Slip Ring and Exciter Brushes: Wound-Rotor Motors, Synchronous Machines and Turbogenerators

Quick Answer

Slip rings — also called collector rings — carry current continuously into a rotating winding without reversing it, so brush selection for rings differs from commutators: contact drop matters less, while current density, peripheral speed, film stability and ring condition matter more. Electrographite grades such as EG4 (nominal 12 A/cm², maximum 60 m/s) suit synchronous machine rings, turbogenerator exciters and many wound-rotor motors; natural graphite is widely used on the fastest turbogenerator collector rings; copper-graphite grades suit low-voltage, high-current rings at lower speed; silver-graphite grades suit signal rings and shaft grounding. The practices that keep ring brushes healthy are consistent: stagger brush positions across the ring width so the tracks overlap and the ring wears evenly; keep spring pressures equal so parallel brushes share current instead of a few carrying most of it; keep ring runout and surface finish within limits; manage brush dust, which is conductive; and on DC collector rings, account for the polarity difference in wear between the positive and negative rings. Shaft grounding brushes are a separate duty: they divert shaft currents — from variable frequency drives in particular — away from the bearings.

A typical slip-ring problem does not start with the brushes. It starts with the dust.

A wound-rotor motor on a crusher runs for years with its brushes in contact. Each brush sheds a little carbon as it wears; the dust settles in the slip-ring enclosure, on the insulation between the rings and on the brush holder supports. Carbon is conductive. Eventually a damp morning, or a dust layer thick enough to bridge between the rings, is all it takes for a flashover across the rings — and the repair now involves the rings, the holders, sometimes the rotor leads, and a stopped crusher.

Slip-ring brushes are simpler than commutator brushes in one way — there is no commutation — and less forgiving in others. This guide covers where they are used, how the grade is chosen and how they are maintained.

Slip Rings and Commutators: What Is Different

Because there is no commutation, the high contact drop that helps a difficult commutator is not needed on a ring. A grade that would struggle on a commutator can be excellent on a ring, and vice versa.

Where Slip Rings Are Used

Wound-rotor (slip-ring) induction motors

Slip rings connect the rotor winding to external resistors or a converter. Adding resistance to the rotor circuit during starting gives high starting torque with low starting current, which is why wound-rotor motors are used on crushers, ball and rod mills, long conveyors, cranes, large fans and other high-inertia loads.

• Brush-lifting and short-circuiting devices. Many wound-rotor motors short-circuit the rings once the motor is up to speed and lift the brushes off. The brushes then wear only during starting and often last for many years. If the device fails to lift, the brushes run continuously at full rotor current and wear fast.
• Continuous-contact designs. Where the rotor circuit is controlled throughout running — by slip energy recovery or a rotor-side converter — the brushes are in contact all the time, and grade and maintenance matter much more.

Synchronous motors and generators with static excitation

The DC field current for the rotor is supplied through two rings, one positive and one negative. Field current is moderate, speeds can be high, and reliability is critical — a lost field trips the machine.

Turbogenerator collector rings and exciters

Large turbogenerators run at 3,000 or 3,600 rpm, so collector rings run at high peripheral speed, and the brushes carry the full field current of the generator. Older and smaller units use a DC exciter — a commutator machine on the generator shaft — whose brushes see high speed and continuous duty. Brush makers' application data for the excitation brushes of thermal-plant turbogenerators is typically about 8–10 A/cm² at 35–50 m/s, compared with under 20 m/s on hydro generators. EG4 brushes are produced for turbogenerator exciters and excitation slip rings on units below 200 MW.

On many turbogenerators, brushes are replaced while the unit is running, one at a time, using insulated tools and plug-in holders designed for removal under load. Before a brush is pulled, the currents in the brushes on that ring are checked with a clamp meter: removing a brush that is carrying most of the ring current can overload the rest or trip the unit. Online change is only safe with trained staff, correct personal protective equipment and a written procedure — the brush gear is live at field voltage.

