3450 rpm motor with a small pulley, or 1725 with a large pulley?

[Allan Speers]

I need a new 3 HP motor for my Powermatic 100 planer. The motor compartment & mount allow for either 3450 or 1725, and a manual for a 1965 version of this planer gives pulley sizes for both speeds.

The newer manual for my 1987 version calls for a 3600 rpm motor ( ! ) with no mention of any other rpm.

Obviously, both will get me to the proper head rpm, and I suppose I should use a 3450 to keep things more "stock" (is there such a thing as a 3600?) -but it seems to me there are three minor reasons to choose 1725 instead, even though the bigger pulley will cost a lot more:


1: The wider diameter pulley would give less strain to the belts. I suppose I could get link-belts, but the vintage guys over at OWWM are vehemently against this,and they've kind of swayed me. Plus, why spend the money if I don't have to?

2: The wider diameter pulley would give more contact area to the belts.

3: The significantly larger pulley needed on a 1725 rpm motor is a lot heavier, so it should be more stable AND add a small amount of smoothness and resistance to speed change. Very minor, of course, but it's surely a factor.

So it seems logical to me to go with 1725., except that the company chose to go with 3600, when they decided to offer / recommend only one speed, and I'm wondering if there was a good reason, or was it just convenient somehow?

Also, the smaller pulley for a 3450 would save me $45.

Your thoughts?

[jim mills]

Did you ask the guys over at OWWM? Bet you'll get some good advice.

[ian maybury]

I'm not sure Allan, but based on the topic never coming up in basic mech design discussions think there's not all that much practical difference other than RPM - which is determined by the number of poles and frequency of your power. They seem to be more or less interchangeable (depending on required RPM), and to be of similar overall efficiency and performance. Your choice seems basically to be between a 2 or 4 pole motor, since the choice of belt pulley delivers the same or a similar cutter speed in both cases.

There are probably some subtler differences. A two pole (higher RPM) motor might depending on pricing policy be a shade cheaper, while the lower RPM 4 pole is probably a bit quieter and delivers its torque more smoothly given the additional pulses. Think I may have seen something suggesting that the number of poles can make some difference when a VFD is used to control the drive - could be that the 4 pole responds better at the lower end of the speed range but don't know for sure.

[Pat Barry]

You trade off power vs rpm by going with a larger drive pulley. I would go with the higher rpm motor

[David Kumm]

What are the two pulley diameters for the motors and distance apart from the drive pulley? The owwm guys are correct about link belts. You want VX power transmission belts. Dave

[Dennis Aspö]

You trade off power vs rpm by going with a larger drive pulley. I would go with the higher rpm motor

Doesn't the higher RPM motor also have less torque than a slower motor?

As I see it, assuming equivalent HP motors, you deliver the same amount of energy into both motors and you get the same amount of work out, but in one motor you get more out in the form of torque. And in the other, more work is delivered in the form of higher RPMs.

I would go with the motor that requires the least size difference between the pulleys.

[roger wiegand]

A two pole synchronous motor runs at 3600 rpm with a 60hz electrical supply.

[ian maybury]

Power = torque X RPM (but the units of measure have to be correctly chosen for the maths to work properly) Torque is the twisting moment e.g. 20 lbs ft. (20lbs applied at the end of a foot long lever)

3HP is 3HP whether it's at either RPM. The torque output of each motor will however be different at the different RPM in accordance with the above formula.

Presuming that the pulley set used with each motor is chosen/sized to deliver the same RPM at the cutter head in both cases (the faster running motor is fitted with a higher reduction ratio set), then the power and torque available at the cutter will for practical purposes be the same. There could in practice be some difference if the pulley sizes don't deliver precisely the required ratio in both cases, but it's probably a stock reduction ratio/requirement so this may not be the case. In that case the HP will be the same at the cutter head in both cases, but the set delivering the slightly lower RPM will deliver proportionally more torque. Again per the formula.

Choice of belt type may influence the situation a little in that lighter/smaller pulleys will be easier to start, but store less angular momentum - which probably doesn't matter much. A higher friction/rated belt will transmit more torque without slipping. A more flexible/efficient belt will absorb fractionally less power while running - and may be significantly quieter and smoother running. Presuming no problem with pulley or belt wear, it'd be wise to go for a higher rated belt if possible if slipping ever has been an issue.

Chances are that choice of the slower motor and a high quality belt system could smooth/quieten the machine quite a bit, but somebody no doubt has direct experience and can comment on the likely reality...

[Rod Sheridan]

Alan, stick with the 2 pole motor, they're normally less expensive than a 4 pole...............Regards, Rod.

[Pat Barry]

Doesn't the higher RPM motor also have less torque than a slower motor?

I think we would need to review the specs to find out since all we have is a HP spec.

All I'm saying is that when you put the bigger pulley on the drive side in order to get a higher ultimate RPM at the cutter then it will take more power from the motor to accomplish this. Ever ride a 10 speed bike. Its far easier to pedal that bike at a given speed with the small sprocket engaged on the pedal than it is with the large sprocket. Easier to pedal with the small sprocket but it requires much higher pedaling RPM to get the same speed. This is the torque relationship as well. Step up the 1725 RPM motor and be prepared for the motor to take longer to get the cutter up to speed, proabbably need to take lighter cuts, easier to lose cutter head speed, etc.

