# RPM & Torque

I’m designing a peristaltic pump driven by a stepper motor, how do I calculate motor torque from “holding torque” and “detent torque” and what’s the sweet-spot RPM operating range of these stepper motors?

what stepper motors ?

holding torque is the load torque that the motor can "resist" when in a non-rotating mode (with relevant pole coils energised)

detent torque is presumably the torque that the motor can apply when transferring between adjacent pole positions. It may be less or greater than holding, depending on winding design.

jackrae,

Any stepper motor, I need to know the best operating RPM range and how to calculate operating torque before I can select a motor that will handle the operating load, correct, or am I missing something here?

I’ve looked at a bunch of stepper motors and have yet to find one that lists and operating rpm and most list two very different torque specifications “holding” and “detent”, like these?

So is operating torque an average of the two, and what’s the operating RPM, I just don’t know?

A simple and quick check on wikipedia defines detent torque as the torque required to move the rotor between adjacent poles with no power required. In a stepper motor with permanent magnets there is a tendency for the rotor to lock onto pole pieces due to magnetic attraction (even with no power applied). The torque required to overcome this lock is the detent torque.

The example stepper you quote has a rated torque of 68ounce-inches (Oz-In). When you apply conversions to transfer from Imperial to Metric this works out as 48Newton-Centimetres (N-Cm), which is the holding torque. On the basis that holding torque is the capability of the motor then this is what the motor can exert to move the load.

As to motor speed, that is determined by the pulse rate you apply to the motor - it is a STEPPER, in other words it steps between poles and the step rate is defined by the step frequency you apply. In the example motor it is specified as requiring 400 steps per revolution so if you apply 5000 steps per second, the motor rotates at 5000/400 = 12.5 revs/sec or 750RPM

However there is a pay-off of torque versus rotational speed. If you go to the shown link you will find a graph of how this varies, and it looks like somewhere around 50RPM is your required “sweet spot”
http://www.velmex.com/pdf/mc/Vexta%20PK245%20tech%20data.pdf

jackrea,

Thanks so much, the graphs in the link are great information. My math suggests that I need a Start-up Torque of 4,111 (gf-cm) and a Running Torque of 2,056 (gf-cm) for this pump application with variable speed, since the stepper motors seem about the same torque at the sweet-spot rpm range of 25-50 rpms, that will help determine tube and platen diameters.

Thanks again

Stepper motors don't have a "speed" they are variable speed motors based on the number of steps called for by their controller. They have more torque at low speed and lower torque at high speeds. Where steppers are used for speed, it is generally because they work reliably in an open-loop configuration as long as the driver can supply sufficient current ti drive the motor. If you want high RPM then you would be better served going with a brushless servo, but then the cost goes up.