What Are The Differences in CNC Spindle Motors?

Today’s CNC routers use a few different types of spindle motors to power their intricate cutting and carving. Each motor type has its advantages and disadvantages. In this article, we’ll compare the different motor types and what they’re used for.

There are three predominant types of motor used in modern CNC spindles:

Universal motors
Variable frequency drives
DC brushless motors

Universal Motors

Universal motors are the most commonplace motors found in CNC routers. Their chief advantages are that they are plentiful and affordable motors. These are often just off-the-shelf routers found at your local hardware store.

They have drawbacks, however. A universal motor gains all its power at high RPMs. At slower speeds, torque drops, greatly lessening the universal motor’s utility. As a result, this high RPM air cooled motor ends up being very loud. They also have brushes that wear and have to be replaced regularly. The commutators also wear down eventually and cannot be repaired. Off-the-shelf routers just aren’t designed for continual loading and ultimately have a very low hour rating.

PROS	CONS
• Economical • Plentiful	• Brushes wear out • Commutators wear out • Narrow power bandLow power-to-weight ratio • Noisy from fan, high RPMs, and the friction of brushes on commutator

Variable Frequency Drives

Variable frequency drives (VFD) are a type of induction motor, varying the frequency to control speed. Like universal motors, variable frequency drives work best at high RPMs. Generally, while VFD are bigger and heavier than universal motors, they also have a relatively low power to weight ratio. These motors’ efficiency losses are worse at low RPMs, and VFD can get very hot as a result. More powerful VFD motors require water cooling. Unlike universal motors, VFD do not require brushes, which means they run longer and are less noisy.

PROS	CONS
• Brushless • Generally more affordable than brushless DC motors • Water cooling makes for very quiet motors • Simple control system	• Bulky • Water cooling requires pumps and tubing • Lower power-to-weight ratio than brushless DC motors

DC Brushless Motors

The DC brushless motor is notable for its flatter power curve, which allows superior torque up to 22,000 RPM and all the way down to 4,000 RPM. The key to the performance of a DC brushless motor is the permanent magnets within their rotors, which provide very high pullover torque. Their primary advantage is their high power-to-weight ratio. They can be air-cooled or water-cooled, depending on the size and application.

PROS	CONS
• Brushless • High power to weight ratio • Field oriented control system • Much more rugged and reliable	• Generally more expensive • Control is generally more complicated

Why RAVEN Uses DC Brushless Motors

DC brushless motors were chosen to power RAVEN’s CNC spindle due to their comparatively small size and weight, as well as their ability to deliver the necessary torque along a wide range of RPMs. RAVEN was designed to be compact and tight to keep the system very rigid, and the DC brushless motor supplied everything we needed to maintain fast speeds and stiffness for repeatable accuracy. Larger spindle motors required larger compromises everywhere in the machine. With DC brushless motors, RAVEN CNC is compact and fast without sacrificing performance.