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Vfd-single Phase Motor


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Hey guys, first off thanks to all who participate in forums like this one.


I have a unique situation (aren't they all) with a very elaborate saltwater aquarium setup. What I wish to do is control the speed electrically of the main pump motor.


The motor is a Baldor 1/3 HP single phase motor, TEFC 1725rpm capacitor start/capacitor run (2 caps). I have a vfd from Control Resources Inc, it is the Stratus model single phase input, single or 3 phase output.


After days of research, I have this thing working on the motor, but the more I read, the more I am weary about its practicality, energy usage, and motor reliability. I also had a unsettling experiance with 400Hz from the drive (which I corrected), which made the motor run very very fast. I have been confused by too much reading, with slip, flux, iron loss, windings, harmonics being terms that did not haunt my sleep until recently... I also know more about Modbus now that anyone should have to.


My questions for the experienced motor technology guys reading this forum are:


1.) does this vfd pose a reliability/longevity issue with my motor?

2.) how does the frequency affect the performance of the motor? I can set the max frequency as a seperate paramenter, so what does this offer/where should it be set. I have the "max" set to 60Hz right now.

3.) at a lower rpm will this motor be more efficient, or less (it will be running 24/7)?

4.) at full speed (expected to be 16hr/day) will it be less efficient?

5.) what is the feasability of replacing with a 3 phase motor, and keep all the performance characteristics of the pump? is it as simple as matching the shaft & rpm??? The Baldor motor was obviously special made for them (MDM Sequence), as the numbers do not "match" anything I can find specs on, but the nameplate list all the specs as you would expect.

6.) could a contactor be used to "switch in" the vfd, and use line voltage for full speed, or would this cause harm to the vfd/motor?

7.) harmonics & filters?? what is this all about? should I be concerned with such a small motor? the vfd will be only a few feet from the motor.


Thanks for the help in advance, if anyone needs help with their home automation/control systems let me know.



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Before going any further, you need to realize that Capacitor start motors are not good candidates for using electronic variable speed drives of any type. The starting capacitors' charging current will look like a short circuit to the transistors in the VFD and eventually damage it. Although they are only on-line for a moment or two during start-up, the problem with variable speed is that there is a centrifugal speed switch in the motor which switches them out of the circuit at about 90% speed. When you use a VFD and it drops the speed below 90%, the caps are now permanently in the circuit, causing damage to both the VFD transistors and the capacitors.


So the other issues are moot points unless you solve that major hurdle. Either change your motor to a PSC (Permanent Split Capacitor) or change to a 3 phase motor and use a VFD that converts the 1 phase supply to a 3 phase output. Most small VFDs are capable of doing that without derating.

"He's not dead, he's just pinin' for the fjords!"
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Hello Scott


I would expect that you may have problems over a period of time with this motor operating off an inverter.

The single phase motor with a start winding and a start capacitor, includes a centrifugal switch which cuts in when the motor is operating below full speed. This connects the start capacitor into circuit. With the start capacitor in circuit, a higher current is drawn through the start winding which is intermittently rated. Also, the start capacitor is typically intermittently rated. If the start capacitor remains in circuit, you can expect a capacitor and/or a winding failure unless the motor has been designed to allow for this usage. Operation below rated speed will result in the capacitor remaining in circuit continuously.


In addition to the problems with the start winding and capacitor, the output of the inverter will probably be a high frequency PWM waveform which will put much higher stress on the capacitor and cause capacitor failure.


Your questions.

1) I would expect problems over a period of time.

2) a three phase induction motor can operate over a wide frequency range, but generally, not so with a single phase motor. As you reduce the frequency, you must reduce the voltage to prevent overfluxing the iron. VFDs do this automatically. The power rating of the motor reduces linearly with frequency. Above rated frequency, the power rating of the motor remains constant, but the torque output drops.

3) At a lower frequency, an induction motor will remain with a relatively high efficiency. Altering the speed does not significantly change the motor efficiency, but it can alter the pump efficiency if the flow is restricted.

4) It will do more work. Look at the efficiency of the pump, not the motor.

5) Yes, provided that the speeds match and it is physically practical to do, I would recommend changing to a three phase motor.

6) You would need more than one contactor and I would suggest that the potential problems outway the advantages.

7) I would not be concerned except for the potential EMC issues that could create interference with radios, cordless phones etc. Use a screened cable between the motor and the VSD with the screen well bonded to the VSD and to the motor will help to reduce this.



Best regards,

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