DC Motor Soft Start: Understand and Creating Your Own

What is a DC Motor Soft Start?

A DC motor soft start is a method of gradually increasing the voltage or current supplied to a DC motor when it is first turned on, rather than applying full power immediately. This helps to reduce the initial current surge and mechanical stress on the motor, as well as any connected gears or belts. Soft starting can extend the life of the motor and connected components, and provide smoother acceleration.

DC motors draw a large inrush current when first energized, often 5-7 times the normal running current. This high current is needed to overcome the motor’s inertia and generate the initial torque to start spinning the rotor. However, this current surge puts stress on the motor windings and can cause voltage sag on the power supply. Repeatedly starting the motor at full voltage will eventually degrade the windings and brushes.

A soft start circuit limits the initial voltage or current, then gradually ramps it up to full power over a set time period, usually a few seconds. This reduces the starting current and torque, providing a gentler acceleration. The motor still receives enough power to start spinning, but with less mechanical shock.

Benefits of Using a DC Motor Soft Start

There are several key benefits to using a soft start circuit with DC motors:

1. Reduced inrush current – By limiting the initial current, a soft start prevents the large surge that occurs when starting at full voltage. This reduces stress on the power supply and prevents voltage sag that could affect other components.

2. Extended motor life – High starting currents generate heat in the motor windings and can cause arcing on the brushes. Over time this degrades the insulation and brush surfaces. Soft starting reduces this wear and tear, extending the motor’s operational lifespan.

3. Decreased mechanical stress – The high starting torque of a motor can shock connected gears, belts, and mechanisms. Gradually accelerating the motor applies power more smoothly, minimizing jolts and vibration. This reduces strain and wear on mechanical components.

4. Smoother acceleration – Ramping up the motor speed provides a less abrupt start and more linear acceleration. This is particularly beneficial for vehicles, conveyors, and precision positioning systems where sudden movement is undesirable. Soft starting allows for more controlled, gradual motion.

5. Lower start-up power consumption – Although a soft start draws out the acceleration time, it actually consumes less total power on startup than full voltage starting. This is because power is the product of voltage and current. By keeping the voltage or current lower, the overall power is reduced even though it is applied for a longer time.

Types of DC Motor Soft Start Circuits

There are a few different methods used to achieve a soft start in DC motors. The two most common approaches are voltage ramping and PWM current limiting.

Voltage Ramping Soft Start

A voltage ramping soft start works by gradually increasing the DC voltage supplied to the motor over a set time period. This is typically accomplished using a series resistor or NTC thermistor that initially limits the voltage, then allows more voltage to flow as the resistance decreases.

One simple voltage ramping circuit uses an NTC thermistor in series with the motor. An NTC thermistor is a resistor whose resistance decreases as its temperature increases. When power is first applied, the high resistance of the cold thermistor restricts the voltage to the motor. As current flows through the thermistor, it heats up and its resistance drops. This allows the voltage to the motor to slowly increase until full voltage is applied.

The time constant of the voltage ramp can be set by selection of the thermistor resistance and dissipation constant. A larger resistance or dissipation constant will result in a slower ramp. Thermistor soft starts are simple and low-cost, but the start time can vary significantly with ambient temperature.

Another voltage ramping method is to use a series resistor that is gradually shorted out by a relay, contactor, or MOSFET. The resistor limits the initial voltage, then is bypassed after a time delay to apply full voltage. The delay time is set by an RC timing circuit or microcontroller. This provides a more consistent soft start, but requires additional components.

PWM Current Limiting Soft Start

Pulse Width Modulation (PWM) is a technique for varying the effective voltage supplied to a load by rapidly switching the power on and off. The ratio of on-time to off-time, known as the duty cycle, determines the average voltage. A low duty cycle will result in a low effective voltage, while a higher duty cycle approaches full voltage.

PWM can be used to create a soft start by gradually increasing the duty cycle from a low value to maximum over a set time period. By keeping the frequency constant and ramping up the pulse width, the average current to the motor is limited at first, then increases to full power.

A microcontroller is typically used to generate the PWM signal and control the ramp time. The ramping can be linear, or follow any profile programmed into the microcontroller. This allows for flexibility in tailoring the soft start to the specific application needs.

Current limiting is achieved by measuring the motor current with a sensor and using the microcontroller to keep the current below a set threshold. If the current exceeds the limit, the duty cycle is reduced to decrease power to the motor. This provides more precise control and protection compared to open-loop voltage ramping.

PWM soft starts offer the best performance in terms of consistency and control, but require more complex circuitry and programming. They are well suited to applications with strict requirements or variable load conditions.