Doesn't the higher RPM motor also have less torque than a slower motor?

As I see it, assuming equivalent HP motors, you deliver the same amount of energy into both motors and you get the same amount of work out, but in one motor you get more out in the form of torque. And in the other, more work is delivered in the form of higher RPMs.

I would go with the motor that requires the least size difference between the pulleys.

[David Kumm]

I think you will find that both motors, geared up or down properly with pulleys will deliver the same hp and torque to the head. The only advantage of one over the other is the contact area of both pulleys. A small pulley set close to a large one has less contact area on the small side to it is at a mechanical disadvantage, slight though it may be. That is why I wanted to know the pulley sizes and distance apart. Dave

[Allan Speers]

I think you will find that both motors, geared up or down properly with pulleys will deliver the same hp and torque to the head. The only advantage of one over the other is the contact area of both pulleys. A small pulley set close to a large one has less contact area on the small side to it is at a mechanical disadvantage, slight though it may be. That is why I wanted to know the pulley sizes and distance apart. Dave

I'm pretty convinced at this point that it doesn't matter too much either way (Powermatic probably went with just the 3450 & small pulley because it costs less.) But FWIW:

The head pulley is a 2.7" The 3450 motor pulley is 3.4" in 1965, 3.75" in 1982, and the 1725 motor pulley (only spec'ed in the 1965 manual) is 6.5"

The belt is either 65" or 66"

It's interesting (I guess) that the upped the head rpm a little in the 1980's.

[Allan Speers]

What are the two pulley diameters for the motors and distance apart from the drive pulley? The owwm guys are correct about link belts. You want VX power transmission belts. Dave

Dave, when you say "VX power transmission belt" are you talking about those GOGGED belts? How could I possibly use one, my planer uses v-pulleys, not gears. Am I missing something?

What about notched v-belts, which I've seen recommended on some threads? This is for a 3 HP planer with a 2-slot pulley.

Oh, and if you're talking about something different from the stock V belts, would I need to change my pulleys?

[ian maybury]

I'm not sure why you're talking of changing belts and pulleys Allan. I'm reluctant to be prescriptive not knowing the machine. The simple fix (unless you are self improving/hot rodding the machine in some way) is likely to drop in a motor running at the same RPM as the original - that way presuming the pulleys and cutter head are also as original and in good order (which should be checked) you should get the correct cutter head speed, and not need to buy belts or pulleys.

The lower RPM (4 pole) motor would probably be a bit more expensive, but should also make a little less fan noise and may start a little more easily - none of which matter much if at all in a jointer, which may be why both have been specified. Cutter heads normally run faster than the motor, so the slower motor all else being equal would require a higher ratio pulley set which due to reduced belt contact area may theoretically have slightly less torque transmission capability, and of course would cost extra if as you say the pulley has to be bought new. If it's clearly the original set up then whatever is in there should (unless worn) be fine though.

It's been implicit so far, but watch out to get the cutter head RPM right. It may be stamped with a max RPM number not to be exceeded (which if done might risk flying metal), so best to make sure that it and the motor and belt combo match the original spec and also that they will as installed deliver the recommended/stock RPM, and that that is within the permitted range. The RPM a given head in principle requires likely depends on how many knives it has, and the diameter - a larger diameter head with more knves will tend to run lower RPM. In practice be guided by the maker's spec, but be careful to match the right pulley set with the right motor, and make sure the pulleys are installed in the correct locations - it's not unknown on old machines for people to get stuff mixed up and overspeed the head. (different shaft diameters may make that impossible here)

Changing the belt and pulleys unless they are worn or sub standard in some way is likely gilding the lily. Some cheap belts can be rough runnning and stiff (often the stuff on budget Eastern machines like drill presses), and would be well worth changing - but all being equal if it's transmitting the drive smoothly without noise, vibration or slippage and the head is running the recommended RPM then short of the machine being known to have a problem it's probably fine.

There may be some fancier alternative belt types available now that might be a moderate improvement, but they might well require new pulleys. I'm out of touch and not familiar with what's out there in that regard. The problem with new pulleys froma generic source is that typically not only are the new items required, they often need boring and keying to suit. Which costs quite a bit extra again....

The plated motor speed number may vary a shade depending on how exactly the maker calculates the speed rating, but in principle there are only two stock motor speed options. Either the 3,450 (2 pole) or 1725 RPM. (four pole) They may well be directly interchangeable (be the same size, have the same shaft overhang, diameter and keyway size - and have the same mounting), but that would need checking and getting right too if you move away from the stock motor.

[David Kumm]

I'd do the 3450 as the pulleys are closer to the same diameter. VX belts are not cogged, just have a little hollow on the sides but fit normal V pulleys. They just do a better job of transmitting power. Not a deal breaker if you don't find them. I like optibelts as they are made to be used in sets without matching. Dave

PS Saw you are posting over at owwm. VX and AX are basically the same.