Category Archives: Motors

Electric Motor Sans Magnets

Although there are electric motor designs that do not use permanent magnets, they typically work with an AC or alternating current supply. As such, these induction motors, as is their popular name, are not suitable for EVs or electric vehicles running on batteries, and are therefore, DC or direct current systems. Magnets in EV motors are permanent types, typically made of rare-earth elements like ferrite, samarium-cobalt, or neodymium-boron-iron, and are heavy and expensive.

The extra weight of the PM or permanent magnet EV motors tends to reduce the efficiency of the drive system, and it would be advantageous if the weight of the EV motor could be reduced somehow. One of the ways this can be done is to use motors that did not use heavy magnets.

A Stuttgart-based automotive parts manufacturer has done just that. MAHLE has developed a highly efficient magnet-free induction motor that works on DC systems. They claim the new motor is environmentally friendly and cheaper to manufacture as compared to others. Moreover, they claim it is maintenance-free as well.

According to a press statement from MAHLE, the new type of electric motor developed by them does not require any rare earth elements. They claim to have combined the strength of various concepts of electric motors into their new product and to have achieved an above 95% efficiency level.

The new motor generates torque via a system of contactless power transmission. Its fine-tuned design not only makes it highly efficient at high speeds but also wear-free.

When working, a wireless transmitter injects an alternating current into the receiving electrodes of the rotor. This current, in turn, charges wound copper coils, and they produce a rotating electromagnetic field much like that inside a regular three-phase induction motor. The rotating electromagnetic field helps to spin the rotor, thereby generating torque.

The magnetic coils take the place of permanent magnets in regular motors. MAHLE typically leaves an air gap between the rotating parts of the motor to prevent wear and tear. According to the manufacturer, it is possible to use the new concept in many applications, including subcompact and commercial vehicles.

MAHLE claims to have used the latest simulation processes to adjust and combine various parameters from different motor designs to reach an optimal solution for their new product. Not using rare earth element magnets allowed them to make lighter motors and has gained them a tremendous advantage from a geopolitical perspective as well.

Electric vehicles, and therefore PM electric motors, have seen a recent boom. But PM electric motors require rare earth metals, and mining these metals is not environmentally friendly. Moreover, with the major supply of these PM electric motors coming from China, automakers outside of China, are understandably, uncomfortable.

Although MAHLE used the latest simulation processes to design their new motor, the original concept is that of induction motors, invented by Nikola Tesla, in the 19th century. Other automakers have also developed EV motors sans permanent magnets, the MAHLE design has a rather utilitarian approach, making it more sustainable as compared to others

Differences Between Brushed and Brushless Motors

Motors govern our lives in multiple ways. They are the basic machines assisting us from simple transportation to sophisticated movement of a large variety of tools. There are many types of motors, both for operating on alternating current and direct current supplies. Of the motors operating on direct current supplies, there are two major categories—brushed and brushless—with differences in their construction, structure, and operation affecting their performance.

Both brushed and brushless motors operate using the principles of EM or electromagnetic induction, converting electrical energy to mechanical rotary movement. Both types of motors allow electricity to pass through copper windings, thereby creating interacting electromagnetic fields that cause the rotor to rotate and produce mechanical energy. However, their design concepts are different, making them differ in performance, cost, and maintenance.

Of the two, the brushed motor is the older design, having been available for over a century. These have a simplistic structure with two coils, one on the stator and the other on the rotor. A pair of carbon brushes delivers power to the coils on the rotor. Typically, brushed motors have four major parts—stator, rotor, commutator, and brushes.

The stator is the stationary part of the motor. It contains the stator windings or permanent magnets. The rotor, as the name suggests, is the rotating part, attached to the shaft. It has several rotor coils that, when powered, create an electromagnetic field to interact with the EM field of the stator. The commutator is a sectioned metal ring to ensure each rotor winding receives power as it rotates. It helps in reversing the polarity of the current through the rotor windings every half turn of the rotor. Brushes are stationary carbon electrodes that feed power to the rotor windings through the commutator.

As current passes through the stator and rotor windings, depending on their relative positioning, their EM fields either attract or repel each other. This makes the rotor turn, and thereby, changes the commutator connection to the brushes. The current flow now passes through a newer rotor coil and propels the rotor further in the same direction as before. This goes on until the rotational friction balances the EM interaction, at which point the motor’s rotational speed stabilizes.

