A manual motor starter is a device used in industrial applications to control the starting and stopping of electric motors. This device plays a crucial role in ensuring the safety and reliability of electrical systems in various industries. The working principle of a motor starter is based on the concept of magnetic switches which act as control switches for the motor. The article will delve into the working principle, applications and basic components of a manual motor starter providing a comprehensive understanding of this essential electrical component.
What is a Manual Motor Starter?
The manual motor starter is a protection device that integrates circuit breaker and overload relay functions. It protects the electrical motor from overload, short circuits and phase loss. It can be used as a disconnector by using its handle and it isolates the motor from the main supply.
Other common aliases for a manual motor starter include:
- Motor-protective circuit-breaker (MPCB)
- Manual motor protector (MMP)
- Manual motor controller (MMC)
- Manual starter protector (MSP)
- Motor circuit protector (MCP)
By using a manual motor starter and a contactor combination, the remote control function is provided by the contactor. Protection and isolation functions are provided by the starter.
Electrical motors should be protected against overloads and short circuits. Moreover, motors should be isolated from the main supply. To be able to do that, there are lots of circuit devices available such as contactors, overload relays, circuit breakers and switch disconnectors. Each device has a different duty in a motor circuit.
How Does a Manual Motor Starter Work?
After detecting an overload or short circuit, the manual motor starter disconnects all phases from the supply and isolates the motor from the supply. In addition, they increase the device’s reliability by reacting very quickly. It protects load-side circuits against damage.
Similar to molded case circuit breakers, standard manual motor starters are equipped with two releases:
− An adjustable, inverse time-delay overcurrent release for overload protection (Thermal protection)
− A fixed, instantaneous release for short-circuit protection (Magnetic protection)
The tripping characteristics of the inverse time-delay thermal over-current release apply to direct current (DC) and alternating current (AC) with frequencies of 50/60 Hz. For three-pole loads and currents of between 3 – 8 times the set current, the tolerance of the tripping time is ±20 %.
The tripping characteristics of the instantaneous short-circuit releases are based on the rated operational current Ie, which, in the case of the manual motor starter is the same as the upper value of the setting range. Lower current settings result in a higher multiple for the tripping current of the instantaneous short-circuit releases. The tripping characteristic curves are valid for the cold state; and the warm state, while the tripping times of the inverse time-delay thermal over-current release have a higher spread.
Phase loss sensitivity is a characteristic of inverse time delay and thermal over-current releases. A strong imbalance between phases can damage motors and other loads. They are designed to detect these faults and trips to prevent load-side circuit and motor damage.
Manual Motor Starter Applications
It is a reliable, cost-efficient solution for motor protection in many industrial applications such as:
- Heating Ventilation Air Conditioning (HVAC)
- Water and wastewater treatment
- Conveyor systems
- Packaging machines
Manual Motor Starter Wiring
During normal operation, the device should be symmetrically loaded on all three poles to prevent early tripping due to phase loss sensitivity. 3 phase devices can directly be connected to the main poles. To protect single-phase or direct-current devices, all three main poles have to be energized and connected in series.
Manual Motor Starter vs Circuit Breaker
There is not much difference between the two products as a working logic. Both of them provide thermal and magnetic current protection. Manual motor starters are mainly designed for motor circuits and are more compact. They also provide phase loss protection.
Generally, MMSs are produced up to 100A. Although the thermal currents can be adjusted from the potentiometer, the protection limits of the magnetic currents are fixed. Short circuit breaking capacities do not exceed 100kA. The accessories that can be attached to MMS are limited. For example, you cannot install a mechanical interlock between two MMS and operate them opposite to each other (changeover).
Circuit breakers (MCCB and ACB) have a more comprehensive thermal-magnetic protection capability. They can reach a current of up to 6300A and a short circuit breaking capacity of up to 150kA. Thermal and magnetic currents can be adjusted using dip switches or electronic microprocessors on the front. To ensure selectivity, time delay functions can be used. The variety of accessories is much more.
Manual Motor Starter vs Contactor
The manual motor starter is a protection device but the contactor is a control device. The contactor doesn’t have any protection function.
Manual Motor Starter vs Overload Relay
The manual motor starter has both thermal and magnetic protection. The overload relay has only thermal protection. It is not possible to isolate the load with an overload relay. An overload relay needs a device like a contactor to isolate the load. MMS can be used as a cam switch to start the engine. Also, the remote control is possible with accessories that can be attached to it.
Accessories of Manual Motor Starters
Major accessories of manual motor starters are:
1. Auxiliary contacts
Auxiliary contacts remotely indicate the state of the contacts in the starter. The auxiliary contacts can be used for signaling, electrical locking or relaying. They change position with the main contact of the starter. They open and close a separate circuit depending on the position of the device. Auxiliary contacts are available in various versions as normally open or normally closed contacts.
2. Signaling contacts
Signaling contacts signal the tripping of the starter. Like auxiliary contacts, signaling contacts are also available as normally open or normally closed contacts.
3. Shunt release
The shunt trip opens the motor starter when the control voltage rises above 0.7 times the rated voltage. Tripping occurs when a supply current is applied.
4. Under-voltage release
The undervoltage release opens the motor starter when the control voltage drops below the trip threshold. The under-voltage release releases the starter or prevents it from being switched on when its voltage supply is interrupted. This can be used in emergency switching circuits or can prevent an automatic restart after voltage interruption.
Manual motor starters are often built together with contactors for different starter combinations. Three-phase busbars with associated feeder terminals ensure a quick and safe connection for several manual motor starters.
6. Handles and shafts
With this solution utilizing a door coupling rotary mechanism it is possible to operate a manual motor starter in the back of a switch cabinet from the outside.