Driver for Universal Motors

Motors from washing machines are powerful and easy to recycle, though they need a specific driver to get the best from them. With a Phase Angle Control and Closed loop speed control we can achieve this.

Washing machine motors can be easily recovered from old machines, often are in good conditions, are powerful, they can spin in both directions, and can be driven to control their speed with good torque from standstill. You can also squeeze out much power if a good ventilation is provided. Since they are fitted with a tachometer they can be driven to rotate at a constant speed even when the load changes.
However to get the best from them a specific driver is required.
In this article, the theory of operation of these motors is presented and how the Phase Angle Control used to drive them works.
Finally a full driver that I've made is presented.

With a power of about 500W I will use one of these motors to temporarily drive the spindle of my mini lathe.

In the video is featured an RPM-counter that I've made, follow this link to watch the video of the project:
https://www.youtube.com/watch https://www.youtube.com/watch

Warning, mains voltage could cause shocks, burns and severe electrocute. Wear safety goggles, do not wear dangling jewels that may fall in contact with the circuit, and watch where you put your hands.

The Versatile Universal Motor in Washing Machines

Washing machines are indispensable appliances in modern households, simplifying the tedious task of cleaning clothes. Among the various components that contribute to their functionality, the motor is a critical element that drives the machine's drum and agitator. The most commonly used motor in washing machines, especially in the more budget-friendly models, is the universal motor, named for its unique ability to operate with both alternating current (AC) and direct current (DC).

Understanding the Universal Motor: A Versatile Workhorse

The universal motor is essentially a type of brushed DC motor with windings designed to function efficiently with the grid's frequency and voltage. Its primary advantage lies in its ability to work seamlessly with both AC and DC power sources, making it an ideal choice for applications where versatility and cost-effectiveness are paramount.
In a universal motor, the rotor and stator windings are connected in series, which ensures that the generated magnetic field remains coherent between the two, regardless of the direction of the current. This arrangement allows the motor to operate indifferently in both AC and DC modes with a negliglible compromise in performance.

Simple Speed Control with Voltage Variation

One of the most intriguing features of the universal motor is its ease of speed control. By simply varying the voltage applied to the motor, the rotational speed of its shaft can be adjusted. This characteristic makes the universal motor an excellent choice for appliances like washing machines, where different wash cycles may require varying levels of agitation.

Voltage Control with Phase Angle Devices

Since universal motors operate with mains alternating current, controlling the voltage is essential for adjusting the motor's speed effectively. To achieve this, phase angle controlled devices, such as light dimmers, can be employed. These devices alter the effective voltage supplied to the motor by delaying the point in each AC half-cycle when the voltage is applied. As a result, the motor's speed can be regulated without complex control systems, making it a cost-effective solution for appliance and power tool manufacturers.

A Versatile Circuit for Universal Motor Speed Control (and more)

While Phase Angle Control circuits offer a straightforward means of regulating voltage, they may not be sufficient to accurately adjust the voltage in proportion to the motor's actual speed.
This is particularly problematic for washing machine motors, as they have the potential to reach very high speeds.
To ensure precise speed control, these universal motors are equipped with tachometers, which measure the actual rotational speed of the motor.
The challenge lies in developing a circuit that can automatically adjust the provided voltage in direct relation to the measured speed, ensuring the motor operates at the desired speed and avoiding potential malfunctions.
And here it comes the circuit I've designed.

Introducing the Multi-Purpose Circuit

The newly designed circuit serves a broader spectrum of applications, making it highly versatile. While its primary goal is to control universal motors, its functionality extends to various other areas, including oven control.
I think this circuit should be of particular interest for those who are new to electronics as it features some classic circuital building blocks using operational amplifiers (op amps). In particular it contains the key blocks you may find in every closed loop controller: The error amplifier, a current limiter, and a synchronized pulse generator to drive an external unit based on a TRIAC to control the mains current phase angle.
And the circuit's adaptability and ease of implementation should make it an appealing choice for electronics enthusiasts.

