# How can this 3-phase PM Synchronous AC servo motor operate with single-phase?

There is a AC synchronous servo motor which can work both as 3-phase and single-phase. Here is a general datasheet.

These can work both with 3-phase and single phase.

I couldn't find what would be the advantage to use it as 3-phase instead of single phase for an application.

Edit:

I made some simulations to clarify my question. Here is what happens to the output DC link of the "same 3-phase rectifier" with 3, 2 and 1 phase inputs:

3-phase inputs:

2-phase inputs:

1-phase and neutral input:

As you see above, the DC link's average and ripple changes at all of the above variants.

But the AC servo motor's driver can take all of these three combinations as input.

How can we interpret these results?

• I'd like for you to tell me where in the data sheet you linked you saw that the motor could run from a single phase supply. – Andy aka May 9 '18 at 15:13
• You are correct. Actually it was its driver manual. Driver had many inputs AC and DC. Driver then has one cable coupled to the motor for its stator I guess and another for encoder. I gave no option that driver would convert 1 phase input to 3 phase. How does the driver know what the input is? – user16307 May 9 '18 at 15:18
• How does a lightbulb know what the input is? – Andy aka May 9 '18 at 15:24
• Isnt the driver converting Ac to Dc first? If so, converting 3 phase Ac to Dc link must be different than converting 1 phase to DC link. Dont they require different circuitry? Very confused – user16307 May 9 '18 at 15:27
• @Andyaka Pls see my edit – user16307 May 9 '18 at 22:57

## 2 Answers

It is not the motor that can work on either single-phase or 3-phase, it is the control unit. The control unit supplies 3-phase to the motor in all cases. The advantage of using 3-phase is that using predominately 3-phase equipment in a factory make it easier to balance the total factory load among the three phases. There is also some advantage in reducing the harmonic content of the load current drawn from the mains. There may be some convenience advantage to having three smaller power conductors rather than two larger conductors.

For both 3-phase and single-phase input, mains is rectified and filtered to provide DC that is then converted to controlled 3-phase AC. For a controller that will accept either single-phase or 3-phase, the rectifier needs to have a high enough current rating to supply the required power using only 4 of the six diodes. The filter capacitor needs to be adequate for un-filtered DC with higher ripple voltage.

• Is creating controlled freq. 3 phase from 3 phase mains easier than creating controlled freq. 3 phase from a single phase mains? – user16307 May 9 '18 at 15:22
• See revised answer. – Charles Cowie May 9 '18 at 15:57
• How come the very same driver take both 3-phase and 1-phase inputs for obtaining DC link? Don't they require different circuitry? One is taking 3-phase the other is 1-phase, yet the same driver. It is a bit confusing. – user16307 May 9 '18 at 16:06
• If you leave one of the three lines disconnected, you have single-phase. With a missing phase, a 3-phase rectifier is a single phase rectifier with two extra diodes with no input. – Charles Cowie May 9 '18 at 16:12
• Please see my edit. – user16307 May 9 '18 at 22:57

It would have been really nice if you had said where in the 66 page datasheet it says the motor can work as single-phase or three phase. Any reference to 'phase' is in the context of three phases.

Servo motors are driven from some sort of VFD. Your comments indicate that you are aware that the VFD output is generated by chopping the DC bus voltage and feeding out on the three phases.

simulate this circuit – Schematic created using CircuitLab

Figure 1. Generating VFD DC bus voltage from 3-phase or single-phase.

The DC bus voltage is generally obtained by rectifying the mains. This can be done using a three-phase bridge or a single-phase bridge. The advantage with 3-phase is that one of the phases is always 'up' and as a result a reasonably constant DC supply can be generated with little or no DC bus capacitors.

Figure 2. DC from 3-phase has low ripple.

For the single-phase a large capacitor is required to supply power while the mains input voltage falls below the DC bus voltage. (See typical full-wave rectified supply waveform.)

For the 3-phase supply the peak DC voltage will be $\sqrt 3 V_{ph-ph}$.

For the single-phase supply the peak DC voltage will be $\sqrt 2 V_{in}$. Note that this will result in a much reduced voltage if run from phase-neutral.

• I edited my question, please have a look at it. The DC link voltage is different average and ripple in al cases. What can we conclude from these? Is that what's gonna happen in real? For the same torque will the 3-phase input draw less current per phase? – user16307 May 9 '18 at 22:59
• Your 3 and 2-phase circuits give the same peak voltage but with worse ripple when one phase is lost. Your 1-phase gives the expected result as explained in my answer. Your question still claims that the motor can work as single-phase and you haven't clarified where in the document you think it says this. It can't. It's a 3-phase motor. – Transistor May 9 '18 at 23:07
• Yes, I learned it from your and Cowie's answer. But the inverter will be working at very different DC link voltages for the case 1 and 3. What is the consequence of this? – user16307 May 9 '18 at 23:14
• The VFD output voltage will be reduced. Why do you not fix the errors in your question? – Transistor May 10 '18 at 3:13
• Charles Cowie says in his comment below: "your simulation does not show what really happens." Are my simulations wrong and do not show what really would happen? – user16307 May 11 '18 at 12:05