Brushless DC Motor Drives

ELECTRICAL DRIVES, SYNCHRONOUS MOTOR AND BRUSHLESS DC MOTOR DRIVES

Brushless DC Motor Drives(Trapezoidal PMAC):

The cross section of a 3-phase 2 pole Brushless DC Motor Drives is shown in Fig. 7.15. It has permanent
magnet rotor with wide pole arc. The stator has three concentrated phase windings, which are displaced by
120° and each phase winding spans 60° on each side. The voltages induced in three phases are shown in Fig.
7.17(a). The reason for getting the trapezoidal waveforms can now be explained. When revolving in the
counter-clockwise direction, up to 120° rotation from the position shown in Fig. 7.15, all top conductors of
phase A will be linking the south pole and all bottom conductors of phase A will be linking the north pole. Hence
the voltage induced in phase A will be the same during 120° rotation (Fig. 7.17(a). Beyond 120°, some
conductors in the top link north pole and others the south pole. Same happens with the bottom conductors.
Hence, the voltage induced in phase A linearly reverses in next 60° rotation. Rest of the waveform of phase A
and waveforms of phases B and C can be similarly explained.

An inverter fed trapezoidal PMAC motor drive operating in self-controlled mode is called a brushless dc motor.
Brushless DC Motor Drive for Servo Applications:
A Brushless DC Motor Drives employing a voltage source inverter (VSI) and a trapezoidal PMAC motor is
shown in Fig. 7.16(a).

The stator windings are star connected. It will have rotor position sensors, which are not shown in the figure.
The phase voltage waveforms for a trapezoidal PMAC motor are shown in Fig. 7.17(a). Let the stator windings
be fed with current pulses shown in Fig. 7.17(b). The current pulses are each of 120° duration and are located
in the region where induced voltage is constant and maximum. Further, the polarity of current pulses is the
same as that of induced voltage. Since the air-gap flux is constant, the voltage induced is proportional to speed
of rotor.
During each 60° interval in Fig. 7.17, current enters one phase and comes out of another phase, therefore,
power supplied to the motor in each such interval

Torque developed by the motor

The waveform of torque is given in Fig. 7.17(c). According to Eq. (7.31) torque is proportional to current Id. It
can be shown that a dc current Id flows in the dc link. Regenerative braking operation is obtained by reversing
phase currents. This will also reverse the source current Id. Now power flows from the machine to inverter and
from inverter to dc source. When speed is reversed, the polarity of induced voltages reverse. With current
polarity shown in Fig. 7.17, the drive gives regenerative braking operation, and when current direction is
reversed motoring operation is obtained. The current waveforms shown in Fig. 7.17(b) are produced as follows.
During the period 0∘ to 60∘ , iA = Id and iB = –Id. The current iA enters through the phase A and leaves through
the phase B. When transistors Tr1 and Tr6 are on, terminals A and B are respectively connected to positive and
negative terminals of the dc source Vd. A current will flow through the path consisting of Vd, Tr1, phase A,
phase B and Tr6 and rate of change of current iA will be positive. When Tr1 and Tr6 are turned off this current
will flow through a path consisting of phase A, phase B, diode D3, Vd and diode D4. Since the current has to
flow against voltage Vd, the rate of change of iA will be negative. Thus, by alternately turning on and off Tr1 and
Self Controlled Synchronous Motor Drive
ELECTRICAL DRIVES, SYNCHRONOUS MOTOR AND BRUSHLESS DC MOTOR DRIVES

Self Controlled Synchronous Motor Drive:

A Self Controlled Synchronous Motor Drive employing a load commutated thyristor inverter is shown in Fig.
7.10. In large power drives wound field synchronous motor is used. Medium power drives also employ
permanent magnet synchronous motor. The drive employs two converters, which are termed here as source
side converter and load side converter. The source side converter-is a 6-pulse line-commutated thyristor
converter. For a firing angle range 0 ≤ αs ≤ 90∘, it works as a line-commutated fully controlled rectifier delivering
positive Vds and positive Id, and for the range of firing angle 90∘ ≤ αs ≤180∘ it works as a line-commutated
inverter delivering negative Vds and positive Id.

