At Rotero, we mainly supply small motors and drives. The information below should also be viewed in this context. The values quoted do not apply to larger drives because they are designed very differently (partly due to the final price of the product).
When old machines need to be modernised, we are regularly asked whether we can offer an alternative to the old DC or AC motors. Naturally, this would have to be a motor without carbon brushes, due to its service life. And, oh yes, it would need to be more accurate and respond more quickly. After all, the machine’s output needs to be increased slightly. A brushless motor like this has a much higher efficiency.
People are usually very surprised when a servo motor or BLDC motor is recommended that is two to three times larger than the current motor. Not to mention the surprise about the cost!
When the above situation occurs, the cause is usually the inertia of the component being driven. A motor without control and feedback is generally very capable of driving large masses without the user noticing much in the way of speed variations or a slightly higher motor temperature. Certainly with DC motors, it is sometimes said that “the motor is much too heavy, because it hardly gets warm”.
Brushless DC motor
A brushless motor is usually equipped with Hall sensors to monitor the position of the rotor and thus the rotational speed. In servo motors, an angle encoder is mounted on the motor shaft to monitor the movement even more accurately. There are also systems that combine these two types. A small piece of electronics is required to run the motor. This is needed to energise the coils in the stator at the right moment so that the rotating permanent magnets in the rotor keep moving. The position of the rotor is therefore continuously monitored during movement. And that is where the difficulties begin…
In principle, it works as follows: as soon as the load changes slightly, the speed of the rotor will also change and thus run ahead or behind the excitation of the coils. If the deviation of the rotor position becomes too great, the motor will no longer be able to run properly. To prevent this, the electronics intervene immediately and attempt to brake or accelerate the rotor. However, the deviation must be corrected before the next magnetic pole passes, which means there is little time for the correction to take place. The coils will therefore need to be energised very strongly. Unfortunately, there is a maximum to the force that can be delivered for this action. Given a certain mass inertia, there is therefore also a maximum to the acceleration or deceleration that the motor can achieve. In other words, for a given motor, there is a maximum to the mass inertia that a brushless motor can “handle”.
Brush DC motor
In this case, a DC motor with brushes will run for some time below or above the desired (or set) speed, and after a while the deviation will be eliminated. This type of motor will always run at the speed that corresponds to the applied voltage. When the load changes, the speed changes slightly and the motor will deliver more or less torque to eventually return to the correct speed. See also the article “How do I read the characteristics of a DC motor?”. In practice, this increase or decrease in torque can take many revolutions of the motor. The result is usually only a slight change in motor temperature.
The aforementioned point about the longer service life of a brushless motor (compared to a motor with brushes) is certainly true. The service life of a brushless motor is definitely longer than that of a motor with brushes. This service life (if used correctly) is entirely dependent on the service life of the ball bearings used. The service life of ball bearings is determined by the deterioration of the grease used. This, in turn, is strongly influenced by the temperature. Generally speaking, the service life of the bearings in a brushless motor will be approximately 30,000 hours. The service life varies with temperature, as shown in the graph below.
As you can see in the graph, a slight temperature increase of 15 degrees is already responsible for halving the lifespan. The reverse is also true. So cooling definitely makes sense!
However, when a gearbox must be used, its service life will often be the determining factor. In general, gearboxes are specified to have a service life of 5,000 or 10,000 hours. If the timing belt on the motor shaft is tensioned a little (too) tightly, only part of this service life will remain.
Assuming a service life of 3,000–5,000 hours for the carbon brushes of a DC motor (which is approximately achieved by high-quality motors), one may question the purpose of a brushless motor. This is particularly relevant considering that many applications take several years to reach 3,000 operating hours.
To illustrate the above argument about dimensioning in relation to mass inertia, consider the following example:
A small conveyor belt is used to transport pots of glue to a workstation where the pots are fitted with lids. Until now, a fairly standard DC brush motor has been used to drive this conveyor belt.
- The diameter of the idler rollers is 50 mm.
- Required speed is 1 m/sec (i.e. rotational speed = approx. 382 rpm)
- Start/stop operation
- The load capacity when the tyre is filled with pots is 10 kg.
After some calculations, we arrive at the following “key figures for application”:
- Mass inertia on the motor shaft (or on the gearbox shaft) = 90 Kgcm2
- Torque required for starting in 0.2 seconds = 2.3 Nm
- Torque required for rotation is 1.4 Nm
For these types of requirements, a 60W – 3,000 rpm DC motor with a gearbox with a ratio of 1:7 or 1:8 would be sufficient (data for Kählig motor M63x60).
When choosing a brushless motor (BLDC), we see in the catalogue of Oriental Motor (BLH series) that such a motor can only handle a mass inertia of approximately 22 Kgcm2. We then have to switch to a 200W motor with gearbox from the BLV series. This combination is approximately 3 times larger than the motor with brushes.
Of course, there are smaller BLDC motors on the market that can power this conveyor belt effectively, but the point remains that it is not simply possible to replace a brush motor with a brushless motor. This often requires all kinds of modifications to the installation.
Text: Gerrit Minnée Illustrations: Oriental Motor
