CA1246662A - Apparatus for stopping a grinding mill - Google Patents

Abstract

IMPROVED APPARATUS FOR STOPPING A GRINDING MILL
ABSTRACT OF THE DISCLOSURE
In a system having a large load driven by at least one electric motor through a clutch, when the system is to be stopped, the power is removed from the motor and the clutch opened or released. The motor and load will continue to move until stopped by friction and it is desirable to stop them more quickly. When the motor is freely rotating and the load oscillating, as in a grinding mill, the clutch may be pulsed at spaced intervals of time for a short duration to a level where the clutch is partly closed and this will slow both the motor and load.
Alternately the motor may be electrically braked by placing a low resistance across the rotor windings and applying DC to the stator windings of the motor. When the motor is stopped it can be used to brake the load by locking the motor with an application of DC to the rotor and stator windings and periodically pulsing the clutch to partly close the clutch for short durations of time. In accordance with the invention the motor is not locked during the pulsing of the clutch but is caused to rotate very slowly in order to cause the slip rings carrying DC power to the rotor to rotate beneath the associated brushes to prevent generation of heat in a localized area beneath the brush.

Description

~246~

Case 2886 IMPROVED APPARATUS FOR STOPPI~G A GRI~DI~G MILL
The present invention relates to the stopping of a system which has a large load driven by one or more dynamoelectric machines, and in particular it relates to the stopping of a large load, such as a grinding mill, driven by one or more electric motors each coupled to the load through a respective clutch.
Canadian Patent No. 1,172,735 - Eastcott et al, i~sued August 14, 1984 to Canadian General Electric Company Limit0d, describes a means ~or stopping th0 motion of a drive system having a grinding mill driven by one or more synchronous motors through a respectiv0 clutch. When power is removed from the motors and the clutches are opened or disengaged, the motors will continue to rotate for some time and the mill will oscillate. It is desirable to stop the motion in a minimum time. The aforementioned Canadian patent describes a system including a clutch actuating means, which is arranged to apply a predetermined amount of pressure to each clutch for a predetermined time interval where the pressure is sufficient to partially clo5e the clutch but insufficient to fully close and locX-up the clutch, and an enabling means which controls th~
clutch actuating means. As the enabling means pulses or actuates the c}utches at the predetermined pressure ....

~Z'~6~

Case 2886 for the predeteLmined intervals, both the motor or motors and the mill will slow down and then at least one o~ either motors or the mill will b~e substantially stopped~ The energy is dissipated in the clutches as heat, and the rate at which the heat is generated can be controlled by controlling the predetermined pressur~ and the predetermined interval.
If the motor or motors should stop first, then as discussed in the patent, the motor can be lock~d by applying DC field excitation to the rotor and applying DC to the stator. This will lock the rotor and prevent rotor rotation~ The pulsing of the clutches can then be continued until the grinding mill i5 stopped. If the grinding mill is equipped or inching, that is for very slow rotation and usua11y rotation in small discrete steps, then there will be a supply of DC available which can be applied to the stator. ~lternately when the power is removed and the clutches opened to permit ~ree rotation, the motors can be braked by applying first ~ low level of DC to the stator and placing a low resistance across the rotor field winding until the motor ~tops. The rotors can then be locked as explained, and the clutches pulsed to stop the grinding mill.
There is, however, a difficulty which arises when a motor is locked because the ~ield excitation is normally applied to the field winding on the rotor through rotor slip rings and brushes which contact the slip rings. ~en the rotor is locked, the rings do not rotate a~d the axea where the brushes are in contact with the rings tends to overheatO This may cause distortion of the rings and excessive wear.
This problem is overcome in accordance with the invention by permitting a very small a~ount of rotation of the rotor of each synchronous machine. If the grinding mill is equipped for inching, then it 6~f~
Case 2886 will have available some means for providing low frequency AC power to the stator winding. A
convenient way of providing low frequency power is to have a DC power source and to use switches, referred to as commutating switches or commutating contactors, to switch the DC to provide a stepped AC current waveform which very approximately simulates a sinusoidal waveform. Such a system for providing a stepped waveform is described in "Spotting Equipment for Synchronous and Wound Rotor Motors", a booklet published by tha General Electric Company, April 1982, designated GET-1722D, (a revision of an earlier booklet publishPd November 1966), pages 3 and 4 and particularly Figure 2. The terms "inching" and "spotting" may be considered as synonomous.
r~le frequency o~ the stepped DC wave used to simulate an AC wave for supplying the stator windings when inching of a motor is required, is usually of the order of one percent of line frequency or about 0O6 cps. It may, however~ be preferable to use even a lower frequPncy in the present invention as only a very slow rotation is required to dis-tribute the heat generated in the slip rings. A frequency of the order of one tenth the inching frequency has been found to be satisfactory. It is, of course, possible to obtain a lower frequency by slowing down the sequence in which the commutating contactors operate.
It i9 therefore a feature of this invention to provide an improved apparatus for ~topping a grinding mill or other large rotating equipment driven by at least one electric motor through a clutch.
It is another feature of the invention to provide an improved apparatus for ~topping a grindin~
mill driven by at least one synchronous motor through a clutch, by pulsing the clutch while providing ~or a ~ery slow rotation of the motor to prevent localizaed , . ~ .

