By Guest Author, Vincent Hua
Picking an appropriate stepper motor size for the CNC router can get baffling, and more research eventually leads to more confusion. Besides, there are several CNC routers and stepper motors available in the market, such that even the experts might have a hard time picking the appropriate size.
If you are just as baffled as described above, this article is for you, as it will give you a walkthrough of the different factors that you must account for to make an informed decision.
Tips to Consider When Picking a Stepper Motor for Your CNC Router
Whether you are on a tight budget or have a substantial amount dedicated to picking a stepper motor, this article will educate you about the crucial factors you must consider when browsing for the ideal stepper size for your CNC router.
1. Current and Voltage
Some of the most crucial factors to account for are the current, voltage, maximum step pulse rate, and micro-stepping of the motor.
You must ensure that the current does not vary with the stepper motor and must be supplied with the rated current. It is worth noting that under-saturating a motor will lead to a nonlinear dropoff in torque below 50% of the rated current, while oversaturation will cause a nonlinear relationship between heat and torque output.
Choose the driver with numerous step resolutions so that you can test different micro-stepping settings, which deliver the smoothest possible motion.
It is advisable to run a motor at its rated current, which will technically be operating at RMS current and deliver a lower current value to the machine. The motor can stop at a full step location and output 100% motor current for as long as it stays halted.
Mathematical Calculations for choosing a Power Supply
Use the following empirically derived formula to choose the power supply voltage –
- 32 * SQRT(motor inductance) = maximum voltage.
This equation implies that a 2mH motor would be 32 * 1.41 = 45VDC maximum. Therefore, any power supply rated for 45VDC or less would do just fine.
- The applied current should be the sum of motor phase current multiplied by 0.7 to obtain a minimum rating.
This statement implies that a machine with four 2.5A steppers would require a minimum of 7A current for smooth functioning. Any current value applied beyond this total would be redundant and will not yield any positive results, apart from future-proofing the machinery for later upgrades.
2. Load Requirement
It is paramount to establish the load time between frictional and inertial loads. If you seek purely inertial, pick a type of transmission that matches motor torque to the load torque.
Choose a machine that measures the force required to move the load at the required speed and acceleration. You can conveniently measure the rotary force with a torque watch or record the force’s measurement on the linear axis.
It is also imperative to figure out the utilization of the supply voltage. Use the equation to calculate how much power is required by your motor to operate –
32 * SQRT(motor inductance) = maximum voltage.
You can assume 70 percent efficiency at all stages, implying that a motor rated for 1.41 times the minimum torque desired at any speeds you require running it on is the best approach to be on the safe side.
3. Issues With Torque
Most issues associated with the stepper motor revolve around torque, with the two most typical being inertia mismatch and the absence of adequate torque speed.
Inertia mismatch between the load and motor leads to the stepper motors not accelerating fast enough. To resolve this, ensure that the size of the motor gets matched scrupulously to that of the load size. Overlooking this factor could lead to the motor subsequently having a hard time accelerating bulky, unstable load, eventually leading to system failure.
Another issue is a lack of adequate torque compared to speed in stepper motors designed to perform at their designated speed ranges. Therefore, using a high-speed stepper motor on a low-speed application will lead to enormous power consumption. This scenario would lead to motor stalling due to the absence of enough torque.
You can also refer to the motor’s speed-torque curve to figure out the performance yield of the motor and whether it would match your standards. The motor-speed curve describes the maximum torque supplied by a stepper motor at a given speed. Any torque or speed required value that exceeds this curve will lead to the motor losing synchronism.
4. Positioning Accuracy and Resolution
The positioning accuracy and resolution of a stepper motor system are influenced by several factors, including the selected drive mode (full-step, half-step, or micro-stepping), the stepper angle (the stepper motor full-step length), and the gear rate.
Several combinations can help the user obtain the required resolution. Owing to this, the resolution issues of a stepper design can conveniently be dealt with after you have identified the motor size and drive type.
Picking the Correct Size of Stepper Motor
Picking a stepper motor that is not the ideal size typically shows symptoms in different ways. For instance, under-sizing the size of the stepper motor leads to deficient deceleration and acceleration, poor performance, and extreme heat.
Under extreme circumstances, the stepper motor will get positioned improperly, lose pulses, or malfunction when placed under high acceleration or deceleration and heavy loading.
On the other hand, overloading causes the stepper motor to yield higher RFI/EMI and makes it run even louder. This scenario might lead you to sustain extra expenditures for a driver and motor and would occupy more machine or panel space than required.
Besides, considering that this is an open-loop system, the matching of rotor-inertia to load-inertia is pivotal, irrespective of whether an encoder gets added, but the inertia mismatch can never surpass the magnitude order. A sizeable mismatch leads to the stepper motor missing position, losing pulse, utilizing excessive current, or sometimes stalling altogether.
The above article can help shed light on the fundamental tips you need to consider before finalizing a stepper motor for your CNC router. Getting an inappropriately sized motor can cause various issues, including a complete CNC machine breakdown.
Picking the right motor size will provide several operational benefits, such as delivering optimum efficiency without over-consuming current. If you still require advice on what stepper motor size to choose, discuss it extensively with the seller to get some recommendations.
About the Author:
Vincent Hua is the Marketing Manager at TSINFA. He is passionate about helping people understand high-end and complex manufacturing processes. Besides writing and contributing his insights, Vincent is very keen on technological innovation that helps build highly precise and stable CNC Machinery.