Can I safely overspeed a Baldor motor?

[loesch]

Rather having to continuous change belts on my 16" floor model Jet drill press, I purchased a new 1HP/240v/3-phase Baldor motor ($75 on ebay). It's inverter rated and reversable. To power it, I got a new Teco VFD from driveswarehouse. The VFD can be programmed up to 200Hz (default is 60Hz), which pushes the motor from it's rated 1780 RPM to around 5900 RPM. My question is how much faster can one safely run a good (Baldor) ball-bearing motor. Could I push it to 3000 RPM (~100Hz) since a drill press is typically less than an hour per job? The motor is a TEFC so heating should only be a problem at the really low RPMs. Any help would be appreciated.

[Al_The_Man]

If its vector rated then you probabally will be OK to go up to 5000+, I usually run non-vector motors up to 3600rpm. But a call to your local Baldor rep. should clarify it.
You should be able to go to 3600 with no problem.
Al.

[Geof]

Do it the bushman's way; buy a second motor as a spare and run the first one up to the top speed whenever you need it.

[NC Cams]

Whatever speed you run at, you'll cut service life of the motor bearings by a factor that EASILY EXCEEDS the inverse of the ratio of the new speed versus the previous rated speed.

Reason: the bearings will run at a much higher velocity and thus tend to heat up the grease therein much more. Thus, grease life can/will be drastically shortened.

Keep in mind that whatever factory imbalance tolerance that is allowed, will greatly be amplified by overspeeding. Thus, whatever was "tolerable" before, could become intolerable now. Result: drastically shortened life potential.

You also won't know if the motor is able to cool itself properly at the higher speed potential. Elevated temps geometrically tend to cut life expectancies.

A well epoxied armature should not toss windings. However, overheat one and run it DRASTIALLY above the rated speed, you could toss windings, balance weights or even other components that are not speed rated for the new ultraspeed.

The ultimate speed potential of the bearings is dependant on the size of the bearing. Simply specifying 'ball bearing' doesn't assure that the speed of the bearing can arbitrarily be doubled. This is especially true if the armature runs thru a range where it could go into an imbalance due to bizarre magnetic vectoring issues that can develop (saw it happen).

Geof's method is as good as any.

Unless someone has already run the same motor you're using at the speeds/loads you're going to encounter, a "generic" yes/no or maybe answer to the viability of the planned overspeeding effort is not something that I personally would count on.

[Al_The_Man]

Although vector rated motors aready anticipate the use of overspeed as in VFD use etc and Baldor should be able to give this rated rpm, The nameplate speed is the BASE speed which is the rpm at 60hz, Baldor sell them as variable speed and many of their Vector 4 pole motors (1745rpm) are rated up to 0-133hz which is close to 4000rpm.
Al.

[JRaef]

Originally Posted by NC Cams Whatever speed you run at, you'll cut service life of the motor bearings by a factor that EASILY EXCEEDS the inverse of the ratio of the new speed versus the previous rated speed. Reason: the bearings will run at a much higher velocity and thus tend to heat up the grease therein much more. Thus, grease life can/will be drastically shortened. ... balanced snipped

While I don't dispute this issue, keep in mind that for a motor manufacturer it makes no sense to have a different bearing system for a 4 pole (1725 RPM) motor and a 2 pole (3550 RPM) motor, so they use bearings rated for the higher speed. This is of course not an absolute truth, especially when you get over 100HP, but it is about 99% for small to mid sized motors. So that means that most 1725 RPM AC motors can be run at 120Hz as far as the bearings go.

Something else important to consider however is torque. As you raise the frequency you cannot keep raising the voltage beyond your input voltage, so what happens is that your V/Hz ratio no longer remains constant. The V/Hz ratio is responsible for creating torque in the motor, so as the ratio diminishes, so does the torque output, at a non-linear rate. This means that at 90Hz you are left with about 67% FLT at the shaft. Not only that, but the slip changes and cooling efficiency drops as well, exposing the motor windings to additional thermal stress.

So even though the bearings may be capable of it, you should never operate a 60Hz AC motor above 90Hz (or 50Hz motor above 75Hz) without consulting the manufacturer first.

[NC Cams]

The SPEED factor is not the issue of concern as you run a bearing, it is a combination of running accuracies and heat that goes along with higher operating speeds.

Yes, the same SIZE bearing may be used for a 1750 versus a 3500 rpm motor. However, the LIFE calculation does change as you increase/decrease instantaneous loading and/or increase/decrease speed. However, should the grease be negatively affected by a higher than normal temp rise (it happens regularly), the grease fails and the bearing does a like nose dive shortly thereafter.

