External Rotor Motors are used extensively by Fantech. These are compact, lightweight motors specifically designed for fans and ventilation products.
To increase the life of three phase External Rotor Motors when controlled by a VSD, it is recommended that All-Pole Sinusoidal Filters be fitted to remove unwanted harmonics.
| Product codes of Fantech fans with 3 phase External Rotor Motors fitted. |
| CD..D/V |
CD..S |
CD..VGL |
CD..VGL-BFC |
PCD..ER |
| WCD |
TCD |
TWCD |
TILD |
FLD..ER |
| JISU |
JWCD |
|
Harmonics
Clean electrical power from the mains consists of a sinusoidal wave with a frequency of either 50 or 60 Hz.
There are many ways an electrical power supply can become “dirty,” and is caused by the introduction of a series of harmonics. Figure 4 shows the introduction of a third harmonic and resulting waveform. This is an example where harmonics are introduced to the waveform from a VSD.
A VSD contains switches that turn on and off very rapidly. This switching is called Pulse Width Modulation (PWM) and occurs in 100 nanoseconds (10 -9 seconds) providing an efficient way of controlling motor and fan speed. A side effect of PWM is the introduction of high frequency harmonics.
Why can a VSD damage motors?
Electric motors work at their optimum when a clean electrical signal is supplied. When VSDs are used:-
- Components in the electrical motor can form a resonant circuit when certain electrical frequencies are present. When harmonics are present, electrical frequencies can be supplied to the motor that will excite the resonant circuit increasing voltages and therefore stress across the windings.
- Motor windings are sized to withstand heat generated by the fundamental electrical frequency due to current flow. When higher frequency harmonics are present the temperature in the windings increase.
- VSDs can create a current through the bearings (known as electro discharge machining), which will break down the grease in the bearings and introduce pitting on the contact surface of the bearings.
Sinusoidal filters
A Sinusoidal filter is a device which filters out high frequency harmonics. A more specialised filter called an All-Pole Sinusoidal Filter also prevents current flowing between the three phases and back to earth. The high switching frequency of the VSD can cause this type of current flow and it is therefore recommended that an All-Pole Sinusoidal Filter is used.
Another filter, that is available on the market, is a dV/dT (or Switching) filter which lowers the voltage rise-time of the VSD but still lets through the PWM waveform. They have their uses, but these filters are not equivalent to an All-Pole Sinusoidal Filter and should not be used as an alternative.
Why is an External Rotor Motors different to a conventional motor?
When VSDs were first introduced, conventional motors began to have issues with insulation and bearing failure. The design of motors and the selection of materials they were made from changed to withstand the extra stresses placed on them by a VSD.
Some of the potential changes made to a motor to better handle these stresses include:
- The amount of insulation on the winding wire was increased to allow them to handle higher voltages and temperatures.
- A sheet of Mylar was placed between the coils which significantly increases the strength of the insulation system.
- Insulated bearings were used.
The first 2 modifications take up more space within the motor body, therefore due to an external rotor motor being more compact, and having less space available for these changes, they are unable to be constructed to include these additions and handle the increased stresses.
Conclusion
While using a variable speed drive can greatly improve the efficiency of your air movement equipment, using VSDs with an All-Pole Sinusoidal Filter on the External Rotor Motor will return the motor to the expected life of one without a VSD. Maintaining the life of the motor will reduce service calls and replacement costs which would have outweighed any expected efficiency gains with the use of the VSD.
Fantech products more compatible for VSD use, or our Electronically Commutated (EC) motor range are other potential solutions for speed controlling three phase fans. Using a voltage controller such as the 5 speed TL3 model, provides you with an alternative to speed controlling an External Rotor Motor with a VSD.
We have simplified this article to make the concepts more easily understood. For those interested in more detail a paper on this topic is available at www.fantech.com.au
Note: Illustration of External Rotor Motor courtesy of Ecofit
