Knowledge Document mmc PWM: size of additional motor chokes Summary: Problem: mmc PWM: How big should additional motor chokes be? Resolution: • Motor chokes protect the motor from overheating due to too large a current ripple. The choke reduces theripple caused by the PWM: the additional heat production is lowered. • In special cases motor chokes may be necessary to guarantee the stability of the current control loop (see„minimum terminal inductance“). • Motor chokes prevent the peak current limitation to be tripped unmeant of the amplifiers DEC 24/1, DEC M24/2, DEC 24/3, DEC 50/5 and DECS 50/5 operated with low-inductance motors at high voltages. • Motor chokes are strongly recommended when operating DC motors with CLL disks with amplifiers withoumotor chokes. Influencing factors are: • The PWM frequency fpwm: The higher the frequency the smaller the current ripple. • The PWM scheme used by the amplifier: 4-Q amplifiers with 2-Level PWM (ADS, DEC70/10 and MIP50) show a higher current ripple than 1-Q amp4-Q amplifiers with 3-Level PWM scheme • The supply voltage VCC: The smaller the supply voltage the smaller the current ripple • The effective inductance Ltot: The higher the inductance Ltot the smaller the current ripple. • The load with respect to the max. continuous (nominal) current Icont of the motor (see catalog): The smaller the load, the higher the current ripple may be without overloading the motor. maxon PWM amplifiers: minimum terminaAmplifier PWM frequency PWM Scheme built-in chokeLint (phase to phase) inductance to fpwm guarantee a stablcurrent control 50 kHz 2-Level (4-Q) 150µH - ADS 50/5 ADS_E 50/5 ADS 50/10 50 kHz 2-Level (4-Q) 75µH - ADS_E 50/10 DEC 24/1 39 kHz 1Q 300µH (=150µH + 150µH) - DEC Module 24/246.8 kHz1Q- - DEC 24/3 39 kHz 1Q - - DEC 50/5 39 kHz 1Q - - DEC Module 50/5 46.8 kHz1Q--50 kHz 2-Level (4-Q) 50µH (=25µH + 25µH) - DEC 70/10 DECS 50/5 50 kHz 1Q - - DES 50/5 DES 70/10 EPOS 24/1 #280937, #302267, #302287 50 kHz 50 kHz 50 kHz 50 kHz 50 kHz 50 kHz 100 kHz50 kHz50 kHz 50 kHz 50 kHz 60 kHz 60 kHz 60 kHz 3-Level (4-Q) 3-Level (4-Q) 3-Level (4-Q) 3-Level (4-Q) 3-Level (4-Q) 3-Level (4-Q) 3-Level (4-Q) 3-Level (4-Q) 3-Level (4-Q) 3-Level (4-Q) 3-Level (4-Q) 3-Level (4-Q) 2-Level (4-Q) 3-Level (4-Q) 320µH (=160µH + 160µH) - 300µH (=150µH + 150µH) 660µH (=330µH + 330µH) 30µH (=15µH + 15µH) 50µH (=25µH + 25µH) 94uH (=47uH + 47uH)20uH (=10uH + 10uH)30µH (=15µH + 15µH) 44µH (=22µH + 22µH) 50uH (=25uH + 25uH)1000µH 320µH (=160µH + 160µH) 20µH (=10µH + 10µH) - > 400µH - - - - --- - -- > 60µH @ 24VDC> 90µH @ 48VDC > 35µH @ 24VDC> 90µH @ 48VDC EPOS 24/1 #317270 EPOS 24/5 EPOS P 24/5 EPOS 70/10 EPOS2 24/2EPOS2 Module 36/2EPOS2 24/5 EPOS2 P 24/5 EPOS2 50/5 EPOS2 70/10 MIP 10 MIP 50 MIP 100 Calculation of the current ripple: PWM scheme: 1Q maximum current ripple Peak to Peak: max, current ripple motor voltage = Vcc/2 occurs at: calculation of Ltot: 2-Level (4-Q) 3-Level (4-Q) motor voltage g = 0V (standstill) motor voltage = ±Vcc Remark: The effective motor inductance for a rectangular PWM excitation is only ab30-80% of the value given in the maxon catalog Lmot (measured at a freqof 1kHz, sinusoidal) • At current ripples the motor can be loaded up to approx. 90% of Icont (catalog value).• At current ripples the use of an external motor choke is recommended according to thefollowing formula. Calculation of additional external inductance: PWM 1Q 2-Level (4-Q) scheme: 3-Level (4-Q) estimation formula: motor choke: no additional motor choke necessary additional motor choke recommended Unicenter Service Desk页码，3/3
Note: All Units used are SI Units.