Motor selection: Getting the balance right

Balancing torque, speed and inertia for optimal

application performance. Howard Horn, product manager at Thomson, Industries, looks into what to consider when specifying motors for motion control applications


hen selecting a motor for motion control, it is important to start by

thinking about the application. Choosing a servo or stepper motor – whether DC or AC synchronous, or some other type of motor – without detailing your application requirements, can be a wasted exercise or could lock you into one type, limiting your options for cost and performance optimisation. Belt-driven conveyors, index tables or rotary

devices, robotic arms, gates and automated guided vehicles, are typical motion applications. So, before you start looking through motor specs, you must determine the load on all axes, the degree of precision required, and the amount of control you need over the process. Every mechanism is a little bit different, and there are many ways to calculate the motor requirements. Hand calculations are good for simple systems and most manufacturers provide motor sizing tools for their various products. Another critical parameter is the amount of

available bus voltage. Voltage correlates with high speed and lower machine cycle time. If your application calls for more speed than your available voltage can support, you will need a different winding and rotor configuration. Another factor that needs to be addressed

early in the selection process is the location of the motors, as installation in dirty or extreme temperatures will negatively impact performance and drive up maintenance costs. Having the right expertise available is important because, if you do need outside assistance, the later you bring it into the

process, the more costly it will be and the more cycles you will have spent up to that point.

SPEED AND TORQUE Once application requirements for load, precision, voltage and other variables have been determined, you can now turn to identifying the motor speed, torque and inertia you will need to meet them. Figure 1 shows the relationship between

torque and speed of a servo motor. The motor must have the right speed and torque to hit the operating point defined by your control requirements. If your application requires movement at speed X with Y torque, the average of the maximum and minimum operating points must fall within the continuous duty curve specified by the motor manufacturer. The speed, torque and voltage relationship

is similar for stepper motors (Figure 2), although you should be more conservative in designating operating points. As a general rule of thumb, the operating range of a stepper should be about half of the max of the speed and torque to ensure that you don’t jeopardise accuracy by missing steps. You can push servos more closely to the limits

of their operating range because they rely on closed feedback loops that monitor the motor position against its target position and make constant course corrections. With most steppers in use today, there is no feedback on missed steps, which presents more of a safety risk. Some of the newer stepper motors and related drives, however, have built-in feedback control or other

means of detecting missed steps (Figure 3).

INERTIA Once you have established the speed and torque characteristics, the sizing process is not complete until you match the inertia of the load and motor. The inertia of the load is its weight in kilograms per meter squared. The inertia of the motor involves both the rotor and the shaft, and motor manufacturers will supply that number. If the ratio of load inertia to the motor is too large, say more than 10 or 20 to 1, the load drives the motor instead of the motor driving the load. Poor inertia ratio is a common problem in

motor selection. While it is generally a good practice to seek the smallest-sized motor possible, focusing only on motor speed and torque without proper attention to inertia can cause major problems. A servo system may not have the response you would expect; it could overshoot its position target and then return with too much force, wobbling as it tries to fix on its final position. On the other hand, picking too large a motor

with too much inertia would give you response but with inefficiency. A motor that is too large also reduces the overall machine efficiency and raises operating costs. The larger the motor, the higher cost for breakers, cables and other support infrastructure, and this can cascade upwards throughout the application design. As an example, consider a multi-axis system

with an XYZ gantry. If you put too large a motor on the first axis, and it is a carry-to- carry axis, you are just adding more mass that must be carried to the other two axes. This means that the whole gantry system must be bigger to support it. Aprecise measurement of inertia is

also necessary to determine whether complementary gearheads, pulley systems or other mechanical interventions are required to optimise motor operations. Gearing is needed for about half of all applications and is especially true for belt-driven systems because belt drives have such a large pitch. Each revolution of the input shaft produces

much greater output than a single rotation of a ball screw. Where the typical pitch on a belt- driven system, for example, might be 150mm, a comparable ball screw pitch might be only in the 5mm to 25mm range. The smaller the distance the shaft moves for each motor turn, the less inertia the motor sees through the mechanism, and the less likely is the need for gearheads to achieve a more favourable ratio, according to the following equation: Inertia ratio with gearhead = Inertia Ratio

without gearhead / gearhead Ratio^2 An accurate calculation of the inertia of each

component is key to selecting the right motor or motor and gearhead combination.

Figure 1: Servo motor torque vs. speed. The average of the maximum and minimum operating points must fall within the continuous duty curve specified by the motor manufacturer


Figure 2: Torque vs speed - stepper motors. The operating range of a stepper should be about half of the max of the speed and torque

ADVANCES IN TECHNOLOGY Where 10 or 15 years ago we might be shooting for a five or 10 to one reflected inertia ratio, with today’s more advanced drives, we can get closer to 25 or 30 to one. This is thanks to the high


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