6~6~
Case 2886 overheating of the motor slip rings.
The invention therefore provides a control system for stopping ~otion of a drive system having a large load driven by one or more electric motors each coupled to the load through a respective clutch, each said motor having a stator including stator windings and a rotor including rotor field windings, said field windings being connected to slip rings having a surface for engagement with brushes to conduct current to said field windings, said control system comprising a DC supply for providing a DC voltage, commutating contactor means connected to said DC supply and to said stator windings for providing a stepped DC
voltage approximating a low frequency sinusoidal waveform to said stator windings to cause a slow stepped rotation of said rotor, and control means for actuating said commutating contactor means in response to said motor being stopped and said load continuing to have motion, to provide said s~epped DC voltage to said stator winding of each said motor and to provide a DC voltage to said field winding via said brushes and slip rings of each said motor, and to control each said respective clutch to periodically partially engage each said clutch for a short interval to brake the motion of said load, whereby said slow stepped rotation of said rotor moves said respective slip rings under said respective brushes for preventing localized overheating of the area of the 91ip rings beneath each brush.
3Q For a better understanding of the nature of the present invention reference will be made by way of e~ample to the accompanying drawings, in which:
Fig. 1 is a simplified schematic drawing showing the invention as used in a grinding mill driven by two motors, and Fig. 2 is ~ simplified block schematic ~, .

Case ~886 diagram of the control.
Referring to Fig. 1, thera is illustrated diagramatically a large ring gear 10 which is attached to a grinding mill ~not shown). The ring gear 10 (and the grinding mill) are driven by two pinions 12 and 14 whose teeth respectively engage the t~eth o~ ring gear 10 in continuous meshing relationship. Two shafts 16 and 18 respectively connect pinions 12 and 14 through air actuated dry clutches 20 and 21 to shafts 22 and 23 of synchronous 3notors 24 and 25. Operation or actuation of clutches 20 and ~1 is by air pressure respectively supplied by piping 26 and 27 through controllable air pressure valves 30 and 31 from air supply 28.
The synchronous motor~ 24 and 25 receive AC
electrical p~wer for normal operatio~ over conductors represented by 32 and 33 from an AC power source 34.
This AC electric power is supplied to the stator windings of each motor 24 and 25. The conductors or lines 32 and 33 each have a breaker or switch 35 and 36 respectively, shown in the closed position. A DC
power supply 37 is provided to supply DC ~ower to the rotor field windings of synchronous motors 24 and 25 over conductors represented by 40 and 410 Thus, for normal operation of the grinding mill, switches 35 and 36 are closed to supply the stators of motors 24 and 25 with AC power at a standard frequency and the motor ~ields are provided with field excitation ~rom DC
field supply 37. The motors 24 and 25 rotate driving clutches 20 and 21 which are closed or engaged to cau~e shafts 16 and 18 and pinions 12 and 14 to rotate driving the grinding mill. When two motors are used, as shownj they must by "synchronized", that is, the load must be controlled or adjusted so that it is equally shared between the two motors. This may be done in a number o~ way~, for example by clutch , . ..

