Replacement motor for table saw

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mdf

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Topic # 230385 21-Feb-2018 21:42

My table saw is 68 years old. It has what I assume is the original motor and the more I use it the more I realise it is just too underpowered. I was cutting down some nice oak the other day (there will be a post on that project at some stage) and I took about 4 passes to get through a one inch block without it stalling out and still got a few too many burns than I would like. Of course, I neglected to write down the motor specs when I last had it out, and don't want to have to pull the stand to bits and retension the belt right now, but I seem to recall it's only a 1/2 hp motor. If I am remembering right, google suggests that is woefully underpowered.

I've done some preliminary investigations, but was finding very little searching for "induction motor". It turns out there are way more hits for "electric motor" - see these ones for example. This time yesterday I knew very little about motors beyond induction vs universal/DC. From 24 hours of reasonably intensive going-down-a-rabbit-hole-ing, it _seems_ likely that these are induction / asynchronous AC motors, but can anyone that knows what they are doing confirm?

Would also be keen on people's thoughts regarding required horsepower/KW, and whether 4 pole vs 2 pole is worth the additional investment? The current motor seems to be geared at around 2:1, so would hazard a guess that it is 4 poles to get an approx 3000 RPM at the blade.

Any suggestions on where to buy also welcome. I'm in Wellington and was hoping to get something local, by the ones linked to above seem to have non-ridiculous shipping even though (I guess) they will weigh a tonne.

Aredwood

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Reply # 1961905 21-Feb-2018 23:11

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4 pole means it will run at around 1400rpm and 2 pole means it will run at 2800rpm. Note that the speed does vary a little with load, but that shouldn't be important for your application. Those above speeds are the so called syncronus speeds. The real no load speeds will be slightly less. The linked motors are induction motors.

I'm assuming that you don't need the ability to vary the motor speed? If you do, get a 3 phase motor instead. And get a variable speed drive as well. The VSD allows you to run the 3 phase motor on single phase power. Single phase induction motors don't like phase angle motor speed controllers.

kiwigander

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Reply # 1961921 21-Feb-2018 23:36

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I'll venture some thoughts here - happy to be corrected by the pros.

Firstly I suspect the only kind of motor you'll find that you'll want to buy will be an induction motor.

My table saw is a reasonably basic model from one of the big machinery houses in Auckland. 254 mm blade. The motor is a 3 hp beast that'd make quick work of your 1 inch oak block. The downside is that it requires a 15 A powerpoint.

I suspect that if you're presently managing to do anything at all with a 1/2 hp motor you would be happy with a 2 hp motor, which could be plugged into a regular 10 A powerpoint.

The motors of most of the table saws at this level seem to be 2 pole (2850 or 3000 rpm). There's a Jet with a 4000 rpm motor - not sure how they achieve that.

TheMantis

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Reply # 1961999 22-Feb-2018 08:22

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The synchronous speed of an induction motor is determined by the formula N=(F*60)/P; where N=speed in RPM, F=frequency in Hertz, and P=pairs of magnetic poles in the motor stator. A degree of slip (expressed as a % or as full load speed on the data plate) exists between the synchronous speed and the actual rotor speed. Without this slip the rotor would stall (or not start at all) and the motor would quickly overheat and burnout (if not suitably protected). As the motor is progressively loaded to rated load the rotor speed will fall to match the slip speed: i.e at no load the actual rotor speed will be very close to the synchronous speed (but always less than).

Synchronous speeds for motors with 1,2, and 3 pairs of poles are: 3000RPM, 1500RPM, and 1000RPM. After gearing, a motor with a single pair of poles would give approx. 6000RPM at the blade while a motor with two pairs of poles would give approx. 3000RPM at the blade. Thus your assumption is correct. As for a suitable size of motor? That depends on things like the current motor size, motor mounts available on the saw bench, electrical supply available (10A or 15A), etc.

To correctly determine the actual current draw on a single phase load the following formula is used I=P/(V*p.f); where I=current in Amperes, P=full load power consumption in Watts, V=NZ nominal supply voltage of 230V, and p.f=power factor of the load (in single phase induction motors this can be quite low, 0.65-0.85). The 2.2kW single phase motor in the link requires a 15A socket once power factor is considered, the others will work with a 10A socket.

The single phase motors in the link are capacitor start/capacitor run which, as a general rule, make them ideally suited to the intended application. The slip speeds given do seem a bit on the high side and the general construction of the motors seems very consumer grade.

It is generally not cost effective to perform speed control on a single phase induction motor. As already mentioned, by using the correct VSD single phase can be converted to three phase. This then enables the use of, generally, far more efficient and reliable three phase induction motors whose speeds can be easily controlled.

MikeAqua

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Reply # 1962014 22-Feb-2018 09:06

When I replaced the motor in an old table saw that I use for cutting small firewood I did just what @Aredwood suggests and bought a three phase motor and a VSD. Worked really well and also allowed speed control.

That was about 10 years ago and at that time I could get a three phase motor cheaper than a single phase. I have no idea if that's still the case

Mike

mdf

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Reply # 1962076 22-Feb-2018 09:35

Thanks all! I had no idea I could use a three phase motor on single phase! I can gear whichever motor I get correctly to get the right spin speed on the saw, and I don't need to adjust after that (I never use it for metal cutting), but you can definitely pick up cheaper three phase motors second hand. Any steers on where/what I should be looking for in a VSD (and ballpark pricing?).

