Treadmill Motor Speed Controller Circuit

In this post I have explained a simple, accurate, high torque treadmill motor speed controller circuit which may be effectively installed in similar units for acquiring PWM controlled variable speed feature. The idea was requested by Mr. Samuel.

Technical Specifications

I've a treadmill whose power failed completely...it had been imported from china and it's like they can't help after negotiating with them..guarantee is only meant in their x-try.

So, am asking, how would you assist me in designing a power supply that will control speed and change of direction of the treadmill movement as well. I'm and forever will be glad for your work.

Looking into the specs of the unit, the switching relays are specified with 10A ratings. I also had a view of the motor and it was written 180Volts on it.

This is the information i got sir. They also had a cautionary notice that the T.Mill shouldn't be run beyond 2hrs continuously. I hope I've given the best for the best.Thanks sir. Stay blessed now and forever! best moments!

Using a Single IC 555 Astable Multivibrator in PWM Mode

Audio/Video Representation

This is a PWM based speed controller circuit for a high voltage DC treadmill motor. The main controller is the 555 timer, which generates the PWM signal and drives the IRF740 power MOSFET. The motor is supplied from a separate +145V DC supply, while the 555 control circuit works from the +12V supply.

12V control supply

The +12V supply first passes through R3 330 ohm. D1 9.1V zener is used to limit the supply voltage for the 555 circuit and C1 47uF provides filtering.

So the 555 gets roughly 9V supply and the high voltage 145V motor supply is kept separate from the low voltage control section.

555 PWM generator

The 555 together with R1 4.7K, R2 47K and C3 100nF forms the PWM oscillator.

The output is taken from pin 3 of the 555 and is given to the gate of IRF740 through R4 10 ohm.

RV1 10K is used as the speed control. Its wiper is connected to pin 5 of the 555. When RV1 is rotated, the voltage at pin 5 changes and this changes the PWM duty cycle.

When the PWM duty cycle is increased, the IRF740 remains ON for longer time and more average power is given to the motor. So the motor speed increases.

When the duty cycle is reduced, the motor receives less average power and the speed comes down.

With R1 4.7K, R2 47K and C3 100nF, the basic operating frequency is around 2.7kHz. The actual frequency can change somewhat because pin 5 is also being used for controlling the PWM.

Soft start

C2 10uF is used for the soft-start function.

When the circuit is switched ON, C2 is initially discharged. Since C2 is connected to pin 5 of the 555, the control voltage changes gradually as C2 charges.

Because of this, the PWM does not suddenly jump to a high level. The motor power increases gradually and the motor starts more smoothly.

This is useful in a treadmill because the motor can take a very high current during starting, especially when the belt is already loaded.

MOSFET gate drive

The PWM output from pin 3 of the 555 reaches the IRF740 gate through R4 10 ohm.

R4 limits the instantaneous gate current and also helps to reduce gate ringing.

The 1N4148 provides a faster path in one direction, allowing the MOSFET gate to charge or discharge faster.

The 15V zener connected between the gate and source of IRF740 protects the MOSFET gate from excessive VGS voltage and switching spikes.

Motor switching

The IRF740 is used as a low-side power switch.

The motor is connected between +145V and the drain of IRF740. Therefore, when the MOSFET turns ON, current flows from +145V through the motor and then through IRF740 to ground.

When the MOSFET turns OFF, the motor current cannot suddenly stop because the motor is an inductive load. The UF4007 then provides the freewheeling path for this motor current.

The UF4007 is a fast recovery diode and is more suitable for this PWM application than the normal 1N4007.

But its actual current and voltage ratings still have to be suitable for the motor. A treadmill motor can draw very high current during starting and heavy loading.

RC snubber

R7 100 ohm and C4 100nF are connected across the IRF740 switching section and form an RC snubber.

When IRF740 switches OFF, the inductance of the motor wiring and other parasitic inductances can produce a fast voltage spike and ringing at the MOSFET drain.

R7 and C4 absorb and damp these high frequency spikes.

So this RC snubber helps to reduce switching stress on the IRF740 and makes the switching waveform cleaner.

Current limiting

The current limiting section works in a different way from the voltage protection section.

Rs is the current sensing resistor and it is connected in the source path of IRF740.

When motor current flows through Rs, a voltage develops across this resistor. The value of Rs can be approximately selected using:

Rs = 0.6 / Maximum Current

For example, if the required maximum motor current is 10A:

Rs = 0.6 / 10

Rs = 0.06 ohm.

The voltage developed across Rs is applied through R7 100 ohm to the base of the LEFT BC547. When the motor current becomes high enough to produce around 0.6V across Rs, the LEFT BC547 starts conducting.

Now the important part is that this BC547 does not directly pull the IRF740 gate to ground.

