Belt Grinder Motor Guide

three phase TEFC induction motorI get a lot of questions on the blog about what motors work for grinders. The truth is, many motors will work, but some are better than others. Then there are the motors that have their challenges. Armed with a little information, you can decide if a motor will suit your belt grinder application.

The focus of this essay will be Alternating Current  (AC) induction motors like you'd find on an old-school table saw. (I have a DC treadmill motor coming as another topic.)

AC Induction Motors

The first thing we look at when understanding motors is the different parts and what they are. The figure below shows a motor and the important parts. 

Although all these parts are important, there is nothing more important to us than the nameplate.


When it comes to figuring out a motor, the most of the important stuff is found on the motor's nameplate. The nameplate is the label or metal plate that gives the manufacturer's name, model number, voltage, current, speed and horsepower as well as other details that will be useful in deciding whether a motor will work for your belt grinder.

This is our example motor nameplate:

The nameplate has a lot of info on it. Let's digest some of the important pieces. I will draw a red rectangle on the parameter so you can see what it might look like on your motor.


The first and foremost thing to look at when it comes to AC induction motors is phase number. If your motor is a single phase, PH 1 or similar, you have to use single phase (household) voltage to run the motor. This also means that controlling the speed of the motor is pretty much out of the question. That doesn't imply the motor is useless, only that the grinder's belt speed will be fixed unless you add counter-shaft with sheave and a belt to adjust the belt speed.

If the phases parameter is 3 or PH 3, then you will need three phase (industrial) voltage supply, a VFD or a phase converter to run the motor. We'll discuss that later. In our example we have a single phase motor, so let us continue down that path.


Motor manufacturers put the voltage (V, VOLTS, U etc.) ratings on the nameplate so you know what supply to connect it to.  Motor manufacturers tend to use 115 scale, that is 115/230/460 volts. Just remember that for our purposes 110, 115 and 120 are all the same thing; and that 220, 230 and 240 are all mean the same thing. It's like low and high, but wait! What if the motor is 230/460? Well, high is still the higher of the two voltages.

Nomenclature Note: I use 120 V or 240 V when describing supply voltage. Why? I use these numbers as they are based on 12/24/48/120/240/480/600 and 208 (square root 3 times 120). This is recognized as standard for supply. However...some people (like my Dad) still use 110 V. Others use 115 V and that's fine. What's important to remember is that 120 means the same things 110 or 115.

The National Electrical Manufacturer's Association (NEMA) suggests that a motor's voltage range is plus or minus 10 % of the nameplate voltage. Therefore, a motor's nameplate that states 115 volts can be supplied a voltage between: 104 V and 126 V. A 230 V motor can be supplied with a voltage between: 207 V and 253 V. You can see the voltage supply numbers are pretty loose when taken within the 10% plus or minus range.

Dual voltage: When a motor voltage rating has a slash in in, this means the motor can be configured to run at different voltages, quite often 115/230 V. NOTE: The motor does not automatically know what voltage it will be operating at. You must change the wiring in the terminal box to change voltage the motor is to run at.

Be careful when looking at a motor's voltage ratings. Some motors will not work conveniently with your available supply. As a rule, single phase motors are either one voltage 115 V.  or dual voltage 115/230 V.

Three phase motors may come in a variety of voltages such as 208 V, 230 V, 460 V or 575 V in North America and 220 V or 380 V in Europe/Asia. If you are wanting to run a three phase motor with a VFD, check that the nameplate on the motor states 230/460 V. This is common for NEMA motors. However, some motors are 575 V only! A 575 V three phase motor would require a VFD that is not easy to run from home without special equipment.

Amps (Current)

The Amps number is the current needed to run the motor. It's stated in Amperes (A, amps, Amperes, I,  FLA, Full Load Amperes, etc.) The current on a motor nameplate is very important as it lets us calculate what gauge (thickness) of wire to use as well as what amperage of circuit breaker is needed to operate the motor.

