Stepper Motors: Performance Curve

Motor speed results from step clock.

Speed, as well as current level, is not effected by load torque.

Hence load can’t be sensed by current measurement.

However, special feature in driver circuitry can be able to detect an end stop hit by measuring and evaluting the current shape.

If the Torque load exceeds the Max.torque Tmax, the stepper motor can’t start or will stop running.

Motor must be designed for a rated torque considering a safety margin to Tmax.
Reason is, that Tmax is effected by manufacturing tolerances (winding resistance, air gap and magnet strength).

Additionally, torque decreases at higher ambient temperature and with warmer winding after longer time of operation.

JE specifies Tmax for normal room ambient temperature and cool motor winding.

Motor torque depends on clock (step frequency). The higher the clock, the lower the torque.

This curve is called “Performance Chart”.

There are two different Charts, showing performance for two different clock commanding methods:

Pull-in range

Driver circuitry switches on/off full clock at once. fA is max. permissible clock.
Full load torque acting already during start.
At load torque T1 max.clock is f1.

Pull-out range (slew range)

The motor starts at no load and is loaded after having accelerated.

Or the Driver circuitry electronics provides an acceleration and deceleration ramp.
Max.permissible clock allowed is fB.
At load torque T1 max.clock allowed is f2.
Driver control circuitry must be more complex, usually a microcontroller is needed.

Current controlled Chopper driver operation allows a higher clock rate.

Lower duty cycle operation allows a higher load torque.

Duty cycle of 40% permits a torque increase by about 40% (assuming that stator iron doesn’t get into saturation).

However, motor to be designed with a special winding (wire diameter increase, smaller number of turns).

The performance is effected by operating conditions:

It decreases with higher temperature and with lower supply voltage.

Holding torque

Stepper motors have a Holding torque:
Torque resistance against any motion attempt from output shaft, at standstill (step frequency zero), windings being switched ON.

Stepper motors also have a Detent torque (Cogging torque):
Torque resistance against any motion attempt from output shaft at standstill (step frequency zero), windings being switched OFF.
It is caused by interaction of the permanent rotor magnetic field with stator claws (reluctance).
Detent torque value is much lower than Holding torque,

Example: Stepper motor UCD1

Holding torque 1.9 cNm
Detent torque 0.26 cNm