Motor Type Selection

Parameters that define a motor type are the mechanical output power, the shaft bearing system, the commutation system used, and the possible combinations with gearheads and sensors. The most important criteria include the speed and torque requirements, and the commutation system.

Some of the formulas used in this article use subscripts that need to be explained. The variables on the motor shaft (output) are identified with the subscripts Mot. For example, n_Mot stands for the required motor speed. Parameters that describe the characteristics of the motor have no special additional subscript. For example, n_o stands for the no-load speed of the motor.

We’ll discuss speed and torque requirements first. The maximum speed that occurs on the motor shaft n_maxMot should be below the maximum permissible speed of the motor n_max, or n_maxMot < n_max.

As a rule, expected useful life decreases as motor speed increases because of the greater load on the bearings and the higher mechanical wear of the brush system on DC motors. Noise generation increases as well. These effects are particularly pronounced at speeds above the maximum permissible speed. The effective torque M_rmsMot that is required must be less than the rated torque of the motor M_N, or M_rmsMot< M_N.

Rated torque is derived from the rated current I_N, which is selected to equal the maximum continuous current of the motor (from a thermal point of view). Note that the low friction losses of maxon motors are included in the permissible continuous torques of the motors. The friction losses of the graphite brushes and the iron losses of brushless EC motors depend on speed. The result is a visible rounded boundary of their continuous operating range.

The motor must be able to produce the maximum torque for the application. This means that during startup the motor’s stall torque at the rated voltage M_H should not be exceeded. On larger motors with a correspondingly higher stall torque, brief peak torques up to approximately four times the rated torque M_N can be handled without any problems: M_maxMot < 4 • M_N or M_axMot < M_H.

A more detailed analysis would take into account the extent and duration of the overload, as well as the winding and ambient temperatures. In very dynamic applications, the additional torque needed for motor acceleration must be included in the calculation of the operating points: M_Mot,a = J_rot • p/30 • DnMot/Dt_a(duration Dt_a, rotor moment of inertia J_rot). The effective load must then be recalculated.

There are influences on the rated torque. Take, for example, the rated torque in a maxon motor. M_N represents the maximum permissible continuous torque. Under standard conditions at 25^oC the motor reaches the maximum permissible temperature. The rated torque varies as a function of the ambient temperature T_Amb and mounting conditions; a higher ambient temperature results in a less efficient dissipation of heat and consequently a lower continuous torque.

Similarly the details of the thermal coupling have an influence on the thermal resistance R_th2 between housing and ambient and thus on the rated torque. As the heat dissipation improves, like at a lower R_th2, the rated torque of the motor increases. The thermal resistance R_th2 can easily be reduced by one-half by forced cooling or thermal coupling to a heat conducting (metal) heat sink.

The selection of a suitable commutation system is an important issue. The first choice to be made is between mechanical commutation (DC motor with brushes) and electronic commutation (EC motor or brushless DC motor, BLDC). Considerations relating to life expectancy, reliability, simplicity of actuation, and maximum speed play a role in this process. Furthermore, special environmental conditions such as operation in a vacuum or the ability to withstand sterilization must also be taken into consideration. The most important characteristics and differentiating features are presented in the comparative tables 1 and 2.

Comparative Table 1:

Comparison of the basic characteristics of DC and brushless (EC) motors.

Comparative Table 2:

Comparison between the characteristics of precious metal and graphite brushes.

Examples: Selection of motor type (commutation and bearing system)

Linear axis for positioning

Production appliances are generally equipped with brushless motors that guarantee a long operating life with minimum maintenance. Because these drives are precision-controlled, the additional costs for the electronic commutation has only marginal significance in the selection process. Typical operating conditions are start-stop motions in both directions of rotation with cycle times of a few seconds and round-the-clock duty.

Evaluation, selection of commutation, and bearing system:

• Very long life expectancy required in operating hours and number of duty cycles
• Cyclical motion in both directions of rotation
• Operating life can be achieved only with brushless EC motor and preloaded ball bearings – generally in combination with feedback sensor

Billboard drives

Billboards where ads rotate and change at specified intervals attract attention and make it possible to display several ads in a single space. Typical operating requirements are: around the clock start-stop operation with a cycle time of 10 seconds (e.g. 2 seconds on, 8 seconds off). Over an operating life of 10 years, this comes to approximately 30 million duty cycles and a total on-time of 17,500 hours.

Evaluation, selection of commutation, and bearing system:

• Required operating life very long
• Cyclical motion in both directions of rotation
• Life expectancy can probably be achieved only with brushless commutation (EC motors) and preloaded ball bearings
• A combination with a ceramic gearhead must be considered for the low speeds of the load
• Note on the control system: The motor Hall sensors feedback used for commutation might be sufficient for positioning, given a high enough gear reduction ratio

Motor winding selection

Finding the appropriate winding for a selected motor type means understanding the electrical characteristics, which vary within the winding series. The objective of the winding selection process is to achieve an optimum match between the electrical and mechanical power aspects.

The speed-torque line describes the possible operating states (working points) of a motor at a given applied motor voltage U_Mot. On DC and EC motors, this characteristic line is represented in the standard diagram (speed vs. torque) as a linear relationship that runs from the no-load speed n₀ (torque 0) to the stall torque M_H (speed 0). It follows the equation: n = k_n • U_Mot – Dn/DM • M, where D>n/DM is the slope of the speed-torque line and k_n is the speed constant of the motor.