Motors are dynamic loads; that is they don't draw the same amount of current all the time. When a motor starts it can draw up to five times the normal running current. This is why the lights seem to blink when the old fridge compressor motor kicks in.

The nameplate current is the running amps of the motor at the rated horsepower.

Dual current? If there is a slash in the current rating like 7.6/15.2 it means the current ratings are for two different voltages, in this example 230/115. Yup, when the volts goes up the amps come down.

HP (Power)

The power of an induction motor is expressed as horsepower (hp) or kilowatts (kW). Horsepower is the commonly used unit in North America, whereas the rest of the world uses the International System of Units' (SI) kilowatts unit.

One horsepower equals 0.746 kilowatts if you need to do the conversion.

Induction motors are made from fractional horsepower ratings like 1/3 hp to thousands of horsepower. For a decent 2" x 72" belt grinder, we're looking for ideally 1 to 3 horsepower, with a preference for 1.5 to 3 horsepower. The motor needs to have enough power to perform efficient knife grinding and general metal shaping tasks.

A technical publication I read a few years ago from the German abrasives manufacturer Klingspor recommends 1 hp to 5 hp per inch of belt width. As we are working with 2 inch wide belts, this range is 2 hp to 10 hp! Luckily, empirical evidence suggests that 2" x 72" belt grinders are quite useful, (although a little under-powered) at 1 hp; extremely capable at 1.5 hp and awesome at 2 or 3 hp!

RPM (Speed)

The speed of an induction motor is based on the number of poles (or windings) the manufacturer builds into it. The more poles the motor has the slower it will spin at a given frequency. Two pole motors are fast, whereas four pole motors are half that speed. In North America, you will see motor nameplate speeds that are approximately 1740 RPM or 3450 RPM at rated load.

On occasion we find even slower six pole motors which will spin just under 1200 RPM and eight pole motors that spin around 900 RPM.

The six and eight pole motors are not very useful for a direct drive belt grinder as they run too slowly and would require very large drive wheels to keep the belt speed in an acceptable range. They may be be better suited to other applications like honing, sharpening or polishing where slower speeds are advantageous in reducing heat from friction.

The synchronous speed of a motor is often stated by the manufacturer. Example 1800 RPM or 3600 RPM. This is a consistent way of describing motor speed without factoring loading or slip.

Here are the synchronous speeds at 60 Hz for common induction motors.

AC motor speed by poles

For 50 Hz supply, the table above would read 3000, 1500 and 1000 RPM.

Don't worry if your motor nameplate shows something a little bit less than 3600 or something less than 1800. Nameplates show the RPM when the motor is under loaded conditions. Every induction motor needs to slip, that is, be "out of sync" with the supply voltage. When a motor spins freely without load, the slip is small. As you increase the pressure on the belt, the motor slows and the slip increases. only under load will the RPM will be 3450 or 1740.

There are some less common single phase motors that are both two and four pole motors in one frame. These motors can run near 3600 or 1800 RPM and are usually found in hot tub water pumps.If you can overcome the pump shaft problem, you could easily have a single phase low/high speed grinder at the flip of a switch.

The RPM of a motor has a big influence on how fast the belt will be moving. For a direct drive setup, meaning the drive wheel it attached to the motor shaft, this is very important.

The diagrams below show direct drive wheels of three different sizes, 4" (100 mm), 5" (125 mm) and 6" (150 mm) and their respective belts speeds in Surface Feet per Minute when being driven by a four pole motor at 60 Hz.

And the same drive wheel options being driven by a two pole motor at 60 Hz.

SF (Service Factor)

The Service Factor sometimes labelled as SF is a multiplier which indicates how much a motor can go over its rated horsepower for a short period of time. For example, a Service Factor of 1.4 would mean the motor can temporarily operate at 140% of its current rating. This is not meant to be continuous, rather the SF is helpful for calculating the current to bring a load like a flywheel or conveyor system up to speed as it may require larger service, cables, switches and so on. In the case of our nameplate, the SF A (Service Factor Amperes) is stated as 7.6 / 15.2, which means this motor can pull 7.6 A @ 230 V or 15.2 @ 115 V for short periods.