When Self Controlled Synchronous Motor Drive operates at a leading power factor, thyristors of the load side
converter can be commutated by the motor induced voltages in the same way, as thyristors of a line-
commutated converter are commutated by line voltages. Commutation of thyristors by induced voltages of load
(here load is a motor) is known as load commutation. Firing angle is measured by comparison of induced
voltages in the same way as by the comparison of line voltages in a line commutated converter. Converter
operates as an inverter producing negative Vdl and carrying positive Id for 90∘ ≤ αl ≤ 180∘. For 0 ≤ αl ≤ 90∘ it
works as a rectifier giving positive Vdl. For 0 ≤ αs ≤ 90∘, 90∘ ≤ αl ≤ 180∘ and with Vds > Vdl, the source side
converter works as a rectifier and load side converter as an inverter, causing power to flow from ac source to
the motor, thus giving motoring operation. When firing angles are changed such that 90∘ ≤ αs ≤ 180∘ and 0∘ ≤
αl ≤ 90∘, the load side converter operates as a rectifier and the source side as an inverter. Consequently, the
power flow reverses and machine operates in regenerative braking. The magnitude of torque depends on (Vds
— Vdl). Speed can be changed by control of line side converter firing angles.

When working as an inverter, the firing angle has to be less than 180° to take care of commutation overlap and
turn-off of thyristors. It is common to define a commutation lead angle for load side converter as

If commutation overlap is ignored, the input ac current of the converter will lag behind input ac voltage by angle
αl. Since motor input current has an opposite phase to converter input current, the motor current will lead its
terminal voltage by an angle βl. Therefore, the motor operates at a leading power factor.

Lower the value of βl. higher the motor power factor and lower the inverter rating. The commutation overlap for
the load side converter depends on the subtransient inductance of the motor. The motor is provided with a
damper winding in order to reduce subtransient inductance. This allows operation with a substantially lower
value of βl. The damper winding does not play its conventional roles of starting the machine as an induction
motor and to damp oscillations, because rotor and rotating field speeds are always the same as explained later.
In a simple control scheme, the drive is operated at a fixed value of commutation lead angle βlc for the load
side converter working as an inverter and at βl = 180° (or αl = 0°) when working as a rectifier. When good
power factor is required to minimize converter rating, the load side converter when working as an inverter is
operated with Constant Margin Angle Control. If commutation overlap of the thyristor under commutation is
denoted by u, then the duration for which the thyristor under commutation is subjected to reverse bias after
current through it has fallen to zero is given by
For successful commutation of thyristor

where tq is the turn-off time of thytistors and ω the frequency of motor voltage in radians/sec. Since u is
proportional to Id, for a given Id, βl can be calculated such that the thyristor under commutation is reverse
biased for a duration γmin which is just enough for its commutation. This in turn minimizes βl and maximizes
motor power factor. Since γ is kept constant at its minimum value γmin, the control scheme is called constant
margin angle control.

The dc link inductor Ld reduces the ripple in the dc link current Id and prevents the two converters from
interfering with each other’s operation. Because of the presence of inductor in the dc link, the load side
converter when working as an inverter, behaves essentially as a current source inverter of Fig. 6.45, except
that thyristor commutation is now performed by motor induced voltages. Consequently, the motor phase current
has six step waveform of Fig. 6.45(b). Because of the dc current through Ld, the ac input current of source side
converter also has a six step current waveform.

