Why the Motor Matters

Our background is high tolerance CNC machining, precision and performance is our goal. Before getting in the 2x72 belt grinder market we did a lot of research on what it takes to make a quality belt grinder, and what features are most important in a high quality knife grinder. By far the most important single component of the grinder is it’s motor. Without the torque to keep the belt running smooth and lag free it doesn’t matter how nice the frame is. Without solid bearings and an accurate drive shaft in the motor, it doesn’t matter how accurate your tracking and drive wheels are.

For this reason, one of the first things we did was to buy most brands of 2HP electric motors, both import and domestic, single and three phase, and put them through their paces to find the best options available. We will publish our testing results as soon as our initial rollout is complete and we get a little time, but in the meantime our main conclusion is buy American, specifically Leeson, Baldor, or Marathon.

Electric motors

The motors we supply are 2 horse power 1725/1800 rpm three phase 56c face mount 4 pole electric motors, and use a variable frequency drives (VFD) to convert from the 110v or 220v single phase power of a normal house or shop wiring to the 3 phase power the motor requires. At the same time the VFD gives smooth and accurate variable speed control. We chose the 4 pole motors despite there lower rpm because we can double their speed by increasing the hertz from 60 to 120 with the VFD and maintain full torque up to the original 1800 rpm of the motor with a slow drop off as it increases to 3600 rpm. But as you increase speed you gain inertia in the form of rotational momentum to help offset the loss of torque. Compared to a 2 pole 3600 rpm motor where you would lose torque throughout the entire rpm range rather than just half of it.

2 pole vs 4 pole

The standard AC motors that everyone is familiar with have 2 poles, that is two electromagnets on opposite sides of the motor. At any single point in time while the motor is running, one pole is north and the other is south. By alternating the poles back and forth, the changing magnetic field causes the motor to spin. The fields are switched twice per rotation, always trying to repel the close side of the motor’s armature while at the same time attracting the opposite side, in a constant game of tug-a-war. A four pole motor has twice as many magnetic poles, one every 90° instead of 180°. The same basic principles apply but instead of switching twice per rotation, the magnetic field is switched 4 times. This is like a game of tug-a-war with four ends to the rope instead of just two. Just like a 4 cylinder engine vs a V-8, the more poles a motor has the more torque it produces.

Variable Frequency Drive

A Variable Frequency Drive (VFD) uses electronics to change the frequency of the AC current being supplied to the motor. Since the speed of the motor is directly linked to the frequency of the AC current being supplied, changing the frequency is the best way to change the speed of the motor. Standard household current is 60 Hz, so by increasing the frequency to 120 Hz, you can double the speed of the motor, and decreasing the frequency will decrease the speed of the motor.

Speed/Torque Considerations

Most AC Drives can have output frequencies of 120 Hz or greater. However, the output voltage is limited to the line voltage. A drive supplied by 220 volts cannot output more than 220 volts. Therefore, as frequency is increased above 60 Hz, the output voltage remains constant but the volts per hertz ratio decreases, resulting in a loss of torque, but only at the highest speeds.
So by using a 4 pole 1725/ 1800 rpm motor you keep full torque through the critical low speed range where the added power is needed most.

To compare this with a 2 pole 3600 rpm motor, you only have full torque close the rated speed of 3600 rpm. As you decrease speed on a 2 pole motor, you also decrease the torque. So in the critical lower speeds you have the least amount of power and a greater chance of belt lag.

For this reason, we feel the full torque in the lower rpm range is the most important consideration and a 4 pole 1800 RPM motor is a win win given the options!

Import vs domestic

Import motors have major issues with shaft concentricity, shaft diameter inconsistencies, and problematic compatibility with low speed VFD. For these reasons we chose to only offer USA made motors.

Direct Drive vs Drive Shaft

The most common drive setup for a belt grinder is a pair of bearing blocks which supports the drive shaft with the drive wheel on one end and a single or step pulley on the other, like our Mod-3 grinder. The motor is then connected with a V-belt to the motor pulley. This has the benefit of being very flexible when it comes to motor choices, and with a pair of step pulleys it makes a simple low tech way to get rudimentary speed adjustment. This is usually the cheapest motor setup, and for this reason a very popular choice among knife makers and blacksmiths.

The downside is that you pay a significant price in power and performance. All of the extra components (two bearings, two pulleys and a V-belt) add a significant amount of extra friction, reducing torque by an incredible 30-40%! That means the big awesome 2HP motor you bought will only effectively be a 1.2 HP motor when compared to a 2HP direct drive motor. (What a bummer.) And that’s assuming your using 220v for that motor, if your only using 110v then you drop to 1.5 HP, which means it’s equivalent to a 0.9 HP direct drive motor. (Ugh, that just sucks.) As if that’s not bad enough, (wait, it gets worse?!) all those extra linkages add places for vibrations and misalignments to creep in, taking your smoothly running grinder and downgrading it yet again.

For these reasons, we have decided to only sell direct drive motors with VFDs for our complete machines, but we understand that for many the cost is too great. For that reason we offer the Mod-3 grinder chassis as an entry level option.