Motor controls, and specifically motor starters, are the foundation of many electrical control solutions. To help you gain a better understanding of what a motor starter is, how they work, why we use them, and different types of starters, we’ve put together this post,the first in a series of articles focused on the basics of motor starters.
WHAT IS A MOTOR STARTER?
A motor starter is an electrical component designed to start and stop a motor safely. Like a relay, a motor starter switches power on and off. Unlike a relay, a starter also provides overcurrent and low voltage protection.
A motor starter has four main functions:
- Start a motor safely.
- Stop a motor safely.
- Reverse the direction of a motor.
- Provide low voltage and overcurrent protection to the motor.
The motor starter is generally comprised of two components that work together to control and protect the motor.
- Electrical contactor: starts and stops the power supply to the motor by making or breaking the contact terminals.
- Overload relay: monitors the current for an overload condition.
WHY WE USE A STARTER WITH A MOTOR
Electric motors are classified as an inductive load, which means it requires an enormous amount of power to get up to speed, then a relatively small amount of power to maintain that speed. This initial surge of power is called inrush current. Although short, it can be several times the steady-state current. Motors are rated for this inrush current and can be started and stopped without damage.
However, applications that require multiple starts/stops (typically more than 4 per hour) should use a VFD variable frequency drive, as the repeated inrush from starting across-the-line could overload and damage the motor.
With larger motors, such a large amount of current can cause dips in voltage of the supply line that could damage other equipment attached to the same line. Accordingly, most larger motors are started via a different method (RVSS soft starter, or VFD variable frequency drive, discussed separately). If a number of smaller loads are set to come online together, care should be taken to stagger their starts, via on-delay timers or programming of the control system.
HOW A STARTER WORKS
A motor starter can be manual or electromechanical. In applications where a manual starter is used, an ON/OFF lever or button must be manually changed from the ON or the OFF position. This will turn off or on the power source by making or breaking the contacts.
A disadvantage of manual starters is that during a power failure, they would resume operation as soon as the power comes back online, potentially creating a safety hazard. Manual starters do not allow for automatic control, however, so industry best practices limit the use of manual starters in favor of automatic starters.
In an electro-mechanical starter, the contactor acts much like a relay. The control circuit provides voltage to a coil, which energizes and magnetically pulls in the contacts completing the circuit to the motor and allowing it to draw power. This isolates the control and power circuits completely, allowing for low voltage (typically 120V) control to a higher voltage (typically 208V or 460V) power circuit.
The overload relay protects the motor from overloading. Since current is what generates heat, the overloads monitor the current going to the motor to see how much heat is being generated.
An overload can trip for many different reasons, it could be running above full load amps, the inrush current could be drawn for too long, or one of the phases could be pulling more than the others. If an overload condition exists, the overload relay is tripped, automatically de-energizing the coils in the contactor and shutting power off to the motor. This is done to prevent damage to the motor.
Unlike a manual starter, the contactor contacts in an electro- mechanical starter move back to their normal position following a power failure or overload, which keeps the motor powered off when power flows again.
In applications where an automatic restart could lead to a safety hazard, a start pushbutton can be added to prevent automatic restart of the motor when the power comes back online. This would also prevent multiple loads from coming online at the same time after a power failure, leading to voltage dips and potential equipment damage, as discussed previously.
Palmer DCS offers a wide of options for magnetic motor starters that meet the time demands of busy HVAC services companies, mechanical and electrical contractors and more. We approach every job with quality, flexibility, and reliability, offering more than 30,000 configurations for NEMA and IEC magnetic motor starters. To download our Enclosed Magnetic Starter Solution Guide, visit our website at nationalmotorstarters.com.
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