Tag Archives: Motors

Industrial Motors for Machine Automation

Industrial engineers use different types of motion control devices for improving the production rates and efficiencies on the floor of automated factories. Three major types of motion control devices are in demand for machine automation—stepper motors, servomotors and variable frequency drives (VFDs).

In general, stepper motors along with their drives, and controllers are widely used as they offer simple implementation, beneficial price/performance ratios, and high torque at low speeds. This motor is essentially a brushless DC version, moving in equal fixed steps during rotation, and only a single step at a time. Not requiring tuning or adjustments, stepper motors provide very high torque at speeds below 1000 RPM. They are cost-effective, as their prices are substantially lower than the cost of comparable servo systems. Since the torque they produce decreases as they speed up, it makes their operation difficult. Therefore, the work done by stepper motors becomes impractical at speeds in excess of 1000-1500 RPM.

Servomotors come with a motor, drive, a controller, and a device for positional feedback. For variable load applications, engineers prefer them to stepper motors, as they deliver high torque when rotating at speeds above 2000 RPM. Servos require adjustments and tuning, making them more complex to control compared to stepper motors. Including maintenance costs, their positional feedback arrangement can push their prices well beyond those of stepper motors.

Costing less than stepper motors or servomotors, VFD systems include an AC motor and a drive, but are unable to provide positioning. However, they can be good for applications requiring speed control on variable loads. For applications where the motor need not run continuously at full load, a VFD system can save considerable amount of energy. Another feature of VFDs is their soft-start capability, allowing a limit to high inrush currents.

In a stepper motor system, the controller regulates the position of the step, the torque generated by the motor, and the speed of the motor as it moves from one step to another. The driver operates on the control signals the controller generates by modifying and amplifying these signals to regulate the direction and magnitude of the current flowing into the motor’s windings. This way, it drive rotates the shaft of the motor to its desired position, and holds it in position with the required torque for the required time.

Controllers for stepper motors can be either open or closed loop types. Open-loop controllers are simpler, not requiring any feedback from the motor, but are less efficient. Open-loop controllers operate on the assumption the motor is always at the programmed step position and is producing the desired torque.

On the other hand, closed-loop controllers always operate with feedback based on the effective load on the motor. Therefore, the performance of the closed-loop stepper motor controller is similar that of a servo motor, and makes the operation more efficient.

Making a stepper motor rotate through each of its steps requires energizing the several windings within the motor in a specific sequence. Typically, stepper motors rotate 1.8 degrees per step, necessitating 200 steps to make a complete revolution.

Integrated Motors Simplify Motion Control

With machines getting more robust, smaller, less expensive and more reliable, engineers are facing the challenges of designing newer types of motion control. One way of addressing such motion control challenges, without being an expert in mechatronics is to use integrated motion control systems. Typically, these solutions combine the motor, the drive and the system components within a single unit. The system components include the intelligence or motion controller and input outputs all onboard. The use of an integrated solution allows the designer to focus more on the development of the machine and less on solving compatibility issues between various system components. The integrated motion system usually has all the components within a complete unit and sized for proper use. The decision to use an integrated motion system or an integrated motor usually depends on several factors. Major among them are requirements based on machine size, cost, reliability, modularity and distributed control.

With integrated motors, engineers can reduce the amount of space a machine needs. This is mainly the result of consolidation of components resulting in elimination of cabling. For example, an integrated motor may replace a drive and motor housed in separate enclosures, eliminating one of the enclosures. The panel space required reduces significantly for an integrated motor, while for a multi-axis system the real estate reduction can be substantial. However, an existing machine design must contain adequate space to house the integrated motor as this type of motor is larger than conventional motors.

Using integrated motors results in definite cost savings in contrast to using conventional components. One of the major saving in expenses comes from the absence of cabling that is no longer required with integrated motors. For example, the conventional drive may be located in a centralized cabinet with the motor a distance away on a long conveying machine. This arrangement needs considerable power cabling and feedback wiring between the motor and the drive. With the integrated motor, the drive being directly on the motor, much of the cabling is eliminated contributing to cost reduction.

