Motor Speed Controllers – How They Work (PMDC BLDC and AC, Torque and Speed Control)

PSoC® 3 Sensored BLDC Motor Control - AN53595

For reference (mostly my own) my post about how PMDC motor controllers work is here: How a (PMDC) Controller Works (so you can see).

I’m trying to get my head around how controllers actually work, beyond a simple analogy of pulse-width-modulation, because I know it’s a whole lot more complicated than that.  Also, its becoming clear to me that not only do different types of motors have different power-curve characteristics, but you can’t separate out the controller’s part in that.  That’s to say, yes, motor designs have difference basic traits, but the controllers that drive them do as well – largely because of what the controller can (and has to do) to make the motor spin.  Clear as mud?

How about an analogy back to gas motors?  A small block turbocharged 4 cylinder with fuel injection delivers power in a different way than a big-block V8 with a 4-barrel, but may, actually, deliver the same amount of power in the end.  I can control a lot more on the little 4 cylinder because of the turbo and the EFI, right?  On the V8 I can just dump more gas into them 4 barrels.  Kind of like that – a brushless AC motor has a lot more you can do with a typical controller.  As usual, an analogy that probably raises more questions than it answers.  Oh well.  I tried.

There are some fundamental differences between how a motor controller works for a PMDC brushed motor and a brushless motor.  Any motor controller has to work with some sort of device that knows where the motor is (in it’s rotation) and knows when to apply current.  A PMDC motor uses brushes and a commutator to do that – physically touching a contact that gets broken and “made” to turn on current.  A brushless motor doesn’t have that, and instead uses sensors to determine the position.  I’m going to try to fathom this without going into all the different types of brushless motors and just, for the moment, talk about brushless DC motors.

So I went in search of information about controllers and found it on the DigiKey site: Using Closed Loop Control in BLDC Systems. That entire page is pretty intimidating, but gave some really basic explanation that I kind of got my head around.  Also some basics about the differences between brushed and brushless motors.  Here:

The advantages of BLDC motors over brushed DC motors are:

  • High efficiency
  • More reliable and no arcing on commutation – no brushes to maintain
  • Higher speed and power to size ratio
  • Heat is generated in stator – easy to remove
  • Lower inertia – no commutator
  • Higher acceleration rate

BLDC motors are more efficient than brushed DC motors. For the same input power, a BLDC motor converts more electrical power into mechanical power than a brushed motor because of the absence of friction due to brushes. In a brushed motor design, the brushes are used to change the poles of the electromagnet in order to keep the motor spinning. Due to this lack of brushes, there is nothing to mechanically handle the polarity changes. As a result, an electronic controller is required to continuously switch the phase of the winding which will keep the motor spinning. To do this, the stator windings are energized in a particular sequence as seen in Figure 1. BLDC motors have three phases. To move the motor, two phases are driven during each commutation cycle. One phase is driven high (VMotor) and the other is driven low (GND). The remaining phase is left floating. With every commutation step, the motor will move 60 degrees. Upon completion of all cycles, the motor will have moved a complete 360 degrees.

Figure 1: BLDC sensor output vs. commutation timing.

Figure 1: BLDC sensor output vs. commutation timing.

To implement this sequence, it is important to know the rotor position. This is done by using sensors, such as Hall Effect sensors (sensored control), or by sensing back EMF (sensorless control). Hall Effect sensors are embedded in the stator. When the rotor magnetic poles pass near the hall sensors, they supply a high or low signal, indicating that the north or south poles are passing nearby. The position of the rotor is derived from the exact combination of the three hall sensor signals.

Sensored BLDC is introduced in this application note. Three position sensors provide the current position of the rotor. Position sensors toggle each at 180 electrical degrees of electrical rotation. A timing diagram of sensor output and the required motor driving voltage is shown in Figure 1. The optional use of pulse-width modulation (PWM) provides speed or torque control as shown on Phases A, B, and C in Figure 1. The duty cycle of the modulated output control signal (PWM) is varied to change the speed and torque of the motor.

I highlighted the two points that were interesting.  First, with a three-phase motor, you’re pushing two phases, and letting the third float.  Second, you can control the speed and torque.  Remember, volts = speed, current = torque, so I’m going to translate that as, we can control the voltage as well as the current of the pulses.  OK, I feel like we’re getting somewhere.

And finally I got to the mother lode: Motor Control Basics: Drive Operating Modes via Carotron – a motor controller company.

Here’s a synopsis:

1.)  DC Drives – Torque Control:

To control DC motors torque, a DC drive will regulate armature current.

csag-d1

The armature voltage is unregulated allowing the motor to operate at whatever speed is necessary to achieve the set current /torque level.

2.)  AC Drives – Torque Control:

An AC drive uses complex processing of motor voltage, current, frequency and rotational position to give it torque regulation capability.  TORQUE mode operation usually requires encoder feedback.  Even evaluation of an inverter drive’s torque regulation ability is not a straightforward task.  Do not assume that an inverter and motor operating in “torque” mode will produce a linear and proportional output torque versus reference.  Complete torque control may be dependent on the use of an external torque reference circuit or control that has flexibility and adjustability to compensate for any drive/motor shortcomings.

3.)  DC Drives – Velocity (Speed) Control:

To regulate DC motor speed, the drive will normally control the armature voltage.  How well it does this depends on what feedback signal is used to represent the motor speed.

csag-d3

Common selections for some DC drives are as follows:

  • AFB – Armature feedback
  • TFB – Tachometer feedback
  • EFB – Encoder feedback

(be sure to see the source link for more on this subject.)

4.)  AC Drives – Velocity (Speed) Control:

AC Inverter drives can have several selectable control methods.  Some examples are:

  • V/F Control (V/F, voltage/frequency – also called Volts-per-Hertz control)
  • V/F Control with PG or Tachometer Feedback
  • Open Loop Vector
  • Closed Loop or Flux Vector

The V/F, voltage/frequency, Control method – also called Volts-per-Hertz control is the most common inverter control method.  Requiring no feedback device, it is suitable for general purpose and multiple motor applications.

V/F Control with PG Feedback gives the better speed regulation of a closed loop system.

Open Loop Vector, sometimes called sensorless vector, utilizes a more complex control algorithm to give precision speed control, quick response and higher torque at low speed.

Flux Vector or closed loop vector requires encoder feedback and gives precise speed and full rated torque control over a wide speed range – sometimes even at zero RPM.

Inverters and their motors can also be operated in a “Constant Horsepower” profile where motor speed can be extended beyond the base speed rating with torque capacity de-rating.

OK, I feel like this is a start.  The next step on the quest is going to be looking at typical motors and the controllers you can get, and seeing what types they are, and thus, what you can do with them.  Stay tuned.

Advertisements

One response to “Motor Speed Controllers – How They Work (PMDC BLDC and AC, Torque and Speed Control)

  1. Pingback: The Controller Thread: Last Installment (Alltrax SPM Controllers) | The Electric Chronicles: Power in Flux·

Comments?

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s