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COMPARISON OF MOTORS WITH
AND WITHOUT BRUSHES
Strengths
The limitations of the electronically commutated motor (ECM) have their roots in the life of the bearings and the load-bearing capacity of the amplifier chosen. There is also a thermal limit and a maximum current with a view to demagnetization.
Fig. 5.10 identifies these limitations compared with the limitations of mechanically commutated motors; the hatching shows the limits for ECM motors; the vertical line is determined by the maximum speed of the bearings and/or heating caused by eddy currents; the horizontal line is determined by the maximum current in relation to demagnetization.
The hyperbola in fig. 5.10 is the line of maximum output power for the motor with brushes. Along this line the shaft power is constant, because it is given by Pshaft= T. For the ECM the load point with maximum output power Pshaft is the point at the extreme top right. The conclusion is that in the absence of the commutation limitations ECMs can be loaded more heavily than motors with brushes.
The advent of power electronics and the development of new magnetic materials such as SmCo and NdFeB ran in parallel and both are used in ECMs. The use of these magnetic materials means that ECMs have smaller volumes than mechanically commutated motors, which generally use AlNiCo magnets.
The strengths of these electronically commutated motors compared with mechanically commutated motors are:
- Life greater than 30,000 hours
- May be excited for a prolonged period in the same rotor position (positional servo!)
- Substantially higher predictability of operational reliability
- With an appropriate winding suitable for 300 V, make a mains transformer unnecessary
- No carbon contamination and sparks
- Not sensitive to impurities in the ambient air
- Simple mechanical structure
- Measurable winding temperature so that, with a temperature cut-out, the motors can be loaded to the thermal limit
- The dissipation in the windings can be found in the stator, so the thermal resistance from source to cooling environment is significantly lower. This means that more dissipation in the coils is permissible and the motor has a higher power density.
- The thermal capacity of the stator is high, so power peaks take more time to reach the maximum permissible temperature
- More compact construction due to the absence of brush/commutator combination
Points To Consider
IRON LOSSES
Both versions of the electronically commutated motor have a rotating permanent magnet inside the iron stator, normally a SmCo or NdFeB. These strong magnetic materials are responsible for high magnetic inductions in the stator. As a test the stator temperature of an EC motor was measured at 5,000 rpm. In this case the EC motor was driven by another motor by way of the shaft. The EC motor was disconnected from its power supply. This showed that the motor temperature rose 50 C simply because of any current losses resulting from the rotating rotor field.
Another approach to this phenomenon is the plotting of the current required for a particular torque as a function of the speed. Fig. 5.11 shows an increase of 50%; the iron losses are the cause. In contrast to the situation with mechanically commutated motors, here these losses are a significant factor for the load-bearing capacity of the motor.
The T10, T30, T50, T80 and T100 lines in Fig. 5.12 are given for an ECM as an indication. The message below is that in the case of the ECM the maximum permissible copper dissipation falls with speed, because the increasing iron losses use a substantial part of the permissible dissipation. At the same time the motor torque is used in part to overcome the iron losses, so that there is less available for the shaft.
We can regard any current losses as a viscous friction; a good approach is to say that:
Peddy = dw2, so that the total losses are:
Pdiss + Peddy =
d (Td + w.d)2 + (1 - d)* + w2.d£Qmax
Ploss = Peddy + Pdiss £Qmax. If we employ a duty cycle d, then Td is related to the duty cycle d and the rotational speed w as follows:
Td (w,d)=
The value of the damping d is most easily determined by measuring the current as a function of the speed in the case of an unloaded motor: d = K.I(w)/w.
But these high losses are not fundamental to electronically commutated motors! If we want a brushless motor with a speed greater than around 5,000 rpm, we can achieve this by using lower strength magnets; we can do this by reducing the magnet volume or by using ferroxdure magnets. But we will then have to accept that the steepness of the motor (the factor S) will be lower.
We have already discussed the fact that the current form from PWM amplifiers leads to iron losses and additional ohmic losses. It is certainly the case with ECMs that the motor specification goes with a particular amplifier. Current ripple reduction leads to better performance.
Summarizing, the following are the causes of the motor constant falling with increasing speed:
- The increasing iron losses resulting from the rotor rotating faster, so that there is less torque available at the end of the shaft;
- The lagging of the current in a stator coil in relation to the EMF because of self-inductance. With the EC-DC motor this effect is related to the finite supply voltage; with the AC synchronous motor there is additionally the phase shift with increasing frequency of the current amplifier.
Remark: when a PWM-amplifier is used and a considerable ripple on the current remains present due to the lack of sufficient inductance, one will notice a further performance reduction as discussed in 3.6.3.
Sinusoidal or trapezoidal EMF and the amplifier
Depending on the type of brushless motor, the EMF is sinusoidal or trapezoidal. If these motors are operated with a square-wave or sinusoidal current, there will be a torque ripple of not less than 13%! This can be demonstrated (numerically, for example) by calculating the torque of a DC brushless motor, with the EMFs from Fig. 5.4 that is fed with three sinusoidal currents.
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