I've got an electrician rewiring some of the garage at the moment. We discussed putting in three phase power but it wasn't going to be cost effective (wiring to the house was fine, but from house to garage was going to be a pain). I should ask him about a 15 amp circuit though, give some more flexibility.

Finally, some of the ones I've managed to find online don't include plugs - see for example https://www.trademe.co.nz/business-farming-industry/industrial/engines-motors/electric/auction-1551548802.htm (extremely cheap; almost too cheap so I'm suspicious). Is that even legal? Assuming that's the case, are these expected to be hardwired in by an electrician? I'm happy to have a suitably qualified professional do this, but I like the ability to unplug things and move them around when needed.

TheMantis

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Reply # 1962079 22-Feb-2018 09:44

Another thing to consider is: If a larger motor was installed can the exisiting pulley/belt transmission system handle the increased forces/stresses. A pulley/belt transmission can only transfer a specific amount of mechanical power and there are many factors that determine this limit. Factors to consider:

1. Angle of lap of the drive belt (how much of the belt is in contact with the drive pulley).

2. Speed of the belt (determined by the RPM of the motor and the diameter of the drive pulley)

3. Friction factor of the belt material

4. Tension on the belt

5. Belt dimensions and type (vee, flat, round, etc)

6. Physical properties of the belt (what is it made of)

The maths for determining the maximum power transmittable on a given pulley/belt transmission can be quite complex but there are online calculators that can help. If the exisiting transmission system can't handle the power then you will get belt slippage and/or breakage.

All good quality mechanical equipment should have a built in safety factor to compensate for unpredictable use, manufacturing variations, etc. It is quite likely that the table saw was overbuilt given the age mentioned so it would be reasonable to assume that a larger motor (within reason of course) can be installed without significant issue.

It is perfectly normal for electrical equipment to be supplied without a lead. I wouldn't consider a table saw an appliance so unless you hold a current and appropriate EWRB practicing licence you will need an electrician to install the motor.

MikeAqua

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Reply # 1962080 22-Feb-2018 09:45

If you know someone with access to trade pricing, you can get a really sharp price on three phase motors and VSDs.

Mike

nunz

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Reply # 1967254 2-Mar-2018 19:44

mdf:

Thanks all! I had no idea I could use a three phase motor on single phase! I can gear whichever motor I get correctly to get the right spin speed on the saw, and I don't need to adjust after that (I never use it for metal cutting), but you can definitely pick up cheaper three phase motors second hand. Any steers on where/what I should be looking for in a VSD (and ballpark pricing?).

I've got an electrician rewiring some of the garage at the moment. We discussed putting in three phase power but it wasn't going to be cost effective (wiring to the house was fine, but from house to garage was going to be a pain). I should ask him about a 15 amp circuit though, give some more flexibility.

Finally, some of the ones I've managed to find online don't include plugs - see for example https://www.trademe.co.nz/business-farming-industry/industrial/engines-motors/electric/auction-1551548802.htm (extremely cheap; almost too cheap so I'm suspicious). Is that even legal? Assuming that's the case, are these expected to be hardwired in by an electrician? I'm happy to have a suitably qualified professional do this, but I like the ability to unplug things and move them around when needed.

Possibly getting around the legal requirements for certification of electrical equipment that came out a few years ago. no power cord = not electrical equipment needing certification.

nunz

larknz

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Reply # 1967292 2-Mar-2018 20:53

You can't use a 3 phase motor on single phase. If you go to a 3 phase motor you will need a 3 phase supply, the cost of which would be prohibitive in most cases.

larknz

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Reply # 1967307 2-Mar-2018 21:54

nunz:

mdf:

Thanks all! I had no idea I could use a three phase motor on single phase! I can gear whichever motor I get correctly to get the right spin speed on the saw, and I don't need to adjust after that (I never use it for metal cutting), but you can definitely pick up cheaper three phase motors second hand. Any steers on where/what I should be looking for in a VSD (and ballpark pricing?).

I've got an electrician rewiring some of the garage at the moment. We discussed putting in three phase power but it wasn't going to be cost effective (wiring to the house was fine, but from house to garage was going to be a pain). I should ask him about a 15 amp circuit though, give some more flexibility.

Finally, some of the ones I've managed to find online don't include plugs - see for example https://www.trademe.co.nz/business-farming-industry/industrial/engines-motors/electric/auction-1551548802.htm (extremely cheap; almost too cheap so I'm suspicious). Is that even legal? Assuming that's the case, are these expected to be hardwired in by an electrician? I'm happy to have a suitably qualified professional do this, but I like the ability to unplug things and move them around when needed.

Possibly getting around the legal requirements for certification of electrical equipment that came out a few years ago. no power cord = not electrical equipment needing certification.

All equipment needs an SDoC, it's irrelevant whether or not it has a cord and plug.

richms

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Reply # 1967314 2-Mar-2018 22:37

larknz: You can't use a 3 phase motor on single phase. If you go to a 3 phase motor you will need a 3 phase supply, the cost of which would be prohibitive in most cases.

That's what the VFD does. Not all motors expose the needed terminals to change it to 230v tho without much more disassembly vs just having jumpers in the terminal block.

Richard rich.ms