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LESSONS LEARNT FROM A DUAL “SLIP ENERGY RECOVERY” DRIVE SYSTEM ON
A SAG MILL
P Warner, E Gilfillan, Principal Engineer, Anglo Technical Division, Anglo American plc group,
P.O. Box 6158 Marshalltown 2107 South Africa; e-mail: [email protected]
Resource Manager, Anglo Technical Division, Anglo American plc group,
P.O. Box 6158 Marshalltown 2107 South Africa; e-mail: [email protected]
Abstract
A full investigation into on a dual (2 x 3,6 MW, 11 kV) Slip Energy Recovery (“SER”) drive grinding mill was
prompted by the failure of a pinion during drive commissioning. This paper gives an overview of SER drive principles
and the stress imposed on SER drives during supply disturbances. Also discussed are combined computer simulations
of the total mechanical, electrical and electronic system, which indicted that the failure was caused by a combination of
electromagnetic noise interference (“EMI”) issues such as earthing, signal isolation, screening together with
commissioning procedures and settings. Several other unusual critical aspects of SER drives are also reported on,
followed by a summary of the lessons learnt by the failure.
Introduction
The drive system on a semi autogenous (“SAG”) grinding mill was recently upgraded from a single to a dual variable
speed drive, rated 2 x 3 600 kW. During commissioning of the second drive, a girth gear pinion was ejected within
minutes of starting the mill. Subsequent analysis established that the pinion must have been subjected to more than
300% torque. However, at the time of the failure the mill load was only approximately 75% per drive.
Although no one was injured and the equipment damage was limited, the failure is regarded very seriously. To date the
variable speed system has still not been commissioned, but instead the mill is operating at fixed speed, resulting in
significant under recovery and consequent financial loss. The situation is placed in perspective by the fact that an
Australian Gold Mining company has taken their consulting engineers, mechanical vendor and electrical vendor to court
for lost production and damage to a girth gear of a dual variable speed drive SAG mill.
This paper records some of the results of the investigation by Anglo American plc into the pinion failure.
The mill installation
The SAG mill has a diameter of 6,1 m and a length of 7,770 m. As is standard practice in South Africa for large
grinding mills, drive power is provided by two slip ring induction machines, each wound for 11 000 Volts, 6 pole. Each
machine drives a speed-reducing gearbox, which in turn drives a pinion, which engages a girth gear, mounted on the
mill circumference. The mechanical shaft system can thus be seen to comprise several large inertias connected by
shafts with finite stiffness. Obviously such an arrangement will exhibit several modes of torsional resonance, and the
response to exciting torques can only be determined by computer simulation.
Slip ring induction machines lend themselves to slip energy recovery (“SER”) speed control, particularly if the required
speed range is small. A 15 % speed variation above and below motor rated speed was indicated by process
requirements. Therefore SER was selected for the drive upgrade. SER’s are known to be sensitive to supply
disturbances, therefore the upgrade included the installation of a crowbar across each set of slip rings, to protect the
associated inverter from damage during supply disturbances.
Overview of slip energy recovery drive principles
With reference to figure 1, each motor is part of a slip energy recovery (“SER”) variable speed system, comprising:

A liquid resistance starter (to accelerate the motors and mill to the minimum controllable speed);

Low voltage electronic converter (to control the motor);

A step down transformer to adapt the 11 kV supply voltage to the electronic converter’s operating voltage;

Rotor switchgear (to connect the slip rings to the starter during starting and to the electronic converter in speed
control mode).
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Figure 1 : Block diagram of a slip energy recovery variable speed drive
Rotor current is directly proportional to motor torque and thus the motor / mill speed may be controlled by regulating
the rotor current. Unfortunately it is not possible to control the motor speed by simply connecting the slip rings to the
supply through a transformer, because the rotor voltage and frequency both vary with speed. This is because a slip ring
induction motor is effectively a rotating transformer and as the motor speed increases from zero, the rotor voltage falls
linearly to zero:
V 2  sV 2 n
where :
V 2  rotor volt age
s  slip
ns  n
ns
ns  synchronou s speed

