Gears, Drives & Lubrication
Slip-Ring or Squirrel Cage?
Mathis Menzel, Menzel Elektromotoren, Germany,
discusses the pros and cons of the slip-ring motor
and the squirrel cage motor with soft starter.
Introduction
This article will address an issue that continually arises
when one comes to select a suitable mill drive and has to
decide between two types of motor. As power electronics
continue to become more efficient and affordable, there
is no denying that for some time now slip ring motors
have had to face the ever increasing competition of
squirrel cage motors. In the context of having to choose
one type of motor over the other to power a mill drive,
the alternatives prompt the following question: do the
facts actually recommend laying out a mill drive “the
conventional way,” using a slip-ring motor with starter,
or does the “modern” preference for using squirrel cage
motors including a soft starter harbour certain advantages
that make it the option of choice? In order to be able to
answer this question in the light of factual evidence, the
engineering principles underlying the two types of engines
must first be addressed.
A comparison
The direct comparison given in Table 1 does seem to
suggest a basic motivation to use a squirrel cage motor
rather than a slip-ring one, and there are other factors
corroborating this view. In addition to the lower price,
which is frequently countered by the added costs of the
power electronics, there is the factor of a maintenance-free
operation. Of course, the bearings ought to be regularly
checked and the motor should be monitored for vibrations
and overheating but, as they apply to either type of
motor, these hardly count as extra maintenance. After all,
they could easily be automated and integrated into plant
automation control.
For the majority of industrial applications, the low
price and low maintenance requirements, combined with
an identical range of options, make the squirrel cage the
motor of choice for many companies. When it comes
to mill drives, however, the slip-ring motor has a major
Table 1. General comparison
Slip-ring motor
Squirrel cage motor with soft starter
Requires monthly control of the brushes.
Does not use carbon brushes.
Carbon dust accumulates in the engine’s interior, which needs
to be removed in intervals of 2 – 5 years, in conjunction with a
major overhaul.
No accumulation of carbon dust inside the engine.
Full locked rotor torque when starting up.
Markedly lower starting torque.
Energy loss during start-up in the form of heat is generated
inside the starter, but outside the motor.
Energy loss during start-up in the form of heat is generated
inside the machine itself.
Low starting current in spite of the high locked rotor torque.
Starting current is normally about 4 – 7 times the nominal
current. Any current that is reduced by the soft starter will in
turn reduce the starting torque.
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Gears, Drives & Lubrication
advantage: it provides full start-up torque out of a lockedrotor state. By contrast, the squirrel cage motor shows a
substantially lower start-up torque, as Figure 1 illustrates.
One ought to bear in mind, however, that this will hardly
become a problem unless the short period of the motor
start-up plays a role in the specified application, for, in
terms of basic speed, squirrel cage motor and slip-ring
motors show identical behaviour (once the starter is
short-circuited).
If a squirrel cage motor, used as a mill drive, was put
directly on line (DOL) and caused four to seven times the
nominal current, this would generate the starting currents
shown in Table 3, assuming a supply voltage of 6 kV.
Soft starter devices are used because most power
supplies are unable to handle such currents. Soft starters
will cut the starting current down to the desired value. For
applications with a light-load start-up (e. g. vent or pump),
this represents a superior method to reduce the starting
current. To the extent that the starting current is reduced,
however, the torque available for starting up the mill will
also be lowered, as Figure 1 illustrates.
The motor torque is calculated as follows:
M = (U x I x √3 x cos ϕ x η ) / (2 π x n)
(Nm) (V) (A)
(s-1)
or, simplified:
M = (9.55 x U x I x √3 x cos ϕ x η ) / n
(Nm)
(V) (A)
(min-1)
However, for the acceleration of the motor (and thus of
the mill), only that fraction of the motor’s torque is available
which exceeds the load torque of the mill (accelerating
torque).
Due to the reduction of the starting current and,
consequently, to the reduced
starting torque, the motor’s
torque may actually drop
down below (or very close
down to) the mill’s torque. This
very low excess torque that
is available for acceleration
will prolong the starting phase
considerably where it does not
stall it altogether, as the starting
torque undercuts the required
accelerating torque.
The problem caused by
the enormous quantity of heat
generated in the process is
illustrated by a look at the
physical principles involved.
The heat generated during the
start-up equals exactly the kinetic
energy of the accelerated mass.