SF is not a big concern with belt grinders as grinders are usually started under light loads and quickly get up to speed.

Time (Duty)

The time or duty parameter on a nameplate indicates whether the motor was meant to run continuously or intermittently. Some motors are designed to run all the time, while other special purpose motors are designed to have rest periods where the motor can cool. Continuous Run motors have a code of S1 or CONT. Motors that are not meant to run all the time will have codes like s2 or specify running time in minutes like 30 or 60. You want to have a motor that can continuously run without overheating issues.


The temperature rating of a motor will either be labelled temp or ambient and is usually stated in degrees Celsius.  Temperature is not normally an issue for garage or workshop applications.  Avoid putting your motor in an enclosed area where heat can build up. If you live in a crazy hot place, you may have to derate the motor. An example of derating would be a 2 hp 40°C motor should treated like a 1.5 hp at 50°C ambient. Keep your motor in free moving air to avoid heat build up.

When operating the motors at slower than intended speeds this become a larger problem. See more in the VFD section that follows.

Insulation Class

Wiring inside the motor is insulated and can withstand some pretty warm temperatures. The higher the temperature of the insulation, the better the wiring will hold up when overloaded. Class A is okay at just above boiling, but Class H is 75°C (167°F) better than Class A. Again, for the most part, insulation is not really a concern in belt grinder applications unless you're loading the motor heavily for some period of time and generating enough heat to exceed the insulation class.

This is a table of the insulation classes A through H and their °C and °F temperature ratings.

Hertz (Hz)

The Hertz rating tells us what supply frequency the manufacturer designed the motor for. This is usually 60 Hz for North America and 50 Hz for Europe/Asia. Hertz or frequency rating is directly related to the RPM rating or speed; that is, the same motor will run faster when fed with 60 Hz than 50 Hz. The difference in speed from North America to Europe is 50 divided by 60 or 5/6 or 0.833.

Synchronous speeds on 50 Hz motors will be just under 3000 RPM and 1500 RPM. Note: This can make a difference in the drive wheel or sheave configuration you choose.

The Hertz rating becomes more important for cooling the motor too. Running motors slower that their rated frequency obviously results in the motor's fan running slower. This impacts overall cooling and could cause overheating in the motor. Slowing down a fan a little can cause a large drop in airflow. See Affinity Laws.


The frame code tells us about the physical properties of the motor including the size, mounting hole dimensions and shaft diameter. The frame code does not convey any information about horsepower, however, as a general rule, the larger the frame  number the more powerful a motor can be.

A 56 frame motor will have foot mount holes at 3" x 4-7/8" and 5/8" (0.625") diameter shaft.

A 143T frame motor is larger than a 56 frame and has foot mounting holes at 4" x 5-1/2" and a 7/8" diameter shaft.

A 145T frame motor is larger than a 143T frame and has foot mount holes at 5" x 5-1/2" and a 7/8" diameter shaft.

There are some special flavors out there like the 56J which has a 5/8" shaft, but it's threaded at the end with a 7/16" NF. These are used for attachment of pump impellers. What you are looking for is a 56, 143T or 145T frame motor as these are well suited for a belt grinder.

Motor Enclosures

Totally Enclosed Fan Cooled Motors

Some motors are sealed from dust and moisture, while others are not. The sealed ones are called "Totally Enclosed Fan Cooled" motors or TEFC for short. In an environment where metallic dust is floating around, TEFC motor is the best choice.

TEFC grinder motor with caster for drive wheel

TEFC motors are more expensive that their ODP counterparts because they require more mass, in some cases external fins etc. to keep the motor cool.

