brand or motor i should be looking at for inverter duty vector controlled 400 hz c face mount 30 hp?
Click to expand...

Kid
Basically it is correct.
1. Two 30HP motors one 1800 and one 900 rpm will have differing torques in accordance with
HP = (T x RPM)/5252

The 900 will have about double that of 1800.

Now for the buts
ASSUMING you are not changing machine speed and are doubling VFD output freq to compensate for motor with 1/2 the design speed.
Over baseline (60 Hz) you will start dropping torque as mentioned. This is because motor is in constant HP region.

The bearing argument needs great consideration. I have seen motors run at 400 Hz on a test bench. I have no experience with this for long term loaded operation - generally start to back away over 120 Hz - over revving equipment scares me. A lot of people say this has no impact on bearing life but somehow I have my doubts. Maybe all standard industrial motors have 5,000 rpm bearings heck if I know.

Does existing motor drive a gearbox and if you speed machine up from design 1800 rpm to faster will the gearbox handle the increased speed? Will you have cooling problems?

What kind of torque loading is the machine
constant ie conveyers etc
increasing ie pumps fans etc
decreasing machine tooling for example.
If either of first two and you intend to drive faster than design 1800 rpm you will need more HP
If the last you may be OK.

My gut instinct (based on provided info) is
If it is running OK leave it alone.
Tell the Production God you will not make it run faster than design.

Dan Bentler
 
Dan

The machine is a stretch oven so the torque is pretty much constant changes in the material my cause a increase or decrease in speed/torque.

Basically we need a little more torque but we have size constraints that limit us from increasing hp.

This is an 1800 rpm(1765 data plate ) motor 60 hz 30 hp.

We run lighter products that need the speed of the 1800 rpm motor that is why i was looking to change to 900 rpm and double torque then overspeed to 1800 rpm for the lighter products that do not need as much torque

On many of our motors there is a 120 hz rating so is this base? In other words would i still loose torque when overspeeding a motor of 120 hz rating?

How can i figure the gain in torque if switching from 30 hp to 50 hp

How can i calculate the torque?
 
How can i calculate torque if the hp and rpm is known.

BTW this is a closed loop flux vector setup.

Encoder 1024ppm

Will the encoder need to be changed to overspeed the motor?

More pulses?
 
Sorry i seem so lost guy's

I am trying to wrap my head around all this but right now understanding all the dynamics of it is getting the best of me.
 
The Plc Kid said:
Dan

The machine is a stretch oven so the torque is pretty much constant changes in the material my cause a increase or decrease in speed/torque.
REPLY no idea what a stretch oven is. Is product moved thru oven on a chain type conveyer?

Basically we need a little more torque but we have size constraints that limit us from increasing hp.
REPLY are you running motor at full load amp or having motor overheat problems.

This is an 1800 rpm(1765 data plate ) motor 60 hz 30 hp.

We run lighter products that need the speed of the 1800 rpm motor that is why i was looking to change to 900 rpm and double torque then overspeed to 1800 rpm for the lighter products that do not need as much torque.
ASSUMING
1 this is a conveyer
2 with lighter product
3 thus less mass overall
your torque demand from conveyer should drop.
HOWEVER if your unit weight drops by half AND you are tripling the number of units then torque demand will rise by 50%.

On many of our motors there is a 120 hz rating so is this base? In other words would i still loose torque when overspeeding a motor of 120 hz rating?
REPLY send photo of nameplate please - this is new to me.

How can i figure the gain in torque if switching from 30 hp to 50 hp
HP = (Torque X RPM)/5252.
T = (HP X 5252)/RPM
OR get motor data sheet from mfr. They should give you torque values at varying %load values.

A very rough estimate of %torque could be derived from %FLA.
you may be able to get this from VFD readout
better yet
will your VFD give you a torque value readout?

Dan
Click to expand...
 
I am no motor expert but...I believe if you are experiencing problems with motor overloads when trying to run the 4 pole (1800rpm) motor at low speeds, then your application may benefit from an 8 pole motor.

The encoder won't matter. You program the drive for the number of poles (or possibly just the nameplate base rpm, frequency) and the number of pulses per revolution.

If you run the 8 pole motor at 120Hz, it will reach 1800rpm (minus the slip). I would expect that to be no problem for the motor, but if there's any doubt, contact the mfg. and verify that it's okay to do so.

Using vector control should maximize the motor ability to develop torque throughout the speed range, so I am unsure if you can gain anything by decreasing the base speed. A larger horsepower and/or higher efficiency motor might fit the location and solve your problem though.

Hopefully DickDV will see this thread and offer true expert advice.
 
Golly, thanks Okie (blush)!