~2~L~6~
Case 2886 pulsing as described in Canadian Patent ~o~ 934,679 -Eastcott et al, issued October 2, 1973 to Canadian General Electric Company Limited, or by use of an auxiliary field winding as described in Canadian Patent No. 896,091 - Herzog et al, issued March 21, 1972 to Canadian Gen~ral Electric Company Limited, or by other means~ The system used for load sharing does not form part o~ the invention e~cept that the invention requires controllable clutches in the motor drive and such clutches may be present in a grinding mill drive because they are used for starting and/or to assist in load sharing (where more than one motor is involved). The present invention is for an improvement which is used when stopping such a system, that is when stopping a system having a large load driven by at least one motor through a clutch.
The present invention require~ a source o-E
low frequency AC power. It is convenient to provide an approximation of low frequency power for the stator windings by using a direct current supply in combination with contactors or switches which apply positive or negative voltages in a predetermined sequence to predetermined ones of the three phase windings of the stator of a synchronous motor. This is described in the aformentioned Technical Information Bulleting GET-1722B and is well known in the prior art for use in inching a rotor. Thus there is provided, referring still to Figure 1, a DC supply and commutating contactors 42 which provides on conductors 43 and 44 an output that may be a stepped DC voltage simulating a low frequency AC waveform, or when desixed may be direct current (i.e. a voltage at zero frequency). The frequency of the simulated AC
waveform is controlled by an input at 45.
The switches 35 and 36 are respectively operated with switches 46 and 47 so that when one ~2~ ;2 Case 2886 switch of a pair is closed ~he other is open and vice ver6a. Thus, the stator windings of the synchronous motors 24 and 25 receive power either from the AC
power source 34 or from the DC supply and commutating S contactors 42.
A control 50 has an input 51 which provides a signal representing the po~ition of switches 35 and 36 (and the paired or associated switches 46 and 47).
Control 50 has input signals on cond~ctors 52 and 53, which may be signals from tachometers, with the signals representing the rotation of shaft 18 with pinion 14 and shaft 23 of motor 25, respectively. The difference in the signals provided to input conductors 52 and 53 would represent the slip of clutch 21, the signal at input 52 would represent rotation o~ the grinding mill, and the signal at input 53 would represent rotation of the motor. Thus, the input signals on conductors 52 and 53 would indicate if either the grinding mill or the motor 25 was stationary. The control 50 also has an input 54 for providing start/stop or inching commands as will be referred to hereinafter.
An alternate and perhaps simpler way to determine i~ the motor is stopped is to measure the induced voltage in the rotor~ When the motor is being braked and is not coupled to the load, it is conveniently braked by applying DC to the stator winding and connecting a low resistance across the rotor winding. The machine acts as a generator and the generated power is dissipated in the low resistance. This is a well known w~y to brake a motor~ When the motor is being braked in this manner, the voltage induced in the rotor can be measured at the slip rings. When the induced voltage reaches a predetermined low level, the rotation of the rotor is substantially stopped.
It will be apparent that a motor braked ln Case 2886 this manner, using the same voltage on the stator and thP same resistance, will come to a stop in a certain time that can be ascertained. A simpla timar could be used to indicate when the motor had stopped under these conditions.
The control 50 provides an output at 45 which carries an input signal for DC supply and commutating contactors 42 for selecting either DC as an output or selecting a stepped voltage simulating an AC waveform at a low frequency w~ich is determined by control 50. Also control 50 may incorporate means (not shown~ for controlling switches 35/46 and 36/47 if desired, or the switches may be separately controlled or may operate in response to improper operation of AC power source 34.
The contrvl 50 also provides an output on conductor 55 ~or controlling valves 30 and 31. The output may control valves 30 and 31 to cause clutches 20 and 21 to be open (disengaged), closed (engaged) or partially closed, and they may be pulsed as required that is they may be disengaged or partially disengaged for a predetermined time interval and then re-engaged in a repetitive sequence.
Re~erring now to Figure 2, the control 50, shown outlined in a broken line, will be described in more detail. ~le control 50 provides control for other operations in addition to that of stopping the mill in accordance with the invention and these functions will be described ver~ briefly.
The control 50 provides control signals for inching the motors, that is for slow rotation of the motors 12 and 14 (Figure 1). When inching of the drive is required the switches 35 and 36 are opened and switches 46 and 47 (Figure 1~ are c}osed. An inchin~ input at input 54 of inching control 56 causes a signal on conductor 57 which is applied to a clock i6~Z
Case 2886 _ g _ control 58 and switching control 60. Clock control 58 controls clock 61. Clock 61 provides a series of precisely controlled pulses and the frequency of the pulses is selected or con~rolled by clock control 58.
In response to the signal on conductor 57, clock control 58 provides a signal on conductor 62 which controls clock 6l to provide output pulses on conductor 63 at a frequency suitable for inching.
Switching control 60 also receives a signal from conductor 57 indicating that inching is required.
Switching control 60 then provides signals on conductor 45 suitable for controlling the commutating contactors in DC supply and commutating con~actors 42 (Fig. l).
The input conductor 51 has a signal indicating the switches 35 and 36 are open and this is received by the normal/contxolled circuit 64. A
conductor 65 extends from inching control 56 to normal/controlled circuit 64 and carries a signal indicating inching is underway. The normal/controlled circuit 64 provides an output signal on conductor 56 which is applied to pulsing or lock circuit 67 requiring the clutches to remain locked. Pulsing or locking circuit 67 provides a signal on conductor 68 to clutch drive 70 which in turn provides a signal on conductor 55 operating valves 30 and 31 to maintain the clutches 20 and 21 (Fig. l) locked.
During starting it i5 often desirable to pulse the clutches to balance the load between two driving motors. One system for doing this is described in Canadian Patent No. 9341679 issued October 2, 1973 to Eastcott et al and assigned to Canadian General Electric Company LimitedO The load is measured for each motor and if it is not balanced within certain predetermined limits the clutches are pulsed, that is the clutch pressure is reduced for a "~,.