Open Motors

Unsealed motors are typically labeled as "Open" or "Open Drip Proof". Open or ODP motors are cooled by an internal fan drawing air inside of the motor housing and expelling the air along with the heat. Unfortunately, open motors also allow dust and moisture inside the motor, so they are not the best choice for belt grinders. Metallic dust being sucked into the electrical workings of a motor is a bad thing. Anecdotal evidence suggests that some protection such as dust shroud and periodic maintenance like blowing the motor out with compressed air works to prolong the life of open motors in what is a pretty harsh environment.

open motor open drip proof

On some ODP nameplates it may say "Drip proof in vertical position only," which means the motor was meant to be used in the vertical position like in a drill press where dripping water is not going to be a problem with the electrical parts of the motor.

Inverter Rated

An inverter rated motor has be designed to work with an inverter or VFD. VFDs do not create perfect voltages. The output of a VFD contains noise spikes that can cause damage or puncture to the insulation inside the motor. These problems are more pronounced with long cables going to the motor.

It is always best to have an inverter rated motor for use with a VFD, but is it necessary? For most of Typical three phase motors have insulation rated for 460 V and we normally operate these motors with a VFD at 230 V and we use short motor cables.

Motor Shafts

Motors are made with a few different shaft configurations and are defined by NEMA standards. Smaller frame motors will use a 3/8" shaft with a flat spot for the setscrew to sit. Frame 56 motors have a 5/8" diameter shaft which can be keyed (or threaded in the case of a 56J). Larger frame 143T and 145T have 7/8" keyed shafts. Even larger 180 series motors can come with a 1-1/8" shaft. Be sure to look up the frame and shaft size in the NEMA dimensions table.

The most common motors you'll encounter will have shafts as shown in the graphic below.

See the NEMA motor table for dimension 'U'.

The length of the shaft is important for belt grinder applications. Shaft lengths of 2" or more are preferred as we will need to mount in a 2" wide drive wheel. NEMA 56, 143T and  145T have shafts that "stick out" more than 2 inches. Motors with shafts less than 2" can still be used with suitable extensions.

As a rule, more powerful motors will have larger shaft diameters. The NEMA standard for motors define this. The smaller 3/8" and 1/2" shafts are for fractional horsepower applications while 5/8", 7/8" and 1-1/8" can be found on larger frame motors.

Got Keys?

A way is required to secure the wheel to the motor shaft. On smaller motors the shaft can be D shaped and a setscrew on the wheel or sheave will contact the flat spot and keep the wheel secure.

Larger frame motors will typically have a slot called a keyway.

Both 5/8" and 7/8" keyed shafts use the 3/16" key stock. Smaller 1/2" shafts generally have a flat on the shaft for a setscrew.

A sheave/pulley will have a slotted keyway just like the motor shaft.

Keyways in the shaft and wheel or sheave are mechanically locked with a piece of square steel, yep called a key. Brilliant! Most hardware stores sell keystock.


Many motors do not have easy to use shafts. Motors for specific applications such as pumps have no provision for setscrews or keys. Rather, a threaded shaft end is to be used to fasten an impeller or other type of wheel.

This photo is showing an open motor with a shaft intended for a screw on flywheel.

A reader (thank you Dan Hargrove) sent me this photo of his nice little 1 hp Marathon 56 frame TEFC. But...what the heck kind of shaft is that?

These purpose-built shafts are going to require some adaptation; possibly shims, or possibly grinding and broaching keyways yourself. Read: A lot of work, but if you are getting this oddball motor for cheap it may be worth your time. I'd like to hear your stories of odd motor shafts and how you made it work.

Motor Mounting

Motors are usually mounted with bolts through a mounting foot or through the face (where the shaft comes out) of the motor. With some motors, both foot and face mounting is possible.

C Face

Face mounted motors are called "C Face". Typically there are four threaded bolting points around the face.

The benefit of C face mounting is that the motor is fixed to the grinder frame and alignment issues can be minimized. For NEMA frames 56, 143T and 145T the holes for face mounting are 5.875" diameter or 2.94" radius as measured from the center of the shaft. I put this into AutoCAD and made a graphic below showing the center-to-center dimensions which may work better for some people designing a frame to hold a C face motor.