This means that an amount of
Figure 1. Comparison of squirrel cage and slip-ring motors.
energy identical to the rotating
Table 2. Starting performance
Slip-ring motor
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Torque and power input may be varied in small increments
during the start-up, as well as within wide limits, by varying
the resistor steps inside the starter.
l
By switching on a starter resistor in any phase of the rotor
winding, the start-up torque of a slip-ring motor can be
changed. In fact, the start-up torque can be increased up
to the breakdown torque by the very choice of resistor. The
starters used are the face-plate or drum type, with the resistor
being adjustable in several steps. The start-up resistor is
reduced as the supply voltage is applied during start-up, and
is eventually short-circuited.
The size of the starter, the selection of the number of tapping
positions, and the definition of the tap resistors orient
themselves to the inertia of the starting duty and to the
starting frequency.
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Squirrel cage motor without soft starter
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The build of the rotor defines the torque curve during start-up,
which cannot be varied.
l
When turned on directly, the motor will start up according to
its torque characteristic, the start-up current being a multiple
– normally four to seven times – of the nominal current,
depending on the number of poles and the output.
l
During the start-up, the voltage drops down to a value
proportionate to the load. If the power supply lacks the
required capacity, another starting method must be chosen.
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Start-up with soft starter.
l
The soft starter represents a three-phase a.c. power
controller, using inverse-parallel controllable semi-conductors.
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The fundamental mode of the motor voltage is adjusted via
phase control of varying degree, and thus via torque and
starting current.
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Soft starters of a more recent generation enable the operator
to set a so-called breakaway impulse in addition to the
current limiting. This permits the adjustment of the squirrel
cage motor to the starting curve of the drive motor.
Gears, Drives & Lubrication
Figure 2. Three-phase squirrel cage motor with all six ends
of winding leading outside the motor. The winding is
connected inside the soft starter.
Frame: 560 mm
Rated power: 2250 KW
Rated speed: 2992 rpm
Rated voltage: 6600 V
IC 411 ribbon-cooled cast-iron frame
Weight: 10 500 kg
Application: Fan
Figure 3. Technical data:
Medium voltage slip-ring motors
Quantity: 3 (total order quantity of identical pieces: 8)
Frame: 560 mm
Rated power: 900 kW
Rated voltage: 6000 V
Rated speed: 985 rpm
Protection: IP 55
Weight: 6800 kg
Application: Mill
Table 3. Starting currents of a squirrel cage motor
Installed motor output (kW)
Starting current, DOL (A):
500
240 - 420
1000
480 - 840
2500
1200 - 2100
4500
2200 - 3800
mill’s entire energy is lost in the form of heat during the
start-up process.
The energy consumed during the start-up originates
(by generating heat loss) exclusively in the cage
winding of the rotor; that is, in the machine’s interior. In
fact, it may take no more than two start-up attempts,
executed in quick succession, to cause thermal
overheating or even destruction of the squirrel cage
motor’s rotor cage.
No kind of monitoring method is able to prevent this, as
any of the available detectors invariably monitor the stator
winding. It is impossible to monitor the temperature of the
rotor cage directly. The latter would already be destroyed
by the time the temperature monitoring device inside the
stator winding had triggered an overheating alarm.
Compared to this scenario, the energy dissipation
affecting slip-ring motors is generated in the starter. The
latter is very easy to monitor, for instance by equipping
the cooling agent (usually oil) with thermal detectors, and
shutting down the drive as soon as a certain temperature
threshold is crossed.
Moreover, the start-up process of a starter can
be adjusted in regard to the starting duty, the rate of
acceleration, and the starting frequency. Changes in the
number of starts in succession, for instance, require only
that the starter be laid out for a higher heat capacity.
This enables a slip-ring motor to execute even frequent
start-ups, whereas a squirrel cage motor would have to
have a layout of extreme dimensions in order to be able to
accommodate the generated heat loss.
Conclusion
In closing, it needs to be emphasised that this account is
not intended to flatly endorse either of the two solutions.
Rather, the idea is to stimulate a critical assessment of the
proposed drive system, thus enabling the decision maker
to opt for one or the other solution on the basis of factually
established requirements, such as those of the motor startup. Every scenario is marked by its own specifications,
and here, as elsewhere, a holistic approach that takes all
the aspects involved into consideration will help to make
the right choice every time this kind of selection has to be
made.
The most decisive criteria in selecting a motor to power
a mill drive are the following:
l The required starting torque.
l Starting frequency.
l Available power supply.
l Mass inertia of the mill.
As this discussion of these two types of motors could
not possibly exhaust the complexity of the subject matter,
the company invites readers to contact them via the Editor
to discuss the matter further. ___________________________l
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