The speed-torque line describes how the motor becomes progressively slower as the load increases, until it ultimately comes to a stop at the stall torque. From a torque point of view the motor exhibits the maximum torque at stall, and the faster it turns the less torque it produces.

In motor catalogs, voltage-dependent values are often given at the rated voltage U_N (understood as a reference voltage), which is not necessarily the voltage the motor must operate at. The influence of the applied motor voltage U_Mot on the speed-torque line is a parallel shift: upwards at higher voltages and downwards at lower voltages. Slope remains the same. The no-load speed and stall torque shift accordingly. The no-load speed is easily calculated using the applied motor voltage and the speed constant: n₀ = k_n • U_Mot. Note that this simple relationship between motor speed and applied voltage is valid only at no-load operation.

The characteristic lines of the various windings of a maxon motor, for instance, are represented at their specific rated voltage. Rated voltages are chosen to give a resulting no-load speed that is similar for all the different windings. If we plot all these characteristic lines at the same motor voltage, we get a diagram that shows a group of parallel straight lines. This result is because all the characteristic lines of the windings of a given motor type have approximately the same value of slope or gradient, even at different speed constants (k_n).

The slope or gradient of the speed torque line is a measure of motor performance. A flat speed-torque line with a low value for the slope corresponds to a powerful motor where speed varies only slightly as the load increases. Less powerful motors have a steeper speed-torque line with a correspondingly higher value for the slope.

The slope of the speed-torque line is a constant determined solely on the basis of design variables: the geometric arrangement and dimensions of the winding and magnetic circuit and the resulting magnetic flux density in the air gap. The number of windings and the resistance of the winding also enter the calculation, however, their influence cancels over a series of windings if the total copper cross section in the air gap is equal. Therefore, the slope of the speed-torque line may be considered a mechanical constant. Motors with the same speed-torque line slope will exhibit the same speed response under load.

On larger motors, we find the constant k as a comparative measurement for motor performance, which is given N_m/W^1/2. A higher k value indicates a more powerful motor. The slope of the characteristic line has the same meaning as k, and they are related by Dn/DM¥ 1/k², although Dn/DM is more practical for the calculations.

For example, a speed-torque line slope of 5 rpm/mNm means that at constant applied voltage an increase in the required torque by 10 mNm results in a speed drop by 10 mNm • 5 rpm/ mNm = 50 rpm.

The slope of the speed-torque line of a motor can be calculated from the no-load speed n₀ and the stall torque M_H values listed in the motor data sheets. The following equation applies: Dn/DM = n₀/M_H. If the motor data sheet is unavailable, the slope of the speed-torque line can be calculated from a measurement of the no-load speed n₀ at the motor voltage U and from the motor resistance R: Dn/DM = p/30 • R • n₀²/U. The electrical resistance R of the motor should thereby be determined by means of a current measurement with the motor shaft locked (starting current I_A): R = U/I_A.

The objective of winding selection is to find a motor with the speed-torque line that covers all the working points dictated by the load at a given voltage – the extreme working point. The speed-torque line must extend above this point at the maximum available voltage. All other working points can then be reached by reducing the motor voltage. This extreme working point falls at the end of the acceleration process, where both the torque and speed are at maximum.

For a methodical determination of the appropriate windings, assume that the optimum winding for an application has a speed-torque line that runs directly through the extreme working point (M_maxMot, n_maxMot) at the maximum possible motor voltage U_Mot. On the assumption of an average speed-torque line slope for the motor type, we can calculate a no-load speed n_0,theor as an auxiliary variable: n_0,theor = n_maxMot + (Dn/DM)_avg • M_maxMot.

Note that a more conservative approach takes the slope of the least powerful winding of the motor type being considered (i.e. the steepest slope). Accordingly, n_0,theor becomes greater. This theoretical no-load speed must be achieved with the maximum voltage U_Mot available to the motor. This is equivalent to saying that the speed constant k_n of the selected motor must be greater than the ratio of the no-load speed to the motor voltage: k_n > n_0,theor/U_Mot.

To this point, no tolerances have been explicitly taken into consideration in the load, drive, motor, controller and power supply. We can now make up for this lack of consideration by selecting a winding with a speed constant that is higher by approximately 20%: k_n > 1.2 • n_0,theor/U_Mot = 1.2/U_Mot (n_maxMot + (Dn/DM)_avg • M_maxMot).

The final step is to verify whether the available current for the selected motor winding can produce the required output torque for both intermittent and continuous operation. On maxon motors, the torque produced is proportional to the motor current which can easily be calculated by means of the torque constant k_M. Losses that occur in the motor must also be taken into consideration. These are expressed by the no-load current I₀, which depends slightly on speed. To estimate whether the current available is sufficient, however, this dependency can be ignored. The resulting selection criterion is therefore that in each time cycle the available current I_Mot must be greater than that current required for the production of the load torque (including the no-load current of the motor): I_Mot > M_maxMot/k_M + I₀. The current I_Mot available at the motor is specified by the characteristics of the power supply and the controller.

Motor type selection, including winding selection, in the end, doesn’t take as much calculating as one might think. Having motor specifications handy and knowing a few key motor characteristics in relation to a specific application often provides all the necessary data needed to come up with a useable solution.

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