Bolts for NEMA C face mounting are typically 3/8" NC x 0.75". Of course, always check your motor mounting configuration before buying any fasteners.

Foot Mounting

Foot mounting is where the a plate or cast frame under the motor has holes for mounting. by bolting the foot of the motor to a surface.

The foot is supposed to bolted to a steel plate where the grinder fame is squarely attached to, but this can also be a work bench or slab of thick plywood. The motor will be able to cant slightly in the foot mounts, so care has to be taken to ensure the motor shaft is perpendicular (90°) to the belt path in two dimensions.

Single Phase Motors

Single phase motors have either 2, 4 or 6 windings that are installed inside the motor. The windings are on the opposite or 180° apart. When voltage is applied the motor would simply vibrate and not spin as it doesn't know which way to turn. In order to "nudge" the motor in the proper direction a capacitor is often used. 

The capacitor "bump" is a tell-tale sign of a single phase motor.

Leaving the capacitor powered while the motor is running makes the motor inefficient, so a switch is used to take the capacitor out of the circuit once the motor gets up to speed. The audible click that can be heard when starting and stopping the motor is a centrifugal switch that is taking the capacitor in or out of the circuit.

In some single phase motors you'll see two capacitor bumps. One capacitor is a run capacitor and the other is the starting capacitor.

One of the drawbacks with using a single phase motor on your grinder is that you cannot control the speed of the motor as it is set by the frequency of the supply voltage, say 60 Hz or 50 Hz depending on where you live.

Of course changing speed of the belt can be accomplished with different ratios of sheaves (pulleys) but the motor itself is only one speed. One exception is a two speed pump motor found in hot tubs. This type of motor will be two pole and four pole with the wires coming to the terminal box to to change the speed. The speeds will be for example 1800 or 3600 RPM.


Single phase motors are not always reversible. Motors are said to be rotating forward when the shaft is turning Counter Clockwise (CCW) when viewed from face. That is, looking at the shaft where the wheel will attach.

Some designs of single phase motors are not reversible. This is set by how the motor is manufactured. As nothing is impossible, some people pull the rotor out of the motor and re-install in the other way 'round. How's that for reversing and non-reversible motor. There is always a way if you have the time.

Many single phase motors can be reversed by changing two wires in the terminal box. Look for a connection diagram on the nameplate or inside the terminal box. This will state something like "to reverse rotation interchange red and black leads".

If your motor is non-reversible, it may still be used by re-positioning the motor with respect to the belt.

Single Phase Supply

Most everyone will have single phase 120 V in their workshop or garage. Single phase is the power we get at our homes. Common circuits may let us conveniently run a 1 hp induction motor without any issues as long as we 'dedicate' the circuit to running that one motor; that is, not sharing that circuit with lights and other equipment in the shop.

Every motor nameplate will show the current in Amperes that are drawn when the motor is fully loaded. Starting a motor requires a lot more current. In some cases as much as five times the nameplate current.

Some motors, 1 horsepower at 120 Volts may need a 20 Ampere circuit breaker, even though its nameplate says 14.4 A. 

Single phase also comes in the form of two 120 V lines. Used together they make double the voltage at 240 V. By doubling the voltage we can use smaller wires and circuit breakers and still get the same work done. Using a motor over 1.5 horsepower means you really do need to have 240 V available.

Sizing Your Supply

Understanding what you have for supply is a good place to start when looking for a motor that will work for your belt grinder.

For 120 Volt only supply

A single 120 Volt circuit in North America is at minimum capable of supplying 15 Amperes.
New regulations are making 20 Ampere circuits more common, especially in kitchens.

  • Typically 1.5 horsepower maximum
  • Require a 20 A circuit, preferably dedicated to powering your motor alone.
  • Small VFDs are available with 120 V inputs, normally 1 horsepower or less.
  • Uses NEMA 5 receptacle and plug.