With improvements in motor technology, the concern with reliability in integrated motors is outdated. The major point of concern earlier was heat buildup and dissipation. With reduced components making up the system, the reliability of integrated motors has improved because of the lower number of wire connections used. Better construction technology has improved the efficiency, decreasing the heat generated and the need for dissipation.

Industrial automation today requires modular machines. That essentially means smaller machines focusing on singular tasks combined to form a bigger system responsible for multiple functions. The smaller machines may operate independent of each other. This arrangement is beneficial because it allows engineers to change on modular section and transform the system into another customized machine. The modular concept is beneficial in shipping individual modules to the factory floor as the motor and drive of the integrated motor is placed directly in the machine.

As more and more industrial control is through PLC or Programmable Logic Controls, motor operations and synchronization through digital data signals is the norm. Since each integrated motor has its own controller, a distributed control system provides faster response and greater accuracies.

Efficient Control of Motors at Low Speeds

When a motor is operating at high electrical frequency or high mechanical speed, the back EMF signal generated by the rotating rotor presents an efficient feedback technique for a sensor less motor control.

However, generation of the back EMF requires a minimum frequency and that makes it difficult to control motors running at low speeds. The process of continuously estimating the rotor flux angle at zero and very low speeds, together with stably moving between low-speed and high-speed estimators helps to improve the effectiveness of starting the motor under load without using sensors.

TI or Texas Instruments’ InstaSPIN-FOC software called FAST helps to make this estimation at very low speeds, sometimes below 1Hz. Although the initial rotor flux angle is unknown, FAST estimates this using sensor less techniques. Until it has measured enough back EMF, this estimate remains unpredictable and the estimated angle is incorrect.

However, FAST feeds the control system applicable to the motor and induces motor movement. Enough back EMF is generated with only a small amount of rotor movement and the algorithm can then converge on a reasonable estimate for the angle very quickly. This allows a controlled high-torque drive at low-speeds with excellent operation. Although the start-up performance may not be consistent, this method can start the motor with enough torque for rotor movement.

With increase in the starting load, the torque requirement goes up. The amount of torque the system can generate depends on the current through the motor and the alignment angle between the magnetic fields of the stator and the rotor. For ensuring generation of enough current, the speed controller must necessarily have a maximum output larger than the rated current required to generate the necessary torque.

For example, a motor starting under full load may require 4A of current to produce the necessary torque to move. This requires setting the speed controller’s maximum current output to 6A. When started, the motor will draw a current of 6A in its first electrical cycle for moving the rotor. With FAST providing a valid angle within this first cycle, the control system will quickly regulate the current usage to the required level of 4A.

However, even when there is a stable feedback angle, the rotor may not necessarily align itself properly for generating the maximum torque. In reality, you are simply sweeping the stator field and waiting until the rotor field locks on and synchronizes. If the stator field is not oriented properly, the motor may fail to generate enough torque or even produce torque in the opposite direction. Control systems can improve this situation only by starting with a better starting angle.

The simplest way to control the initial alignment is to inject a DC current in a field-oriented control system. This defines the orientation of the rotor flux. A large enough DC current injected will move the rotor and the load to a known angle. Even though the forced angle is still emulated, the orientation will be proper for correct starting and the rotor will be in the best position for produce torque. The DC current injection may be done manually or programmed through FAST.

A new 32 HD motor for rough operations

Maxon Precision Motors, Inc. based in Fall River, MA, has successfully designed a new 32 HD motor for rough and abusive operations. Known as the EC-4pole 32 HD motor with part number 397798, the motor is considered ideal for operating in conditions existing in deep drilling. Exploration in gas and oil fields is known by the name “downhole drilling” and according to deep drilling technology, it is possible to recover oil and gas even from depths of 2500 meters or more. This 32 HD motor is being offered for this application as it has been specifically designed for the purpose. According to the company, this 4-pole power motor uses a winding technology that is the best among the same class of motors in the field.