Its collector is connected to pin 5 of the 555. Therefore, when the current becomes excessive, the BC547 pulls pin 5 of the 555 towards ground. This changes the 555 control voltage and reduces the PWM duty cycle.

The IRF740 then gets less ON time and the motor current is reduced.

This is especially useful during motor starting and when the treadmill is heavily loaded. R5 22K and C5 100nF are used around the sensing section to keep the sensing point stable and filter unwanted switching spikes.

Voltage protection

The voltage protection works separately from the current limiting circuit.

Zx, R6 100K, R5 and C5 are connected to the RIGHT BC547 which monitors the high voltage condition.

Zx is connected to the +145V rail. When the monitored voltage becomes high enough, Zx starts conducting and current flows through R6 into the base of the RIGHT BC547.

When this BC547 turns ON, its collector pulls the IRF740 gate directly towards ground.

Therefore the IRF740 is switched OFF. So the voltage protection action is....This is different from the current limiting action.

The current feedback controls pin 5 of the 555 and reduces the PWM, while the voltage feedback directly pulls the MOSFET gate to ground. The exact voltage protection level depends on the actual value of Zx, so the cutoff voltage cannot be calculated from the diagram unless the Zx value is known.

C5 and R5 in voltage sensing

C5 100nF provides filtering at the base of the RIGHT BC547. This helps prevent short switching spikes from unnecessarily turning the transistor ON. R5 22K keeps the BC547 base pulled towards ground when there is not enough sensing voltage. So this section gives a more stable voltage protection operation.

Complete working

• In simple way, RV1 controls the speed by changing the 555 control voltage.
• The 555 generates the PWM and this PWM drives the IRF740.
• The IRF740 switches the 145V DC supply to the motor and the PWM duty cycle controls the average power given to the motor.
• C2 provides soft start during startup.
• UF4007 provides the freewheeling path for the motor current when the MOSFET switches OFF.
• R7 and C4 form the RC snubber and reduce voltage spikes across the MOSFET.
• The 15V zener protects the IRF740 gate.
• Rs senses the motor current and the LEFT BC547 provides current feedback to pin 5 of the 555. When current becomes too high, it pulls pin 5 down and reduces the PWM.
• Zx and the RIGHT BC547 provide voltage protection. When the sensed voltage becomes too high, the RIGHT BC547 directly pulls the IRF740 gate to ground and switches the MOSFET OFF.

Using Two IC 555

Here's a simple PWM based motor speed controller circuit which can be used for controlling a treadmill speed right from zero to maximum.

The circuit also provides an instant bidirectional stop and reversal of the motor rotation by a single flick of a given switch.

Another interesting feature of this circuit is its capability of sustaining and balancing optimal torque even at lower speeds ensuring a continuous working of the motor without stalling it during extreme low speeds.

The circuit of the proposed treadmill motor speed controller may be understood with the help of the following points:

Here the two 555 ICs are configured as PWM generator/optimizer for acquiring the required speed control of the connected motor.

Circuit Operation

IC1 works as a frequency generator and is rigged at around 80Hz, any other value would also do and is not anyway critical.

The above frequency from pin#3 of IC1 is fed to pin#2 of IC2 which is wired as a standard monostable. IC2 responds and starts oscillating at this frequency, forcing equivalent triangle wave frequency at its pin2/6.

The above triangle waves is instantly compared by the set potential at pin#5 of IC2 creating an equivalent level of chopped PWM at its pin#3

The preset or a pot positioned at pin#5 of IC2 forms a  potential divider network for a selectable fixing of any voltage from zero to maximum supply voltage at pin5 of IC2. This level is directly translated through optimized PWMs at pin#3 of the same IC as explained above.

The PWMs are fed across two sets of NOT gates via an SPDT toggle switch.

The NOT gates which act as inverters provide the feature of instant toggling of the motors rotational direction by a mere flick of the SPDT switch.

The resultant PWMs from the selected NOT gates finally reach the transistorized bridge network that holds the motor between them for implementing all the specified features discussed above.

These transistors should be rated as per the motor specifications, and the voltage across this bridge should also be as per the motor requirements.

Video Clip:

Simplified Design

If you do not wish to have the reverse forward facility, then you can much simplify the above design by eliminating the lower section of the circuit entirely, as shown below:

The 10K pot can be used for the speed control, while the 220uF determines the soft start feature. Increasing the 220uF value increases the soft start effect and vice versa.

Controlling Through an External Power Supply

The above design could be also modified for enabling motor speed control through an external variable power supply, as shown below.

Pin#5 can be seen driven from an external 0 to 10V variable power supply, for example from a LM317 based power supply

If you do not wish to use an external power supply, the above treadmill speed controller design could be simplified even further, by adding a 1k pot at pin#5 of IC2, as demonstrated below:

The 1k pot will allow you to adjust the treadmill speed from 10% to 90%, and the C4 value could be experimented to add a nice soft start to the treadmill motor during the switch ON.