120 Volt Motor Circuit Examples

For 240 Volt supply

A 240 V supply uses two 120 V lines and offers much more power and a wider variety of connection options. Circuits from 15 to 50 Amperes can be made available. 240 V circuits can do twice the work of a 120 V circuit and this is why they are used to power heavier loads like your stove or welder. 240 Volt circuits can be used in grinder applications on:
  • single phase motors between 1 and 5 horsepower
  • VFDs driving three phase motors from 1 to 5 hp
Using a 240 V supply also means:
  • lower current means smaller wiring and circuit breakers to do the same work.
  • NEMA 6 receptacles and plugs are required.

240 Volt Motor Circuit Examples

Motor Rated Switches

In order to safely turn on and off to your motor you need to use a switch that is capable of handling the voltage and current of the motor. Motors require more robust switches than lights or other simple loads do. When you cut power to a motor, it fights back and produces an arc (spark) that can damage or even weld the switch contacts together.

Motor rated switches will have a horsepower rating on them. Something like 3/4 HP at 125/250 VAC.  
Single pole switches will open/close one contact. This is your basic light switch. These can be used up to 1 horsepower.

Double pole switches will open/close two contacts. These are suitable for switching single phase 240 V motors. 

Three pole switches will open/close three contacts. These are intended for switching three phase motors. 

As motors get larger, a contactor is used to do the heavy switching. A contactor is essentially a relay as it has a coil and contacts that are moved by an electromagnet.


Although this guide wasn't meant to be a wiring guide, it is important to know what kind of wire is suitable for wiring equipment like belt grinders.

The most popular cable for wiring is SJOOW. It is considered portable power cord. It is has a supple rubber jacket that is weather and oil resistant. SJOOW is rated for 300 Volts. The individual wires inside are multi-stranded which helps it be more flexible. You can find it in a wide range of gauges and buy it by the foot at many hardware stores. A similar cable SOOW is rated for 600 Volts and can be used as well.

A more permanent cable is armored cable, known in the trade as BX. It's that familiar spiral armor that we've all seen. On the inside is solid copper wires in thermoplastic insulation a lot like regular NMD house wiring wire.

Conductor is another name for wire. Cable identified as 3/C means it has 3 Conductors. In North America these are normally one black, one white and one green. Cable identified as 4/C may have red, black, white and green wires inside.

Three Phase Motors

Three phase motors, although similar in construction to single phase motors, have windings 120° apart. The motor will always start moving in same direction as the supply determines the rotation. Three phase motors have no need for capacitors. A good thing to note is that any three phase motor can be reversed by swapping any two wires supplying the motor.

Many three phase motors can be used for belt grinders. The biggest obstacle with three phase motors is that most of us do not have three phase voltage in our shops, we have single phase. A simple remedy to the lack of three phase is to use a Variable Frequency Drive (VFD).

Variable Frequency Drives

A Variable Frequency Drive or VFD, sometimes called in inverter, Variable Speed Drive (VSD), Adjustable Speed Drive (ASD) etc., is an electronic device that creates three phases to drive a three phase motor.

The concept of a VFD as a phase converter works great for home and garage use. We give it single phase and the VFD outputs three phase to run a three phase motor.

A VFD must drive a three phase motor. Single phase motors as we know are constructed differently and should not be powered by a VFD.

Of course a VFD is not as cheap as a power switch, however the benefits are plenty. Using a VFD means we have the ability to electronically control the speed, direction, acceleration deceleration and many other operational aspects of a three phase motor making it truly flexible power delivery system. For a knifemaker this means slow speed cool polishing and high speed for aggressive stock removal in the same package.

For 120 V single phase drives, expect a maximum capacity of 1 horsepower.

For 240 V single phase, expect a maximum capacity of 5 horsepower, however some models will only go to around 3 hp. This 240 V range of VFDs are very useful as a single phase to three phase converter applications, perfect for a shop built variable speed grinder.