First, let's clear up a couple of mistakes in the preceding posts. An AC motor's maximum speed is NOT on the nameplate. NEMA motors up to about 250hp (that would be motors with a cast aluminum rotor) can be run up to 90hz (that's 50% overspeed) without asking any questions. The exception would be 3600rpm 2 pole motors which have to be held to a max of 4500rpm. If you don't believe me, then look at the torque/speed curves the motor manufacturers publish for their commodity motors. Generally, they go to 90hz.

Second, while it is true that torque falls off when you run overspeed, if you increased the reduction ratio in the power train to get that overspeed, the increased ratio and the percent of torque reduction exactly cancel each other as long as the motor hp curve from base speed to your maximum speed is constant. The same is true if you choose a lower base speed motor of the same hp. The increase in number of poles increases the base speed torque in exactly the same ratio as the increase in overspeed necessary to get back to the same shaft speed.

The advice the OP was given about increasing the starting torque by dropping the motor base speed and running overspeed is good assuming that the power train ratio on the existing motor cannot easily be changed. Changing the power train, if possible, is usually cheaper as in different pully ratios, etc.

Let's take the specifics we were given: 30hp 4 pole 1800rpm motor. This motor develops 3ft-lbs of torque for each hp so it is good for 90ft-lbs of torque from zero to 1800rpm.

Now, let's substitute an 8 pole 30hp 900rpm motor. This motor develops 6ft-lbs of torque for each hp so it is good for 180ft-lbs of torque from zero to 900rpm. Running this motor up to 1800rpm requires 120hz which is about the limit for an inverter-duty motor. The doubling of frequency (speed) means that the torque will fall off by the inverse of that ratio which would be 1/2 or half. Half of 180ft-lbs is 90ft-lbs which is exactly where we were with the four pole motor. Except now, the starting torque is doubled.

One caution with this. Since torque determines the diameter of a motor (not hp), this 900rpm motor will be much larger and more expensive. Also, all of this extra torque may only serve to destroy the existing power train and connected machine so that needs to be checked carefully.

Personally, and without seeing the application, I would generally recommend dropping down to a six pole motor. It will increase starting torque 50%, only needs to go to 90hz to regain the shaft speed, and will be smaller and cheaper.

Might want to check that option out.
 
The Plc Kid said:
...The OEM of this machine is telling us that if we use a lower rpm motor that we can have more torque on our lower speeds...
Click to expand...
Sounds like the OEM hasn't got a clue.
First of all, since the motor is attached to a gearbox, you're stuck with the motor frame size. 30HP (at 60Hz) is already the highest power output in that particular frame size. And motor torque remains unchanged within a frame size: A 15HP-900rpm motor fits the same frame as a 30HP-1800rpm motor, so there's no torque gain/loss at all.

2nd, the motor is running in the constant torque range (0-60Hz): The motor doesn't lose any torque at lower speeds.

But it seems you would like to solve two problems with one solution: Insufficient torque at lower speeds AND a higher speed setting if the product is smaller.

A different gear ratio (or a smaller sprocket if the conveyor is chain driven from the gearbox) is the way to go, to deal with the torque problem. Then you could consider overrevving the existing motor to get a higher throughput for the lighter products.

Overrevving the motor means decreasing torque above 60Hz.
There is a way around this: Suppose the motor is a star/delta type, 460V being the star voltage. If you replace the VFD by a 50HP version while connecting the motor in delta, the 30HP motor can deliver up to 50HP at 104Hz, which also means (practically) no loss of torque even at 3100rpm (=1800 x 1.73). Base frequency has to be set to 104Hz, base voltage to 460V.

Would you mind explaining why you need "a little more torque"?
At what speed range do you notice a substantial lack of power?
 
Sparkz, in the NEMA world (North America) you don't normally have both ends of the motor coils available in the motor junction box so changing from wye to delta and vice versa is not an option like it often is with IEC motors. Further, a 30hp 4 pole NEMA motor will usually already be wired delta so that cuts that option off.

It seems that you have concluded that, since the motor is C faced, it must be mounted to a gearbox. You are probably right. But it doesn't necessarily follow that a larger motor is out of the question. It also doesn't rule out a sprocket/chain or belt/sheave stage after the gearbox as you mention.

PLC Kid, we need two things from you to be of further help. 1)The motor nameplate data, and 2) a description of the power train between the motor and the driven machine. We especially need to know if there are chain or belt stages anywhere in there.

And, PLC Kid, a third thing---just how much more torque do you think you need? The solution to get 5% more could look (and cost) a lot different than the solution to get 50%.
 
DickDV said:
Sparkz, in the NEMA world (North America) you don't normally have both ends of the motor coils available in the motor junction box so changing from wye to delta and vice versa is not an option like it often is with IEC motors. Further, a 30hp 4 pole NEMA motor will usually already be wired delta so that cuts that option off...
Click to expand...
Bummer! This I did not know, sorry about that. Over here 230/400V-50Hz motors are still just as common as the 400/690V-50Hz types, even for this power range. It surely limits your options...
 

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