PM motor field weakening

Afternoon all,

I've been struggling to understand the exact working of field weakening in order to increase motor speed. My understanding is that to speed up the motor the AC voltage is increased, upto the base speed of the motor. At this point the rotor voltage and stator voltage is equal due to the BEMF equaling the max AC voltage the inverter can supply. To increase the speed further the control system using FOC control, must start reducing the Id current to counteract the BEMF voltage. Now that there is a difference between the stator and rotor voltage, the machine will speed up. Untill the rotor and stator voltage are equal again. Then the id current is reduced again, and this loop continues as the motor speeds up.

My question is, is the BEMF voltage reduced because it must be lower than the AC voltage inorder to prevent damage to the inverter and current flowing the wrong way? or is it simply reduced in order accelerate the motor?

My other question is as the Id current is reduced, is iq also reduced as they are linked? Or does iq stay the same, but the torque is reduced as the rotor field and stator field will gradually be going over 90degrees, with 90degrees being the optimum angle to produce torque?

Jamoclements said:

Afternoon all,

I've been struggling to understand the exact working of field weakening in order to increase motor speed. My understanding is that to speed up the motor the AC voltage is increased, upto the base speed of the motor. At this point the rotor voltage and stator voltage is equal due to the BEMF equaling the max AC voltage the inverter can supply. To increase the speed further the control system using FOC control, must start reducing the Id current to counteract the BEMF voltage. Now that there is a difference between the stator and rotor voltage, the machine will speed up. Untill the rotor and stator voltage are equal again. Then the id current is reduced again, and this loop continues as the motor speeds up.

My question is, is the BEMF voltage reduced because it must be lower than the AC voltage inorder to prevent damage to the inverter and current flowing the wrong way? or is it simply reduced in order accelerate the motor?

My other question is as the Id current is reduced, is iq also reduced as they are linked? Or does iq stay the same, but the torque is reduced as the rotor field and stator field will gradually be going over 90degrees, with 90degrees being the optimum angle to produce torque?

Click to expand...

Iq must be reduced such that Is²=Id²+Iq² remains below the threshold where the resistive load to current would damage the coils. The torque is reduced proportional to the decrease in field strength and the decrease in Iq. Meanwhile the losses increase since faster=now Eddie currents and the Id current still has resistive losses.

Essentially, field weakening applies a current, and thus magnetic field, in phase with the magnetic field in the opposite direction to the magnet polarity thus suppressing it and making the motor behave like a motor with weaker magnets.

It's mathematically equivalent to a phase advance, but this doesn't really help understanding...

The inverter can be destroyed if the control loses synch or faults at speeds where the BEMF is greater than the MOSFET reverse withstand voltage. Essentially, the inverter must continue to suppress the magnetic field until the speed is sufficiently low.

Sorry can you just elaborate on that first part, is2 = id2 +iq2 and this damaging the coils.

Aditionally is I'd and iq related to phase angle and phase current??? With phase current being the desired current and phase angle being the different between Id and iq current???

Many thanks

This might be an uphill struggle if you're not up to speed with Id and Iq but...

The 3 phases of the motor are translated into 2 phases and they are moved from stator to rotor reference frame.

They are 90° to each other (one leads the other by 90 in rotation angle or time, however you prefer to think about it).

You add vectors by Pythagorean summation, that is x²+y²=z²

The losses to resistive heating are I²*R. This applies to the overall current, the real, magnitude of the current which is the Pythagorean sum of Id and Iq.

See
https://www.switchcraft.org/learning/2016/12/16/vector-control-for-dummies
For the best visual explanation I've seen yet

How about a practical explanation? What are the benefits and risks of field weakening for riding and should we enable it or not?

raylo32 said:

How about a practical explanation? What are the benefits and risks of field weakening for riding and should we enable it or not?

Click to expand...

Benefits:
Motor max rpm is increased for same battery voltage and same motor
No increase in power... Below the start of field weakening rpm there should be identical behaviour. Above it, you enter the "constant power zone", so named because the current is primarily limited by resistive heating and as you increase field weakening current you have to decrease torque producing current... But the rpm increases so power remains the same. Realistically though, power decreases since the other losses (Eddie currents, bearing losses...) increase.

Risks:
The field is being actively suppressed to achieve the extra speed. If your controller crashes/trips out while field weakening is active, you need to consider whether the voltage generated by the motor at this new higher speed would exceed the mosfet/other components in the controller's rating.