Torque is rotational force. Some loads require larger motor torque than others. In induction motors torque is inverse to speed, yet horsepower does not change with speed. With this in mind we can think of a four pole (1800 RPM) motor has having twice as much torque as a two pole (3600 RPM) with the same horsepower rating. Or, when the speed goes up the torque goes down for a given horsepower. Does this matter for belt grinders? Not really. Belt grinders are generally low inertia machines. Once the belt is moving at speed the requirement for lots of torque drops. Where we run into problems is grinders with counter-shafts, stiff bearings and large diameter wheels. These all load the motor and in smaller horsepower (3/4 or less) two pole motors, the motor torque may not be enough to overcome the resistance and the motor will be in a stall condition. (This is very bad for the motor.)

One example of a 2 hp 1800 RPM motor will offer 5.8 lb-ft of torque.
Whereas a 2 hp 3600 RPM motor will offer 2.9 lb-ft of torque.

The formula for torque in lb-ft is:

Torque  = horsepower x 5252 / RPM

VFDs can manipulate the torque of a motor and can be made to start into very heavy loads or maintain the same torque over a wide range of speeds. Features like "torque boost" and "constant torque" are programmable in  most modern VFDs. However, there is as always a trade off. As we increase past the normal running speed of a motor, the torque begins to drop off. Fortunately, this is not usually a problem as your belt is up and whizzing by then. The curve in blue below shows how the torque drops off in a motor when the VFD pushes past the normal operating frequency of the motor.

Belt Speed

The last consideration is to make sure you are getting suitable belt speed, (Surface Feet per Minute or metres per second) required to do the grinding you want to do. Belts behave differently at different speeds. Many abrasive manufacturers publish recommended belt speeds for grinding materials with their products and some are surprisingly fast. It's not uncommon for a belt manufacturer to suggest 5000 SFM  (25 m/sec). It can be a little intimidating when a belt is going by your hand at 55 miles per hour.

As a practical range for knifemaking, I'd suggest anywhere from 2500 to 4000 sfm is good. A couple of direct drive examples would get us in that range.

1).  A 2 pole (3600 RPM) motor with a 4" drive wheel will produce a surface speed of about 3700 sfm.

2). A 4 pole (1800 RPM) motor with a 6" drive wheel will produce a surface speed of about 2800 sfm.

This is why 6 and 8 pole motors are not very useful without some counter-shaft ratio to up the speed.

You will need a larger drive wheel to compensate for a slower motor. Wheels cost money and larger wheels cost a lot of money, so looking for a two pole motor or a motor VFD package where you can create the speed you need using a smaller wheel makes sense.

Ideal Motors

We've looked at a lot of motor parameters so far and it's clear that some motors are better than others for a grinder build.

Three Phase
230 VAC
1.5 to 3 Horsepower
2 Pole (3600 RPM)
4" Direct Drive Wheel
Variable Frequency Drive

Choice #2
115/230 VAC
1 to 2 Horsepower
4 Pole (1800 RPM)
Counter-shaft with 4" Drive Wheel

Choice #3
230 VAC
1.5 to 3 Horsepower
2 Pole (3600 RPM)
4" Direct Drive Wheel

Choice #4
115/230 VAC
1.5 to 3 Horsepower
4 Pole (1800 RPM)
6" Direct Drive Wheel

Choice #5
1.5 to 3 Horsepower
2 Pole (3600 RPM)
4" Direct Drive Wheel

Choice #6
1.5 to 3 Horsepower
4 Pole (1800 RPM)
6" Direct Drive Wheel

Suggested Packages
Check out Dealers Industrial for Motors, VFDs and combo packages.

Frequently Asked Questions

Q: Can I change the speed of a motor by changing the voltage?
A: No. An induction motor's speed is based on the supply frequency 50 Hertz or 60 Hertz and the number of poles in the motor.

Q: Can a single phase motor work with a VFD?
A: No. VFDs are meant to drive three phase motors.

