Category Archives: Motors

Different Types of E-Bike Motors

The major difference between electric bikes is the various types of drive systems they use. These include shaft drives, mid-drives, geared and gearless hubs. In addition, there are differences between the motors, chiefly brushed and brushless. Therefore, if you are looking for an e-bike for a specific use, this article will help you to understand and focus on finding the right one.

The shaft drive

This system works more like the arrangement in an automobile, with the motor positioned more towards the center of the bike and driving the rear wheel with a shaft. These are not popular nowadays, because of the customized frames required to support the motor and shaft. The entire arrangement is awkward and difficult to service.

A mid-drive motor

Mid-drive motor systems are used in e-bikes meant for climbing. You will find this design close to the bottom bracket, at the point near the pedals. The system drives the chain forward rather than the wheel, benefitting from mechanical drivetrain systems such as use of gears for going fast or for climbing. Therefore, when approaching a hill, the rider can shift to a lower gear, making it easier to pedal and climb.

The geared hub motor

There are two types of hub motors – geared and gearless. The geared hub motor provides mechanical advantage with smaller and lightweight motors. However, they also produce more friction and hence more noise and wear out faster. A built-in flywheel mechanism unlatches the shaft from the axle while the rider is coasting, preventing addition of any resistance.

The gearless hub motor

The simplicity of the gearless hub motor delivers smooth and quiet performance, much eulogized by shops selling e-bikes. These motors rely greatly on electromagnets and most do not even include a freewheel mechanism. That may be due to the extremely low magnetic resistance to be overcome when the electromagnets are powered off. Usually, such motors are also called direct drive systems, enabling regeneration of electricity from repelling magnets within the motor.

Gearless hub motors are generally larger than other types, because they need to accommodate magnets, ultimately making them weigh more. However, improvements in technology are helping to produce small and lightweight direct drive hub motors nowadays.

Hub motors usually operate even when the rider is not pedaling. Whether geared or gearless, the system can fit in the rear or the front wheel. However, with increased unsprung weight, hub motors can experience reduced traction, limited efficiency and strain the spokes and rims of the wheel.

The drive system you select will affect the overall weight and weight-distribution of your electric bike. The cost will depend on whether you need a customized frame, regeneration and special sensors for shifting gears. Motorized e-bikes provide improved efficiency, help in riding fast, in climbing and in navigating bumps. For lightweight around-the-town transportation, geared hub motors are fine. If you like quieter rides with more power and regenerative braking, go for direct drive hub motors. However, if you ride your bike more in the mountains and do lots of hill climbing, you definitely need mid drive motors.

Increasing the Accuracy of Peristaltic Pumps

There are vast applications of peristaltic pumps because of their simple construction and ease of use. The construction of peristaltic pumps does not allow the liquid being pumped to be exposed to the pump’s mechanism. That helps hospitals using these pumps to circulate blood during bypass surgery as a critical part. Used in heart-lung machines, the design of these pumps prevents significant hemolysis – the rupture of destruction of red blood cells. The chief advantage of the design is the compressible polymer tube through which the dispensing liquid passes.

For fluids that must be isolated from the environment, this simple arrangement works very well. For example, it allows pumping slurries with a high solid content and other aggressive chemicals. However, rollers inside the peristaltic pump produce pulsations as they move an on and off a pressure shoe that compresses the tube. These pulsations prevent accurate dispensing.

Drug development and delivery depends largely on accurate dispensing. For example, accurate dispensing and aspirations are extremely important for addressing safety concerns related to tremendously expensive high-potency compounds such as biotech designer molecules used by leading-edge pharmaceuticals. The proteins and synthetic molecular chains composing these compounds are very fragile and highly susceptible to tear. That calls for short setup times and the dispensing tube meeting or exceeding the safety and contamination concerns. The peristaltic pump finds wide applications because it is able to address the above requirements.