The dc line current Id flows through the machine phase for 120° in each half cycle. Fundamental component of
motor phase current Is has following relationship with Id

For machine operation in the self-controlled mode, rotating field speed should be the same as rotor speed. This
condition is realized by making frequency of the load side converter output voltage equal to the frequency of
voltage induced in the armature. Firing pulses are therefore generated either by comparison of motor terminal
voltages (as induced voltages are not directly accessible) or by the rotor position sensors. Self control is
ensured when firing pulses are generated by the comparison of motor terminal voltages (as induced voltages
are not directly accessible). Alternatively firing pulses are generated by rotor position sensors, which are
stationary and suitably aligned with the armature winding. The frequency of induced voltages depends on the
speed of rotor (or rotor field) and their phase depends on the location of rotor poles with respect to the
armature winding. Hence, signals generated by rotor position sensors have the same frequency as that of the
induced voltages and they have a definite phase with respect to induced voltages. Load side converter
thyristors are fired in the sequence of their numbers with 60° interval. Therefore, for the control of load side
converter thyristors, in all six rotor angular positions are required to be detected per cycle of the induced
voltage. The Hall-effect sensors can detect the magnitude and direction of a magnetic field. Hence, three Hall-
effect sensors can detect the six rotor positions. The sensors are mounted at 60° electrical intervals and
aligned suitably with armature winding.

As stated earlier the load side converter and the current source inverter of Fig. 6.45 perform essentially the
same function.. The only difference between the two is that while the former uses the load commutation, the
later uses forced commutation.

Load commutation has a number of advantages over forced commutation:

1. it does not require commutation circuits,

2. frequency of operation can be higher, and

3. it can operate at power levels beyond the capability of forced commutation.

Load side converter performs somewhat similar function as commutator in a dc machine. The load side
converter and Self Controlled Synchronous Motor Drive combination functions similar to a dc machine. First, it
is fed from a dc supply and secondly like a dc machine the stator and rotor fields remain stationary with respect
to each other at all speeds. Consequently, the drive consisting of load side converter and synchronous motor is
known as Commutator Less DC Motor.

At low speeds, motor induced emf will be insufficient to commutate the thyristors of load side converter,
therefore, at start and for speeds below 10% of base speed, the commutation of load side converter thyristors
is done by forcing the current through conducting thyristors to zero. This is realised by making source side
converter to work as inverter each time load side converter thyristors are to be turned off.

For example thyristors T1 and T2 are to conduct together for 60° electrical. After 60°, source side converter will
be made to work as an inverter, which will reverse Vds and turn-off thyristors T1 and T2. Now the source side
converter operation is brought back to rectification and gate pulses are released to T2 and T3 to turn them on
and make them conduct together for next 60° electrical. Since frequency of operation of load side converter at
low motor speeds is very low compared to source frequency, such an operation can be realized. This operation
of the inverter can be termed as Pulsed Mode. This mode of operation requires rotor position sensors.
Therefore, even when the normal operation above 10% of base speed is implemented by sensing motor
terminal voltages, rotor position sensors will be needed to realize pulsed mode.

The dc supply to the field can be provided from a controlled rectifier through slip-rings and brushes.
Alternatively, brushless excitation system consisting of diode bridge mounted on the rotor and therefore rotating
with the rotor and supplied by a rotating transformer can be used. The field current is controlled by controlling
the input voltage of the transformer by feeding it from an ac voltage regulator. The brushless excitation
eliminates slip-rings and brushes and associated maintenance.
A closed-loop speed control scheme is shown in Fig. 7.11. It employs outer speed control loop and inner
current control loop with a limiter, like a dc motor (Fig. 5.47). The terminal voltage sensor generates reference
pulses of the same frequency as the machine-induced voltages. The phase delay circuit shifts the reference
pulses suitably to obtain control at a constant commutation lead angle βlc. Depending on the sine of speed
error, βlc is set to provide motoring or braking operation. Speed ωm can be sensed either from the terminal
voltage sensor or from a separate tachometer. An increase in reference speed ωm produces a positive speed
error.

βlc value is set for motoring operation. The speed controller and current limiter set the dc link current reference
at the maximum permissible value. The machine accelerates fast. When close to the desired speed, the current
limiter desaturates and the drive settles at the desired speed and at the dc link current which balances motor
and load torques. Similarly a reduction in reference speed produces a negative speed error. This sets βlc for