Proactive maintenance can Save motors during brownouts
May 1st 2006 A sustained voltage sag or 'brownout' could virtually destroy the insulation system of every electric motor running fully loaded during the event.
Chuck Yung* of EASA explains why, and what can be done about it
The dangerously low voltages that accompany brownouts and power interruptions not only reduce motor efficiency during the event, but also may cause a rash of motor failures in the days, weeks and months that follow.
Shutting down every motor during a brownout is seldom an option. Production personnel are rarely eager to scrap batches of product just to protect an electric motor. Still, proactive maintenance management can minimise the effects of the problem in ways that may be more acceptable to production.
Why brownouts pose problems
During a brownout or power interruption, line voltage often drops significantly. That extends the life of incandescent bulbs, but it can be hard on motors. The torque developed by an electric motor changes as the square of the voltage applied. A 10% increase in voltage for example, will boost torque 21% (1.1 x 1.1 = 1.21). Conversely, a motor operating at 90% of rated voltage during a brownout will suffer a 19% reduction in torque (0.9 x 0.9 = 0.81). If the applied voltage were 20% below the rated voltage, the motor would produce only 64% of rated torque (0.8 x 0.8 = 0.64).
Unless the load is reduced, the motor will, in effect, be subject to a 156% overload (1 / 0.64). It will heat up and fail - quickly.
If torque decreases, below the torque required by the load, the motor will stall. At that point, all the energy is being converted to heat, so the temperature of the winding will increase rapidly.
Excess heat is a problem for motors because insulation life is halved for every 10°C increase. At the same time, the temperature of the winding will rise 10 to 15°C for each 10% drop in voltage. That means the insulation life of a motor operating on 10% lower voltage will decrease by 50 to 75%.
A temperature increase of 10°C would cut a motor's theoretical life expectancy from 30 000 to 15 000 hours or less, while a 20°C increase would shorten it to 7500 hours. The winding temperature of a severely overloaded motor can quickly rise 50°C or more, reducing its life expectancy to under 940 hours.
Importantly, the theoretical effects on insulation life in Table 1 apply to motors exposed to such temperatures for any appreciable period, NOT just those operating continuously at elevated temperatures.
That means even brief brownouts (less than an hour) can damage motor insulation systems, causing unexpected failures later.
A sustained brownout could virtually destroy the insulation system of every electric motor running fully loaded during the event.
What happens to efficiency?
Another potentially expensive result of operating on reduced voltage during a brownout is increased energy consumption. Compared with normal operation, an overloaded motor converts more of the supplied power to heat and less to doing work, so efficiency drops and power bills rise - sometimes dramatically.
As Table 1 shows, operating on just 10% lower than nominal voltage can increase winding temperature 10 to 15°C, while reducing efficiency by 0.5 to 1%. This seemingly small change could have a significant impact on the bottom line, given that the incremental cost of electricity during peak periods can soar from. For example, a 75 kW motor at 90% of rated voltage means the winding temperature increases by 10°C, and the efficiency decreases by 0.6%.
Some good news
It is clear that brownouts and power outages can have expensive repercussions, both in terms of damaged motors and higher power bills. Fortunately, proactive maintenance management can reduce or even eliminate the effects of such low-voltage events.
Basic safeguards include monitoring the supply voltage and training personnel to respond quickly if it drops to a predetermined level. For critical applications, install thermal protectors or condition-monitoring devices that can detect abnormally high winding temperatures and shut down the motor. Such modifications can be made easily by your motor maintenance supplier. If shutting down is not an option, it is sometimes possible to reduce the load by throttling back a fan or partially closing a valve during a brownout. The alternative is to rewind or replace motors at an alarming rate.
Lastly, when having a motor rewound, be sure to specify 'class H' insulation and maximum slot fill. The lower the current density (amps per mm squared), the better able the motor will be to withstand undervoltage conditions (and the more efficient the motor will be). Energy efficiency and conservation may help get us out of this mess, so that is important. When considering the repair versus replace decision, work with your repairer to see which makes better economic sense.
Important considerations include annual operating hours, cost of electricity, and capital expense payback requirements for your firm.
Weigh the efficiency of newer available EFF1 and EFF2 designs, and the potential savings in operating costs. Be sure to factor in the actual running time for the motor, and determine whether or not the payback is within a reasonable period. The longer a motor operates, the greater the benefits of improved efficiency.
Since electric motors account for some 70% of industrial electricity use, it makes sense to be proactive in guarding them against the effects of low-voltage events. The benefits for end users include avoidance of higher power bills and substantial motor repair and replacement costs.
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