1800RPM motor is always a 4 pole motor?

Hi,

Normally in 4 poles. You can also reconform by physical view which is dismantle the cover and pull out shaft.

Theory and practical sometimes the same.

tks

magic

Magic,

Thanks for the confirm. Exactly what I was thinking to confirm it as well, but trying to avoid it. Motor is a pain to get too. :(

Hard enough to even read the motor plate.

Thanks, Chris

Hi chris,

appreciate to your fast response. From my outstanding to check that all

1800 rpm motors are in fact 4-pole could be untrue. It could be more or less depand on various technical factors. What i am trying to suggest you that since you have got listed as 145TY-4 attached, possiblely direct to any nearest motor suppliers in your country. more or less could help you.

tks

magic

go cautiously with your application. Not every motor likes running at slower speeds with a VFD. Cooling is very important at lower speeds. I have seen some motors really get hot when running on a VFD at a given speed say below

75% of rated rpm.

If your just ramping up to full rpm then there will be little to worry about unless your over torquing the motor due to a load requirement.

I agree 100% on the speed. For what it is worth this is off a Bridgeport knee mill. It already has the ability to run at low speeds (60 ~ 100 rpm if I recall correctly) and I can only assume that the motor is rated as such.

So it is not like I am redoing it, just using the VFD so that it can be CNC controlled.

is the manufacture. I can find 20 pages of pdf documents how to PAINT the thing, but nothing on the specs of this 1977 motor. No phone number to call either.

Oh well, apart it comes I guess.

Thanks all again, Chris

IF it's an induction motor, then yes, 1800 RPM synchronous speed means four poles on a 60 hz line. Of course, there are 1800 RPM motors that are *not* induction motors.

IF it's an induction motor, don't bother trying to take the thing apart to 'count the poles'. You will find a squirrel cage rotor that has no descernable 'poles'. In fact, if you try counting the cage bars, you'll just confuse yourself to no end. (the number of squirrel cage bars has very little to do with the speed of the machine, or the number of poles).

When running VFD, they usually strive for a constant Volts/Hz ratio. This is okay for driving induction motors. It will not saturate the iron or cause other 'nasty' effects.

But SQLit has a point about cooling. You see, VFD is ideal for certain types of loads, where the power needed to drive them drops off with speed. Preferably loads that follow speed-squared law. So a motor running at half speed, doesn't need to develop rated power, or even half of rated power.

That's important because if it were to have to develop rated power, it would need to draw double the current when at half voltage. If just developing half power, at half speed that means rated torque. And that would mean rated current.

Both of these mean a lot more I^2R losses in the machine than what can be removed by a shaft-mounted cooling fan.

Some large units get around this by using separately driven cooling fans to force the same amount of cooling air over the windings regardless of main motor speed.

One place that VFD really works well is pumps and fan/blowers. The power requirements of such devices tend to follow the 'ideal pump laws'. Cutting the speed in half reduces the shaft hp required to 1/8. So motor cooling remains adequate.

daestrom

Sounds like a good excuse to not take it apart. Good, I like that.

Thanks for the detailed response. Again I hear the questions on fans. To be honest I do not have a clue hot how the original design of the mill or motor deals with cooling in this area. It is closed off, so not easy to check it out.

I was assuming, and still think I am correct, that the motor was designed to run as low as ~ 60rpm. It does currently have a hi/low electric switch as well as a dial to further control speed electronically. Some of these machines have had their motors running longer than most of us have been alive. So the speed issue is really not a concern to me.

With the induction information you mentioned above, it might just be quicker to find a number and call the manufacture.

Thanks, Chris

Older motors are better than the products produced in the last 5-10 years. They were beefier, more copper and steel. And usually the design engineers took to heart the temp considerations. Is your ambient high? above 30 C? If so a thermal couple on the housing wired to the drive as a safety would be all that I would suggest. Not worth the trouble to put RTD's on the bearings and windings? or is it? Heat is the killer of motors, Where I work 40 C is not uncommon as the ambient. I remember setting up a new CH AF-95 once. The factory specs said 40 C max. I called them and asked if this was a joke. They were serious. At the time there was nothing made with a higher temperature rating so I ducted a/c into the cabinet. Clean cool air being forced into the drive kept it cleaner and it is still running now after 10 years. Dirt, oil and contaminates in the air will shorten the life of a drive in a hurry.

I have seen motor drive applications fail miserably. Once I worked around a fan motor that usually ran in the 20 to 30 cycle range. ( half speed basically ) The motor chattered so loud from the drive that the "brain surgeons" put a box over the motor to cut the noise. This of course killed the motor. I asked politely " Since you do not need full speed then change the pulley ratio and run the motor closer to 60 cycles and get away from the noise from the drive. This was awhile back before PWM's this particular drive was a 6 stepper. I got the deer in the head light look and was told then they could not control the motor. I guess changing the scaling was to hard. The rebuild guy was happy. They lost that motor about every 6 weeks.

You sound like your checking your bases twice. Always a good thing when delving into the automation world.

Thanks again for the feedback.

I do agree the 1977 motor will probably outlast any 2hp motor built today, even if it does already have nearly 30 years on it. Probably weighs three times as much too.

Your thermal idea is a good one. It is a high end Siemens 5hp VFD that I got of eBay, and I am sure it will have temp feedback. I will have to check though. That would 100% ensure me that the motor outlives me.

An induction motor is a pseudosynchronous motor. That is it runs at almost synchronous speed. On 60Hz, 2 pole synch speed is 3600RPM, 4 pole is 1800RPM, 6pole is

1200RPM etc. On 50Hz, 2 pole synch speed is 3000RPM, 4 pole is 1500RPM, 6 pole is 1000RPM

You can apply a three phase induction motor to a VFD, but not a single phase induction motor.

When you slow the motor down, it is torque limited. That is it will provide the same torque as at full load. The effective power rating drops with the speed. At half speed, the power rating is halved. If you use a gear box, at half speed, the power is equal and the torque is double. At 10% speed, the maximum torque will begin to drop and the cooling of the motor is very low. It is possible to overheat the motor with no load unless you use additional cooling.

Best regards, Mark Empson

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