If you’re like me and dealing with pumps and motors on a regular basis, it can be useful to learn a few fundamentals about them. I’m learning these, myself, and just sharing along as I go for any interested parties. Here, motor operation and efficiency optimization will be discussed. Before I start, some of you may be thinking, “Now, wait just one doggone minute. Aren’t you a woman? Shouldn’t you be at home baking cookies instead of messing around with motors?” Rest assured, I can still bake a cookie. Behave, and you might get one along with your new motor or motor repair.
A motor engine has two main components: a rotor and a stator. Single phase motors operate according to the principle of induction. When alternating current power is applied to the stator, an expanding magnetic field is created that cuts across rotor conductor bars, inducing current in the rotor. When dealing with single phase asynchronous induction motors, 110-240v, the stator field has no rotation. Hence, it cannot start itself. With the stator in permanent position, +/- poles change position once each cycle. Why are single phase motors asynchronous? Synchronous speed here refers to the speed match between rotor and stator. Having the speed match is impossible in single phase induction motors. A rotor will always lag behind a stator in rotations per minute with induction motors. This lag is called slip. Slip is measured in %. The greater the slip, the greater the torque. It looks like this:
- Slip = [synchronous speed – rotor speed / synchronous speed] * 100%. Motor slip increases as power increases.
- To illustrate: Slip = [3,600 RPM stator – 3,450 RPM rotor]/3,600 RPM stator = 3.6% slip.
Because single phase motors are not self starting, there are four means to start these motors. Each has its set of torque/speed curves along with pros and cons to consider for applications these motors will pertain to.
- Capacitor start/ induction run (CSIR): This type of induction is noisy due to polar operations. CSIR is most commonly found in hard to start applications.
- Resistance start/ induction run (RSIR): Another noisy motor starter. These are cheap motors for low torque, low power applications.
- Capacitor start/ capacitor run (CSCR): This is a best choice and is more expensive. The capacitors are in series. The capacitor balances the motor operations.
- Permanent split capacitor (PSC): This starter offers smooth operation and is a good choice for centrifugal pumps and fans.
Stators are coiled iron or aluminum with silicon steel lamination. Okay, why laminate? To prevent energy losses via hysteresis and eddy currents. Guys, I had no clue what either of these terms meant. Hysteresis is a lag in effect behind its cause. That cause in this case would be energy loss. At least for me, eddy currents are tough to grasp. They’re closed loop reactions back to magnetic fields that created them. What that means in real terms is an energy loss via heat transfer. Copper is used as conducting material in stator coil windings. So, lamination serves to prevent these couple forms of energy loss, preserving motor efficiency. When discussing motor efficiency, we’re really talking about minimizing heat transfer, which is unconverted energy. Put your hand on a hot motor, and this is what we’re talking about. What accounts for efficiency loss?
Pcu1 = losses due currents flowing in stator windings, accounting for 40-45% of motor efficiency losses.
Pfe = losses due to eddy currents and hysteresis in the laminations. These stray losses are small and hard to measure. Iron losses = 30-35% of total losses.
Pcu2 = losses due to eddy currents flowing in the rotor bars and end rings. This accounts for 10-15% of total losses.
Pfrig = friction in bearings and windage losses from the fan. This accounts for 10-15% of losses.
Permanent Magnet Synchronous Motor (PMSM) is a good way to help eliminate loss, but it takes us out of the single phase motors we’ve been discussing thus far. These are also known as Electronically Commuted Motors (ECM). Variable frequency drives (VFD) are required for these types of motors. VFDs can only be operated on three phase motors. These motors are synchronous in that there is no slip between stator and rotor speeds. Without slip, these motors operate at cooler temperatures. These are relatively new to market, since the early aughts. The main difference is that the rotor is permanently magnetized via rare earth permanent magnets. These magnets produce more flux and resultant torque for their physical size as compared to induction types. What this all means is that in terms of reliability, lower operating temperatures reduce wear and tear and maintenance. They extend bearing and insulation life.
This is all I’ve got so far. There is much more to understanding motors, such as learning the concept of electromagnetic fields, the number of magnetic poles within stators, and how they relate to speed. For reference, I took the coursework via Grundfos online training, and got a bit of reinforcement via Wikipedia. Fair disclosure: I took that Grundfos training twice before passing the final exam. The content was dense to this newbie trying to grasp it on a first go.