Speed Control of single phase AC induction motor - Possible options?

Can the speed of an 230V 50Hz single phase Induction Motor be controlled by varying supply phase?

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Joined Mar 14, 2008
When I connect the motor directly to the power supply, the motor starts and reaches max rpm almost instantly, but when I connect the VFD the motor takes approximately 30 secs to reach the "max speed", which is noticeably lower than the true max speed (the one it reaches without the VFD)
Can anyone please give me some advise about how to approach this issue?
Too little info to give an informed answer.
Need to know how the VFD is configured, i.e.
What are the various switch settings (SW1, SW2, SW8, etc.)?

Did you program it or was it programmed from the factory.?
If so, which of these modes was it programmed for?

• Isolated Voltage Control (0–5V / 0–10V) [Default]
• Isolated Current Control (4–20mA)
• Isolated Potentiometer Control (1KΩ-100KΩ)
• Isolated Temperature Control (CRI Thermistor)
• Isolated Modbus Control (J1)
• Non-Isolated Fixed Speed Switch Control (SW2)
• Non-Isolated Potentiometer Control (A/B)
• Non-Isolated Temperature Control (A/B)
• Non-Isolated Contact Closure Control (Z)

If set to the default, what is the input control voltage?


Joined Jul 18, 2013
@cmartinez There are a few VFD's that purport the ability to run 1ph motors, but much depends on the motor and it's application, many work OK at the higher RPM's but once in low speed, especially if a load is suddenly presented they tend to drop out of run.
Hence just about everyone in the DIY CNC hobby etc use a 3ph motor and the cheap Huanyang VFD's ran off of 240v 1ph. supply..


Joined Jun 28, 2018
I will gladly share all the steps followed so far to configure the VFD.

First, operation without the VFD.
The motor runs at a high speed and measures indicate it runs steady at 205.6V @60.1hz and approximately 3.25A, (about 1.1A and 2.1A in each line). All good.

Operation with the VFD
Being single phase, the power lines from the motor were disconnected from the power supply and connected to the W/T3 and V/T2 ports.
The power lines from the power supply were connected to the N and L ports of the VFD. A ground line was added. See page 12 of the manual in controlresources.com/pdf/stratus2.pdf.

SW2 switch settings are as follows:
Switches 2, 4 and 6 ON; switches 1, 3, 5, 7 and 8 OFF.
This leads to configure a single phase motor to be configured by modbus interface with modbus ID=42 (see page 9)

Before plugging the wires to the W and V ports, configuration of the VFD was made to receive input control signal of 4-20mA (isolated current control mode). USB cable was used to connect the VFD to a PC using ModScan64 from WinTech software. Below is the configuration for the holding registers:

Reg-----Descripcion----------------------------------------- Value
40001 Run Control (See Table 3)-------------------------257 (isolated current control mode)
40014 Ramp Up Rate (Hz/sec) 1 – 20-------------------6
40015 Ramp Down Rate (Hz/sec) 1 – 20---------------12
40017 Startup Run Control (See Table 6)---------------257 (isolated current control mode)
40018 Startup Frequency (Hz) 0 – 400------------------60
40019 Max Frequency (Hz) 0 – 400----------------------60
40020 Idle Frequency (Hz) 0 – 400-----------------------20
40022 Max Control (%) 0 – 100----------------------------0
40023 Idle Control (%) 0 – 100-----------------------------100
40029 PWM Carrier Frequency (Hz) 2000-20000-----16000 (default value)

Registers 40018, 40019, 40020, 40022 and 40023 were set that way so the minimal control signal (4mA) means max output (60hz) and the max control signal (20mA) means min/idle output (20hz).

After configuring the VFD, energy was removed, the V and W ports were connected again and then the power was restored. The control signal was at 0% (4mA), the motor started spinning very slowly and after about 30 secs it reached what I call the "max vfd speed" (referring to the maximum speed the motor achieves with the vfd) which is significantly lower than the speed achieved without the VFD.

Measures indicate that in those conditions: V=205.4V average, varying between 204 and 206.5; 53hz, but current is about 4.9A (2 and 2.9A in each line, not constant)

Is worth noting that register 40006, which displays the % of control signal received by the VFD, does not show 0% and 100% for 4 and 20mA respectively. Instead it shows 18% for 4mA and 94% for 20mA. That's why even at max output (4mA) it tops 53hz instead of 60hz. However, If I "scale correctly" the VFD setting regs 40022 to 18% and 40023 to 94%, the signals are interpreted correctly as 0% and 100% and the frequency does go up to 60 hz but the motor changes spin direction back and forth during runtime.

I don't know what else to do in order to configure this VFD. Maybe the PWM frequency would lead to something? What do you think?
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Joined Jun 28, 2018
Have you tried with a higher maximum frequency (40019)?
Yes sir, setting 40019 at 65hz for example yields an output frequency of 56.88hz but when i start the motor it does a non uniform noise and spins like a stepper motor (like discrete movements). I turned it off before doing voltage and current measurements.
At max frequency of 64hz and changing the control signal doesn't change much, it spins more "continuously" but veeeery slow even after a substantial amount of time.
At 60hz it alternates spin direction like I said before.
Apparently, it seems the only parameter set that at least spins normally and in one steady direction is the one in my previous post.


Joined Jun 28, 2018
I'm sorry to re-open this thread but i think it is necessary for people in the future with the same problem as me.

I finally managed to control the motor with this SCR:

The potentiometer is labeled 457k (about 400k measured): as for the circuit, I have very limited electronics knowledge so I can't tell what it is but I know it has a triac (also idk what a triac is). The point is that it controls the speed of the motor and thus the airflow for the vaccuum pump.

Would you be so kind to help me understand what kind of motor is this that can't be controlled the "traditional" way with a VFD, but instead a SCR? I need this information because my final goal is to be able to control the speed with a 4-20 mA signal from a DCS, and for that I may be buying this: http://controlresources.com/ac-fan-control-ac-motor-control-smartfan-nimbus/.

Again, thanks for your assistance.

P.S.: Should I make a new thread with my case?


Joined Jul 18, 2013
As the link mentions, it is a PSC (Permanent Start Cap) motor, these generally are limited to around 1/2hp.
The other small motor that can be controlled this way is the small shaded pole motors, usually limited to fan use.
A Triac is used in order to utilize both half cycles of the AC, it consists of two SCR's back to back. Google it.