Pool motor efficiency

A couple of years ago, Pentair introduced a motor to the pool industry that is a great improvement in efficiency as compared to what we have been calling “energy efficient” motors. This motor is referred to as a permanent magnet synchronous motor (PMSM) and it is used on their IntelliFlo line of pumps.
To truly appreciate why this style of motor is more efficient, one needs to understand what a power factor is.
First, all motors use magnets in order to turn a rotor. They work on the principle that opposites attract. Thus, the south pole is attracted to the north pole and vice versa. If you were to take a large horseshoe magnet and place a boy scout style compass in between its ends, the compass needle would spin to align itself with the horseshoe magnet’s magnetic field. If you could then reverse the polarity of the horseshoe magnet, the compass would spin 18o degrees and realign itself. If you could continuously reverse the polarity of the horseshoe magnet, the compass would keep spinning. This is the basic idea behind how a motor works.
So, how do you reverse the polarity of a magnet? One way is to use an alternating current to create an electromagnet. The following graph shows an alternating current. The current (and voltage) alternates from a maximum positive value down through zero to a minimum negative value and back up in a continual cycle according to the familiar sine wave pattern:

As the voltage and current alternate, so too does the polarity of the magnetic field. By using this type of continually reversing electromagnet, the compass needle (or rotor) will continuously spin inside the magnetic field.
An interesting thing happens when you pass an alternating current through a coil of wire that you use to create an electromagnet. The coil imposes a restriction upon the power. The voltage passes through faster than does the current. In other words, the current is said to “lag” behind the voltage. This means that the peak voltage no longer occurs in sync with the peak current. The peak current lags behind by a  small amount of time. This lag is indicated by the phase angle φ  in the following graph.  

This is very significant. From Ohm’s law, recall that Power equals voltage times amps (P=EI). This means that at any given time, you are to multiply the voltage value in the above graph times the current value to calculate the power value. Notice that when voltage and current are both positive, the power will be positive too. Similarly, since a negative times a negative is also a positive value, when both the voltage and current are negative, the power will again be positive.
But when the current lags the voltage, there will be times when one will be positive and the other negative. In these cases, the power will be negative. (Since a positive times a negative is negative.) This can be seen in the above graph where the power sine wave dips below the zero line. In these cases, the motor is no longer converting electrical power into mechanical power to turn a pump, it is turning mechanical power into electrical power. It has for this period of time, become a generator and is trying to feed power back to your power company. Your power company doesn’t like this, nor do they pay you for it. It pollutes the power in the neighborhood by screwing up their nice sine wave power and you simply pay for power that you don’t use.
To counter this problem, motor companies build “energy efficient” motors that capture this power that induction motors try to stuff back down the power company’s lines and store it in a capacitor so it can be reused by the motor. This is a small advance in efficiency.
A giant step forward was taken with the permanent magnet synchronous motor. With this style of motor, you don’t use a coil to create an electromagnet. You use permanent magnets instead. Since there is no inductive electromagnet, there is no current lag. This means that the voltage and current stay in sync. Since they are in sync, they will either both be positive or both zero or both negative. There will never be a case in which one is positive and the other is negative. Because of this, their product (the power) will always be positive as well.
The bottom line: efficiency
An induction motor is around 20-30 % efficient. An “energy efficient” motor (with a capacitor) is around 40-65% efficient. A permanent magnet synchronous motor (PMSM) is around 90% efficient.
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