Jim Bryan
EASA Technical Support Specialist (retired)
The most stressful time for electric motors is during starting. The speed-current curve in Figure 1 illustrates why. At starting, the motor current is the highest it will ever be. This is referred to as starting or locked rotor current. These different terms describe that when the shaft speed is zero, the current is maximum. Note also the impact of applied voltage to the current characteristics. This will be discussed later.
Many performance parameters of the motor are directly proportional to the current. The parameter of most concern in this article is the heat produced which is proportional to the square of the current as represented by P = I2R. Where P is the power lost in heat (kilowatt-hours [kW•h]) due to the square of the current flow (I2) through a resistance (R). Once the motor has been successfully started, the load current level is reached and the cooling circuit of the motor is able to dissipate the additional heat produced by the starting current. Restarting the motor before this additional heat has been dissipated means more heat in the form of kW•h will be added on top of that which is there. Each subsequent start before the additional heat has been dissipated will add more heat — raising the temperature until some component in the motor reaches its failure point.
The limiting factor as determined by the design is the temperature increase resulting in component failure in a squirrel cage induction motor of one of three components: the winding, the rotor bars or the rotor shorting end rings. Depending on the design, the thermal “weak link” could be any of these.
NEMA guidance on starts
Because of this, thermal protection located in the winding might not be sufficient to prevent rotor bar or end ring damage. For this reason, the National Electrical Manufacturer’s Association (NEMA) and the International Electrotechnical Commission (IEC) have provided limits to the number of times a motor can be safely started in a given amount of time. These limits are defined in NEMA MG1-2011 12.54.1 and IEC 60034-12-2007 8.3 below.
NEMA MG1-2011
12.54.1 Normal Starting Conditions
Design A and B squirrel-cage induction motors having horsepower ratings given in 10.32.4 and performance characteristics in accordance with this Part 12 shall be capable of accelerating without injurious heating load Wk2 referred to the motor shaft equal to or less than the values listed in Table 12-7 under the following conditions:
a) Applied voltage and frequency in accordance with 12.44.
b) During the accelerating period, the connected load torque is equal to or less than a torque which varies as the square of the speed and is equal to 100 percent of rated-load torque at rated speed.
c) Two starts in succession (coasting to rest between starts) with the motor initially at the ambient temperature or one start with the motor initially at a temperature not exceeding its rated load operating temperature.
IEC 60034-12-2007 8.3
Motors shall be capable of withstanding two starts in succession (coasting to a rest between starts) from cold conditions, and one start from hot after running at rated conditions. The retarding torque due to the driven load is assumed to be constant and equal to rated torque, independent of speed, with an external inertia of 50% of the values given in Table 3. In each case, a further start is permissible only if the motor temperature before starting does not exceed the steady temperature at rated load.
Note: It should be recognized that the number of starts should be minimized since these affect the life of the motor.
In layman’s terms, both references mean if a motor is at room temperature, it can be started twice in succession. If the motor has come to the normal operating temperature, it can be started once. Before subsequent starts can be made, it must be cooled to normal operating temperature. As stated earlier, monitoring the rotor temperature is difficult so relying on the winding temperature monitors is necessary. For applications requiring multiple starts, this temperature monitoring becomes very important. Often the shorthand version is used: “2 cold/1 hot starts.” Manufacturers will often limit larger motors (for instance, greater than 200 hp [150 kW]) further by saying a total of nine starts per day or less in certain circumstances. Manufactures may relax these limits during commissioning to allow for alignment or refine balancing procedures. They should be consulted before these procedures are attempted to verify that it is safe.
Note that these limits are based on load inertias (Wk2) listed in NEMA Table 12-7 or Table 20-1 for large motors and IEC Table 3. The inertias were calculated based on the horsepower or kW rating and speed of the motor. The load inertia is an important factor because it will determine how long the load will take to accelerate to full speed. The higher the inertia, the longer the acceleration time and therefore the longer the motor will draw the increased current necessary for acceleration. This increased time at elevated current results in more heating of the motor.
NEMA MG10-2001 Table 7 (see Figure 2) lists allowable starts for motors through 250 hp also based on size and speed. At first these limits seem to be in conflict with those in NEMA MG1, but closer examination shows they are in harmony. The limits in this table are based on the following conditions:
- Applied voltage and frequency must be with the limits set in NEMA MG1-2011 12.44 which is a combined value of ±10% of rated voltage and frequency. For instance, an 8% voltage variation and 2% frequency variation would be a combined 10% variation.
- During acceleration, the load torque is equal to or less than a torque that varies as the square of the speed and equal to 100% of rated torque at rated speed.
- External load inertia is equal to or less than the values listed in NEMA MG1-2011 Table 12-7.
The allowable starts per hour is the lesser of the value in Column A or Column B divided by the load inertia, if known. The values of Column B are nearly identical to the values for the same ratings given in Table 12-7 of MG1. In Table 12-7, the load inertia of a 100 hp, 4-pole motor is 441 lb/ft2, and the same value appears for a 100 hp, 4-pole motor in MG10. This means for the allowable inertia, the safe starts would be one, the same as stated in MG1 12.54.1. If the inertia is known and is less than the value given in Table 12-7, then additional starts might be allowable. If the motor is larger than 250 hp or if the load inertia is not known, the application should use the “2 cold/1 hot start” rule unless the manufacturer is consulted.
Impacted by voltage available
As noted in Figure 1, the starting current will be impacted by the voltage available during starting. When the power supply is limited, it is often necessary to employ some starting method such as reduced voltage, wye start delta run, or soft starting to limit the starting current and avoid voltage sag to other loads on the supply. While this does reduce the starting current, the acceleration time will be extended. So although the heating rate is reduced, the length of time the heating is occurring increases. This results in the same amount of kW•h being injected into the motor. Another way to look at this is to consider the acceleration of the load to be work accomplished.
Whether the load is accelerated in 5 seconds or 30 seconds, the same amount of work has been accomplished; therefore, the heat accumulated is the same.
Conclusion
A wise and experienced motor engineer once said that since a motor has a specific number of starts in its life, you can use those in the first year or spread them out. It is important to realize the stress imposed by starting and to limit the number of starts to achieve the best and longest performance of a motor. Even the limits as defined here represent an extreme in application; if that many starts are not necessary, they should be avoided.
The starting stress can be mitigated by using alternative applications as well. For instance, in the case of flow demand for a pump or fan, variable speed control may be able to provide constant correct flow rather than starting and stopping to adjust the availability of the material. In the case of repetitive operations such as a punch press or positioning a load, a clutch such as an eddy current or fluid clutch may help.
There are some applications that require many more starts per hour than these NEMA or IEC guidelines offer. Elevators for instance may start 40-50 or more times per hour during peak operation. These published guidelines refer to usual conditions. Unusual conditions, such as the elevator example, must be specifically addressed by the manufacturer’s designers.
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