How efficient and reliable are electric motors?
Motor reliability is vitally important in 24/7 continuous processes such as this pump house, and access to the effect of temperature on motors must be made available.
Small departures from ideal operating conditions can substantially affect the performance and reliability of electric motors, says Steve Ruddell.
A major manufacturer of electric motors is calling upon the electric-motor industry to create greater transparency with its technical data by making available information that gives a clear indication of motor reliability, such as running temperature, alongside efficiency and motor noise. Root cause
Steve Ruddell, ABB’s general manager for motors and drives in the UK, acknowledges that winding breakdown is the main reason why motors fail, closely followed by bearing failures. He believes that all too often, customers accept motor failures without understanding the root cause. ‘In our experience, we believe that many of these winding and bearing failures are a direct result of motors running too hot,’ says Steve Ruddell. ‘For example, you may be told that your bearing has run dry. While in some instances this may be down to a poor re-greasing regime, it is also possible that the motor was simply too hot and the grease degraded prematurely.’ While efficiency classifications, as defined by Eff1, Eff2 and Eff3, have helped customers recognise the difference between a motor with poor energy efficiency motor and one with high energy efficiency, ABB is concerned that the classification is also regarded as a measure of the reliability of a machine. Today Eff1 is perceived as high quality. The belief is that you pay a higher price for a motor of higher efficiency motor and that the certification also indicates high quality and high reliability. ‘This is just not the case,’ he argues. ‘Our experience shows that there are some motors that achieve Eff1 status at the cost of significant drawbacks. These manifest themselves in many ways — increased running temperatures and excessive noise being a couple of examples.’ Reliability
Steve Ruddell proposes that ‘reliability’ could be defined as the sum of efficiency plus temperature rise. He argues that each of these elements directly affect each other and, therefore, the quality and reliability of a motor. Eff1 is easy to achieve in all but the smallest motors simply by increasing the amount of active material in the motor — more copper in the slots and smaller air gaps in the design.
| Modern motors have the potential to be efficient and reliable — but small changes in operating conditions can affect both and necessitate more frequent maintenance. |
However, the challenge is that the IEC34-2 sets tolerances for efficiency which are quite wide. ABB fears some manufacturers are declaring efficiencies that are at the top of the tolerance band, while delivering motors close to the lower tolerance level, or at worse outside this band. Meanwhile, the temperature rise of a motor can also be higher than you would expect from an Eff1 motor. ‘Hold a thermal camera up to the motor and you can see how hot it is running.’ Higher temperatures degrade the winding insulation more rapidly. and can lead to premature degradation of the grease in the bearing end shield and shorter re-greasing intervals. ‘Clearly, the cooler the running temperature the better,‘ says Steve Ruddell. ‘Lowering the temperature by just 10 to 15 K can double the life of the winding, yet most catalogues do not give this specific information.’ ABB has observed that some cheaper motors are generating more heat. The normal frame surface temperature in a high-reliability motor running at full load can be as low as 60 to 80°C, yet lower-reliability motors often run in excess of 90°C and have even been recorded at well over 100°C. Temperature
Temperature affects the re-greasing intervals of bearings. ABB assumes that bearings will run at 80°C with an ambient of 25°C. Should the bearing temperature increase by 15 K, the re-greasing interval should be halved. If the temperature falls by 15 K, the interval can be doubled. ‘The problem is that this data is not always highly visible, and so many engineers are probably not aware of this fact,’ says Steve Ruddell. ‘As a result if a catalogue states 10 000 h and the temperature increases by 15 K, then the bearing would need re-greasing in 5000 h. But how is a user meant to know this? End-users with continuous process applications should ask their supplier to provide the winding and bearing temperature criteria from the type test reports.’ Noise
Temperature can lead to problems in other areas. More active materials usually mean more heat being generated. To keep the motor within stated temperature limits, larger fans are employed to provide more cooling air. Larger fans mean more noise. ‘Remember, a 3 dB(A) increase in noise level is equivalent to a doubling of the audible noise of the motor,’ says Steve Ruddell. ‘So if, say, a 200 kW motor is showing 77 dB(A) against one showing a 70 dB(A) rating, then the 7 dB(A) increase in noise equates to the motor being about four times noisier. This should set alarm bells ringing. Higher noise levels could mean that the temperature is higher, which affects the overall motor reliability.’ ABB says that getting the right balance between efficiency, temperature and noise will go a long way to lower life-cycle costs, lowering running costs and increasing the overall reliability of an electric motor. ‘If you use motors in a 24/7 continuous process, then the last thing you need is a hot motor, which eventually fails. The cost to production can be immense. We believe that these users should have access to temperature-rise information and should understand the importance of the noise levels. This would help them choose a motor with greater reliability.’ Statistics
Steve Ruddell and his team at ABB are gathering statistics and data and are keen to hear from any end-user who believes that a motor has failed prematurely through temperature issues or from users interested in learning ways to identify a ‘reliable’ motor. www.abb.co.uk/energy