Wind turbines

Doubly fed induction generators — common in many wind turbine designs — feed the rotor through a slip-ring unit, and wind turbines also use shaft grounding brushes. The slip-ring unit runs in a confined nacelle, and the conditions are hard on brushes: low humidity, condensation during cold starts, oil vapour from the gearbox and, offshore, salt. Long spells of light wind mean low ring current, and high-metal brushes perform poorly at low current density, tending to thread the rings. Carbon dust that turns conductive in damp air can track and flash over. With service visits expensive to arrange, dust extraction, wear monitoring and planned replacement intervals matter more there than almost anywhere else.

Other applications

Rotating equipment that needs power or signals on the rotor — kilns, cable reels, rotating tables, instrumented shafts — uses slip rings of various sizes, often with metal-graphite or silver-graphite brushes for low and stable contact resistance.

Ring Materials and Surface

• Steel — common for larger rings; hard-wearing, works well with electrographite grades
• Stainless steel — corrosion resistance in damp or aggressive atmospheres
• Bronze and copper alloys — good conductivity, used with metal-graphite grades for high current

Surface finish and runout. Rings need a fine, uniform finish and low runout — the brush must follow the ring surface without bouncing, especially at high speed. Rings that are oval, grooved or have a spiral track worn into them are machined before new brushes are fitted.

Helical grooves. Rings running above about 20 m/s usually have a helical groove cut in their surface. The groove breaks up the air cushion that would otherwise lift the brushes at high speed, improves cooling and helps the brushes share current evenly. Typical proportions are a pitch of 8–25 mm, a width of 2–5 mm and a depth of 3–4 mm, with the groove edges chamfered about 0.3–0.5 mm at 45°. The brush's axial width should be a whole multiple of the groove pitch, and the remaining contact area at least about 60% of a plain ring's. Slotted brushes are not recommended on grooved rings. Each machining of the ring makes the groove shallower, and a worn-out groove is a known cause of uneven current sharing, so check its depth at overhauls and recut it before it disappears.

Choosing the Grade for Rings

Metal-graphite grades are kept off high-voltage rings and out of totally enclosed machines, because their wear dust is conductive.

Then run the same checks as for commutators — see our guide to carbon brush grade selection:

Current density. Ring current divided by the total contact area of the brushes on that ring. Example: a wound-rotor motor ring carrying 400 A with three 25 × 32 mm brushes has 24 cm² of contact area — about 16.7 A/cm². That is above the nominal 12 A/cm² of an electrographite grade such as EG4, so either a fourth brush per ring is needed, bringing it to 12.5 A/cm², or a higher-current grade.

Peripheral speed. v = π × D × n / 60. A 300 mm ring at 990 rpm runs at about 15.6 m/s; a 400 mm ring at 3,000 rpm at about 62.8 m/s, which is above EG4's 60 m/s maximum.

Current Sharing Between Parallel Brushes

Several brushes on the same ring are connected in parallel. They share the current according to their contact resistance, and that sharing is rarely perfectly even. The problem is that it can become self-reinforcing: a brush that takes more current runs hotter, its contact resistance can fall, and it takes even more — while its neighbours carry less and their film deteriorates. In the worst case a few brushes carry most of the current, overheat, wear fast and burn their shunts; the remaining brushes are then overloaded in turn, and the sequence can end in a flashover.

Prevention:

• Equal spring pressure on every brush, checked with a spring balance
• One grade, one batch on a machine
• Equal shunt lengths and good, clean connections at every terminal
• Clean rings and holders — contamination changes contact resistance locally
• Helical groove kept to depth on grooved rings — a worn-out groove is a known trigger
• Measure the current in each shunt with a clamp meter during operation on critical machines; a brush carrying far more than its share is the early warning

Polarity on DC Collector Rings

On rings carrying DC field current, the positive and negative rings do not behave identically: the direction of current through the sliding contact affects contact drop, film formation and wear, so the two rings and their brushes typically wear at different rates. Reports differ on which ring suffers more, and the answer depends on grade, ring material and operating conditions, so compare the two rings at each inspection rather than assuming. Periodic reversal of the field polarity, to even out the wear, is a recognised remedy. Where the generator or exciter maker recommends it, follow the OEM's interval and procedure.