Once transistors became more common in electronics, brushless motors started gaining popularity. Brushless motors also have four major parts—stator, rotor, sensors, and control circuits. Here too, the stator is the stationary part of the motor and has several copper coils, which, when powered, generate EM fields. The rotor is the moving part attached to the shaft of the motor. But rather than coils, the rotor has permanent magnets that generate their own EM fields. Hall-Effect type sensors sense the position of the coils with respect to the rotor magnets. The control circuit replaces the commutator and brushes to decide which coils in the stator should be powered next.

Brushless motors are more efficient as compared to brushed motors, and they provide higher torque, faster acceleration, lower noise, and lower maintenance. However, brushless motors are more expensive and heavier.

Electronically Commuted Motors — Higher Efficiency

Restaurant owners have long been facing operational challenges. These include high energy costs, limited kitchen space, and equipment downtime. For addressing these challenges and improving restaurant productivity, the owners have turned to commercial kitchen equipment. Most of such kitchen equipment has an electric motor at heart, whose performance dramatically impacts how the equipment operates and how it mitigates the above challenges.

It is imperative that owners increase their productivity while reducing their costs, considering their profit margin usually falls between three and five percent. This requires a clear understanding of the connection between the motor and the equipment. Doing so not only reduces the operating costs but also ensures a smoother running operation.

Energy costs happen to be a major concern in the restaurant industry. Commercial kitchen equipment is uncommonly hard on the electricity bill, being typically robust and energy-intensive. According to the US Energy Information Administration, consumption in restaurants is typically three times more per square foot than any other comparative commercial enterprise. This is because restaurants use specialized equipment that has a high power demand, and they operate for extensive hours, thereby consuming huge amounts of energy.

Therefore, purchasing and using high-efficiency, higher energy star-rated restaurant equipment is one of the easiest ways to improve the bottom line. However, as a motor is at the heart of each piece of equipment, it offers a greater choice. In fact, restaurant operators can improve on this further by taking a proactive approach and selecting equipment that has an electronically commuted motor or ECM. They can even consider retrofitting existing equipment with ECMs for a more favorable option.

The reason for the above decision is that an ECM operates more efficiently as compared to what a traditional induction motor does when running restaurant equipment such as ovens, walk-in coolers, mixers, and fryers. Depending on the use cycle, equipment with ECM technology can save more than 30% in annual energy costs. This improves the bottom-line savings and improves the profitability of a restaurant.

A microprocessor and electronic control help to run an ECM. Compared to regular induction motors, this arrangement offers higher electrical efficiency. It also offers the possibility of programming the precise speed of the motor. Moreover, ECMs can maintain high efficiency across a wide range of operational speeds.

Apart from the higher efficiency, ECMs are precise and offer variable speeds, which in fans means an unlimited selection of airflow. A properly maintained airflow during changes in the static air pressure brings important benefits to the restaurant, especially for its hood exhausts and walk-in coolers. The higher efficiency of ECMs leads to reduced heat in the refrigerated space, thereby reducing the equipment runtime.

Forward-thinking original equipment manufacturers are re-engineering their designs and products to include ECMs for delivering smaller and more versatile equipment. Compact motors such as ECMs, are gaining wider recognition and appreciation as they improve the power density of their equipment. Compared to equipment with traditional induction motors, those using ECMs offer the same output, but with a much smaller footprint and lower weight.

Three-Phase Monitor Relays Protect Expensive Machinery

Three-phase motors power many industrial and commercial machines. One can find these machines in material handling, water treatment, air conditioning systems, ventilation, heating, marine, machine tools, and aviation applications. However, a range of fault conditions can damage these reliable devices when not addressed quickly. This can lead to a shortened operating lifetime or even a failure, resulting in significant repair costs and downtime.

Phase monitoring relays can detect these faults, notify the operators, and stop the machinery before it develops permanent damage. These relays detect the presence of all three phases, their correct sequence, and that all phase voltages are within the specified range. Should an error develop, the relay opens a set of contacts, initiating an alarm condition, and powers down the machine. There are many types of phase-sensing relays. They can handle a wide range of phase configurations, voltages, and errors.

Among the common failure modes of three-phase motors, are those related to their three-phase power source and their effects on the motor. An imbalance in the phase voltages, or a loss in one of the three phases, can result in the remaining phases driving higher-than-normal currents into the motor. This can lead to a loss of rotational power and excessive vibrations. Likewise, over-voltages and under-voltages can force the motor to draw excess current for driving the same load, and this can shorten the life of the motor. An incorrect phase sequence may cause the motor to reverse the direction of rotation. This can have significantly disastrous results on the load connected to the motor.

Phase monitoring relays monitor the state of the three-phase power source. The three-phase lines that they monitor also power them. Apart from the phase sequence, they also monitor the loss of any phase voltage. Only when all the phases are present, and are in the correct sequence, do the relays activate. Whenever there is a loss of any phase, or the phase sequence is incorrect, the relays de-energize.