Classic Circuital Building Blocks Using Op Amps

At the core of this circuit are classic circuital building blocks that utilize operational amplifiers. These building blocks play essential roles in closed-loop controllers, ensuring stable and accurate control over the motor's speed. The key components include:

Error Amplifier: This block compares the desired speed (setpoint) with the actual speed measured by the tachometer (feedback). It generates an error signal that drives subsequent control actions.
Current Limiter: As a safety feature, the current limiter prevents the motor from drawing excessive current, protecting it from damage and ensuring a safe operation.
Synchronized Pulse Generator: This component drives an external unit, which is a TRIAC-based device, to control the mains current phase angle effectively. By altering the phase angle, the circuit regulates the amount of power delivered to the motor, thereby adjusting its speed.
Zero-Cross Detector: This module detects the zero-crossing points of the AC waveform, which are critical for synchronizing the pulse generation and ensuring smooth and efficient control.
Frequency to Voltage Converter: This block converts the pulses received from a pulse based speed sensor (e.g., an encoder or a hall sensor) into a voltage signal, which is then used by the control loop.
Instrumentation Amplifier: While not utilized in this specific application, the instrumentation amplifier provides the flexibility to amplify other types of sensors, such as thermocouples, for different applications.
Precision Rectifier: This component converts an alternating signal into a direct signal, streamlining signal processing.

Customizable Configuration for Various Applications

The strength of this circuit lies in its modular and customizable nature. These building blocks can be conveniently interconnected to form a tailored configuration that suits the specific requirements of the application at hand. Whether it's speed control for washing machines, oven temperature regulation, or other purposes, the circuit offers a flexible and reliable solution.

schematic — Universal PFC circuit (click to enlarge)

Configuration for a washing machine motor controller

As said before, motors coming from washing machines are fitted with a tachometer, but this is a very cheap winding that generates an alternating voltage at a frequency and amplitude proportional to the rotational motor's shaft speed, thanks to a magnet that spins solidly with the shaft.

So we could use the amplitude of the signal or its frequency to retrieve a signal that is proportional to the motor's speed. In either case the signal needs to be filtered to smooth out the oscillations when the motor spins at the lower speeds.

I rather used the frequency to get rid of possible inconsistencies between motors. Therefore this signal is bought into the zero-cross detector, that has a little bias and a smoothing capacitor to stop noise from generating fake pulses in output when the motor is not spinning.

The signal from the zero-cross detector is then fed into the frequency to voltage converter. This one is the simplest way to make a f to V converter, and I had to adjust the low pass filter to fit the tachometer and the motor's characteristics, limiting the range of operations within 8.3 KHz to 50 Hz. Better performances could be achieved with a dedicated integrated circuit, but this solution uses the same op amps that are used elsewhere, reducing the part type list.

The low pass filter used in the converter slows down the response of the tachometer feedback.
This has some implications with the responsiveness of the motor to the variations of speed. Also because the phase control works on the mains' frequency, a possible source of inconvenience could arise from the intermodulation with the phase control synchronization, which runs at 100 Hz, or 120 Hz in the case the circuit runs on a 60 Hz grid.

It is therefore important to have room to adjust the response of the error amplifier through a proportional and integral control.

The core block is the error amplifier. The speed reference signal, coming from an external potentiometer through a buffer stage, is algebraically summed to the speed feedback coming from the frequency to voltage converter, and a further feedback from the amplifier itself is fed through a couple of trimmers that set the integral and proportional constants.
A further trimmer is used to carry a positive or negative voltage that is summed to set the offset from zero.

The output of the error amplifier then goes to the comparator and a synchronized ramp generator. The synchronization comes from the pulses that arrive from the external TRIAC unit where a zero-cross detector sends a signal each time the mains voltage phase reaches zero.

Important: While the motor and the TRIAC unit work at mains voltage, the control board works at low safe voltage, and the TRIAC unit is opto-isolated on the side toward the control board.

Put everything together

The tachometer must be connected to the board at the zero-cross detector input. Here a voltage divider and a couple of clamp diodes limit the signal.
Then one terminal of the rotor must be connected with one terminal of the stator, and the remaining terminals, one from the stator and one from the rotor, must be connected to the output of the triac unit. To reverse the rotation just swap the stator's terminals. Simple.

While these motors can spin in both directions, they have a preferential way of rotation, that can be understood by looking at the brushes: the motor should preferentially turns away from the brushes and not against them.

The control board must be powered with an insulated source at 12V DC.

For more details and explanation of how the circuit works, please watch the video:

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