Unlike their fractional and integral horsepower cousins that can be “Rated” using norms by such organizations as the National Electrical Manufacturers Association (NEMA) or the International Electrotechnical Commission (IEC) there are no such standards for rating small fractional and sub-fractional horsepower dc and stepper motors.
Each Manufacturer must decide their own rating system
Trying to compare published data from multiple manufacturers is akin to having a conversation in the Tower of Babel. Nobody speaks the same language!
To exacerbate this situation is the fact that today engineer routinely download PDF datasheets from the Internet and/or receive them by email. Important supporting information is usually omitted.
As an example, in the case of the Faulhaber Group, the 8 pages of carefully prepared Technical Information in the published catalog are not routinely attached to a PDF datasheet. This results in important details regarding how the technical figures were arrived at not being readily available to an engineer about to make a decision on which motor to choose.
The following example will illustrate that not being conscious of the differences between how manufacturers “rate” their products can cause compromises in the selection of a small dc motor.
Let’s assume that for a new design a 13 mm diameter motor will be the perfect diameter mechanically. Looking at the Faulhaber 1336 series of motors you notice that the 6 Volt winding is “rated” for 1.75 Watts. Comparing this with a similarly constructed motor from Vendor X, you notice that they have a 13 mm diameter motor of similar length but it is rated for 3 Watts.
The decision seems rather easy You take the motor withmore than 70% power. More "head room". Right? In this example...WRONG!
Calculating Stall Torque of a Mechanically Commutated (Brush) DC Motors
What could possibly be subject to interpretation when it comes to Stall Torque? It is one of the two points on the ubiquitous speed/torque curve.
There are at least 4 methods used to calculate Stall Torque in sub-fractional horsepower dc motors. The key word here is ‘calculate’. Unlike stepper motors where holding torque, pull in and pull out torque are measured, in small dc motors stall torque is calculated.
Here are some methods:
||No Load Current
Theoretical Stall Torque Method
Generally Stall Torque is defined as the Starting Current multiplied by the Torque Constant. Simple enough! This is how many vendors specify Stall Torque. This is the simplest method and gives the most favorable results.
TS = (V / RC) x KT
For a few motor vendors, including the members of the FAULHABER GROUP, it is defined as Starting Current minus No Load Current, multiplied by the Torque Constant. This is the technique for Precious Metal Commutated Motors of the Faulhaber Group.
TS = (V / RC – IO) x KT
This is a conservative approach because the friction torque of the dc motor near zero speed will be lower than at a typical high no load speed. This method is particularly brutal for smaller motors, graphite commutated motors, and conventional iron armature motors.
Graph 1 illustrates this rather well.
Friction Torque and Brush Voltage Drop Compensation Method
For Graphite Commutated Motors the Faulhaber Group, as an example, subtracts a 0.5 Volt Brush Drop from the Applied Voltage before the stall current is calculated and then the no load current is subtracted. The Voltage drop across the brushes is real but the actual value of Voltage drop is subject to many variables beyond the scope of this paper.
TS = ((V – 0.5 Volts) / RC – IO) x KT
This conservative method can easily reduce the calculated Stall Torque by more than 10% vs. the more typical method. Additionally, the lower the motor voltage rating, the more penalizing the calculated results.
Extrapolated Stall Torque
This technique is used particularly by Asian Motor Vendors. From the no load speed and rated speed and torque a slope of the speed torque curve is calculated. This is extrapolated to zero speed and the stall torque is then computed. Any rounding of the no load speed, rated speed and rated torque generate wide variations in the extrapolated Stall Torque. In other words, the numbers seldom match.