E.g:
Your motor is 8kV at 20s and your controller has 100V FETs -84V at full charge so 15V headroom. Your motor spins to 670rpm with no load.
You set a field weakening current that enables 50% extra speed, and your motor now spins to 1000rpm.
You're riding on Flat at max speed and for some reason the controller trips. Field weakening stops, and now the motor is generating 1000rpm/8kV=126V.
The batteries absorb this for a bit by recharging aggressively but the BMS says no no no and turns off. Your wheel might judder and lock during this. If you're very lucky... But most likely...
The 126V generated by the motor is now seen across each of the MOSFETs and they instantly blow, violently.

Should you use it?
If your controller never trips, then I'd say yes definitely use it.
1) definitely use it up to the point where the rpm/kV doesn't exceed the MOSFET or dcdc converter voltage rating
2) if you really never ever trip, consider upping it to the point where your common practical max rpm (after considering wind, voltage drop to motor resistance etc) divided by kV doesn't exceed the ratings
3) if you're willing to risk money and time, and your controller never trips and you have a physical metal fuse that will easily blow in the event of a disaster and a BMS you trust installed... Run it as high as you like.

There's some talk of clamp implementations that clamp all 3 phases together to dead short them and lock the rotor in the event of crashing under heavy field weakening. This unfortunately isn't always reliable... Usually isn't. Say we take the case above, the current generated is roughly 126V/Rmotor which is likely about 50mohm so current in the 2000A region which is sustainable by an 18fet controller for... Maybe 1ms... After which the MOSFETs will vaporise.

There's practically perhaps 10% more headroom than described above due to losses, controllers only using 95% modulation, MOSFET diode body losses, under rating etc... But in the end, you pick your risk comfort zone.

Thanks for the great explanation. My only real opportunity for this is on my TSDZ2 bike running the MBRUSA OSF. The menus give the ability enable or disable it but I don't believe the user has the ability to specify the magnitude of the effect. I assume the developer has set it at a level that the controller can handle but I don't know for sure. I need to do some testing with both settings to see how the bike performs and whether it would be beneficial for me to enable.

I don't know much (anything) about tsdz2, but isn't it a motor that assists the pedals? In which case .. wtf do you want field weakening for, surely it'll just make your pedals spin faster than you can keep up with.

When you have gears, field weakening seems completely futile to me.

As a side note...its my understanding that when a motor is using FW, the power will be less, there will be more waste-heat, and as a result the watts-per-mile will be slightly worse.

No free lunch, but...it seems to be useful in certain situations, especially on flat land in a direct-drive motor

The field weakening in the TSDZ2 firmware is needed because the motor is optimised for lazy 70 rpm pedaling and the controller does not support high enough voltage to bring the RPM Up to sporty pace.

It is Risky as mbrusa said but I think the TSDZ2 can run it somewhat safer due to low inertia in the motors drivetrain, which is disconnected from the rear wheel.

spinningmagnets said:

As a side note...its my understanding that when a motor is using FW, the power will be less, there will be more waste-heat, and as a result the watts-per-mile will be slightly worse.

No free lunch, but...it seems to be useful in certain situations, especially on flat land in a direct-drive motor

Click to expand...

This is broadly correct. Theoretically with FW the power is constant but the reality is there is a drop and the torque, so perceived power, drops rapidly. Heating will increase a lot in many motors due to Eddie current.
Watt hours per mile will suffer mainly as a result of air resistance being proportional to Square of velocity, attached with the higher losses.

Vbruun said:

The field weakening in the TSDZ2 firmware is needed because the motor is optimised for lazy 70 rpm pedaling and the controller does not support high enough voltage to bring the RPM Up to sporty pace.

It is Risky as mbrusa said but I think the TSDZ2 can run it somewhat safer due to low inertia in the motors drivetrain, which is disconnected from the rear wheel.

Click to expand...

Makes sense! Had no idea the tsdz2 was such slow cadence. Never used one :lol:

I mechanically weakened the magnetic field on my axial Flux motor by reshimming the stators further away from the rotor. It made way more power than I needed anyway. This has upped the rpm/volt and appears to have improved efficiency somewhat. Possibly due to running 36v on a 60v capable motor.

https://endless-sphere.com/forums/viewtopic.php?f=30&t=97880&start=25

And

https://endless-sphere.com/forums/viewtopic.php?f=30&t=72692&p=1097701#p1098494

Lebowski's view of field weakening (and I'm inclined to agree with him, I've never really been convinced by the field actually being weakened) is that it's actually just using the inductance to push more current than the BEMF would allow.

Another interesting thing he's pointed out is that the regen current will be limited to the amount that you were using for field weakening in the event of a crash.

This would imply that provided your BMS isn't prone to shutting off, the safe field weakening current would be the larger of the BMS over current limit (for charging over current) or the max BEMF that could be generated without exceeding the MOS/dcdc limit