Case 2886 very short time interval to a value low enough to permit the clutchas to slip~ Preferably the time interval is made slightly longer on the clutch associated with the motor having the greater load.
~en a load balancing signal is received on conductor 72, the normal/controlled circuit 64 provides a signal on conductor 66 requiring load balancing pulses and this is applied to pulsing or lock circuit 67. A
signal is provided on conductor 73 to clock control 58 which, in turn provides a signal to control clock 61 to generate suitahle pulsing signals on conductor 74.
The drive 70 receives the signals from conductor 74 and provides signals on conductor 55 to control the clutch valves 30 and 31 (Fig. 1). It may be convenient to have two clocks. The first clock would then provide timing signals for the commutating contactors and the ~econd clock would provide timing signals for clutch pulsing as these two sets of timing signals are not necessarily related. A signal on conductor 75 carries signals representing the power input to each motor so that drive 70 can provide differential pulsing, that is provide a slightly longer pulse to the valve controlling the clutch associated with the motor having the greater power input~ As was previously mentioned, this i~ not part of the present invention and is included only for a compLete understanding of Fig. 2.
The invention is concerned with the stopping o~ the system, and in particular with the stopping of the ~ill~ If the breakers or switches 35 and 36 should open (and switches 46 and 47 close) either because of a power failure associated with AC power source 34 (all Fig. 1) or because an operator wishes to inititate stopping, there will be a ~ignal on input conductor 51 which i~ applied to the normal/controlled circuit 640 Since the breakers have just opened there .~ , 6~
Case 2886 can be no inching in progress so stopping i5 initiated.
When stopping is initiated, the normal/controlled circuit 64 provides a signal on conductor 66 to pulsing or lock circuit 67 which in turn provides a signal on conductor 68 1:o drive 70 to actuate the valves 30 and 31 (Fig. 1) to release or disengage clutches 20 and 21 ~Fig~ 1). Now there are two ways to stop the system. One way of stopping the system is the way described in the a~orementioned Canadian Patent No. 1,172,735 where the clutches are periodically actuated to a partly closed condition for a short interval of time until the motion of either the motors or the mill is stopped~ If this procedure is used, the normal/controlled circuit 64 provides a lS signal on conductor 66 requiring pulsing, and on conductor 73 to clock control 58 requiring a suitable pulsing frequency. Clock control 58 then provides a signal ~ia conductox 62 to clock 61 which provides on conductor 74 the required pulsing signals for drive 70. As previously mentioned, a second clock (not shown) may be used to provide timing pulse signals to drive 70. Drive 70 actuates valves 30 and 31 (Fig. 1) via conductor 55.
Input conductors 52 and 53 provide signals to decision circuit 76 repre~enting respectively rotation of the mill and rotation of motor 25 (FigO
1). In the fairly unlikely event that both the motor and tha mill stop at the same time, the decision circuit 76 provides a signal on conductor 77 to normaltcontrolled circuit 64 to terminate the stopping oper~tion. I~ the mill has stopped oscillating but the motor is still rotatiny, the decision circuit 76 provides a signal or conductor 78 to the switching control 60 and the nor~al/controlled circuit 64. The normal/controlled circuit 64 provide~ a signal on conductor 66 which terminates the pulsing and on ~2~66~
Case 2886 - 12 ~
conductor 81 to initiate motor braking. The switching control 60 provides a signal on conductor 45 initiating the application of DC to the stator windinys of the motors 24 and 25 ~Fig. lj by DC supply and commutating contactors 42 (Fig~ 1) and the placing of a resistance across the rotor windings If the motor has stopped but the mill is still oscillating, the decision circuit 76 provides a signal on conductor 80 which is connected to pulsing or lock circuit 67, to clock control 58, and to switching control 60.
Clock control 58 provides a signal on conductor 62 to clock 61l and clock 61 outputs a signal on conductor 74 suitable for continuing the pulsing o~ the clutches as be~ore (as the mill has been slowed down somewhat, the duration or level of the pulses may be altered to maintain or restore a higher level of heat dissipation in the clutches), and outputs a signal on conductor 63 with clock pulses suitable for timing a very slow rotor rotation~ Thus, switching control 60 receives the necessary pulse timing signals on conductor 63 and a signal on conductor 80 which requires switching control 60 to output a signal on conductor 45 to cause the commutating contactors in DC ~upply and commutating contactors 42 (Fig. 1) to provided a stepped DC waveform suitable for very slow rotation o~
motors 24 and 25 (Fig. 1). It is the slow rotation of ~he motors that prevents overheatiny of the slip rings and brushes (not shown) in the field supply path to the rotors~ The rotation of the motors desired when the motors are used through the clutcheq to act as a brake for the mill oscillation, is a very slow rotation. That is, the rotation need only be sufficient to slowly move the area of contact between the slip rings and contacting brushes to distribute the generated heat. Consequently the clock pulses which govern this are usually ~t a lower frequency .

