Frequently Asked Questions

Using a CAD model, assuming materials properties are correct, you can extract the system’s inertia. Then you can determine how quickly your load will need to accelerate. Once you have this acceleration rate defined and your inertia, you can determine the torque required purely for acceleration (Torque = Inertia * acceleration). Friction is much more complicated. If there is an existing or similar system you can put a torque wrench on it, if not you need to use information from bearing suppliers, which tend to be conservative. Total torque is typically covered by summing these two. There is occasionally viscous friction (damping) torque, which is similar to windage torque that is present and is proportional to speed as the motor travels through a fluid medium (air, water, etc.).

No. A motor with cogging torque will always have torque ripple when running. A motor without cogging torque (slotless or air-core motor) may still exhibit torque ripple if the phases are not balanced in amplitude and phase relationship, and the torque versus angle curves is not sinusoidal with low harmonic content. It can also happen if current waveforms from the driver are not well controlled sinusoids. If a six state older style motor controller/driver is used, any motor will exhibit torque ripple.

Most motors have a torque constant you can find in a datasheet. This is only first order approximation for the torque output as a function of current. In reality, many motors experience saturation at higher currents levels, which results in less torque output per current input. This is called torque linearity. It is not shown on datasheets, but it can impact system performance and make your control system very non-linear. Only Air-Core motors and slotless motors have good torque linearity up to current levels well above the continuous rated current.

Most motors have a continuous torque rating. This rating was derived under some thermal test conditions at the supplier. Rated torque is the least understood and the most corrupt parameter for any motor. Heat is the enemy of torque. Know the test conditions that a particular motor was rated at and make an engineering judgement whether you can use those ratings. It is recommended to avoid using these ratings unless the test conditions were exactly like your project (never the case). Torque rating is a reference point only. Thermal modeling is difficult and needs empirical input to be useful. Many solid modeling software tools have a thermal FEA module this can help determine the likely thermal resistance of the system you are developing. Peak torque rating should not be used until you know how close the motor torque output is to your needs.

Typical ranges for motors are -50 to +150C. This means that the motor supplier will use materials that are rated above or below these to achieve a safety factor. If there is an encoder in the system the limits may change from operation at 0C to 80C due to limits on the encoder, depending on where it is in the system. Bearings are limited by colder temperatures, as standard lubricants may thicken and solidify. Electrical insulation systems and magnets can be chosen to operate over 200C if the rest of the mechanical systems can handle this temperature.

In vacuum there is no convection, only conduction. This means the physical mount and material used is critical to heat flow. Smaller motors rely on conduction more so than large motors with lots of surface area that use more convection. So, if it is a small motor <50mm diameter then its derating in vacuum would be less. A large motor >50mm would require a more significant derating. General rule of thumb would be to assume 50% of the heat transfer available (under nominal operating conditions). This means that the motor would be derated to 75% of the torque output because power is mainly a function of copper losses which are proportional to the square of the current.

Eddy currents in a motor are related to the rate of change of the magnetic field in the iron. They appear as a damping term in simulation modeling, such as Nm/rad/sec. Motor designers typically laminate the magnetic structure to minimize the flow of eddy currents. Thinner laminations provide a higher resistance to eddy current flow. Any eddy currents present will produce heating in the motor and result in a drag torque that is proportional to speed.

Yes. Hysteresis in the motor laminations is caused by the changing magnetic fields as the motor rotates. Motors with high levels of magnetic flux will have higher hysteresis. Motor or transformer grade steel with high Ni content is typically chosen to minimize magnetic hysteresis. Hysteresis is a friction term if you are doing any modeling. Most motors have hysteretic friction. The higher-performance motors with higher strength magnets have higher hysteresis. There are some trends moving away from laminations to SMC (sintered metal compound) materials.

Linear motors produce motion in a straight line rather than a circular motion, and they provide the power for transporting from point A to point B, such as a conveyor belt or railroad track. Linear motors are also a solution for actions requiring speed control. They are often used in semiconductor, print and science industries, where they help perform measuring and testing tasks.