n  actual rotor speed
V 2 n  nominal (stand still) rotor volt age
(1)
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Figure 2: Rotor voltage and frequency as a function of motor speed.
Above synchronous speed, the slip is negative, the slip ring voltage phase changes (the voltage becomes “negative”)
and the magnitude of the rotor voltage increases. Similarly, the frequency of the slip ring current varies in proportion to
the slip, falling from 50 Hz at stand - still to dc at synchronous speed.
The necessary voltage and frequency conversion of the slip ring power is performed through an intermediate dc link. A
“rotor bridge” rectifies the varying voltage and varying frequency of the slip ring power and feeds this into the dc link.
A “line bridge” inverts the dc link power to a fixed 50 Hz frequency and a fixed voltage for return to the supply
network. In common with most SER’s today, the bridges are both voltage source pulse width modulated (“PWM”)
units.
The motor rated power and stand - still slip ring voltage determine the rotor current for rated torque:
Pn
3V 2 n
where :
I 2n 
I 2 n  nominal rotor current
(2)
P 2 n  nominal motor power
V2n  nominal rotor volt age
The proportionality in the above equation accounts for the power factor of the rotor circuit as well as for and for rotor
losses. Together, these two factors account for less than 5%. The machines on this SAG mill are rated 3 600 kW with
nominal rotor data of 2 608 Volts and 836 Amps. Therefore, in the speed control range (15% below and above
synchronous speed) the rotor voltage is below 391 Volts. The load torque of a grinding mill is approximately constant
with variation of speed. Thus conventional low voltage (480 Volt) PWM inverters may be connected in each motor’s
rotor circuit to perform the voltage and frequency conversion and the drive control. Several inverters are connected in
parallel per motor to deliver 836 Amps (plus a margin).
It is significant that in an SER drive with limited speed turn - down, the power electronics is rated at only a fraction of
the motor power. Indeed, the “lever” effect inherent in SER drives is their principal attraction as it allows a low capital
cost solution. However, a significant challenge for the SER drive designer is to cater for the considerable forces and
energy that is unleashed during faults.
Stresses on an ser during a supply disturbance
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During supply disturbances, an SER drive is subjected to severe stresses. It is somewhat counter – intuitive, but a
supply under voltage causes a large over voltage internally in an SER drive. The relationship between the stresses on
the drive and the extent of eth supply voltage depression is non linear. This can be understood by considering
moderate and severe supply disturbances separately.
Moderate Supply disturbances
The motor’s time constant is larger than that of the converter transformer. Thus the transformer secondary voltage will
tend to follow the supply network with little time delay, while the rotor current will not change instantaneously.
Therefore, during a supply voltage depression, the power that the converter is able to return to the supply is reduced.
For example, if the supply voltage falls to 80% of nominal, the current must increase to 1/80% or 125% of the current
required at 100% supply voltage to return the same slip ring power to the supply network. When the line bridge is
unable to remove from the dc link all the power that is being injected into it from the slip rings, the dc link voltage and
the slip ring voltage will rise. Clearly an abrupt voltage depression will cause the SER converter to trip on over current
and / or dc link over voltage.
Severe Supply Disturbances
In addition to the above mechanism, extra stresses are imposed on SERs during severe supply disturbances. During a
severe supply voltage dip, the motor’s contribution to the fault causes both the rotor and stator current to increase
rapidly to as high as six to ten times rated current. Thus, for a close – in stator short circuit, in addition to the converter
being unable to extract power from the dc link and return this to the supply, the motor’s fault current contribution is
injected into the dc link. Again, the converter will trip on over current and / or over voltage in the dc link.
SER converter reaction to the stresses caused by supply disturbances
Interrupting the large rotor current in the highly inductive rotor circuit will of course cause a dangerously large L di/dt
voltage at the slip rings. However, blocking the converter power semiconductors does not necessarily protect the bridge
from an over voltage on the slip rings. Most SER drives today are based on PWM bridges in the slip recovery circuit.
The standard topology for a PWM bridge includes an anti-parallel or free-wheeling diode across each power
semiconductor, similar to free wheeling diodes across a relay coil. Even if the bridge is shut down, the free – wheeling
diodes are still available for conduction. Should the source voltage (the slip ring voltage) rise excessively, these diodes
will become forward biased and will conduct, rectifying the high source voltage and passing energy to the dc link in an
uncontrolled manner. IGBT bridges typically utilise electrolytic capacitors with a very narrow voltage margin in the dc
link. If the capacitors’ voltage rating is exceeded, they can fail explosively. IGCT (Integrated gate commutated
thyristors) based converters often utilise polypropylene insulated capacitors with considerably higher voltage margins,
but these will also rupture if their ratings are exceeded.
SER drives are unique in variable speed drive topologies in that the converter power is less than that of the motor. In
the case of this SAG mill, a converter rated at 0,8 MW controls a 3,6 MW motor. That is, the motor and its fault power
is 450% of the converter’s nominal power rating. Thus the “lever” effect of SER drives, that is utilised to great
advantage in the project initial capital cost, causes unusually large stresses on the electronic inverter during faults.
Designs to mitigate against damage by supply disturbances
Different SER vendors utilise several solutions to protect the converters from damage during a supply disturbance:

Surge arrestors on the slip rings, together with large ratings for the power semiconductors and dc link
capacitors. However, both site experience and computer simulations show that this may be inadequate for
severe supply disturbances;

Switching the liquid resistor starter to the slip rings in the event of a supply disturbance. However, again both
site experience and computer simulations show that the rotor switch gear may be too slow to prevent damage
to the drive during a severe supply disturbance;

A crow bar to short-circuit the slip rings during a supply disturbance. A solid-state crowbar offers sufficiently
high-speed operation as well as the ability to dissipate sufficient energy during a fault. However, this approach
can cause large torque transients, effectively transferring an electronic problem to the mechanical system.
All of the above approaches are widely used in windmill generators, which are currently by far the largest application
for SER drives. The vendor of this SAG mill’s dual drive adopted the crowbar approach for this grinding mill.
Failure causes
Investigations revealed that the failure of the pinion could be attributed to two principal issues in this rather complex
installation:

Electro magnetic interfere (“emi” or electrical noise) in both design and installation;
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
Commissioning settings and procedures.
These will be discussed in turn.
Failure Cause #. 1: Electromagnetic Interference
EMI issues increase significantly with increasing power and also with the latest power semiconductors. Traditionally,
large power drives have been based on thyristor converters while the vast majority of IGBT drive installations are
below 200 kW. In contrast, these mill drives each incorporate 1 131 Amp IGBT converters. Further, thyristors are
commutated in fractions of a millisecond; IGBTs are commutated in fractions of a microsecond. The consequent large
current, steep – fronted pulses of the inverter PWM outputs results in significant harmonics at MHz frequencies, a
combination that is outside the experience of most drives engineers.
Figure 3: Sketch extract of earthing configuration of leader and follower SER drives
The earthing network for the two drives includes an earth bus bar in each drive cubicle suite, connected by 70 mm2
braided copper cable to a “subsidiary star” point for each drive. In turn, these two subsidiary star points are connected
to the main drive system star point, together with the motors, transformers and liquid resistance starters. However, all
connections to the main earth star point were by conventional stranded 70 mm 2 copper cable. Unlike the braided cable,
this cable has high impedance to MHz harmonics. Thus, for the predominant noise signals, the two drives are
electrically isolated from each other and may be at different potentials. The product of high impendence in the earth
paths and large magnitude of currents at MHz frequencies induce substantial harmonic voltages in the system.
Load sharing between the two drives is achieved in the traditional manner. That is, one drive is selected as the “leader”
and receives a speed reference from the plant plc. The leader drive calculates a speed error, which is used as a torque
reference for both the leader and the second, “follower”, drive. The speed control is relatively slow as the grinding
process does not require a dynamic speed control and the total mechanical inertia is large. In contrast, the torque
control is fast, so as to ensure system stability and to achieve accurate dynamic load sharing. Thus the torque signal is
only passed through a very high frequency filter, allowing much of any electrical noise present to be injected into the
follower drive’s torque control algorithms. The torque reference signal was by screened copper cable without any form
of optical isolation between the leader and follower drive. After the pinion failure, 20% noise was measured on this 010 V cable. Simple re-routing of the cable away from power cables reduced this noise level, but the total solution
included optical isolation of the torque reference signal, replacement of the stranded earth with a braided cable and full
emi measures on cable glands, panel doors etc.
Failure Cause #. 2: Commissioning settings and procedures
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The pinion failure could probably have been avoided if either adequate emi precautions had been taken or if
commissioning settings and procures had been tighter. Although the load on each motor was only 75% of rated
capability, the drives’ instantaneous trip levels were set at the factory default level of 330% of motor rated power. This
is significant as the pinion was capable of withstanding up to only 300% torque. Further, the torque reference between
the two drives was scaled to 330%, thus the 20% background noise measured corresponded to 66% of rated load torque.
That is, the design and installation together caused a torque noise signal to the follower drive of close to the load torque.
Other cautionary measures that could have been taken include:

