Electric motors: Replace or repair?

There are two avenues of thought about whether an electric motor should be replaced or repaired, depending on whether you are a motor manufacturer or a rewind company. A motor manufacturer argues that rewinding a motor loses efficiency and the cost of repair can be as much as the price of a new motor. Rewind companies, disagree, saying a rewind today delivers equal or improved efficiency levels at a competitive cost compared to new. So how does the end-user decide?

Here are three simple steps to help you decide. 

Running hours
When a motor is running for less than 2000h per year, then the efficiency level is not the main concern and purchasing a new motor is not always the best solution: the energy savings gained from a new high efficiency motor are unlikely to justify its purchase price. These low running hour applications are ideal candidates for a high quality repair and rewind. Above 2000h per year, you now need to weigh up the cost of repairing the motor with the cost of replacing it with a new one. 

Cost of repair vs. replacement
When repair costs are 60% or more of the cost to replace, replacing the motor could be the best option. Where repair costs are below 60% of replacement costs, further calculations should be done to compare total overall running costs of the repair and replacement option.

Total overall running costs
Before deciding to replace or repair a motor, the total overall cost of running the motor over its lifetime should be calculated (see formula below). This involves looking at:

• The purchase cost
• The cost of running (energy consumed over its expected life)
• The cost of not running (annual costs associated with maintenance or repair)

The new motor will have a higher purchase price, but lower running costs due to the higher efficiency modern motor designs typically bring. It will also have a lower cost of not running as it will be more reliable.

For the existing motor we substitute the repair cost for the purchase price and evaluate the running costs taking into account the loss of efficiency a rewind may bring. The cost of not running will also be higher as the older the motor the higher the maintenance costs.

Do you measure up?

A total cost of ownership calculation shows whether the repaired or replacement motor would be the most cost effective.

In replacement scenarios for example, a 45kW motor costing £1700, with 94.1% efficiency and running for 6000h per annum would cost £2,105,454 over a 20 year life. This assumes an average electricity price of £0.08 and the downtime cost per annum of around £80,000.

When expressed as purchase price over total cost of ownership, the resulting ratio is very small. For example, if the ratio was one, then this would mean that the purchase price was the same as the cost of ownership. But in the above example, you end up with a ratio of 0.0008, meaning that the running costs of the motor are more important to consider over the purchase cost. In these situations, a more efficient and more reliable motor choice should be chosen to reduce both the costs of running and costs of not running until the ratio is 1 or greater.

Condition monitoring device for low voltage motors

At Hannover Messe this year, ABB displayed its new smart sensor for motors. It’s essentially a 'fitness tracker' for industrial motors that enables plant operators to actively monitor the health of their motors before problems occur. ABB’s smart sensor picks up data on vibration, temperature, sound and other parameters and uses it to reduce motor downtime by up to 70%, extend lifetime by as much as 30% and lower energy use by up to 10%.