Designing a DC Motor Soft Start

When designing a soft start circuit for a particular DC motor and application, there are several key factors to consider:

Motor Specifications

The first step is to gather information about the motor itself. The important specifications for sizing a soft start include:

• Rated voltage
• Rated current
• Stall current
• Starting current
• Winding resistance
• Inductance
• Inertia

The rated voltage and current are the normal operating parameters of the motor at full load. The stall current is the maximum current the motor will draw if stalled or overloaded. This is typically 5-7 times the rated current.

The starting current is the initial inrush experienced when applying full voltage. This is usually slightly less than the stall current. The motor resistance and inductance determine the rate of current rise.

The motor inertia affects the acceleration time and influences the required starting torque. Higher inertia loads need a longer or more powerful soft start.

Application Requirements

The specific application the motor will be used in sets additional requirements and constraints for the soft start design:

• Required starting torque
• Allowable acceleration time
• Cycle frequency (starts per hour)
• Ambient temperature range
• Space constraints
• Cost targets

The required starting torque is the minimum torque the motor must output to overcome friction and inertia to begin rotating the load. This will determine how much the soft start can limit the current and still provide sufficient starting torque.

The acceleration time is how long the soft start takes to ramp the motor to full speed. This is application dependent, but typically ranges from 0.5 to 5 seconds. Longer ramp times provide a gentler start, but may not be tolerable for equipment that needs to start quickly.

Frequently cycling the motor on and off will generate more heat and stress. A soft start can alleviate this, but the design must account for the number of starts per hour the motor will experience.

Ambient temperature extremes affect component ratings and the performance of thermistor circuits. The soft start may need to be de-rated or oversized for reliable operation in high temperature environments.

Space and cost constraints can dictate the complexity and componentry of the soft start design. A simple thermistor circuit may meet the performance needs while minimizing size and cost. A microcontroller-based design offers more flexibility and features, but takes up more space and adds expense.

Soft Start Sizing

With the motor specifications and application requirements defined, the soft start circuit can be designed and components selected. The key parameters to determine are:

• Current limiting value
• Ramp time
• Component ratings

The current limiting value is the maximum current the soft start will allow the motor to draw at any point. This should be set to 150-200% of the rated current to provide sufficient starting torque while limiting stress and heat.

For a voltage ramping design, the resistor value is selected to provide the desired current limiting based on Ohm’s law:

$R = \frac{V_{supply} – V_{motor}}{I_{limit}}$

Where $R$ is the series resistance, $V_{supply}$ is the DC voltage, $V_{motor}$ is the rated motor voltage, and $I_{limit}$ is the current limiting value.

The power rating of the resistor must exceed the maximum power dissipation during the start:

$P = I^2R$

For PWM current limiting, the duty cycle is set to keep the average current below the limit. The microcontroller measures the actual current and adjusts the duty cycle in real-time.

The ramp time is set by selection of the thermistor dissipation constant, RC time constant, or microcontroller program. This will be application dependent, but should not exceed the thermal limits of the motor at the current limit setting. Ramping too slowly can overheat the motor.

All components should be rated for the maximum expected voltage, current, and power dissipation. Appropriate heat sinking must be used for power resistors and solid-state switches. The soft start circuit should include overload protection fuses or circuit breakers.

Implementing a DC Motor Soft Start

Once the soft start circuit is designed, it can be built and integrated with the motor and application. The specific implementation will depend on the type of soft start and componentry selected.

Thermistor Soft Start

A thermistor soft start is the simplest to implement. The main components are:

• NTC thermistor
• Series power resistor (optional)
• Bypass switch (relay, contactor, or MOSFET)

The thermistor is sized to provide the desired current limiting when cold, then allow full current when warm. A series power resistor can be added to increase the resistance and spread the heat dissipation. This resistor is shorted out by the bypass switch after the thermistor heats up.

The thermistor and resistor are connected in series with the motor positive lead. The bypass switch is connected in parallel with the thermistor/resistor combo, and controlled by a time delay relay or microcontroller output.

When power is applied, the high resistance of the cold thermistor limits the motor current. As the thermistor heats up over a few seconds, its resistance drops and the motor current increases. When the motor reaches full speed, the bypass switch activates and shorts out the thermistor to apply full voltage.

The time delay setting will depend on the thermal characteristics of the thermistor and motor. It should be long enough for the motor to reach full speed before the bypass activates, but not so long that the thermistor overheats.

PWM Current Limiting Soft Start

A microcontroller-based PWM soft start offers the most precise control and customization. The main components are:

• Microcontroller with PWM and ADC
• MOSFET or IGBT switch
• Current sensor (shunt resistor or hall effect)
• Gate driver (for high power switches)
• Input and output protection

The microcontroller generates a PWM signal to control the switch, which applies pulsed DC voltage to the motor. The duty cycle starts low and ramps up over the set start time. The ramp profile can be linear or any other shape programmed into the microcontroller.