Some phase monitoring relays also have the capability to monitor the voltage levels of all three phases. This typically uses a true root-mean-square measurement. The relay deactivates whenever the voltage drops below a preset threshold. Some relays also offer adjustable limit settings along with voltage detection. Other relays monitor phase asymmetry along with tolerance. Typically, phase monitoring relays offer a delay before actuation. This prevents spurious activation from temporary voltage levels or asymmetry issues. In some models, the delay is adjustable.

The DPA01CM44 is an example of a three-phase monitoring relay meant for three-wire configurations. The three-phase source powers the relay. Relay models available operate at voltages of 208, 230, 400, 600, and 690 VAC. Although relays for mounting on DIN rails are typical, plugin models are also available. The relay output configuration can be single or dual SPDT contacts.

Normal voltage and phase conditions allow the relay to remain activated. That means, the normally open contacts of the relay output remain closed. Abnormal conditions make the relay operate within 100 milliseconds. The front panel on the relay has status LEDs to indicate relay activation and power on.

What are Axial Flux Motors?

AC induction motors are no doubt the most popular and widely used electric motors today. For DC applications, there are permanent magnet motors. However, newer applications are demanding different types of motors with higher efficiency and better speed-torque characteristics. One such application is the electric vehicle sector, where axial flux motors are gaining traction.

Axial flux motors are not new. For the past few decades, manufacturers have been using these motors for stationary applications like agricultural machinery and elevators. With modifications and innovations over the past decades, axial flux motors are now capable of running airport pods, electric motorcycles, delivery trucks, aircraft, and electric cars.

Induction motors and permanent magnet motors are most often known as radial flux motors, as the flux they generate radiates out perpendicularly, relative to their axle. With extensive development, engineers are aiming to optimize the weight and cost of radial flux motors, but the going has been asymptotic. Therefore, moving to a completely different type of machine like an axial flux makes better sense.

With the axial flux design, a permanent magnet motor can provide higher torque for a given volume than a similar motor of radial flux design can. This is because the axial flux design works with a much larger active magnetic surface area to generate torque rather than the motor’s outside diameter.

Therefore, the axial flux motor can be much more compact, with an axial length far shorter than that of their radial counterparts. Because of their shorter axial length, axial flux motors are more suitable for applications that use a motor inside the wheel. Although these motors are slim and lightweight, they can provide the machine where they are mounted with higher power and torque density than a comparable radial motor can, without resorting to high-speed rotation.

The shorter, single-dimensional flux path also provides the axial flux motors with high efficiency, typically over 96%. This is a tall order for the best 2D radial flux motors available on the market.

Compared to radial flux motors, axial flux motors can be five to eight times shorter, and two to times lighter. Both these factors improve the options for designers of EV platforms.

Axial flux motors are available in two principal technologies—dual-rotor, single stator, and single rotor, dual stator.

In a permanent magnet motor using radial flux technology, the magnetic flux loop starts from a permanent magnet on the rotor. It then passes through the first tooth of the stator, continues to flow radially along the stator, and passes through a second tooth, arriving at the second magnet in the rotor.

In an axial flux motor, using the dual rotor technology, the flux loop begins at the first magnet. It then passes axially through the stator tooth arriving immediately at the second magnet. Therefore, the flux has to travel a much shorter distance compared to that in the radial flux motor. This allows the axial flux motor to be much smaller for the same power, increasing its power density and efficiency. In contrast, the flux has to follow a 2-dimensional path inside a radial flux motor.

What are Motor Starters?

Starting up small motors is usually through a manual starter that can make or break the power supply line to the motor. The method is also known as DOL or direct online start. If the motor gets too hot due to an overload, a thermal protection circuit in the starter opens and disconnects the motor. DOL starters are the most common method of starting and stopping single-phase motors up to 5 HP, 230 VAC, and three-phase motors up to 15 HP, 600 VAC.

Magnetic starters can have controls such as float switches, pressure switches, timers, relays, limit switches, and push buttons, as they have a separate mechanism for closing and opening a set of contacts for the motor circuit. They also include a thermal overload protection device. The mechanism consists of a coil, which, when energized, closes contacts to complete the electrical circuit of the motor. Likewise, de-energizing the coil opens the contacts, switching off the motor.

However, one of the problems with DOL or magnetic starters is both allow the motor to start with a high current. Under normal conditions, motors must start with a current that is nearly 6 to 7 times the rated running current of the motor. This is necessary for the motor to overcome the initial torque due to friction. However, for some motors, the starting current can go up to 9-10 times the rated current.

