According to our electrical team, heat is the number one cause of motor failure. It simply kills them. Temperatures near or above the insulation rating of your motor will cause your motor’s insulation to deteriorate faster, ultimately resulting in premature motor failure.
Defining the starting limit for your pump’s electric motors
Our team is often asked one important question: How often can I start my pump?
The answer will change depending on the size and starting method of the pump (which we will touch on below), but to avoid damage to your pump’s electric motor, it is always best to verify the starting limit with your vendor.
Motor starting limits are usually specified in two ways: number of starts per hour (typically used for lower HP motors) or minimum time frame for one cold and two hot starts (typically used for larger HP motors). These ratings are based on starting a motor across the line, which is when a typical induction motor may see inrush currents of 6-8 times its FLA, generating large amounts of heat in the motor windings. The allowable start number reflects how long it takes for your induction motor to cool off. Larger motors will take longer to cool, meaning that they can safely start less frequently than smaller motors.
H class insulation and heat
Some may think that having a pump with H-class insulation means that the insulation can’t be damaged. In reality, it only means that your motor has a little more headroom on the amount of heat it can safely tolerate. While this will help increase the number of allowable starts, the higher temperature allowance of H Class (180°C) insulation over standard F Class (155°C) insulation is far outweighed by how slowly most motors (especially submersible motors) can dissipate excess heat. Any benefit from higher insulation temperatures comes from the specified allowable number of starts.
Submersible pumps with a cooling jacket
There is also a belief that submersible pumps with a cooling jacket can be started more often. Unfortunately, the answer to this is usually still no. Once again, it comes back to how fast your motor can dissipate its excess heat. For heat to leave the motor windings, where most heat is generated, it has a long way to travel. It must pass through the winding insulation, the steel laminations in the stator, and the motor housing before being removed by a cooling jacket. While this may not sound like a lot, means that some larger motors can take hours to cool. Providing a cooling liquid to the motor housing does very little to increase the speed at which the excess heat disappears.
Solutions to mitigate motor damage from excessive motor starts
Change the starting method
As stated above, one way to increase the number of allowable starts involves starting a motor across the line (direct on line or DOL). Other options include soft starters and VFDs, which can drastically reduce the inrush current to an induction motor—sometimes to only 1.5 or 2 times FLA (as low as one quarter of across the line) if configured correctly. These are the best options for reducing the inrush current; they have the added benefit of reducing the required current from your power system to start your pump. Using a wye-delta starter or a reduced-voltage autotransformer starter can also reduce the inrush current to your motor by up to half that of an across the line start.
Change the number of starts
This can involve anything from increasing the volume of your sump, to simply adjusting the level of your pump up or pump down switches to eliminate backflow oscillations or maximize the range of your sump. Any change in the sump and piping system can reduce the number of starts that a pump experiences will help increase the life of your motor.
We hope this helped answer some of your questions! As noted at the beginning of this blog, best practice is to verify the starting limit of your pump with your vendor.