Speed control for PSC induction motor

Question

I would like to control the speed of my PSC induction motor.

I found a similar topic here: varying run capacitor for speed control of single phase motor

However I would like to modify the speed while the motor is running.

Something based on Temic's U2008B perhaps?

Any suggestions?

Thanks,

Juan

Answer 1

If you want to control a fan blower, then a phase control is most suitable for inexpensive approach. I have seen poultry houses running ventilation on single phase induction motors with U2008 circuit. Also the small fan blowers in the home pellet/wood burners, they all use shaded pole fan with phase control.

In theory, using the phase angle control, reduces the voltage on the motor thus the output torque is decreased with square of voltage. Also the required torque for fan is square related vs speed, so if you combine both voltage control of induction and fan load, then you have a good result.

Also the VFD, when used to power the fan is usually adapted to give a quadratic V/f output characteristics.

IMO, don't spend money on VFD for fan blower if it's not bigger than 2 kW.

Answer 2

The speed of a PSC induction motor with a fan load can be adjusted while the motor is running. Changing the voltage provides a continuous range of adjustment. The diagram below, shows the full-voltage torque vs. speed curve and three reduced-voltage examples. If the voltage is reduced, the torque capability that will be provided at any given slip is proportion to the square of the voltage. If the voltage is reduced to half, the torque at a given slip will be 25% of the previous torque. The slip at any given load torque will increase accordingly as shown.

The motor will operate at the intersection of the torque capability curve (blue) and the torque demand characteristic curve of the load (red). The torque characteristic curve of a fan is shown because this type of speed control is sometimes used for fans and centrifugal pumps. Loads that require the torque to remain constant at reduced speeds or have less reduction in torque requirement are rarely operated this way. This method may work to some extent with those type of loads but it is generally not recommended. Even with variable-torque loads, a three-phase motor with a variable-frequency drive (VFD) would be preferable.

The losses due to slip are proportional to the load torque multiplied by the percent slip.

This can not be done with a single-phase, capacitor-start motor because the motor needs to be operating near full speed for the centrifugal switch to disconnect the capacitor.

Answer 3

I made it this way -

speed control of PSC 1f engine with Raspberry Pi 3

6 elements

DC side:

(1) Raspberry 3, 3V3 GPIO

(2) transistor 2N222, with resistor 220 Ohm at the gate

(3) Solid State Relay SSR 40 - DA

AC side:

(4) PSC motor, 1phase, 0,35 A, 1,5 mF

(5) double choke coil inductor 2x 10mH

(6) Inlet with Line Filter 250VAC 10A Chassis FBNAB2470ZG110

application

(1) Raspberry give 3V3 signal to transistor at the gate (connection through resistor) That opens way to 5V signal from Raspberry to SRR back to Raspberry ground pin.

(2) Reason to use Transistor is that 3V3 signal won't give enough current to the SSR. SSR is trigger by current at 7.5 mA @ 12V. (http://www.fotek.com.hk/solid/SSR-1.htm)

(3) When SSR got 5V current, it let 230 V current to the engine.

(4) Now: when we let power jump up and down, engine starts getting big impusles which in principle is harmfull - both to your engine and power system.

(5) To avoid it, you connect in line a choke coil - to flattern the voltage - bigger : better. Hand made coils should do as well. Stator from other engine works great (photo).

(6) Line filter helps the simmilar way. You can buy one, or with simple capacitor [schematics] make connection between L and N.

Be careful and have fun!

Code#1 python2.7

#!/usr/bin/env python
# -*- coding: utf-8 -*-
import time
import RPi.GPIO as GPIO
try:
    while True:
        # Use BCM GPIO references
        # instead of physical pin numbers
        GPIO.setmode(GPIO.BCM)
        # Define GPIO to use on Pi
        GPIO_RELAY_1 = 12

        # Set pins as output and input
        GPIO.setup(GPIO_RELAY_1,GPIO.OUT)  # RELAY_1

        # Set trigger to False (Low)
        GPIO.output(GPIO_RELAY_1, False)

        # Allow module to settle
        time.sleep(0.1)

        # Send 10us pulse to trigger

        GPIO.output(GPIO_RELAY_1, True)
        print "RELAY_1 ON"
        time.sleep(0.5)
        GPIO.output(GPIO_RELAY_1, False)
        print "RELAY_1 OFF"
        time.sleep(0.2)

        # Reset GPIO settings
        GPIO.cleanup()

except KeyboardInterrupt:
    pass
GPIO.output(GPIO_RELAY_1, False)
print "Finito : Off"
GPIO.cleanup()

Code#2 python2.7

https://sourceforge.net/p/raspberry-gpio-python/wiki/PWM/

import time
import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BCM)
GPIO.setup(12, GPIO.OUT)
frequency = 500
dc_low = 40
dc_mid = 75
dc_high = 100

p = GPIO.PWM(12, frequency)  # GPIO.PWM(channel, frequency (in Hz)
p.start(0)

try:
    while 1:
        print "frequency = ", frequency
        #dc_low
        p.ChangeDutyCycle(dc_low)
        print "dc_low =", dc_low
        time.sleep(120)

        #dc_mid
        p.ChangeDutyCycle(dc_mid)
        print "dc_mid =", dc_mid
        time.sleep(120)

        p.ChangeDutyCycle(dc_mid)
        print "dc_high =", dc_high
        time.sleep(120)
        """
        for dc in range(0, 101, 5):
            p.ChangeDutyCycle(dc)
            time.sleep(0.1)
            print "dc in range(0, 101, 5) =", dc
            print "freqnecy = ", frequency

        for dc in range(100, -1, -5):
            p.ChangeDutyCycle(dc)
            print "dc in range(100, -1, -5) = ", dc
            print "freqnecy = ", frequency
            time.sleep(0.1)
        """
except KeyboardInterrupt:
    pass

p.stop()
GPIO.cleanup()