Tag: motors

How does a typical motor know how fast to run?

Typical induction motors are slaves of the electrical cycle rate of the entering power (measured in hertz).

Our power in the US makes one full rotation from positive electrical peak to negative peak 60 times per second or 60hz (50hz in many other countries)

This means that the generators at the power plant would have to run at 3600 RPM if they only had two poles of power 2 poles (60 cycles per second x 60 seconds per minute = 3600 rotations per minute) in reality, power plants generators can run at different speeds depending on the number of magnetic poles within the generator. This phenomenon is replicated in motor design.

The more “poles” you have in a motor the shorter the distance the motor needs to turn per cycle.

In a 2 pole motor it rotates all the way around every cycle, making the no-load speed of 2 pole motor in the US 3600 RPM.

A 4 pole motor only goes half the way around per cycle, this makes the no-load (Syncronous) RPM 1800

6 pole is 1200 no load (no slip)

8 pole is 900 no load (no slip)

So when you see a motor rated at 1075 RPM, it is a 6 pole motor with some allowance for load and slip.

An 825 RPM motor is an 8 pole motor with some allowance for slip.

A multi-tap / multi-speed single phase motor may have three or more “speed taps” on the motor. These taps just add additional winding resistance between run and common to increase the motor slip and slow the motor.

This means  a 1075, 6 pole motor will run at 1075 RPM under rated load at high speed. Medium speed will have greater winding resistance than the high speed and therefore greater slip. Low speed will have a greater winding resistance than medium and have an even greater slip.

Variable speed ECM (Electronically commutated motor) are motors that are powered by a variable frequency. In essence the motor control takes the incoming electrical frequency and converts it to a new frequency (cycle rate) that no longer needs to be 60hz. This control over the actual frequency is what makes ECM motors so much more variable in ten speeds they can run.

So in summary. There are three way you can change a motor speed.

  • Change the # of poles (more = slower)
  • Increase slip to make it slower, decrease slip to bring it closer to synchronous speed
  • Alter the frequency (cycle rate)

— Bryan

In Residential and light commercial HVAC we work primarily with PSC (Permanent split capacitor) motors. However, there are some other types that are good to be aware of.

PSC (Permanent Split Capacitor)

A common medium torque single phase motor with a run capacitor always in the circuit. This type makes up the majority of HVAC motors (condenser fans motors, blower motors compressors)

CSCR – (Capacitor Start, Capacitor Run)

A higher starting torque motor that uses a run capacitor as well as a start capacitor. The start capacitor is removed from the start circuit shortly after starting using a potential, current or centrifugal relay.

CSIR – (Capacitor Start, Induction Run)

These motors are fairly rare and utilize a start capacitor and no run capacitor.

Three Phase

Three phase motors require three phase power and do not require capacitors.

Shaded Pole

Shaded pole motors are very small, low torque motors. They can only run in one direction and they do not utilize capacitors.

D.C. (Direct Current) 

D.C. Motors work on Direct Current and (generally) utilize brushes to transfer an electrical charge to the armature (rotor) of the motor.

ECM (Electronically Commutated Motor)

This type of motor is a high-efficiency  DC motor that uses no brushes, a permanent magnet rotor and utilizes electronically switched DC power to turn the motor at various speeds.

 

— Bryan


Most motors are designed to set amount of work, usually rated in either watts or horsepower, which is just 746 watts.

Watts law states that Watts = Volts x Amps. If a particular motor need to do 1 horsepower of work at 120 Volts it will draw about 6.22 amps. And yes in an inductive load like a motor it’s not quite as simple as VxA=P but we are keeping it simple here.

A motor designed to do the same amount of work (1HP) at 240v will draw half the Amps (3.11).

This does not make the second motor “more efficient” because the power company charges by the Kilowatt NOT by the amp. 

If you take a load that is designed for a particular voltage and you DROP the voltage it will also decrease the wattage according to Watts law as well as decrease the amperage according to Ohm’s law (so long as the resistance remains the same).

Let’s say you take a 5KW heat strip that is rated as 5Kw at 240v and you instead connect it to 120v.

It would then only produce 1.25 kw and draw 1/4 the amps, this is because while we may call it a “5 Kilowatt heater” it is actually just a fixed resistor designed to do 5 kilowatts per hour of work in the form of heat at 240 Volts. Cut the Volts in half you also cut the amps in half the and you decrease the amount of work done down to 1/4.

Does that make sense?

— Bryan

P.S. – It gets more complicated in inductive loads because the resistance isn’t constant but you need to understand the above first.

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