One simple method of reducing the pulsations from the peristaltic pump is to increase the number of rollers. However, that is not a very practical idea. A pump with three rollers can greatly reduce its fluid-dispensing variance provided it has one roller in the same starting position when starting each dispense – the pump repeats its starting position every 180 degrees.

An integrated motor solution with signal inputs and outputs for roller positioning makes this a possibility. The design allows the pump to dispense volumes made from multiple revolutions plus some fraction of a revolution. With roller positioning, it is possible to take into consideration the fraction of the revolution and ignore the complete revolutions. External valves help with the dispensing of fluid from the peristaltic pump to control the starting position of the next roller without dispensing.

In practice, the motor allows the pump to dispense and then operates the valve, allowing the rollers to be positioned to the same starting point for the subsequent dispense cycle. The process ensures a precise and repeatable quantity of dispensing. Usually, drip retention is also used to bring back the fluid into the tube. This is to prevent a drip of the fluid when closing the dispense valve. Usually, that causes a small amount of fluid to be wasted. This can be prevented by repositioning the next roller in a positive or a negative direction to minimize the fluid waste.

Another method is to use multiple tube peristaltic system. The rollers in this system are intentionally offset and the output of the tubes combined. This effectively increases the total number of rollers, minimizing pulsations. Here, one of the legs becomes the waste tube and the dispense valve is positioned after the combined outlet.

Selecting the Proper Brushless DC Motor

You may have an application that requires high-speed, but quiet operation with low EMI generation and long operating life. For such applications, BLDC or brushless DC motors are what you must be looking at. Among many advantages of these motors, high-speed operation is a special one. As there are no brushes or commutator in the motor, the bearing friction is the only factor limiting their rotational speed.

Absence of brushes and commutator also means there is no arcing within the BLDC motor to cause erosion or EMI. The last factor makes these motors suitable for use in RF applications. With windings on the stator, BLDC motors show superior thermal characteristics over conventional motors and are consequently more efficient. Because the stator is connected to the case, heat dissipation is fast. All such factors means a BLDC motor has virtually non-existent maintenance problems.

The major downside to all the above good characteristics of BLDC motors is their higher cost. BLDC motors can easily cost about twice as much as simple brushed motor and this puts the BLDC technology out of reach for many applications. Apart from the cost of the basic motor, there is the added cost of the control or drive electronics. If not integrated within the motor itself, you will need to find space for mounting the electronics outside, but nearby. You cannot separate the drive and the motor with long cables, as the noise introduced will cause malfunctioning.

A brushless motor also must overcome starting friction, just as brushed motors do. Again, starting friction does not depend on speed, but is the sum total of torque losses. Dynamic friction, proportional to speed, defines the torque losses in BLDC motors. Viscous friction in the ball bearings cause dynamic friction and eddy currents in the stator, originated by the rotating magnetic field of the magnet, adds to it. Nevertheless, the speed-torque curve of a BLDC motor demonstrates excellent linearity.

Directly connecting to a DC supply will not operate a BLDC motor, unlike a brushed DC motor. This is because brushless technology makes use of electronic commutation. Although there is no physical commutator or brushes, the operating principal remains the same, with the permanent magnet rotor initiating motion by chasing a revolving magnetic field induced by a current in the stator windings. A PWM or pulse width modulated signal is necessary to create the on/off signal, which actually creates the motion.

A comparator normally generated the PWM signal, which is a voltage generated because of a sinusoidal command signal superimposed with a saw toothed carrier or chopper frequency. If the command is greater than the carrier frequency, the PWM signal will be high. This is because the low chopping frequency gives the current more time to gain amplitude. The current density governs the rate at which the motor accelerates or decelerates.

To avoid ripples and a shortened motor life, it is important that the switching frequency is high enough. This is usually done by controlling the discrete on/off steps with six semiconductor switches. These send the amplified current through the correct phases, with the necessary switching being done by the semiconductor switches.

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.