Brush Arrangement

• Stagger the brushes across the width of the ring — not all in the same circumferential track — so the brush tracks overlap and the whole ring surface is used. Brushes running in one track cut a groove.
• Space brushes evenly around the ring.
• Provide margin in the number of brushes on critical machines, so one brush can be removed for replacement while the rest carry the current within their limits.

Brush Dust

Carbon dust is conductive. In a slip-ring enclosure it settles on insulation, between rings and on holder supports, and is a classic cause of tracking and flashover.

• Keep slip-ring enclosures ventilated or dust-extracted, with filters maintained
• Clean regularly by vacuum; do not blow dust deeper into the machine
• Inspect insulation between rings and on holder supports, and measure insulation resistance at overhauls
• On wind turbines and other remote machines, use brush wear sensors and plan replacement before the brushes reach their limit

Shaft Grounding Brushes

A grounding brush running on the shaft gives shaft currents a low-resistance path to earth, so they do not pass through the bearings. Sources of shaft current include:

• Variable frequency drives — the common-mode voltage of the drive couples onto the shaft and discharges through the bearing oil film, eroding the races. The pattern of fine grey frosting and fluting on the races is characteristic.
• Magnetic asymmetry in large machines, which induces a voltage along the shaft
• Electrostatic charge from belts, fans or process equipment

The usual protection on VFD-fed motors is an insulated bearing at one end combined with a shaft grounding brush or ring at the other, as covered in our guide to industrial electric motors. Grounding brushes carry very little current, so they need a grade with low and stable contact resistance at low current — silver-graphite or copper-graphite — and a clean, dry contact track on the shaft. A grounding brush on a rusty or oily track does nothing.

Maintenance Checklist for Slip-Ring Brushes

• Brush length — record and track wear rate
• Spring pressure — every brush, all equal
• Brush fit in holder — free without rocking
• Shunt condition and colour — discoloured shunts mean uneven current sharing
• Ring surface — film uniformity, grooving, spiral tracks, ovality and runout
• Helical groove condition on grooved rings
• Brush staggering — tracks overlapping across the ring
• Dust — enclosure, insulation, filters
• Insulation resistance — at overhauls
• Brush-lifting device — on wound-rotor motors, check that it lifts and short-circuits fully
• Polarity — on DC collector rings, compare wear on the two rings and follow the OEM's polarity-reversal interval where one is given

Common Mistakes

1. All brushes running in the same track. The ring grooves.

   Prevention: Stagger brush positions so the tracks overlap across the ring width.

2. Unequal spring pressures. A few brushes carry most of the current and overheat.

   Prevention: Measure and equalise spring pressures at every inspection; replace weak springs as sets.

3. Too few brushes for the ring current. Current density far above the grade's nominal value.

   Prevention: Calculate current density per ring and add brushes or change grade where needed.

4. High-speed rings with a grade rated below the peripheral speed. Bounce, arcing and rapid wear.

   Prevention: Calculate peripheral speed and select a grade rated above it with margin.

5. Brush dust left to accumulate. Tracking and flashover between rings.

   Prevention: Ventilate or extract dust, vacuum regularly and inspect insulation.

6. Brush-lifting device not working on a wound-rotor motor. Brushes run continuously at full current and wear out quickly.

   Prevention: Check the lifting and short-circuiting mechanism at each inspection.

7. Grounding brush on a dirty or corroded shaft track. No effective path to earth, and the bearings keep failing.

   Prevention: Keep the grounding track clean and dry, and check the brush contact resistance.

8. Online brush replacement without a procedure. Brush gear is live at field voltage.

   Prevention: Replace brushes under load only with trained staff, insulated tools, correct PPE and a written procedure.

