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Field weakening in burshless servo motor


ulhasfegade
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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.

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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.

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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.

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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.

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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.

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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.

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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.)

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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.)

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