~L6~
Case 2886 than the pulses which are used to govern inching. For inching the clock control 58 responds to the signal on conductor 57 to provide a clock control signal on conductor 62 so that the clock 61 provides the appropriat~ inching frequency on conductor 63~ For the slow rotation desirable for stopping the mill, the clock control 58 responds to the signal on conductor 80 to provide a clock control signal on conductor 62 so that clock 61 provides a slower pulse control frequency, perhaps of the order of one tenth the inching frequency, on conductor 63.
Previously reference was made to another way to stop the system. While this system is perhaps not quite as efficient, it is nevertheless effective and will be described. When stopping is initiated, a signal is received on conductor 51 as before and the normal/controlled circuit 64 first provides a signal on conductor 66 to open or disengage the clutcheR. In response, pulsing or lock circuit 67 provides a signal on conductor 68 to drive 70, which in turn provides a signal on conductor 55 to close valves 30 and 31 (Fig.
1) which releases or disengages clutches 20 and 21 (Fig. 1). Normal/controlled ~ircuit 64 aLso provides a signal on conductor 81 to initiate motor braking.
Switching control 60 then provides a signal on conductor 45 which ~ctuates DC supply and commutating contactors 42 (Fig. 1) to provida DC to the stator windings of motors 24 and 25 (Fig. 1). At this time a low resistance is placed across ~he rotor windings also initiated by the signal on conductor 45 which turns off the supply 37 (Fig. 1). It is convenient to arrange for the resistance to be placed across the field windings whenever the DC field supply is disconnected from the field windinys. The motors will stop while the mill wilL continue to oscillate. When the motors stop the decision circuit 76 will provide a . ~.,.

Case 2886 signal on conductor 80 which, as be~ore, does three things. Conductor 80 is connected to clock control 58, switching control 60 and pulsing or lock circuit 67. Clock control 58 provides a control signal to clock 61 ~hich outputs on conductor 63 a s~ries of pulses suitable for timing slow rotation and o~
conductor 74 a series of pulses suitable for pulsing the clutches to a partially engaged condition for short intervals. Switching control 60 receives the initiating signal from conductor 80 and it first provides a signal on conductor 45 applying the DC
field to rotor windings over conductors 40, 41 (E'ig.
1) to locX the xotors at the motors~ Control 60 also receives the timing pulses ~rom conductor 63 and outputs control signals on conductor 45 ~or the DC
supply and commutating contactors 42 (Fig. 1) to cauqe the rotors of the mo~ors 24 and 25 (Fig. 1) to rotate very slowly to move the area o~ the motor slip rings (not shown) contacted by the brushes to avoid localized overheating~ Pul~ing or lock circuit 67 receives an actuating signal on conductor 80 and provides a pulse enabling signal on condu~tor 68 to drive 70~ Thu8 drive 70 receives a pulse enabling signal ~rom conductor 68 and a pulse timing signal from conductor 74 and provides on conductor 55 a series of control pulse.s which open valves 30 and 31 (Fig. 1) su~iciently to partially engage clutches 20 and 21 (Fig. 1) for short intervals sufficient to brake the oscillations of the mill without overheating the clutch~s. When the mill is stopped, a signal on conductor 77 from decision circuit 76 terminates the stopping routine.
The control of the motors to provide slow motor rotation when the motors are u~d to brake the mill (rather than having the motors electrically ~, ~6~62 Case 2886 locked) ensures that the motor slip rings do not have areas subjected to excessive overheating.
It is believed that the preceding description will provide an understanding of the invention including any variations which would be apparent to those skilled in the art.

