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Slip-ring motors with brush lifting device

Slip ring motors offer many advantages for heavy duty applications. However, they need regular cleaning and maintenance. In applications with few motor starts and long operating times, brush ventilation can be useful - if it is well designed!

Reduced carbon brush wear, low maintenance requirements

Carbon brush lifting devices reduce carbon brush wear. Where they are used, carbon brushes rarely, if ever, need to be replaced and slip rings do not need to be cleaned of carbon dust as often.

With a slip-ring motor with brush lifting device, the advantages of a slip-ring rotor (smooth starting with high starting torque and low starting current) can be combined with the advantages of a squirrel-cage motor (low maintenance, operation without permanent brush wear).

The main advantages of a brush lifting device:

  • Less wear on slip rings and brushes
  • Less production stops for maintenance
  • Fewer carbon brushes required
  • Lower spare parts and service costs
  • Increased uptime and motor life

Brushes can be smaller and last much longer, often over the life of the motor. The slip ring chambers need to be cleaned less frequently. Another advantage is that lifting the brushes eliminates contact resistance and friction, resulting in a slight increase in efficiency. Finally, the brushes are returned to their original position after the motor has stopped.

Slip ring motors with brush lifting device or permanent contact brushes?

MENZEL slip ring motors are available either with permanent contact brushes or with a brush lifting device system. Permanent contact brushes have lower purchase costs. However, since they are in constant contact with the slip rings, both components wear out over time and must be maintained and replaced.

Brush lifters are slightly more expensive to purchase initially. However, they reduce wear on brushes and slip rings and help extend maintenance intervals, reduce service costs and increase reliability.

You should discuss whether it is worth investing in a brush lifting device with an electric motor expert. The choice depends on the particular application and the planned maintenance programme. Our engineering team will be happy to advise you at any time!

Slip ring motors with brush lifting device are part of our standard offer for industrial customers. We offer complete service as a one-stop-shop-solution: consulting, project planning, manufacturing and commissioning of entire motor systems with brush lifting device.

Functional principle of a brush lifting device

The functional principle of a brush lifting device with short-circuiting of the rotor is easily explained:

When the rotor has reached its rated speed, the rotor winding is first short-circuited via the short-circuit ring and then the carbon brushes are lifted off the slip rings with the brush lifting device. As soon as the rotor comes to a standstill, the brushes are put back on.

Once the carbon brushes are lifted off, the contact resistances between the slip rings and the carbon brushes are avoided. This means that no current heat losses can occur and there is no longer any friction between the carbon brushes and the slip rings. Wear of the carbon brushes and slip rings therefore only takes place during run-up.

Normally, the brushes simply stay in place. Due to the combination of conductive material such as copper with compressed carbon dust, the brushes are continuously worn and carbon dust is released through the sliding contact. To minimise brush wear and maintenance requirements, the rotor windings can be short-circuited and the brushes lifted off the slip rings after starting and after the motor has reached rated speed. The motor then runs efficiently at a single speed and the brushes are not needed again until the next motor start.

Why MENZEL electric motors?

MENZEL offers several of its own motor series and also specialises in customer-specific adaptations and theidentical reproduction of existing motors. The company keeps various motor variants up to 15 MW in stock at all times and adapts them at short notice for a wide range of applications and locations. The entire power range extends up to 20 MW and beyond. In addition, MENZEL offers comprehensive testing services for electric motors, generators, transformer-inverter-motor systems, gearboxes and frequency converters, no-load tests up to 10 MW, load tests up to 1.8 MW and bifrequency heating tests up to 5 MW.

MENZEL has been around for almost 100 years. Due to decades of working for different industrial sectors, our engineers know the requirements and expectations of their customers very well and deliver solutions very quickly and flexibly, even for unusual requirements.

Slip ring motors compared to squirrel cage motors

Applications include mills, crushers, conveyors, fans and pumps. Slip-ring motors are typically used for heavy-duty applications with continuous operation (S1 mode) and slow starting, which require high starting torques and high nominal torques.

In contrast, squirrel cage motors (which do not require motor brushes) are more common in variable speed applications and are preferred in most applications today.

However, there is still a stable demand for slip-ring motors, which, in addition to their good performance characteristics and robust design, can withstand voltage fluctuations and can therefore be used without any problems in regions where grid stability cannot be relied upon. A disadvantage is the constant wear of the motor brushes, so that they have to be replaced at regular intervals, which has to be integrated into the maintenance scheme of the plant.

Our slip-ring motors: Easy maintenance, reliable performance

Decades of experience repairing and replacing brush stroke motors have taught MENZEL how not to do it: Elaborate, cramped installations often led to failures and made access for maintenance and servicing difficult. Consequently, as with its motor series, MENZEL relies on a modular design with easily accessible parts for cleaning and, if necessary, replacement. The main modules

  • Slip ring body with short circuit ring
  • Brush rocker
  • Short-circuit and brush lifting device

can each be customised. The slip rings are dimensioned according to the rotor current and voltage. The hub diameter is dimensioned to fit the motor shaft. The carbon brushes are selected with suitable brush holders depending on the application and installation location.

We only use high-quality carbon brushes and advise our customers on the correct care

Mistakes are easy to avoid:

  • Only use suitable, high-quality brushes
  • Replace carbon brushes when they are half worn out
  • Always replace the whole set of brushes at the same time
  • Never mix different batches of brushes

The design of the short-circuit and brush lifting devices allows for easy assembly and maintenance. The slip rings and shorting bush are on the same hub, which facilitates mounting on the motor shaft and high-precision alignment. Wear parts such as carbon brushes are easily replaceable. The multiple contact bushings and flat plugs on the slip ring body are also easily accessible and replaceable. When the brush rocker is positioned by a servo motor, the limit switch is adjusted so that the rollers are completely removed and the roller bearings also show no wear. The servomotor has a very large travel, which ensures sufficient distance between the contacts for easy replacement without dismantling the unit.

Design details and the high-quality processing of suitable materials ensure safe and reliable operation. The slip rings are made of stainless steel. Bronze for the short-circuit socket ensures excellent sliding properties, which can be greatly extended by re-greasing. The short-circuit ring is made of silver-plated copper for low contact resistance. Multiple silver-plated shorting contacts provide a robust, safe solution for non-selective contacting with minimal contact resistance. Silver-plated copper blades on the slip ring ensure smooth and safe insertion into the short-circuit contacts. As a backup solution, the mechanism can be operated absolutely safely with a handwheel - both for switching on and for releasing the brushes.

A proven design from an experienced supplier is a must

The technology has proven to be trouble-free in various low, medium and high voltage applications that MENZEL has realised in the past. However, the savings in brushes and maintenance are offset by significant costs for the brush lifting device, which are similar for different motor sizes. For this reason, the technology is generally only used on motors of size 630 and larger. A proven design from an experienced supplier is a must, as failure of the brush lifting device itself would defeat the purpose.

MENZEL offers modular design adaptable to your individual requirements. Contact us for a personal quote!

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