ulhasfegade Posted December 23, 2014 Share Posted December 23, 2014 We have a brush less servo motor which is rated for 400 rpm which we are running on Geobrick controller. We want to increase its rpm. The limitation is the back EMF generated by the motor. Can we use field weakening above 400 rpm to reduce the back EMF and increase the speed of the motor? I could not see any section in the manual which deals with field weakening of brush less servo motors. Link to comment Share on other sites More sharing options...
ulhasfegade Posted December 23, 2014 Author Share Posted December 23, 2014 We have a brush less servo motor which is rated for 400 rpm which we are running on Geobrick controller. We want to increase its rpm. The limitation is the back EMF generated by the motor. Can we use field weakening above 400 rpm to reduce the back EMF and increase the speed of the motor? I could not see any section in the manual which deals with field weakening of brush less servo motors. Link to comment Share on other sites More sharing options...
curtwilson Posted December 23, 2014 Share Posted December 23, 2014 You have a couple of options here. The easiest is to use the Ixx56 term, "commutation delay compensation". Most people just use this at very high commutation frequencies to compensate for the delay between input and output of the commutation algorithm. However, if you use values higher than what is necessary just to compensate for these delays, you create a "phase advance" that has the effect of field weakening at higher speeds. The second strategy is to use negative values for Ixx77 magnetization current at high speeds. You would have to implement this functionality in a PLC program, setting Ixx77 as a function of motor speed, making the values increasingly negative at higher speeds above "base speed" (the speed at which the back EMF is close enough to the supply voltage that you can just get the desired torque). If you do aggressive field weakening, you must remember that if your algorithm fails for any reason, including a fault, the amplifier will be exposed to the full (unweakened) back EMF of the motor at that speed. People have destroyed their amplifiers this way. Link to comment Share on other sites More sharing options...
curtwilson Posted December 23, 2014 Share Posted December 23, 2014 You have a couple of options here. The easiest is to use the Ixx56 term, "commutation delay compensation". Most people just use this at very high commutation frequencies to compensate for the delay between input and output of the commutation algorithm. However, if you use values higher than what is necessary just to compensate for these delays, you create a "phase advance" that has the effect of field weakening at higher speeds. The second strategy is to use negative values for Ixx77 magnetization current at high speeds. You would have to implement this functionality in a PLC program, setting Ixx77 as a function of motor speed, making the values increasingly negative at higher speeds above "base speed" (the speed at which the back EMF is close enough to the supply voltage that you can just get the desired torque). If you do aggressive field weakening, you must remember that if your algorithm fails for any reason, including a fault, the amplifier will be exposed to the full (unweakened) back EMF of the motor at that speed. People have destroyed their amplifiers this way. Link to comment Share on other sites More sharing options...
ulhasfegade Posted December 24, 2014 Author Share Posted December 24, 2014 Is there any way to calculate maximum field weakening allowed so that amplifier current always within the specified limit. e.g. my motor parameter are Back emf= 415.15 V/1000 rpm which gives 145 V for 350 rpm for Geobrick. Phase winding resistance is 4.15 ohm. Hence if we ensure minimum back EMF of 93.75 V will provide the safe limit for the amplifier (240*0.65-102)/4.15< 15 amps. With this limit, amplifier will be safe in all the cases. Link to comment Share on other sites More sharing options...
ulhasfegade Posted December 24, 2014 Author Share Posted December 24, 2014 Is there any way to calculate maximum field weakening allowed so that amplifier current always within the specified limit. e.g. my motor parameter are Back emf= 415.15 V/1000 rpm which gives 145 V for 350 rpm for Geobrick. Phase winding resistance is 4.15 ohm. Hence if we ensure minimum back EMF of 93.75 V will provide the safe limit for the amplifier (240*0.65-102)/4.15< 15 amps. With this limit, amplifier will be safe in all the cases. Link to comment Share on other sites More sharing options...
curtwilson Posted December 24, 2014 Share Posted December 24, 2014 Because PMAC closes a current loop to maintain the desired current level, I would not worry about overcurrent problems here. The worst case for possible overcurrent is at zero velocity, when there is no back EMF. The bigger worry in field weakening a permanent magnet motor is what happens when the field weakening is removed (as with some kind of fault) when the motor is at very high speed. Can the amplifier take the full voltage of the back EMF? For instance, at 700 rpm, your motor would produce 290V. (Is this RMS? I think so.) Link to comment Share on other sites More sharing options...
curtwilson Posted December 24, 2014 Share Posted December 24, 2014 Because PMAC closes a current loop to maintain the desired current level, I would not worry about overcurrent problems here. The worst case for possible overcurrent is at zero velocity, when there is no back EMF. The bigger worry in field weakening a permanent magnet motor is what happens when the field weakening is removed (as with some kind of fault) when the motor is at very high speed. Can the amplifier take the full voltage of the back EMF? For instance, at 700 rpm, your motor would produce 290V. (Is this RMS? I think so.) Link to comment Share on other sites More sharing options...
ulhasfegade Posted December 28, 2014 Author Share Posted December 28, 2014 Yes it is RMS value. Link to comment Share on other sites More sharing options...
ulhasfegade Posted December 28, 2014 Author Share Posted December 28, 2014 Yes it is RMS value. Link to comment Share on other sites More sharing options...
ulhasfegade Posted January 6, 2015 Author Share Posted January 6, 2015 Yes it is RMS value. What is the maximum back emf Geobrick can withstand without any problem? Link to comment Share on other sites More sharing options...
ulhasfegade Posted January 6, 2015 Author Share Posted January 6, 2015 Yes it is RMS value. What is the maximum back emf Geobrick can withstand without any problem? Link to comment Share on other sites More sharing options...
curtwilson Posted January 6, 2015 Share Posted January 6, 2015 The shunt circuitry turns on at 385VDC, which corresponds to 272VACrms. An external shunt resistor (15ohm, 300W) is recommended if there is a possibility of going above this. Link to comment Share on other sites More sharing options...
curtwilson Posted January 6, 2015 Share Posted January 6, 2015 The shunt circuitry turns on at 385VDC, which corresponds to 272VACrms. An external shunt resistor (15ohm, 300W) is recommended if there is a possibility of going above this. Link to comment Share on other sites More sharing options...
ulhasfegade Posted January 7, 2015 Author Share Posted January 7, 2015 We are using 6 axis Geobrick and user GAR10 (10 ohm 300 watts) resistor. Do we need to change it to GAR15(15 ohm 300 Watts)? Link to comment Share on other sites More sharing options...
ulhasfegade Posted January 7, 2015 Author Share Posted January 7, 2015 We are using 6 axis Geobrick and user GAR10 (10 ohm 300 watts) resistor. Do we need to change it to GAR15(15 ohm 300 Watts)? Link to comment Share on other sites More sharing options...
curtwilson Posted January 8, 2015 Share Posted January 8, 2015 I think that resistor will also be fine. You should calculate what the flow through this resistor would be if the field weakening failed at your maximum RPM. Link to comment Share on other sites More sharing options...
curtwilson Posted January 8, 2015 Share Posted January 8, 2015 I think that resistor will also be fine. You should calculate what the flow through this resistor would be if the field weakening failed at your maximum RPM. Link to comment Share on other sites More sharing options...
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