Monitor the stability of the two drives;

Reduce the mill charge (and then to reduce the torque limits on both drives)

De-sensitise the load sharing on the two drives (for example, set the lead drive to assume 80% of the load and
the follower drive the remaining 20% load. In this case, a 20% oscillation of the load sharing would amount to
only 4% of the total load torque.)
Other possible problems encountered
While the pinion failure was being investigated, other issues were uncovered that could also cause problems. Since the
variable speed drives were not operated after the failure of the pinion, these other issues uncovered did not result in
failures. However, they were resolved before the drives were returned to service to prevent any further problems.
Possible Problem #1: Excessive Torque caused by Crowbar operation
In the original crowbar design the slip rings were short-circuited if the associated inverter tripped (caused for example
by a supply disturbance). To investigate the transient torques the performance of the total drive system: the various
inertias, shaft stiffnesses, motor and converters was simulated with the computer programme EMTP. Simulation
studies established that shorting the slip rings would induce 400% torque in the associated pinion and 190% in the
opposite pinion. The solution was to include a resistor in the crowbar circuit to limit the current and hence the transient
torque. This approach is used in SER windmill generators in Europe. The resistor must be carefully selected as an
incorrect value can cause excessive fault torques or generate excessive slip ring voltages.
Regarding excessive slip ring voltages it should be noted that, as stated previously, most SER drives today are based on
PWM bridges in the slip recovery circuit. The standard topology for a PWM bridge includes an anti-parallel or
freewheeling diode across each power semiconductor, in a similar manner to a free wheeling diode across a relay coil.
Even if the bridge is shut down, the free – wheeling diodes are still available for conduction. Should the source voltage
rise excessively, these diodes will become forward biased and will conduct, rectifying the high source voltage and
injecting energy into the dc link. IGBT bridges typically utilise electrolytic capacitors with a very narrow voltage
margin. If the capacitors voltage rating is exceeded, they rupture explosively. IGCT (Integrated gate commutated
thyristors) based converters often utilise polypropylene insulated capacitors with considerably higher voltage margins,
but these also rupture if their ratings are exceeded.
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Figure 4: Computer simulation of the transient torque developed in the two pinion when the crowbars are fired
into a short circuit. The peak values of 400% and 190% are in excess of the 300% mechanical limit.
Possible Problem #2: Excessive torque caused by asynchronous firing of the crowbars
In the original system design the crowbars were fired independently of each other. The computer model discussed
above was used to analyse the effect of sequential timing of the crowbar firing. It was shown that asynchronous
triggering could result in excessive torques. The solution was to synchronise the crowbar triggering and tripping of the
11 kV stator breakers.
Possible Problem #3: Spurious firing of the crowbars
An intertie was installed to ensure synchronous operation of the crowbars and to trip both 11 kV stator breakers.
However, the intertie caused spurious operation of the crowbars without tripping the 11 kV breakers. The problem was
eventually determined to be caused by emi. The steep - fronted pulses of the inverter PWM output are strongly
reflected at the slip ring connections as the transient impendence of the cables is very different from that of the rotor
winding. This ringing causes of the order of double the dc link voltage to be imposed on the slip rings and cables, that
is some 2 x 680 Volts or 1 360 Volts. Snubbers were installed on the output of the rotor bridge and across the crowbar
thyristor to prevent spurious triggering. However, the intertie connection still caused spurious triggering on powering
up the drive, which the simulations had shown could cause excessive torques on the pinions.
An analysis of the crowbar firing circuit showed that the sensing circuitry was referenced to the mid point of the slip
rings. The cables “ring” at around 45 kHz in response to being subjected to a steep - fronted PWM pulse train of some
4 kHz. Thus the zero volt points (“earths”) of the leader and follower drive’s crowbar trigger circuits are at several
hundred volts and some 45 kHz to true earth and to each other. The intertie between the crowbar trigger circuits of the
two drives comprised a screened co-axial cable with a capacitance to its screen of some 50 pico Farad / meter. Thus the
20 m long intertie cable can be modelled as a 500 Volt, 45 kHz source coupled by a 1 nF capacitance. The crowbar