Reversing any two phases of a three-phase induction motor results in the motor reversing its direction of rotation. Adding an extra set of contacts to a basic starter can turn it into a reversing starter. Appropriate electrical and mechanical interlocking mechanisms must also be present for safeguarding the motor operations.

A soft starter applies a low voltage to the motor, ensuring a low starting current and torque. The torque gradually increases as the soft starter begins to apply higher voltage.  Semiconductor switches such as thyristors, inside the starter, accomplish the gradual increase in the voltage that the starter applies to the motor.

A slow start is essential to prevent stress on the internal components of the motor, and to the machinery, the motor is driving, especially belts and gear drives. The soft starter also features soft stopping. This is essentially helpful for stopping conveyor belts and pumps, where a sudden stop may cause water hammering in the pipe system.

Multispeed induction motors have multiple windings that require special starters. For instance, two-speed motors with separate windings need starters with two built-in standard starters within a single enclosure with mechanical and electrical interlocks.

Consequent-pole two-speed motors need a three-pole starter unit or a five-pole starter unit. The design of the motor winding determines whether the three- or five-pole unit makes a slow-speed or fast connection.

Delta-type multi-speed motors require different power circuits for the currents circulating within the unconnected and inactive windings. Two-speed motors with separate open-delta windings require a pair of four-pole starter. For each speed, a different four-pole starter is necessary. Therefore, very complex starters are necessary for motors with open-delta windings capable of running at three or four speeds.

Stepper Servo Motors

Although many designers prefer to relegate stepper motors to the realm of low-cost low-performance technology, a new technique is bringing the step motors a fresh lease of life. This new drive technique is the stepper servo, and it uses the generic stepper motor, yet extracts significantly more performance out of it. The technique requires adding an encoder and operating the motor effectively as a commuted two-phase brushless DC motor.

While the inclusion of an encoder makes the stepper servo idea non-suitable for low-cost applications, designers are increasingly considering the technique an alternate approach to applications requiring a brushless DC motor.

This is because the cost of a stepper servo motor is considerably less than a comparable brushless DC motor, while the former actually outperforms brushless DC motors in areas of torque output and acceleration. Therefore, designers are considering the stepper servo motor as a candidate for high-speed applications such as coil winding, point-to-point moves, textile equipment, high-speed electronic cams, and more.

Stepper motors are easy to use, making them popular. They maintain their position without external aids such as encoders. Neither do they require a servo control loop when designers use them for positioning, as other DC motors do. Their brushless operation, high torque output, and low cost are their biggest advantages. However, their limited speed range, noisy operation, and vibrations are their main disadvantages.

Being a multi-phase device, stepper motors require the excitation of multiple coils and driving control waveforms for their operation. The usual configuration for stepper motors will have 1.8 mechanical degrees for a full step of 90 electrical degrees—making it 200 full steps for every mechanical rotation. Other stepper motors may have 7.2- or 0.9-degree configurations in place of the customary 1.8.

A stepper servo motor has an encoder attached to the shaft. For a typical 1.8-degree stepper motor, the resolution of the encoder must be of the order of 2000 counts per mechanical rotation. The encoder verifies the final position of the rotor through a traditional step motor control scheme.

The stepper servo motor operates more like a brushless DC motor, with the actual encoder position commuting the phase angle, instead of the commanded position. The phase angle and amplitude of the driving waveform need to vary continuously depending on the output from a position PID loop. This allows the motor to servo to the commanded position.

The presence of the encoder frees the stepper servo from losing steps—the encoder determines the location. The motor operation is now more efficient, causing much lower heat generation. Traditional stepper motors require driving at large currents adequate for handling worst-case motions.

Traditional stepper motors always have problems achieving positional accuracy. With the encoder driving the stepper servo motor to its location, these vagaries of position do not arise. The encoder frees the stepper servo motor from the restrictions of the 1.8 degrees per step of the regular stepper motor. Simply increase the resolution of the encoder to get better positional accuracy.

The addition of the encoder also produces a smooth acceleration to the desired position without the customary bouncing and noise.

Intelligent Phase Control for BLDC Motors

Many applications use BLDC or Brushless DC motors for powering several types of high-speed equipment. These include industrial machines, data center cooling fans for servers and home vacuum cleaners. One of the challenges designers face is to ensure the motors operate effectively and reliably. Now, Toshiba is making it easy for designers to do this with its intelligent phase control motor controller.

