Meaning and impact of "Cogging torque" and "Ripple torque"

Topic:

Situation:

• Iron-core motors (e.g. EC-i, EC flat, EC frameless, IDX) are driven with a motor controller (e.g. ESCON) in "Current control" mode with a constant motor current.
• When touching the motor shaft or measuring the output torque, especially at low speeds, repetitive, significant "torque drops" within each motor revolution are noticeable, even though a constant motor current is flowing.
• Even when the motor is de-energized and its shaft is turned manually, it seems to have a kind of preferred rotor positions with some holding torque.

Example:

• Feedback based on a customer's application:
  "Testing an "EC fameless 45 flat" (P/N: 574404) a torque drop or preferred rotor position can be observed approximately every 10 degrees of motor shaft turn."

Questions:

• What is the cause of the noticeable "torque drops" resp. preferred rotor positions in case of iron-core EC motors?
• Does the motor controller influence this behavior too or can it be improved by some motor controller configuration?

Solution: 

The "EC frameless" like any iron-core motor (e.g. EC-flat, EC-i, or IDX motor) has some so-called "detent" or "cogging" torque which is present at certain rotor positions. The "cogging raster" depends on the number of pole pairs and stator teeth.

Cogging torque 

• "Cogging torque" is the torque required to move the motor shaft from one of its preferred cogging rotor position to the next.
• Cogging's periodicity corresponds to the "LCM" (= least common multiple) of the number of rotor poles and number of stator teeth.
• If a motor is powered up (i.e. a current flow present), cogging torque simply adds to the electrodynamic torque.
• The effects of cogging torque can be well "felt" or observed in the following cases:
  • Manual movement of the motor shaft (= rotor) without applying a motor current (i.e. without motor windings connected or with the power output stage disabled). It feels like the motor shaft tries to catch preferred rotor positions and keep these. 
  • "Current control" and demanding a constant motor current. In this case repetitive drops in output shaft torque can be felt and measured when the motor shaft is rotating.
  • Speed control / regulation with very low reference speed (e.g. less than 5 rpm). The speed control reacts to small speed drops at the rotor's cogging positions and tries to compensate them by a short adjustment of the motor current to reach the specified target speed again. The control loop rate as well as the encoder resolution (for speed measurement) are important factors influencing how strongly cogging effects can be perceived by a user. At speeds below 1-2 rpm, a gradual (instead of continuous) movement might be seen.
• Cogging torque cannot be easily compensated by the motor controller because cogging torque will strongly depend on the motor design. Using "FOC / Sinusoidal commutation" (by an EPOS4 plus encoder) does still not result in cogging torque compensation. "Sinusoidal commutation" and "Cogging torque compensation" are different aspects of motor current control.
• maxon does not offer controllers with "cogging torque compensation" as a standard feature. This might be possible by a customized controller firmware targeting a concrete EC motor of a specific system design.

Example: "EC frameless 45 flat" (P/N: 574404)

The "EC frameless 45 flat" (P/N: 574404) has 12 electrical poles and 16 magnetic poles (= 8 pole pairs). The LCM of these two values is 48. This results in a "distance" of 7.5° (= 360°/48) in between consecutive preferred cogging rotor positions. This value fits to the initially mentioned observation of "... torque drops can be observed approx. every 10 degrees or so".

If a constant current is applied to the motor windings all the time (based on current control), the electrodynamic generated torque is also used to overcome the cogging. This finally "feels" like repetitive torque variations at the motor output shaft every 7.5°. 

Additional notes:

Block commutation -> Ripple torque

The ESCON's control is based on so-called "Block commutation" (see by video "Commutation of a multi-pole EC motor"). Block commutation shows a ripple of 14% of the electrodynamic torque under the assumption that current is constant over the whole commutation interval and the commutation takes place exactly at the right time. 

Remarks:

• "Ripple torque" is an additional effect independent of the formerly mentioned "Cogging torque". 
• "Ripple torque" is due to block commutation. It will be present in case of block commutation by any EC motor type (independent if it is iron-core or iron-less).
• There is no "Ripple torque" present in theory in case of a motor with an encoder and a motor controller (like maxon's EPOS4) with so-called "Sinusoidal commutation" or "FOC - Field Oriented Control". In practice some slight "Ripple torque" will be still observed due to Hall sensor and motor winding tolerances. 
  Find more details about "Sinusoidal winding commutation" in these Support Center document:
  • EPOS4: Sensors & start procedure of sinusoidal commutation
  • Strength of BLDC (EC) motor with sinusoidal commutation
• Using a "FOC" or "Sinusoidal commutation" just gets rid of "Ripple torque" but does not(!) compensate the initially explained "Cogging torque" which will be still present.

Control parameter tuning -> Mandatory!

Although "Regulation tuning" will not help to overcome the "Cogging torque", control parameter tuning is required to get best control results in general, i.e. a highly dynamic control's response in order to be able to react on slight speed drops (e.g. due to cogging torque or any external forces applied to the motor shaft). Control tuning can be processed quite easily with the "Regulation tuning" wizard in the ESCON or EPOS Studio.

Important:
Current control" is the lowest level control loop which is active whenever the power stage is "enabled" independent if torque, velocity, or position control is active and what operating mode is configured.  "Regulation Tuning" of "Current control" is mandatory. Do not miss it.

Conclusion:

"Cogging torque" ...

• ... depends on the motor type and its design.
• ... appears as slight short torque drops at certain rotor positions when an iron-core motor is operated with a constant motor current (in current control mode).
• ... will be mainly noticed in the following situations:
  • At very low speed operation (less than 10 rpm) as slight speed oscillations.
  • When the motor shaft is turned manually by hand (without a supply voltage present) as a kind of "locking" at certain rotor positions.
• ... cannot be easily compensated by the controller. 
• ... does not depend on the commutation type. It is a motor specific effect and can be observed in case of block commutation and sinusoidal commutation (resp. FOC - Field Oriented Control") as well. 

"Ripple torque" ...

• ... is due to the commutation type of the motor controller.
  • In case of block commutation there is an add-on "Ripple torque" of 14% present. 
  • In case of sinusoidal commutation there is no "Ripple torque" present is theory. In practice a small torque ripple (of a few percent) will be measurable due to Hall sensor and winding tolerances.