Q: Is it possible to get 230 V from a VFD that is powered with 115 V at its input?
A: Yes. Some fractional horsepower VFDs will step up the voltage from 115 V to 230 V.  However, these are not usually available over 1 horsepower.

Q: Will my motor run more efficiently on 240 V?
A: No. The power of the motor stays the whether wired for and supplied with 120 V or 240 V.

Q: What size of drive wheel should I use with my motor?
A: This depends on the RPMs of your motor. Four pole ~1800 RPM should have a 5" or 6" drive wheel, whereas a two pole motor works quite fine with a 4" drive wheel.

Q: I have a jack shaft with step pulleys. How can I predict what the belt speed will be at?
A: There is a tool to help you with the calculations. Belt Speed Calculator

Q: I have an open motor. What can I do to prevent steel dust from getting inside?
A: There is not a lot you can do. Air will always be sucked through the motor to cool it. You can make a shroud or hood as long as the motor has air moving through it so it doesn't overheat. You can also blow the motor out with compressed air periodically.

Q: My motor bogs down really easily and sometimes won't start.
A: Check that you have the correct voltage setting on the motor itself. A motor wire for 230 V that has 115 V applied will rotate, but it will perform poorly.

Q: Can I use a regular light switch to turn a single phase motor on and off?
A: Yes, up to about 1 horsepower. It is always recommended to use a switch with an appropriate horsepower rating marked on it.

Q: I have a single phase motor that doesn't have a horsepower rating. How do you figure it out?
A: Multiply the voltage and the current, then multiply by 0.85, then divide by 746. This will closely approximate the horsepower. Example: 115 V x 9.5 A x 0.85 = 928. 928 / 746 = 1.24 hp.

Q: Can a washing machine motor work for a belt grinder?
A: No. It is an open motor and is generally too low horsepower to be useful in a belt grinder application.

Q: I have a motor from a hot tub. Can this be used for a decent belt grinder?
A: Yes, but the shaft on the motor will likely have no keyway and will have a threaded end for the impeller. It will take some fiddling, but it can be used.

Q: Should I buy an inverter rated motor?
A: It certainly would be better, but not necessary for short cable runs.

Q: My motor is 115/230 Volts, but I don't have 230 V in my garage. Can I get the most of the motor at 115 V?
A: Yes. The motor will produce the same power at 115 V. However it will be drawing twice the current. Make sure your wiring, switch and breaker are suitable.

Q: The motor I have says "non-reversible" on the nameplate. Can I still use it?
A: Yes. You will likely have to physically re-position the motor to take advantage of it's rotation.

Q: I have a single phase motor that just sits and hums. What is wrong with it?
A: This could be a bad starting capacitor or centrifugal switch. Have somebody check it with a multi-meter.


I hope that you have discovered a few things about AC induction motors and how they relate to a DIY grinder build. Please join the conversation below and ask a question if you have one.

All the best and happy grinding,



  1. Dan your blog is the best thank you so much for sharing your knowledge this helps me alot im glad i did not purchase the motor i had pick out before reading this


    1. Hi Eric,

      Thanks for stopping by. If you have any questions or suggestions, please let me know. All the best with your grinder build.


  2. Holy crap!
    You put a ton of time into this post, not that you were slacking on any of your other post.
    But this a whole bunch of useful information.
    Excellent job man, thanks for all the effort you put into this.

    1. Hi Justin,
      Thanks for your kind words. I hope that the it helps someone find a good motor and saves them money. ;-)


  3. Echoing everyone else, thank you for posting a lot of this information. I've been referencing your site for a while as I'm building a 2X72, and it's been invaluable.

    1. Thanks for your kind words. If you have any questions, just give a shout.


  4. I am new to knife making. Therefore, I have searched for advise only to find a bunch of BS.

    Thank you for providing trust worthy and reliable information that I find necessary to get started.

    Thank you. Thank you. Thank you