Supply from Kasko Makine

Kasko Mak branded EG4 electrographite carbon brushes are produced for:

• Turbogenerator exciters and excitation slip rings on units below 200 MW
• Synchronous motors and slip-ring (wound-rotor) induction motors
• DC generators, converters and single-armature converters
• Rolling mill drive motors and marine propulsion motors
• Low-power DC motors and general-purpose electrical machines

Grade data: nominal current density 12 A/cm², maximum peripheral speed 60 m/s, contact voltage drop 0.9–1.9 V per pair, specific resistance 6–16 µΩ·m, friction coefficient 0.25 maximum, nominal spring pressure 20 kPa, dimensions and test methods to GOST 12232-89.

Construction: 25 × 32 × 40 mm standard and other sizes to order, to drawing or from samples; single or twin tamped copper shunts; forked, ring, flat-blade or bare terminals; top rubber cushion, contact shoe or radial or bevelled contact face.

Engineering support: Send the ring diameter and material, speed, ring current and voltage, number and size of brushes per ring, and whether the brushes run continuously or only during starting. We will check current density and peripheral speed against EG4 and advise if a different grade family suits the duty better.

Send your slip ring data for a brush selection and quotation — we reply within 24 hours. Reach us at info@kaskomakine.com or WhatsApp +90 (537) 521 1399.

Continue Reading: Carbon Brush Series

• Carbon Brushes: Types, Grades and How They Work
• How to Select a Carbon Brush Grade
• Commutator and Carbon Brush Problems: Diagnosis Guide
• Replacement Carbon Brushes: Dimensions, Shunts and Terminals

Frequently Asked Questions

Q: What carbon brush grade is used on slip rings?
A: It depends on the ring duty. Electrographite grades suit moderate- to high-speed rings on synchronous machines, turbogenerator exciters and many wound-rotor motors, and natural graphite is common on high-speed turbogenerator collector rings. Copper-graphite grades suit low-voltage, high-current rings at lower speed, and silver-graphite grades suit signal rings and shaft grounding. The choice is then checked against current density per ring and peripheral speed.

Q: How is current density calculated for slip ring brushes?
A: Divide the ring current by the total contact area of the brushes on that ring, using each brush's tangential thickness times axial width. A ring carrying 400 A through three 25 × 32 mm brushes has 24 cm² of contact area, giving about 16.7 A/cm². Adding a fourth brush brings it to about 12.5 A/cm², close to the nominal value of an electrographite grade such as EG4.

Q: Why are slip ring brushes staggered?
A: If all brushes on a ring run in the same circumferential track, they wear a groove into the ring while the rest of the surface is unused. Staggering the brushes across the ring width makes the tracks overlap, so the whole surface wears evenly and the film forms uniformly. It is a simple arrangement that extends ring life between machining.

Q: Why do some slip ring brushes overheat while others do not?
A: Parallel brushes on a ring share current according to contact resistance, and the sharing can become self-reinforcing: a brush carrying extra current runs hotter and can take even more. Unequal spring pressures, mixed grades, poor connections and contamination are the usual triggers. Equal spring pressures, one grade per machine, clean rings and checking shunt currents with a clamp meter prevent it.

Q: What is a brush-lifting device on a wound-rotor motor?
A: It is a mechanism that short-circuits the slip rings once the motor reaches speed and lifts the brushes off the rings. External rotor resistance is only needed during starting, so lifting the brushes eliminates their wear during running. If the device fails, the brushes stay in contact at full rotor current and wear rapidly, so it should be checked at every inspection.

Q: What does a shaft grounding brush do?
A: A shaft grounding brush gives shaft currents a low-resistance path to earth so they do not discharge through the bearings. On VFD-fed motors the drive's common-mode voltage would otherwise erode the bearing races, producing frosting and fluting. The usual protection is an insulated bearing at one end and a grounding brush at the other, with a silver-graphite or copper-graphite brush on a clean, dry shaft track.

Q: Can turbogenerator collector brushes be changed while the unit is running?
A: On many turbogenerators, yes — plug-in brush holders allow brushes to be replaced one at a time under load. The brush gear is live at field voltage, so it must only be done by trained staff with insulated tools, correct protective equipment and a written procedure. Brush currents are checked first, because removing a brush that carries most of the ring current can overload the others or trip the unit.