Product category:
Industrial Motors
News Release from: NMB-Minebea | Subject: Brushless DC motors
Edited by the Engineeringtalk Editorial
Team on 10 February 2003
Brushless DC motors gather momentum
Although more expensive, brushless DC motors offer a number of key advantages in both industrial and automotive applications.
DC motors are still the most popular drives in both industrial and automotive applications Continuous improvements in the brush materials have led to significant improvements in the service life of the mechanical commutator
This article was originally published on Engineeringtalk on 4 Mar 2002 at 8.00am (UK)
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The electronically commutated BLDC (or brushless DC) motor is the other alternative.
The additional cost for the electronics of the BLDC is compensated by the almost unlimited service life.
The following article aims to differentiate between the two types and point out the strong points of the BLDC motor.
In a DC motor the winding is on the rotor, whereas in the BLDC motor it is in the stator.
As an advantage the winding on the outside offers the best heat flow over the whole motor surface.
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In contrast, the winding of a DC motor is located internally.
Direct contact to the housing of the DC motor is only via the motor shaft and bearing.
Apart from the inconvenient heat transfer, the heat removal by convection from the rotor of the DC motor is also impeded by the fact that the magnets hamper the heat transfer, because of their significantly lower heat conductivity compared with metal.
The BLDC motor can remove the waste heat significantly better which makes it superior to the DC motor, especially in a design which requires encapsulation to achieve higher IP protection classes.
The BLDC motor offers a further advantage for drives with higher speeds: a balancing of the rotor is only necessary at much higher speeds.
A comparison of the dimensions shows that the brush apparatus of the DC motor requires approximately one third of the length of the motor.
The comparable BLDC motor is approximately shorter by this proportion.
In its dynamic performance the BLDC motor is unbeatable.
There are no differences between DC and BLDC motor regarding the motor characteristic: a linear decreasing torque at increasing speed results in a parabolic power curve with a maximum at half the idle speed.
The efficiency, however, increases with speed up to the maximum and then declines sharply.
This declining speed depends on the losses in the iron of the motor and can be estimated to approximately 75% of the idle speed.
The maximum torque is also available at standstill and is called the "stall torque".
For a given idle speed and operating voltage the magnitude of the torque only depends on the resistance in the winding.
A second prominent point of the motor characteristic is the idle speed which is determined by the voltage constant or also the counter-acting electromotive force (EMK).
A possibility for moving this point is the winding number: higher winding numbers lower the speed, lower numbers increase it.
The torque characteristic will then either be rotated around the point of the stall torque or, respectively, moved in parallel to the original characteristic.
While influencing the characteristic is possible without changing the mechanical construction of the DC or the BLDC, the BLDC motor offers a further simple possibility, namely choice of the pole numbers.
A greater number of poles lowers the speed, a lower pole number increases it.
If ring magnets are used there is no mechanical difference, whatsoever, for different pole numbers.
For the DC motor the two-pole design is predominant and, as an exception, the four-pole design is used.
A greater pole number, however, needs a modified mechanical design of the winding or the brush apparatus.
In this case the BLDC motor clearly offers a greater flexibility.
Any number of pole-pairs between one and eight can be used in a nine-flute motor just by modifying the winding design.
The position of the maximum efficiency shows that these motors should operate in the "upper speed range".
The motor design is, therefore, usually chosen in such a way, that the motors can operate at this position for 100% of the duty cycle.
At lower speeds with higher power output, the duty cycle has to be reduced.
The BLDC motor electronic uses power switches instead of the brushes that are prone to wear.
Three-phase motors commonly used for industrial applications need six of these switches including their control logic.
Control of a BLDC motor needs information on the rotor position.
For simple requirements a sensor with three Hall elements suffices.
For higher requirements encoders or resolvers are used: first to recognise which winding has to be charged, and secondly to be able to carry out a position-related current introduction.
Due to this continuously available feedback the BLDC motor typically operates in a "closed loop".
The speed can be controlled precisely and also the position, depending on the resolution of the position monitor.
For applications, which do not require a highly dynamic startup and only have to cope with small load changes during startup, the three-phase motor can also be used "sensorless".
No electronic components, which could be a temperature-limiting factor, are needed inside the motor.
As the induced voltage of one phase is continuously measured, the motor still runs above a minimum speed in closed loop operation.
However, only one speed and torque control can be carried out "sensorless".
Ventilator and pump drives are ideal industrial applications for such sensorless-operated external-rotor motors.
For a long time this technique has been used for hard disc drives.
If the DC motor is to be speed-controlled in both directions of rotation, electronics are necessary, which operate the motor in an H-bridge.
The four power switches of the H-bridge of the DC motor are now matched with six switches, which are necessary for the operation of a three-phase BLDC motor.
If the DC motor, however, is operated speed-controlled, the installation of sensors for speed monitoring is necessary.
The sensors, which are installed anyway in the BLDC motor for the rotor position monitoring, do not incur additional costs.
The difference in cost for this kind of operation is, thus, further reduced in favour of the BLDC motor.
In each iron-containing permanent-magnet motor a resisting torque occurs, which can be felt when the shaft of the nonenergised motor is rotated.
This, in general, undesired behaviour can be suppressed by slanting the magnetic or electric pole.
The BLDC motor with ring magnet offers an easy way of eliminating this resisting torque by appropriate magnetising.
Between the DC and BLDC motor an essential difference exists in this behaviour.
A "smooth torque" is achieved in the BLDC motor by a combination of mechanical and electronic measures.
The motor is designed in such a way that it generates a sinusoidal induced voltage and the phases are energised sinusoidal and synchronous to the induced voltage.
The matching sinusoidal energising, however, requires a finer resolution of the rotor position.
Instead of the Hall elements encoders or resolvers are used.
This additional expenditure for sinusoidal energising of the phases also has a significant advantage in that it avoids faults due to sudden switching on and off of the phase currents.
If one assumes that DC and BLDC motors of the same power and same diameter can be manufactured for the same price, then the additional costs for the BLDC drive are only due to the electronics.
But the costs for the electronics can, depending on the power, actually exceed the motor costs.
The cost difference, is therefore largest for applications where a DC motor is only connected by a mechanical switch to a voltage supply.
The use of a BLDC motor must be justified by the demand for a long service life.
A typical example would be a powerful battery-driven screwdriver for the professional user.
The cost difference is lower in applications where DC motors are also electronically controlled, and brush motors are increasingly being replaced in such applications by "sensorless" BLDC motors.
Users have recognised that it is worthwhile, with respect to long service life alone, to dig a little deeper into their pockets.
The BLDC motor is predestined for use in explosion-protected applications.
The BLDC motor of pump drives does not have to be sealed to protect the brushes against liquid in a closed circuit.
The rotor and bearing can even run in the liquid medium.
The BLDC motor has a large market potential in automotive technology.
The trend towards electromechanical power steering is unthinkable without the BLDC motor.
The same applies, for instance, to electromechanical brake, electromechanical switchgear, active undercarriage control etc.
Also the planned change to 42V poses no problems whatsoever for BLDC motors.
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