Claims (6)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A control system for stopping motion of a drive system having a large load driven by one or more electric motors each coupled to the load through a respective clutch, each said motor having a stator including stator windings and a rotor including rotor field windings, said field windings being connected to slip rings having a surface for engagement with brushes to conduct current to said field windings, said control system comprising a DC supply for providing a DC voltage, commutating contactor means connected to said DC supply and to said stator windings for providing a stepped DC voltage approximating a low frequency sinusoidal waveform to said stator windings to cause a slow stepped rotation of said rotor, and control means for actuating said commutating contactor means in response to said motor being stopped and said load continuing to have motion, to provide said stepped DC voltage to said stator winding of each said motor and to provide a DC voltage to said field winding via said brushes and slip rings of each said motor, and to control each said respective clutch to periodically partially engage each said clutch for a short interval to brake the motion of said load, whereby said slow stepped rotation of said rotor moves said respective slip rings under said respective brushes for preventing localized overheating of the area of the slip rings beneath each brush.

2. A control system as defined in claim 1 wherein said control means comprises a clock for providing a first train of timing pulses, and a switching control for receiving said timing pulses and providing a timing control signal to said commutating contactor means to time the operation of the commutating contactors to set the frequency of said low frequency waveform.

3. A control system as defined in claim 2 wherein said control means further comprises a drive for controlling the operation of said clutches, a further clock providing second timing pulses to said drive for determining the duration of each interval the clutch is actuated and the period of time between successive intervals, and said drive actuating said clutches to a partially closed condition sufficient to brake said motion of said load for the duration of each said interval.

4. A control system as defined in claim 3 wherein said control means comprises a decision circuit for receiving signals from at least one of said motor and said load for determining a condition where said motor is slowly rotating in response to said low frequency signal, and when said load is stopped, and providing a signal when both conditions occur to terminate further stopping operation of said control system.

5. A control system for stopping the motion of a drive system having a grinding mill driven by at least two three phase, synchronous electric motors each coupled through a respective dry clutch to drive said mill, each said motor having a stator including a three phase winding and a rotor including a field winding, said field winding being connected to slip rings having a surface for engagement with brushes to conduct DC current to said field windings, said control system comprising a DC supply for providing a DC voltage, commutating contactor means connected to said DC supply and to the three phases of said three phase winding for providing DC voltages to various ones of said three phase windings to simulate a low frequency stepped waveform to cause a slow stepped rotation of said rotor, control means for actuating said commutating contactor means in response to said motors being stopped and said grinding mill being in motion to actuate said commutating contactor means to provide said low frequency stepped waveform to said three phase windings of each said motor, to provide a DC
voltage to each said field winding via said respective brushes and slip rings, and to control each clutch to periodically partially engage each clutch for a short interval to brake the grinding mill without overheating the clutch, whereby said slow stepped rotation of said rotor moves the respective slip rings under the contacting brushes to change the area of the slip rings contacted by the brushes and thereby prevent localized overheating of said slip rings.
6. A control system as defined in claim 5 wherein said control means comprises a controllable clock for providing at least a first and a second set of timing pulses at selectable frequencies, a switching control for receiving said first set of timing pulses and providing a timing control signal to said commutating contactor means to time the operation of the commutating contactors to provide said low frequency waveform at a predetermined frequency, a decision circuit for receiving from at least one of said motors a signal representing motor speed and from said mill a signal representing mill

Claim 6 continued:
speed, said decision circuit providing a first output signal to said switching control representing a condition with said motor substantially stopped and said mill in motion, said switching control being actuated in response to said first output signal, said decision circuit providing a second output signal representing a condition with said motor slowly rotating and said mill substantially stopped, said switching control being responsive to said second output signal to terminate operation of said commutating contactors and to disconnect the DC supply from said stator windings.