How dramatic is the affect of heat? For every 10 deg rise about 200 Deg F for mineral based grease, you can figure on cutting grease life in HALF. Thus, should temp get to 220 or 230 (not THAT hot in some folks minds), your grease life is now about 1/4th that you'd normally see at a more mundane temp.

If you run some load life calcs, you can see that life is inversely proportional to speed and varies to a much greater degree as you change load up or down. Grease life issues, however, can make load/life calcs totally irrelevant.

From my bearing engineering days, I recall that the guys who did the motor bearings for the various tool suppliers were regularly, no constantly, evaluating new combinations of preload, fits, clearances, greases and grease fills. This iterative loop was reduplicated when they changed speeds and/or load conditions of the motors.

All sorts of weird things developed when they changed something - sometimes the bearings were unphased and other times they had nighmares.

Suffice it to say: what may be totally satisficient for 1750rpm operation may be marginal or even DEFICIENT for 2750 or 3500 rpm use. It had something to do witht he perverse nature of inanimate objects.

The bearings will tolerate the published RPM (or even higher levels) if the "off conditions" they encounter are slim to none. Add any form of "off condition" to the mix and something that lives fine at 1750 may now give you fits at 1775. Been there, done that.

Thus, to arbitrarily say that ANY bearing in ANY motor can be run at ANY arbitrary speed is not something I'd sign up to/for. As a SWAG, yes, most bearings are CAPABLE of a certain speed potential. However, the environment the bearings are forced to survive in has more of an effect on bearing life than the design of the bearing than perhaps anything else.

[SuperChuck]

I do not have the details in front of me to post, but a very simple way to find out the operating ranges of motors is to look at the NEMA standards.

In order to be called an "Inverter Duty" motor, there are certain torque, speed and cooling requirements in order for the motor to wear that badge. Depending on the poles of the motor, as mentioned before, I think there is a speed factor of 2.

One thing to realize is that torque plateaus and then begins to decline at higher RPM's and despite the ability of the VFD to control torque and boost it to some extent, the motor HP wil remain about the same. Motor HP is dependant on RPM, and Torque goes down with speed. There is a chance that the motor could bog down at higher speeds unless you allow for more HP.

If the motor is overrated (5 hp in a 3 hp application) and you are able to safely drive it at 120 Hz(double speed) based on the NEMA standards, then you will need the additional HP at higher RPM.

There's no magic bullet or free ride, when you overdrive the speed, you need more reserve HP. The same as if you changed the pulley ratios or gearing to get a speed increase. Whether its a mechanical advantage or an electronic one, there is some tradeoff.

The beauty of the VFD in my experience has not been to overdrive the frequency, but to improve the low end and starting torque, while varying the speed. In my application, we are using 350 HP NEMA design C motors on VFD's to allow us to start high inertia loads and speed them up within the standard operating range of the motors. This allowed the removal of slip clutches and other mechanical methods of starting.


Instead of overdriving the motor, take a larger than rated inverter duty motor, change the pully ratios of the press to allow for 60 hz(safe range for the motor) to now operate the equipment at the desired top speed(RPM). When you do not need the RPM, the inverter duty motor is fine running at 30 hz all day long, because its designed to operate under those conditions.

SC

[petek]

what hp are you using i have a couple of baldor mtrs rated for 6000 rpm for small $$$
Pete

[in2steam]

Originally Posted by NC Cams Whatever speed you run at, you'll cut service life of the motor bearings by a factor that EASILY EXCEEDS the inverse of the ratio of the new speed versus the previous rated speed. Reason: the bearings will run at a much higher velocity and thus tend to heat up the grease therein much more. Thus, grease life can/will be drastically shortened. Keep in mind that whatever factory imbalance tolerance that is allowed, will greatly be amplified by overspeeding. Thus, whatever was "tolerable" before, could become intolerable now. Result: drastically shortened life potential. You also won't know if the motor is able to cool itself properly at the higher speed potential. Elevated temps geometrically tend to cut life expectancies. A well epoxied armature should not toss windings. However, overheat one and run it DRASTIALLY above the rated speed, you could toss windings, balance weights or even other components that are not speed rated for the new ultraspeed. The ultimate speed potential of the bearings is dependant on the size of the bearing. Simply specifying 'ball bearing' doesn't assure that the speed of the bearing can arbitrarily be doubled. This is especially true if the armature runs thru a range where it could go into an imbalance due to bizarre magnetic vectoring issues that can develop (saw it happen). Geof's method is as good as any. Unless someone has already run the same motor you're using at the speeds/loads you're going to encounter, a "generic" yes/no or maybe answer to the viability of the planned overspeeding effort is not something that I personally would count on.