trigger circuit from point of connection of the inter tie to the outputs (gates of the thyristors that operated the crowbar
and 11 kV stator breaker) was modelled and the transfer functions are plotted below. The model predicted that the
system electrical noise will apply approximately 2,5 V to the crowbar trigger thyristor and less than 0,2 Volts to the gate
terminal of the thyristor used to trip the 11 kV breaker. Thus the analysis confirms the worst case that indeed occurred:
the crowbar is triggered spuriously (and imposes a short across the slip rings, causing a high transient torque) but the 11
kV stator breaker is not tripped, leaving the motors energised until other (slower) protection equipment trips the drives.
The solution developed by Anglo American was to have inexpensive modifications (less than R 10,-- worth of
components) made to the crowbar control printed circuit card.
Figure 5: Extract of the crowbar control circuit, with a 500 V signal generator coupled by the intertie’s
screened cable capacitance, to model the noise induced.
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Figure 6 : Transfer function of a portion of the crowbar control circuit fro a 500 V noise signal injected
through a 1 nF capacitance. At the frequency of the ringing on the inverter output cable (45 kHz)
approximately 2,5 Volt is applied to the thyristor that controls the crowbar while less than 0,5 Volt is applied
to the thyristor that trips the 11 kV breakers – a situation that can lead to excessive torques.
Possible Problem #4 : Under rating of crowbar power semiconductors
It is important that the power semiconductors in the crowbar circuit are selected to withstand the rotor current surges
that flow during (external) network disturbances. As discussed above, under severe supply disturbances (for example a
close-in stator short circuit) an SER converter is subjected to substantial transient currents, and care must be applied to
ensure that the crowbar components are adequately rated.
Possible Problem #5 Thermal Fatigue of IGBT Power Semiconductors
IGBTs in the modular packages used in LV inverters are susceptible to thermal fatigue failure of both the bonding of
the aluminium wire strands used to connect the emitters of the individual transistors and thermal fatigue failure of the
solder junction of the silicon chip and the substrate. 1 Thermal fatigue has been determined to be the cause of premature
device failures in IGBT urban traction drives in Europe. Urban traction duty causes the IGBT temperature to be cycled
during the repetition of acceleration away from a station, cruising at full speed, deceleration into the next station, pause
in the station, acceleration again. This problem is serious because the failure mode of IGBTs is to open circuit, and the
release of the large quantity of energy stored in the dc link capacitors is released in the resultant arc within the failed
IGBT module. Thus IGBT failure in large drives usually results in the IGBT plastic module rupturing catastrophically. 2
Typical industrial IGBT stator - fed variable speed drives, such as a fan, pump or conveyor, does not subject the IGBTs
to a large number of thermal cycles. However, in an SER the IGBTs are subjected to pronounced thermal cycling when
the motor is operated close to its synchronous speed. This is because at these speeds the rotor frequency is very low
(close to dc) and thus the inverter current is slowly cycled through the bridge arms.
In summary, to prevent premature catastrophic device failure, an SER that utilises IGBTs should be de-rated and should
have a “skip speed’ around motor synchronous speed.
Lessons learnt
1
Presently only two manufacturers offer IGBTs in press pack housings, to address this problem. However this
expensive mounting is not used in LV applications.
2
In contrast, thyrsitors and IGCTs typically do not explode on failure; they usually fail to short circuit and the fault
energy is contained in the ceramic press pack housing.
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In conclusion, the following lessons have been learnt in applying SER drives to control the speed of large grinding
mills:

Dual drive mills are considerably more complex than single drives;

It can be very costly to implement unproven technology in a production environment;

In a large grinding mill, as in any large installation, it is essential that the electronic, motor and mechanical
systems be fully integrated;

Supply disturbances cause considerable stresses in SER drives and it is essential that measures taken to protect
the drive should not transfer the problem to the mechanical system;

A torsional analysis is important in a large drive system, particularly in a dual drive mill;

In the event of any uncertainty in the combined system’s fault performance, torque limiting couplings should
be considered;

Dual drives should have a synchronous response to faults;

Emi issues of earthing, bonding, screening etc are more important on high power than low power applications
and in IGBT than thyristor drives;

Signal isolation is critical.