The current sensor measures the motor current and feeds it back to the microcontroller ADC. The microcontroller samples the current and compares it to the set limit. If the current is too high, the duty cycle is reduced to bring the average back down. This provides real-time current limiting throughout the start.

The MOSFET or IGBT switch must be rated for the full motor voltage and current. High power switches may require a separate gate driver to provide the necessary drive current. The switch is connected in series with the motor positive lead.

Input protection includes a fuse or circuit breaker to prevent overdrawing from the power supply. A flyback diode across the motor protects the switch from inductive voltage spikes.

The microcontroller code will initialize the PWM and ADC, then start the ramp function. The current limiting runs in the background, continuously sampling the current sensor and adjusting the duty cycle. The ramp completes when the duty cycle reaches 100%, applying full voltage to the motor.

Example DC Motor Soft Start Circuits

Here are a couple example soft start circuits for common DC motors and applications:

12V DC Motor Soft Start for Automotive Use

This simple and low-cost thermistor soft start is suitable for a 12V DC motor in an automotive application, such as a power window or seat adjustment.

Components:
– 12V DC motor
– 10 ohm NTC thermistor, 3 A
– 12V SPDT time delay relay
– 15A fuse

The 10 ohm thermistor limits the starting current to about 1 A when cold. As it heats up over 2-3 seconds, the resistance drops and the motor current increases to the full 3 A rating.

The time delay relay is set to activate after 3 seconds, bypassing the thermistor. The 15A fuse provides overcurrent protection.

Circuit Diagram:

   +--------------+
   |  12V Battery |
   +------+-------+
          |
         (15A)
          | 
   +------+-------+
   |  10 ohm NTC  |
   +------+-------+
          |
          +-----+
          |     |
   +------+    ++-+
   | Motor +---+  |
   |      ----+ D|  
   +------+    +--+

D = 12V SPDT Time Delay Relay, 3 sec

24V DC Motor Soft Start for Industrial Conveyor

This PWM current limiting soft start provides precise control and customization for a 24V DC motor driving an industrial conveyor.

Components:
– 24V DC motor, 10 A
– Arduino Uno microcontroller
– IRF540N MOSFET
– ACS712 Hall effect current sensor
– 1N5408 flyback diode
– 20A fuse

The Arduino generates a 10 kHz PWM signal to control the MOSFET switch. The duty cycle ramps up linearly from 25% to 100% over 4 seconds.

The ACS712 current sensor measures the motor current and feeds it back to the Arduino ADC. The Arduino samples the current every 10 ms and limits the duty cycle to keep the average below 15 A (150% of rated).

The 1N5408 flyback diode protects the MOSFET from inductive spikes. The 20A fuse provides overcurrent protection.

Circuit Diagram:

   +--------------+
   |  24V Supply  |
   +------+-------+
          |
         (20A)
          |
       +--+--+
       |     |
       | IRF |      +--------+
       | 540N|      | ACS712 |
       |     |      |Current |
       +-+ +-+      | Sensor |
         |          +---+----+
         +------------+ |        
         |            | |        
   +-----+-+          | |       
   | Motor |-+      +-+-+-+--+
   |       |-+------+ Arduino| 
   +-------+    D   |        |
                    +--------+

D = 1N5408 flyback diode

Arduino Code:

const int MOTOR_PIN = 9;    // Motor PWM pin
const int SENSOR_PIN = A0;  // Current sensor input
const int MAX_DUTY = 255;   // 100% duty cycle
const int RAMP_TIME = 4000; // 4 second ramp

int duty = 0;

void setup() {
  pinMode(MOTOR_PIN, OUTPUT);
  pinMode(SENSOR_PIN, INPUT);

  // Initialize PWM 
  TCCR1B = TCCR1B & B11111000 | B00000010;  // Set PWM frequency to 10 kHz
}

void loop() {
  // Ramp up duty cycle over time
  for (int t = 0; t < RAMP_TIME; t++) {
    duty = map(t, 0, RAMP_TIME, MAX_DUTY/4, MAX_DUTY); 
    analogWrite(MOTOR_PIN, duty);
    delay(1);

    // Check current and limit if needed  
    int current = analogRead(SENSOR_PIN);
    if (current > 185) {  // 15 A limit
      duty = MAX_DUTY/2; 
    }
  }

  // Hold at 100% duty
  while(1) {
    analogWrite(MOTOR_PIN, MAX_DUTY);  
  }
}

This ramping function increases the duty cycle from 25% to 100% linearly over 4 seconds. The current is checked every 1 ms, and the duty is cut in half if the limit of 15 A is exceeded. After the ramp is complete, the motor is held at 100% until