Are you talking about a dc motor or ac, ac induction rotors are laminated, they don't have armature coils to toss, they normaly have a aluminum end capped with fins onto it each end(no epoxy), its possible for them to become unlaminated from overheating, but I have never seen it. More likley they will shift or warp hitting the stator coil lams, this can happen from both locked rotor for excsive times and over speeding from bad bearings. Balancing is done on that end plate, typicaly if needs to be balanced they remove wieght via drilling not adding, tweaking the end casting can also happen dependant upon design as some have fins that stick out away from the lams.
Also most motors with ball bearings will have 5k rpm top end unless specially fitted, thats your typical 62xx series bearings which almost everyone uses.
Loesch
Friction and windage from the fan on the back end of that motor will cut its ablity to overspeed quite a bit, I would be impressed if you got 120 Hz on one HP, its entirely possible to go higher but the motor more then likely just won't.

At low RPM the motor will become hotter then normal, baldor seemst to better here then most others, you will have to call them and find out but I would that an inverted rated insulation motor should be able to handle 25Hz for cont. duty at FLA(full load amps) and less Hz for shorter periods. Just remember you loose torque with speed loss and and it flat lines above namplate speed the motor will preform the best at or near namplate speed. So boring that big hole won't be as easy with the drive only putting out 15Hz, if geared or belted correctly it should not be a problem. Even when you go to vector/sensorless control, the gains in torque are still not outstanding when using a regular motor.


chris

[petek]

problem is if its not rated for it it means it hasn't been balanced for it and probably the bearings are not as good as in a motor that has a higher rated rpm. Alot of motors are not rated for vfd use most of which can still be used with a vfd but usually at nothing greater than a 20:1 speed range (standard 4 pole motor) you need a constant torque motor to be able to run a motor slower than 20:1 of rated name plate speed or you run the risk of overheating the motor. when you run a motor faster than name plate rpm the torque curve starts to drop. In the machining world (spindle) thats ok. ex. on a milling machine when you have a face cutter in (large diam.) your running the spindle slower probably in the optimal torque range of the motor (less than name plate but greater than 20:1) when your cutting with a 1/4 in end mill in alum. you have the spindle cranked W.O.T. but theres no real load on it, so you don't need the torquo motor mass is enought to over come the load
20:1 is the standard rating for a vfd rated motor
constant torque motor are usually 1000:1
vector motors (blower cooled)are usually 1000:1 and some are 2000:1
this pertains to name plate rpm and less
all motor loose torque going faster than name plate
Pete

[in2steam]

I won't say that you are wrong, but when I worked at Lesson, all the rotors were balanced on the same machines, they used the same bearings, and for the most part even used the same small parts. So to say that the motor is balance for one speed vs another is a farce, in reality many 2 and 4 pole motors use the same rotor, they just had more poles in the stator and were rated for HP acordingly. Manufacturing something like a motor is done on a mass scale, you use what you can common between them, when the girls on the build line did that on the 48 & 56 frame motors they had about a 12 drawer bulk bin, thats all they needed.

I have no idea what you are talking about with the speed ratio thing 20:1 in relation to what? When you say a constant torque motor, are you talking about a AC servos? thats like comparing apples to watermelons, they are both round and juice and thats about it? Your application is either constant torque like in a mill or belt system or constant HP which is on processes like involute pumps for hydraulics. The motor can be designed to suit that but they are not stuck in that use by an means. The drive would be more likely to be consdiered a CP or CH style, most now are both, typipcally the more you pay the better it is esp at vectoring. Vectoring is well suited to AC servos, as they have an armature, you have a greater control of torque over a broader speed range, but again the motors and drives are entirely different.

A true "vector" 3 phase motor in the higher horsepower range cost as much as some peoples homes, and have encoders, and carefully matched drives which are tuned to that motor. I don't know about AC servos, but AC 3 phase motors flat line there torque at or above name plate, it does not drop, but it may seem like it and out near the breakdown area it very well may drop. 3 phase motors need a change in votlage for more torque at a given frequency, the drives can fool the motor to a ceratin extent, esp on the lower then nameplate area(vectoring and torque boost), but higher you can only gain so much before begin to saturate the motor.

chris