Tech papers
Motor Type Selection
POSTED 10/13/2011
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 noload 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^{o}C 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 onehalf 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.
DC motors  EC motors  
Operating life 


High speeds of rotation 


Actuation 


Motor connections 


Maximum efficiency 


Loses 


Costs 

Comparative Table 2:
Comparison between the characteristics of precious metal and graphite brushes.
Precious metal brushes with CLL  Graphite brushes  
Use for long operating live 


Applications 


Additional characteristics compared 


Construction 


Brush material, brush resistance 


Contact response 


Lubrication 


Commutator 


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 precisioncontrolled, the additional costs for the electronic commutation has only marginal significance in the selection process. Typical operating conditions are startstop motions in both directions of rotation with cycle times of a few seconds and roundtheclock 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 startstop 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 ontime 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 speedtorque 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 noload speed n_{0} (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 speedtorque line and k_{n} is the speed constant of the motor.
The speedtorque 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, voltagedependent 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 speedtorque line is a parallel shift: upwards at higher voltages and downwards at lower voltages. Slope remains the same. The noload speed and stall torque shift accordingly. The noload speed is easily calculated using the applied motor voltage and the speed constant: n_{0} = k_{n} • U_{Mot}. Note that this simple relationship between motor speed and applied voltage is valid only at noload 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 noload 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 speedtorque 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 speedtorque line with a correspondingly higher value for the slope.
The slope of the speedtorque 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 speedtorque line may be considered a mechanical constant. Motors with the same speedtorque 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/k^{2}, although Dn/DM is more practical for the calculations.
For example, a speedtorque 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 speedtorque line of a motor can be calculated from the noload speed n_{0} and the stall torque M_{H} values listed in the motor data sheets. The following equation applies: Dn/DM = n_{0}/M_{H}. If the motor data sheet is unavailable, the slope of the speedtorque line can be calculated from a measurement of the noload speed n_{0} at the motor voltage U and from the motor resistance R: Dn/DM = p/30 • R • n_{0}^{2}/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 speedtorque line that covers all the working points dictated by the load at a given voltage – the extreme working point. The speedtorque 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 speedtorque 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 speedtorque line slope for the motor type, we can calculate a noload 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 noload 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 noload 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 noload current I_{0}, 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 noload current of the motor): I_{Mot} > M_{maxMot}/k_{M} + I_{0}. 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.
Try maxon precison motors easytouse motor selection program at: http://www.maxonmotorusa.com/downloads.asp