“How do I control the speed of my belt grinder?”
The best way to control the speed of a belt grinder is by using a VFD connected to a 3-Phase motor, this will provide a large range of possible speeds with minimal loss of torque.
This answer is deceptively simple, and creates a set of fresh questions. We believe that we have answered these questions with what we consider the best, most cost-effective VFD for belt grinders on the market. However, we want to be able to support as many use-cases as possible, including full DIY machines and lower cost replacements, as well as explaining how we chose our VFD.
VFD stands for Variable Frequency Drive, and are also commonly known as inverters. As the name suggests, they can supply the motor with power at different frequencies. By definition, a motor naturally wants to align itself with the frequency of the power supply. Regular mains power is usually 50Hz, or 60Hz in North America. The RPM of a motor is proportional to the frequency. A VFD can increase or decrease the frequency of the mains power supply to change the rotational speed of the motor without reducing the amount of torque delivered.
The other low-cost solution that is sometimes explored for motor speed control is to reduce the voltage using a TRIAC or variable transformer. However, this is not suitable for use with belt grinders. The reason this method reduces the RPM of the motor is because it reduces the overall power of the motor. The maximum torque that can be delivered by a motor is proportional to the square of the voltage. This means that reducing the voltage by 50% results in a 75% reduction in torque. Compare this to a correctly-configured VFD, which will can delivery 100% of the rated torque down to extremely low speeds. It can even be increased beyond its rated frequency, with a corresponding drop in torque.
How does a VFD differ from other forms of motor control?
I don’t have 3-Phase Power, can I still use a VFD?
Can I use a VFD with a Single-Phase Motor?
Yes, but not with ours! VFDs for single-phase motors are available, but typically are only available up to 1.5HP, and are extremely expensive compared to the equivalent 3P model. The same is true of VFDs that take a 110V input. If you’re not sure if you need speed control, I recommend saving the money and buying a single-phase motor. If you find that you need speed control in the future, the most economical way is almost always to buy a 3P motor and sell your old one. Motors hold their value very well, and we make almost nothing on the motors we supply as part of our kits, meaning that you would be getting a good deal.
Speed control is most important if you are using a belt grinder to work on the edge of a blade after heat treatment. Speed control is not mandatory for knifemaking or applicable to many applications. Choose what is right for your application, and never be pressured into buying expensive equipment if you’re not sure you will need it!
Absolutely. In fact, the VFDs we sell will only take a single-phase, 220-240V input. The VFD will convert it to 3-Phase. What’s more, for a 2HP motor, the current required will work with regular 3-pin plugs in the UK, EU and regular 240V outlets in NA.
What about these Chinese VFDs?
To the displeasure of engineers and Siemen’s salesmen across the world, the market has been flooded in recent years by Chinese VFDs, offering 1.5Kw units in the sub-£100 range. Our verdict? They can be okay, in the right situation, and if you manage your expectations. But they can also be a false economy. We have worked with many of these over the years, and have advised customers on buying them. However, there is a reason we have never sold these units. Here are some of the main issues:
Quality Control
Ever seen companies who buy a generic item from China and resell it for 4x its original value? Often, the reason for this is the burden of quality control. Factories will sell units that they are less sure about direct to consumers via places like Aliexpress, or disseminated out via other platforms like Wish or eBay. These will often be shipped without any real form of testing or QA, largely because there’s very little accountability to the end user, and thousands of miles between you. The best units and quality assurance testing may be reserved for bigger B2B contracts where there is an ongoing business relationship and greater accountability.
IMany of these units will fail on first startup. If you intend to buy one of these, I recommend buying through a platform like eBay that has good consumer protection, just in case you run into a DOA unit. Generally speaking, if they survive the first hour, they will survive extended use, but do not expect them to work forever. Chances of success increase if you buy a 2.2Kw version for a 2HP motor over a 1.5Kw version.
Ingress Protection and Ventilation
These units do not have any protection against dust as they are designed to be installed within a large electrical cabinet with suitable ventilation. An environment with metal dust will destroy any unit that is not rated for such environments. When looking at IP ratings, such as IP67 (the rating of our VFD!), the first number refers to mechanical protection from objects and dust, while the second refers to protection against water. The first number is the one that’s relevant to us, and a belt grinder VFD would require at least an IP5x rating to protect from metal dust.
The obvious fix is to mount the VFD in an enclosure, but it’s not so simple! You’ll notice the big heatsinks, and often fans, on the back of VFDs. They produce a not-insignificant amount of heat, and will quickly cook if mounted in an enclosure with no ventilation. To achieve proper cooling, you should cut two holes in the enclosure for an inlet and exhaust fan, which will require filters to stop them from carrying dust into the enclosure. This can often be achieved with computer cooling fans and purpose-built filters. Additionally, you will need to wire remote controls to adjust the speed if the VFD is mounted in an enclosure.
These VFDs can often be acquired for approximately £70-80. The equipment to fully enclose and cool them would cost approximately £60. With this cost, and ongoing quality concerns, this is not something we would recommend to our customers, because we believe we have a much better alternative:
Is it ad time already?
Yes, and we think this one will actually help! We have been really excited to work with Sanyu EU on this new VFD. To summarise, this VFD is:
IP56 Rated, using a big integral cast aluminium heatsink that is assisted by the motor fan
Mounts directly to the motor, replacing the standard connection box
Made in Poland using high quality parts, including Omron components, with an excellently designed circuit board
Includes connections for mains power
Available with a full set of external controls and heavy data cable, with a convenient mounting bracket for the CORVUS grinder, physical speed dial, forward and reverse switch, and emergency stop
We genuinely think this VFD setup is the best option out there for belt grinders, offering many of the features of the premium KBAC drives, at a price that is accessible to hobbyists as well as professionals!
