Motor Control Methods Explained
Controlling an AC motor isn’t one size fits all. The method you choose impacts energy usage, equipment lifespan, and overall system performance. Terminology for motor controls can vary based on geography, application, or the length of time someone’s been in the electrical industry. The methods to control an AC motor generally break down into three main styles. To help with a basic knowledge of motor controls, we’ll cover each category, what else it may be called, and why it may be the preferred method for your situation.
Direct Start
Direct starting is the simplest way that you can operate an electric motor. This method uses a contactor, basically as a large “on/off switch” for motors. It’s used to avoid complication or additional cost when starting a motor but offers no control over inrush current or speed.
This method has some major downsides for many motors. To start a motor with this method, the motor draws a large amount of current, called inrush current, for a fraction of a second. Even though it’s a short time, it’s a large amount of energy that must be available, up to 700% of the motor’s rated amperage. This inrush is also tough on the motor, leading to stress and potential damage down the road.
When to Use Direct Starting
- Small motors
- Low cost for motor failures
- Little concern for inrush
Direct Start – Also Known As:
Across the Line or ATL. This name comes from the starting method using the energy coming from the utility or incoming line.
Direct on Line or DOL. This is similar to ATL, getting its name from the method using power directly from the line with as little interference as possible.
Soft Starting
Soft starting an AC electric motor is the next step up from a direct start. It uses a device to ramp the motor up at a slower rate. When the motor is up to speed, the device typically goes into a bypass mode, dropping the motor “on the line,” meaning it’s running similarly to the direct start method until the next time it starts. A soft starter does not allow for the ability to continue to control the speed of the motor once it has reached the end of its ramp.
The benefit of a soft start is that it’s a simple way to reduce the inrush. Where the inrush on a motor across the line can be up to 700%, a soft starter will reduce that by about half, operating in the 300 to 400% range.
When to Use Soft Starting
- Mechanical stress concerns
- Limited need for speed control
Soft Starting - Also Known As:
Soft Starter or Soft Start. You’ll also occasionally see both written as a single word, either softstarter or softstart. All of these versions of the name have a relation to the starting method offering a softer mechanical blow to the motor compared to direct starting.
Reduced Voltage Soft Starter or RVSS. This is the technical name for soft starters, and is often used in specifications or other engineering documents. It is a more descriptive name of how a soft starter works.
Speed Control
Speed control comes from a device most commonly called a variable frequency drive (VFD), although it has many other names. A VFD has several benefits over direct and soft starting. It limits inrush current to 100% of the motors rated amps while having the ability to give full torque to the motor through the entire starting process. It also allows for speed control after the motor is up to speed, giving the ability for process control and energy savings.
The downside to a VFD is that it is more complicated than the other methods, which can also show in a higher price. While your exact application and situation can change how wide the complexity gap is, VFDs typically require more setup and programming than other methods.
While VFDs save mechanical harm to the motor, they introduce electrical power quality side effects, such as harmonic distortion, reflective wave phenomenon (dV/dt), and common mode. While these issues are possible to remedy, power quality filters increase system cost and complexity. In cases where speed control of an AC motor is needed, these are usually minor side effects that are easily overshadowed by the benefits of a VFD.
When speed control isn’t needed, power quality issues may not be worth dealing with. In some cases, it’s important to eliminate as much inrush as possible, so even when speed control isn’t needed a VFD may be the right solution. In these cases, it may be good to look at bypasses or synchronous transfer capabilities to try and get the best of both worlds.
When to Use VFDs
- Process control
- Focus on energy savings
- Strict limits on inrush current
Speed Control - Also Known As:
Variable Frequency Drive or VFD. This is the most common name for the device that allows you to control the speed of an AC electric motor. The name comes from the way a VFD functions, varying the frequency of the output electrical waveform to change the motor speed.
Adjustable Speed Drive or ASD. Another common name, similar to VFD. This name focuses on the effect the device has on the motor (adjusting the speed) rather than the output waveform (variable frequency).
Variable Speed Drive or VSD. This name is less common than VFD or ASD, but still is occasionally used. Similar to ASD, the name comes from the ability the device provides to vary the speed of the motor.
AC Drive or Drive. This is typically a simplified name and is not often used in specifications or technical documents. These names both come from the fact that the device drives and controls the motor, and one of them specifies that this is in an alternating current (AC) system. DC drives also exists, but the functionality is different and is almost always referred to specifically with the “DC” description.
AC Speed Controller. This is a more general term that is similar to AC Drive. Rather than referring to the device (the “Drive”) it references the benefit to the system (speed control for AC motors).
Inverter. This name comes from the output section of a VFD, called an inverter. The IGBTs in this section "invert” the DC within the VFD back to a simulated sine wave, called pulse width modulation or PWM. Be aware that other electrical devices are also called inverters. For example, photovoltaic (PV) solar systems use an inverter to change the DC from the panels into usable AC for the grid, but these do not offer the ability to control the speed of a motor. Because of potential confusion, this term is not typically used in specifications or technical documents.
Which Is Right for You?
Just knowing the basics and the names isn’t always enough. Choosing the right motor control method can be confusing and mistakes can be expensive. Even though we’re VFDs.com, we specialize in all motor controls and our experts are here to ask the right questions and recommend the best method for your application.
Motor Control Methods Comparison:
| Inrush Current | Setup Complexity | Speed Control | Side Effects | Cost | |
| Direct Start | 600-700% | Minimal | No | Mechanical stress on motor | Minimal |
| Soft Starter | 300-400% | Basic | Ramp during start only | Reduced mechanical stress on motor |
Basic |
| VFD | 100% | Moderate to High | Full speed control | Power quality byproducts (harmonics, dV/dt, and common mode) | Moderate to High |
Explore Motor Control Solutions
Now that you’ve seen the differences between direct starting, soft starting, and variable frequency drives, the next step is choosing the approach that fits your equipment, process, and inrush requirements. Start with the solution type below based on your application.
Variable Frequency Drives
For process control, energy savings, and low inrush starting.
Soft Starters
For applications that need reduced mechanical stress without ongoing speed control.
Need help choosing the right solution? Contact us here, or call us at 1-800-800-2261