While other manufacturers also offer intelligent phase control devices, they usually meet a specific design need. Toshiba’s TC78B016FTG has a driver rated for 40 VDC and 3 A maximum. The fully integrated motor control driver requires a power supply ranging from 6 to 36 VDC, and provides a sine wave output drive. ON resistance of the driver is only 0.24 ohms, representing the total of low and high sides. This typically reduces the self-heating of the device during operation and allows driving 1 to 1.5 A loads without a heat sink.

TC78B016FTG uses a simple speed control mechanism using pulse width modulation. It has several built-in protections, and these include protection from over-current, thermal runaway, and motor lock. Toshiba offers the TC78B016FTG in a 5 x 5 mm VQFN32 package.

Other controllers from Toshiba include the TC78B941FNG and TC78B042FTG. These intelligent phase controllers allow users to tailor the power requirement of an application by selecting a proper MOSFET and its gate driver for the design. Toshiba offers these devices in SSOP30 and VQFN32 packages respectively. Both measure 5 x 5 mm.

Another controller from Toshiba is the TC78B027FTG, which incorporates a gate driver, for which the user can select the proper MOSFETs according to the application. This controller also has a one-Hall drive system for the user to drive a less expensive one-sensor BLDC motor. Toshiba offers the device in a VQFN24 device measuring 4 x 4 mm.

Conventional drive technology adjusts the phase or lead angle of the voltage and current it feeds to the motor for achieving high-level efficiency. However, high-speed rotation prevents the magnetic drive from reaching maximum power, as phase lag delays the voltage applied to the coil from rising until the current has increased to a maximum.

Intelligent phase controllers avoid the above situation by advancing the rotor by a certain angle from the calculated position. This is the new lead angle that depends on the BLDC motor’s characteristics, its rotational speed, and load conditions.

Designers try to achieve optimal efficiency over rotational speeds ranging from almost zero rpm at motor startup to several thousand rpm at high speeds. As this requires several characterizations for adjusting the phase, they achieve optimal efficiency only for a limited range of speeds. Intelligent phase controllers allow BLDC motors to rotate at high speeds with uniform accuracy and efficiency.

Compared to earlier technologies, the approach taken by Toshiba is different. Rather than adjust the phase difference between the voltage and current to the motor at different points in its operating range, Toshiba automatically and continually adjusts the phases of voltage and current the controller feeds to the motor. Intelligent phase controllers from Toshiba thereby achieve the highest possible efficiency for the entire operating range of the motor.

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.

Difference between AC, DC, and EC Motors

People have been using different types of motors for ages. Primarily, motors can be broadly classified into AC and DC types, depending on the power source they require to operate. However, the basics of operation remain the same for all types of motors. Current running through a wire generates magnetic fields around it, and if there is another magnetic field present such as from an external magnet, the two interact to generate a mechanical force on the wire capable of moving the wire. This is the basic principle on which all motors operate.

AC and DC Motors

AC induction motors have a number of coils controlled and powered by the AC input voltage. This input voltage also creates the stator field, which then induces the rotor field. Another type of AC motor, a synchronous motor, can operate with precision supply frequency.

An AC motor operates at a specific point on its performance curve, which coincides with the peak efficiency of the motor. If forced to operate beyond this point, the motor runs with a significant reduction in efficiency. As the magnetic field in an AC motor is created by inducing a current in the rotor, AC motors consume extra energy from the input. This makes the AC motors less efficient than DC motors are.

DC motors generate their secondary magnetic field using permanent magnets rather than windings. They rely on commutation rings and carbon brushes to switch the direction of the current and the polarity of the magnetic field in the rotating armature. The interaction between the magnetic field from the fixed permanent magnets and the magnetic field from the internal rotor induces rotation in the rotor.

Although DC motors run at high efficiency, they suffer from specific losses. The initial resistance in the rotor, brush friction, eddy current losses cause the motor to lose efficiency.

EC Motors

To achieve higher energy efficiency and control the energy output, engineers have designed the EC or electronically commuted motors. They combine the best of both AC and DC motors by removing the brush and slip ring system of commutation, and replacing them with solid-state devices. This electronic control allows them to operate with a higher efficiency.

EC motors are also called brushless DC motors, and they are controlled by external electronics, which may be an electronic circuit board or a variable frequency drive. Permanent magnets are on the rotor, while the fixed windings are on the stator.

The circuit board keeps the motor running by switching the phases in the fixed windings as necessary. This supplies the armature with the right amount of current at the right time, resulting in the motor achieving higher accuracy and efficiency.

EC motors offer several benefits. Absence of brushes eliminates sparking and increases the life of the motor. As electronics controls the power to the motor, there is less wastage with better performance and controllability. This allows even small EC motors to equal the performance of larger AC or DC motors. Heat generation in EC motors is also lower than that generated in AC or DC motors.