Construction Guide

Power Supply Section

15V DC Supply:

Ensure that the input power source delivers a stable 15V DC, because it is critical for the 555 timers and the IRF540 MOSFET operation.

Use a well-filtered power source to avoid the noise interference.

Zener Diode (D1 - 2.7V):

Provides a fixed reference voltage for the base of T1 (BC557 transistor) which gives stable operation.

PWM Generator (IC1 - 555 Timer)

The first 555 timer (IC1) is configured as an astable multivibrator, to generate a pulse-width modulated (PWM) signal.

Pin Connections:

Pin 1 (GND): Connect to the ground.
Pin 8 (Vcc): Connect to +15V supply.
Pin 4 (Reset): Connect to +15V to enable the IC.
Pin 5 (Control Voltage): Add a 10nF capacitor (C2) to ground to stabilize the PWM signal.
Pin 3 (Output): Outputs the PWM signal to the base of T1 (BC557 transistor) via R4.

Frequency Control:

Use R1 R2, and C1 to set the frequency of the PWM signal.

Formula for frequency:

f = 1.44 / [(R1 + 2R2) * C1]

Where:
R1 = 180kΩ
R2 = 100kΩ
C1 = 100nF

PWM Adjustment (1kΩ Potentiometer):

The 1kΩ potentiometer varies the duty cycle of the PWM signal allowing the speed control of the treadmill motor.

Soft Start and Motor Driver (IC2 - 555 Timer)

The second 555 timer (IC2), is configured as a monostable multivibrator to implement the soft start feature.

Pin Connections:

Pin 1 (GND): Connect to the ground.
Pin 8 (Vcc): Connect to +15V supply.
Pin 4 (Reset): Connect to +15V to enable the IC.
Pin 5 (Control Voltage): Add a 10nF capacitor (C3) to ground.
Pin 3 (Output): Drives the gate of the IRF540 MOSFET.

Soft Start Capacitor (C4 - 10nF):

This capacitor determines, the ramp-up time for the soft start feature.

Larger values of the C4 increases the soft start time.

Adjust C4 using the formula:

t = 1.1 * R6 * C4

Where:
R6 = 10kΩ
C4 = Soft start capacitor

Motor Connection:

The motor is connected to the drain of the IRF540 MOSFET with the source connected to the ground.

D5 (1N4007): Protects the MOSFET from back the EMF generated by the motor.

Working Principle

PWM Speed Control:

The duty cycle of the PWM signal generated by IC1 provides the average voltage applied to the motor.

Adjusting the 1kΩ pot, changes the PWM duty cycle effectively, controlling the motor speed.

Soft Start:

When the circuit is powered ON, the IC2 gradually increases the gate voltage of the MOSFET due to the charging of the C4.

This ensures that motor starts smoothly without jerks.

Overvoltage Protection:

The 2.7V zener diode (D1) ensure the PWM output voltage doesnt exceed safe levels for the transistor T1.

Relevant Calculations

Frequency of PWM Signal (IC1):

f = 1.44 / [(R1 + 2R2) * C1]

For the given values:

R1 = 180kΩ
R2 = 100kΩ
C1 = 100nF

Substituting:

f = 1.44 / [(180k + 2 * 100k) * 0.1µF]
f ≈ 34.3 Hz

Soft Start Time (IC2):

t = 1.1 * R6 * C4

For the given values:

R6 = 10kΩ
C4 = 10nF

Substituting:

t = 1.1 * 10k * 10n
t = 0.11 ms

If a larger value for the C4 is chosen (e.g 1µF):

t = 1.1 * 10k * 1µF
t = 11 ms

Assembly Tips

Use a Heat Sink for the IRF540:

The MOSFET can heat up under heavy motor loads so Attach an appropriate heat sink.

PCB Design:

Keep the ground connections of the IC1, IC2 and the motor driver separate to avoid noise.

Testing:

Test the PWM output with the multimeter or oscilloscope before connecting the motor.

Safety Precautions:

Ensure proper insulation for all tthe high-current connections to prevent short circuit.

Using A Dimmer Phase Chopper Circuit

As rightly suggested by one of the dedicated readers of this blog, Mr. Ivan, a 180 V treadmill motor can be simply controlled through mains phase chopping concept, normally incorporated in all commercial dimmer switches for regulating home fan speed.

Shown below is a modified dimmer switch circuit design which can be effectively used for regulating a 180 V treadmill motor from zero to max:

Please make sure to use a non-polar capacitor for the one shown between the bridge rectifier.

Use the following type, 10 in parallel

10nos of 0.47/400V in parallel will make 47uF/400V non polar capacitor which may work like a decent filter capacitor for the motor.