Maxon Motor Australia provides some advice on selecting from their range of motors, gear heads and encoders.

As with most motion control applications, the continuous torque and speed will need to be known.

The following information would be appropriate if a customer wanted to find a motor combo for an application that has a 0.9 Nm continuous torque and a speed of 10 rpm.

Since price is always an issue, the smallest gear head that can accommodate the required 0.9 Nm torque should be selected. A safety margin of 50 percent over this continuous torque would suffice.

In this instance, the smallest gear head a customer should choose would be the GP22C. The 2.0 Nm torque is sufficiently high enough above the rated torque that this would be an acceptable choice.

The ratio chosen depends on the maximum input rpm of the gear head chosen. In this case, it would be less than 8000 rpm.

Since the load needs to run at 10 rpm with a 10 rpm margin, the gear ratio would be 8000/20 or 400 or less.

To provide for a healthy speed margin of around double the required speed again, making it around 40 rpm, a ratio of 231:1 is acceptable.

The efficiency of gear heads needs to be determined before selecting a motor, and for a 231:1 gear head, the efficiency is 49 percent.

The efficiency is factored in along with the ratio for determining the output torque.

The amount of torque that is needed out of the motor can be determined by taking the required torque, 0.9Nm, divided by the ratio 231:1 and the efficiency (without the percentage), 0.49. This should result in a value of 7.9 mNm, so motors should be considered that have a continuous torque of 7.9 mNm.

In addition, the motor selected has to have enough speed to run at 10 rpm. 231:1 x 10 rpm yields the answer here; the motor needs to be 2310 rpm.

For the purposes of this example, the motors that can connect to the GP22C and an encoder are the A-max 19, A-max 22, RE-max 21, and RE-max 24.

The RE-max 21 and RE-max 24 are DC brush motors, costing less than their brushless counterparts and with less wiring.

DC motors tend to be less linear in speed and torque performance than brushless, and brushes will need to be replaced.

Brushless motors can come with or without Hall sensors. The purpose of these sensors is to indicate the rotor shaft position to perform commutation. Without these sensors, the amplifier and drive has no way of knowing what stator poles to energize.

In recent advances, amplifiers and drives can run without hall sensors (a.k.a. sensorless) but rather using current sensors integrated into the power phases to detect the rotor flux phase.

Sensorless drives and amplifiers cost less then the hall versions, but the disadvantage is the odd occurrence where the motor may turn a few degrees upon power energizing of the drive to determine shaft position.

To meet the 9.7 mNm torque and 2310 rpm, the smallest appropriate DC brush motor is the RE-max 24.

The motor’s maximum continuous torque is 9.94 mNm which is significantly higher then the required 7.9 mNm and the required 2310 rpm is much less then the 6800 rpm limit.

The torque constant and the speed constant of the motor needs to be determined to deduce the voltage and current necessary to run the motor and eventually determine the size of amplifier drive that is needed.

A torque constant of 21.3 mNm/A. 7.9mNm is required from the motor. The amount of current draw will be 7.9mNm/21.3mNm/A or 371mA.

The maximum continuous current on the RE-max 24 is 498mA which is greater than what is required.

The speed constant is 448rpm/V. 2310 rpm is needed out of the motor.

The amount of voltage required is 2310rpm/448rpm/V or 5.16V which is far less then the 12.0V afforded by the RE-max 24.

The size of power supply needed to run the motor in this example would be 12 Volts DC and at least 0.5 amps. To accommodate any peak torque current the motor may draw, as well as the current required by the drive or amplifier, that figure is doubled to 1 amp.

RE-max 24 motors allow two different types of encoders, with each encoder having an additional option to include the index pulse.

Either an encoder with a line driver output or one with a single line output can be selected.

A line driver output has independent commons for the A, B, and I (if included) signals providing for more wiring but also providing better noise immunity than a single line output encoder with shared commons.

The index is optional and is usually used to perform homing sequences if required.

The recommended amplifier, drive or controller for this motor is dependent on the usage of the motor.

If the motor is to be controlled using an external potentiometer, an LSC 30/2 should be selected, but if single or multiple position moves are to be performed, the EPOS 24/1 or MIP should be selected.