The performance in terms of the volume rate, in proportion to its weight along with the quality and security it is able to generate, is excellent due to the motor providing inertia free motion. The durability of the motor is astoundingly long. This factor alone has been the main feature for this motor, which has enabled the motor to cater to the application of deep drilling in a very significant manner. With wide ranging options for different types, as required by the user, the motor is able to meet the requirements for the purpose.

The motor has a power rating of 220W in air and graduates to 480W in oil due to the high heat flow that is generated in the process. The motor functions in an ambient temperature range reaching up to 200°C and more, with atmospheric pressures of up to 1700 bar. The notable aspect of this motor is its capability to bear vibration up to 25Grms along with impacts up to 1,000G. That means an impact that is effectively 1000 times the acceleration due to gravity at the earth’s surface. The operational efficiency of the motor is exceptionally high. The manufacturer claims an efficiency of 89% in air and 80% in oil.

The EC-4pole 32 HD motor from Maxon is 3-phase AC operated at a nominal voltage of 48V. The nominal speed of the motor is 5710RPM with the starting current at 47.5A, for an efficiency of 89%. It weighs only 860grams and the rotation is clockwise. The motor rotor produces inertia of 128 gcm² with a mechanical time constant of 2ms. The terminal resistance of the motor is 1.01 Ω. The maximum permissible speed is 12000RPM and the motor can take up a maximum axial load of 16N.

Maxon Precision Motors, with a global reputation in the field, expects the EC-4pole 32 HD motor to perform well in the field meeting the rigorous standards set for the deep drilling applications. With a wide range of options available to meet the specific needs, the 32HD motor will be able to function in rough operating conditions, where the depth is more than 2500meters. This motor makes it possible to recover gas and oil at a high level of efficiency. This highly durable motor performs in spite of multiple vibrations and severe impacts it undergoes.

What is a brushless DC motor?

Most electrical appliances have an electric motor that rotates to displace an object from its initial position. Various motors are available in the market such as servomotors, induction motors, stepper motors, DC motors (both brushless and brushed), etc. The choice of a motor depends on the requirements of an application. Most new designs favor brushless DC motors, also referred to as BLDC motors.

The working principle of brushless DC motors is similar to that of brushed DC motors, but their construction is very close to that of AC motors. Like all motors, a brushless DC motor too has a stator and a rotor as its major parts.

The stator of a brushless DC motor, similar to the stator of an induction AC motor, is made up of laminated CRGO steel sheets stacked up to carry the windings. The stator windings follow one of two patterns, star and delta. Motors with stators wound in star pattern produce high torque at low RPM compared to motors whose stators are wound in a delta pattern. For motors required to run at very high speeds, the stator core has no slots, as this lowers the winding inductance.

Lack of slots in the lamination stack means the stator has no teeth, which reduces the cogging torque. Teeth in the stator align with the permanent magnets in the rotor, holding the rotor in a stationary position. When starting to move, additional torque, known as the cogging torque is required to make the rotor break free. However, slotless cores are more expensive as a larger air gap is necessary and that means more winding to compensate.

A typical brushless DC motor has its rotor made out of permanent magnets. The number of poles in the rotor depends on the requirements of the application, as more number of poles gives better torque. However, this reduces the maximum possible speed. Torque produced in a brushless DC motor also depends on the flux density of the material of the permanent magnet; higher flux density material produces higher torque.

Brushless DC motors are popular due to several advantages they offer over other types of motors. Compared to brushed type of motors, a BLDC motor produces higher torque because it has no brushes where power may be lost. Lack of brushes also means higher operating life and lower maintenance. Compared to AC motors, the rotor construction is simpler as it has no windings.

The cost to performance ratio of brushless DC motors is the lowest among all the types of motors available. One reason for this is the stator of a BLDC motor is on its outer periphery, which makes it dissipate a larger amount of heat. Additionally, commutation of brushless DC motors is simpler through electronic switches. That makes it easier to control the speed of BLDC motors.

Whether you are looking at single-speed, adjustable speed, position control or low-noise applications, brushless DC motors are the clear winners over all other types. As they are easier to control, maintaining speed of brushless DC motors is simpler with variations in load. A brushless DC motor generates very low amounts of EMI and audible noise.