When to Use Soft Starters

What is a soft starter?

Motors require a lot of energy to get started, and then they’ll require even more to get up to their maximum speed. To get a motor started, it will drain up to eight times the usual working power from your power source. As you can imagine, this is a lot of strain and stress put onto your motor!

A soft starter is designed to lessen the burden and protect the motor and other components of your system from overheating. They can also prevent electrical damages from occurring thanks to lowering the initial inrush of current.

Rather than the motor being thrust a whole load of current at initial startup, the soft starter works to gently allow more current to flow to the motor, a little at a time. Not only does this protect the motor from the inrush of current, but it also helps the motor accelerate smoothly.

Modern soft starters will use solid-state devices, such as Silicon Controlled Devices (SCRs), to control the initial current. These devices start up as soon as the motor is turned on and restrict the amount of current passing through to the motor.

Once the motor is started and is ready to accelerate, the SCRs will begin to relax and allow more power to get through to the motor. The SCRs will turn off once all of the currents have reached the motor, and the motor will remain at a constant speed until you want to turn it off.

As you turn the motor off, the SCRs will turn back on and work to slowly power down the motor. Again, this protects all of the components of your system from being damaged from the current moving too quickly around the system.

What are soft starters used for?

There are many uses for soft starters, from starting an AC motor or regulating a water plant facility.

Below we are going to be looking at just a few of the applications in which you can use a soft starter, but bear in mind that there are many other applications suitable for a soft starter.

For an AC Motor

An AC motor has traditionally been started by using the maximum amount of current, voltage, and torque all at the same time. This pressure put onto the motor was enough to get it starting, but not without a price.

Likewise, when the motor is turned off, all of these things are immediately stopped. This traditional method is one of the main reasons that AC motors have a shorter lifespan and are prone to overheating.

This is because the initial inrush of current is around 6 to 8 times the motor’s rated current. Imagine all of that power racing towards the motor with it having nothing to protect itself with. No wonder the motor gives up prematurely – it’s overworked and overtired.

A soft starter can eliminate these issues altogether. As we’ve mentioned earlier, the soft starter works to reduce the initial inrush of current and torque. Instead, it leaves the AC with a much gentler way to start without all of the damage.

Can you use a soft starter for every AC motor?

Some AC motors require something a little stronger than a soft starter. If the power needed to start the motor is too high for a soft starter to deliver, a hard starter might be better to use.

These offer the motor an even bigger inrush of current to get it started, but in a much safer way rather than leaving it uncontrolled.

Moving back to soft starters, these are best used in AC motors when you’re living off the grid and away from mains power. When you have a small generator and want to power up an AC unit, you’ll need a soft starter to protect both the generator and the AC motor.

RV owners particularly enjoy using soft starters because there is less worry that their generators and AC units are going to be left with a shortened lifespan or being prone to overheat. So, soft starters should not be used on every AC motor, but they can be invaluable in RVs and motorhomes.

Other Applications for Soft Starters

Soft starters can be used anywhere that needs a helping hand keeping the startup and cooldown gentle to prevent damaging the components in your circuit. Once the motor has been started up, the speed of the motor will then need to remain constant and not change.

Take a water plant facility for example. Water enters the plant at a certain speed, with a demand for water coming out of the plant that remains consistent. The water pump has the job of keeping the water moving in and out of the water plant facility.

You can install a soft starter onto the pump’s motor to prevent the inrush of current lessening the lifespan of the motor and pump. The soft starter will allow the motor to accelerate steadily until it reaches its maximum speed.

As the pump does not need to work any harder or slower due to the demand for water exiting the plant always staying the same, a soft starter can be installed without any issues.

If the demand was fluctuating, you might want to install a variable frequency drive (VFD). This device is similar to a soft starter but it allows the speeds to change, so the pump would be able to work harder or slower depending on the water demand at that time.

What is the difference between a soft starter and a variable frequency drive?

A VFD is quite similar to a soft starter in the way that it works, but there is one big difference between the two. A VFD can control an AC motor that needs to run at different speeds. As soft starters can only be used on applications that require the motor to remain at the same speed, a VFD works for the applications that need a flexible motor speed.

VFDs adjust the frequency of the motor, allowing the RPMs to be altered. It does this by converting the AC to DC using diodes. A capacitor will then clean the DC before the transistors change the DC back to AC. This allows the VFD to alter the frequency of what is being supplied to the motor. In turn, this will alter the RPMs and therefore the speed of the motor.

Take a fan as an example. If the fan only had a one-speed setting, a soft starter would be the device for the job. However, if the fan had a variable speed to keep up with the fluctuating temperatures, you would use a VFD.

What is the difference between a soft starter and a DOL starter?

We’ve heard all about soft starters by now, so let’s take a look at another method of starting your motor. Rather than gently feeding current to the motor like a soft starter, this method does quite the opposite.

A Direct On Line (DOL) starter allows the motor to be turned on as soon as you press the ignition switch. There is a direct connection between the power supply and the motor, so there is nothing in the way of an instant power-up.

DOL starters are the quickest way to get a motor up and running, but they are often only reserved for small loads only. This is because too big of a load would require too large of an initial inrush of current to get the motor started.

There is often thermal or electrical overload protection within a DOL starter to protect the motor from a shortened lifespan or overheating. Many people like DOL starters because it is considered the cheapest and easiest way to start a motor, but you have to be careful.

A motor needs an initial inrush of current that is up to 10 times the usual running current of a motor. DOL starters will require all of this at their disposal, so make sure that your power source is up to the job.


Soft starters are best used when the motor needs to stay at a constant speed once it has been turned on. Soft starters can also be used when you want to preserve the motor and your appliance’s lifespan due to a gentler method of starting it up.

However, soft starters will not fit the bill of every application out there. If your motor needs to change speeds all the while it’s running a VFD would fit better. Similarly, if your motor needs a larger starting current to power on, a DOL could be the ideal option.