Electric Motors - Control

[Otispunkmeyer]

Just wondering if there is anyone on here a bit knowledgeable on the subject of controlling AC Synchronous motors (the permanent magnet type like you find in EVs.

I am trying to understand how they are controlled really, speed control or torque control.

Am I right in thinking that whilst achievable in many different ways, the main way to do it is adjust the driving frequency and voltage?

And am I right in thinking the torque is produced when there is slip between rotor speed and the speed of the magnetic field going round? So if you move the field ahead of the rotor, it pulls on the rotor and the strength of the pull is current/voltage based?

Same for regenerating, slowing the field puts the brakes on as the rotor is now ahead of the field.

is that about right?

[anonymous-user]

Google the following:



Space Vector Modulation

Field Oriented Control

Park and Clarke Transform (nee Direct-quadrature-zero transformation)


More home work will be issued next week! ;-)

[Otispunkmeyer]

Max_Torque said:

Google the following:



Space Vector Modulation

Field Oriented Control

Park and Clarke Transform (nee Direct-quadrature-zero transformation)


More home work will be issued next week! ;-)

Already there man! I found a great YT video by a guy from Texas Instruments. Goes through the whole thing.

I then somehow ended up on Fuzzy-Logic models for regenerative braking!

[anonymous-user]

Ok cool, if you have specific questions ask! ;-)

[Otispunkmeyer]

Ok, i guess I really wanted to know.. simply... if the battery pack in an EV is 400V say, then is that 400 V applied to the motor (well once converted to AC again) all the time and the various methods you mention caused current draw to change or does the voltage and current both change?

To be honest I forgot why I needed to know now, but I have some data where I have motor current and battery pack power draw. I can also work out motor power from dyno data so will P = IV get me the motor voltage?

[anonymous-user]

Otispunkmeyer said:

Ok, i guess I really wanted to know.. simply... if the battery pack in an EV is 400V say, then is that 400 V applied to the motor (well once converted to AC again) all the time and the various methods you mention caused current draw to change or does the voltage and current both change?

To be honest I forgot why I needed to know now, but I have some data where I have motor current and battery pack power draw. I can also work out motor power from dyno data so will P = IV get me the motor voltage?

For a fixed DC link voltage (in reality there is no such thing, as all batteries have voltage sag under load and at different temperatures and StateOfCharge etc) the supply voltage to the inverter is fixed, lets say at your 400v value.


The inverter has two states for it's power switches ON and OFF. In order to minimise losses and to avoid as much heating as possible, it transitions between those two states as fast as possible (typical 1uS) because between fully ON (low resistance) and fully OFF (theoretically infinite resistance) the switches are in a linear mode, where huge losses can occur (if you drop say 400V across a switch with say 100Ampes flowing through it, you get 40kW of power loss)

So the forwards voltage we can apply to the motor is 400v. The back emf (the voltage developed by that spinning motor) depends on how fast it is spinning at any particular moment. Lets imagine the back emf is 200volts, that means there remains 200V to push current through the windings, against the impedance of those windings (which are primarily inductive, rather than resistive)


So, the inverter switches on it's power switches, connects one end of one phase winding to the supply +ve, and the other end of the phase winding to supply -ve. in our example, the 200V greater votlage forces current into that phase, and that current generates a torque on the rotor. And as power = speed x torque, the power consumed is that torque multiplied by the rotational speed at the given moment we apply that voltage.


The critical fact with any inverter driven motor is that the inverter is always applying full voltage to the motor, but it can do that in very small pulses so the AVERAGE applied voltage can be varied (the inductance of the motor damps those short pulses into a more constant current, and hence a more constant torque which is what we realy care about. By modulating the length and/or duty cycle (percentage time ON) of the driving waveform the inverter can apply a complex voltage profile, in order to try to keep a smooth, DC current flowing through the motor.





In effect, the inverter using the inductance of the motor acts as a step down/ step up converter.

If we ignore any losses for the moment, If the motor say has 200V backemf at its current speed and is drawing 100 amps, it's using 20kW (200 X 100), so for a supply voltage of 400V, the supply will have 50 amps being taken from it (400 x 50 = 20,000). At some point, the motor speed reaches the point where it's voltage is the same as the supply, and here motor phase current and supply current are identical. If the motor exceeds the battery supply voltage (requires field weakening to do this) then battery current is larger than the motor phase current (without field weakening, the motor would turn into a generator as the direction of current reverses)

[annodomini2]

Known as Pulse Width Modulation (PWM)

[Otispunkmeyer]

Max_Torque

Champion. Thanks for that. I think a lot of the sites i read were trying to say the same thing. Just not as succinct!