Topic:

When it comes to motor control and motor operation, the terms "current ripple" or "torque ripple" are sometimes used. 

• What do all these different "... ripple" terms mean?
• How can the various types and causes of ripple be reduced?

Solution:

It is necessary to distinguish between different aspects which can cause ...

• ... "current ripple"
  resulting in motor heating up even if there is no mechanical load mounted.
• ... "torque ripple" and "cogging torque"
  resulting a less uniform rotation of the motor shaft.

I.) Current ripple caused by PWM switching

Almost all modern motor controllers use PWM (= pulse width modulation) in their power stage, which continuously regulates the motor voltage so that the motor follows the commanded motor current, speed or position as precisely as possible. Simply explained, the motor receives alternating, extremely short voltage pulses with differing pulse lengths at a high frequency of 50 ... 100 kHz (i.e. a new pulse every 0.02 ... 0.01 milliseconds). The length (= so-called PWM duty cycle) of each pulse determines the resulting average motor voltage, which must match with the motor's operating point. The PWM duty cycle is adjusted within each current, velocity, and position control cycle. 

The PWM pulses cause some current ripple in the motor due to rising and falling induced voltage. The level of current ripple depends on the supply voltage level of the power stage, PWM frequency, PWM type, PWM duty cycle, and motor's winding inductance.

Countermeasures:

Motor chokes integrated in the power stage or added externally can reduce the PWM motor current ripple and reduce heating of the motor (which also happens without a mechanical load). 

• The maxon "Compact" and controller types in an enclosure from the EPOS4, ESCON2, and ESCON product lines have integrated motor chokes. In most cases, it is not necessary to add motor chokes. 
• For 3rd party controllers or maxon's "Module", "Micro", and "Nano" controller types, it may be necessary to add motor chokes. 

maxon provides a simply "Rule of thumb" formula to determine whether an external motor choke needs to be added and what inductance is required. Adding a motor choke reduces the current ripple caused by PWM. High current ripple also means the motor heats up, depending on the speed and PWM type (2-point or 3-point). This effect is the same regardless of whether block or sinusoidal / FOC commutation (for brushless motors) is in use.

Cross reference:

• PWM, PWM scheme (2 level, 3 level), Current ripple, Motor heating
• PWM power stage: Current ripple & Motor chokes

Important to keep in mind too:

• The formula for calculating the recommended additional inductance is only a rough "Rule of thumb" based on worst case assumptions, i.e. a maximum consideration of current ripple.
• The calculated add-on inductance is no precise inductance value that must be added. Ultimately, it depends what level of motor heating is acceptable. Motor heat is influenced by the PWM type, supply voltage, and typical speed level during operation (to determine whether the motor is operated at a maximum PWM current ripple at a speed in the worst case scenario). 
  • With a 2-point PWM, the highest PWM current ripple (heating of the motor) occurs at zero speed (= standstill control). Users are often surprised that the motor heats even without any load when at standstill (in case of 2-point PWM). 
  • A 3-point PWM is the better choice (and mainly standard for any modern power stage). With a 3-point PWM, there are no PWM pulses and no current ripple at standstill if the motor does not have to driven against an external force.
• The selected motor choke must be able to maintain its specified inductance value at typical PWM frequencies of 50 ... 100 kHz PWM. 
  • A choke's specified inductance value of a choke usually refers to a low frequency of 1 kHz and sinusoidal waveform (and not a 50-100 KHz PWM). 
  • The inductance value of most "standard" chokes can drop significantly at high PWM frequencies (e.g. to 30% of the specified value). 
  • The "Hardware Reference" of maxon controllers lists recommended motor chokes if you want to develop your own based electronic circuit board (= a so-called "Motherboard") for a "Module", "Micro", or "Nano" controller type.

II.) Torque ripple caused by commutation type:

In brushless drives (= EC, BLDC motor), commutation (i.e. switching between the motor windings for continuous rotor movement) is performed via a so-called electronic winding commutation processed by the motor controller. There are two basic commutation types that have different effects on the "torque ripple" within one motor shaft revolution. 

The commutation type in use is the one which causes a torque ripple (or not).

• With so-called block commutation, the simple, sudden fast switching of the windings results in a motor torque ripple of 14% each time the winding changes, i.e. multiple times within one motor shaft revolution. 
• Sinusoidal / FOC theoretically eliminates torque ripple completely or reduces it to a very low level (approx. 1-2%) which is due to mechanical and sensor tolerances.

Cross reference:

• Different methods for winding commutation of BLDC / EC motors
• Strength of BLDC (EC) motor with sinusoidal commutation

III.) Cogging torque in case of iron-core motors:

Applies to many brushless drives from third-party manufacturers, as well as the maxon "EC-i", "EC-flat", "ECX-flat", and "EC frameless" motor series.

Due to their design, iron-core motors have a cogging torque that is easily noticeable (esp. for powerful EC-i and EC-flat motors) when the motor shaft is turned manually. It is as if the motor shaft wants to move to or hold certain positions (similar to a stepper motor). The effects of cogging torque are often greater and more relevant than the "torque ripple" which is caused block commutation.

• The cogging torque does not depend on the use of block or FOC commutation. It is present in almost equal strength for both commutation types.
• To reduce this effect of iron-core motors, special cogging torque compensation by the motor controller is required, which applies an adjusted torque (= motor current) at the corresponding rotor positions.
  • Since the compensation of the cogging torque must strictly match to the motor characteristics for perfect results, and since the required algorithms are quite complex and add some remarkable processor load, none of maxon controllers (nor most of the competitors) offer such a feature as standard. 
  • maxon has developed and tested algorithms for "cogging torque compensation". The implementation of these algorithms (subject to a fee) can be offered as an application and motor specific customized additional feature based on a concrete project and defined motor.

Cross reference:

• The effects described in "II.) Torque ripple" and "III.) Cogging torque" are also explained in the following public Support Center document:
  • Meaning and impact of "Cogging torque" and "Ripple torque"