The Ultimate Buyer's Guide for Purchasing Synchronous Motor Starting Methods

Dear Farhad47

My immediate reaction was to say soft start too, but then I got to thinking.

The selection of the type of synchronous motor and the most appropriate starting method depends on more factors than just those in the info you have given. These are quite large motors so it is difficult to be specific without knowing more information. For example:

[ul][li]Are these motors still to be purchased, or are you trying to adapt some existing motors to a new application? [/li]

[li]If they are new motors that you are specifying, put the onus on your suppliers to meet your performance criteria - tell them what to achieve, not how to do it. If you're buying the fan separately, give the fan details to the motor suppliers (e.g., torque speed curves, inertia, etc.) Tell them of your electrical system constraints such as fault level, max permissible voltage dip, intermediate transformer details, etc. If you're buying the fan and motor as a package, the fan supplier will still need to know your system constraints to pass on to his motor subcontractor. [/li]

[li]Have you considered the life cycle cost advantages that you might get from a fully rated variable speed drive? If your process demands variable flow rates from your fans, the improvement in part-load efficiency of a VSD over a constant speed drive can be startling. The incremental capital cost of a VSD vs. a conventional installation can be recovered (in many cases) in two to three years, depending on the amount of part-load running involved. Also, if a fully-rated VSD is an option on this basis, you may be better considering a variable speed induction motor drive instead. [/li]

[li]If they are existing motors, the motor construction may guide you to your solution. If you have synchronous induction motors, you are probably stuck with 2). For reactor starting an SIM, you will probably still have to load the starting winding, which will need the starting resistor. If you just short-circuit the starting sliprings and brushgear, you will likely damage your brushgear and sliprings due to the high current that will flow. You might be able to apply a soft start to an SIM, but you will likely not get away from brushgear totally. [/li]

[li]If you have a conventional brushless synchronous motor, you could probably use either reactor or soft-start. For reactor starting, you would have to study whether you can get enough motor torque to accelerate the fan using the current limitation allowed by the reactor. If you can't get enough torque, you might try autotransformer starting as another option. [/li]

However, my immediate reaction these days would be to say soft start, though you would probably have to modify the brushless exciter (to replace the exciter dc field stator with an induction motor type stator) to allow motor field current to be available at standstill. Again, you might consider a full VSD conversion, but you would need to study the effects of the VSD on the motor (e.g., harmonic currents increasing heating, harmonic torques exciting shaft system and fan critical frequencies, need for separate vent fans) and the supply system (e.g., harmonic currents/voltages causing power quality issues for your other plant and your network service provider). [/li][/ul]
There is no hard and fast rule that says one solution is better than another for any motor of this type and size. Each application is different, and it really does depend on more than just the motor size as to which is the best solution. You need to address all of these peripheral issues, or have your supplier address them before homing in on your final solution.

PS I do not work for a motor or drives supplier, so I'm not touting for business here - I've just been there and done that in the past. Farhad:

Are you sure that these are synchronous machines?
SM's do not slip, and their rated speed (at assumingly your 50Hz system) should be RPM, not .
If ther are, here are a few more pointers:

1. Varying the speed of a SM is more complicated than that of a squirrel cage induction machine. A normal VSD is not enough. You would need a unit called 'Load Commutated Inverter' that controls line voltage and frequency and also controls the field. Unless your fan application requires speed varying (e.g. induced draft) I'd stay away from the LCI.
2. Starting your motor, brush-type or brushless depends on many mechanical and electrical factors. Here are some:
a. Back power (available MVA) to consider for voltage drops. Will your motor starting collapse your bus voltage?
b. Amortisseur winding design (this is the squirrel cage induction part of the SM), are they designed for full or reduced voltage start?
c. Your coupling to the fan drive, can it handle full starting torque? The coupling will actually twist during start!
3. Your starter design will depend on your motor design. Brushless being the simplest and only requires an external minimal DC supply source, normally up to 125VDC at less than 10Amps. But the brush-type requires a large field discharge resistor, a large DC power supply, and a large DC contactor with make before break sets of contacts, along with optimum phase angle field application circuitry.
4. The most economical starting method is a reactor start. You get more motor torque for less line current draw with an autotransformer. For more cost savings, and if your motors are on the same electrical bus, you can have a starting bus and a running bus in your switchgear.

Induction motors are popular in the industry, since they are simple to build and economical. The following are some essential things you need to consider when buying an induction motor.

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What is an Induction Motor?

An induction motor or an asynchronous motor is an AC motor that works on the principle of electromagnetic induction. That is, the rotor obtains torque under the action of the rotating magnetic field generated by the stator, and then the rotor rotates. Since the rotor speed is not the same as the speed of the rotating magnetic field (synchronous speed), the induction motor is also known as asynchronous motor.

The rotor is a rotatable conductor, which is divided into squirrel-cage type and wound type according to the winding form. The stator is the stationary part of the motor which provides a rotating magnetic field. The rotating magnetic field is realized through the alternating current. When the current flows through the coil of the rotor, the properties of the magnetic poles are cyclically changed, which is equivalent to a rotating magnetic field. Depending on the the input supply, induction motors can be categorized into single-phase and three-phase induction motors.

Single-phase induction motors are squirrel-cage types, which are mainly used for small loads, such as fans, washing machines, refrigerators, air conditioners and other household appliances.

Three-phase induction motor rotors include squirrel-cage type and wound type. They are widely used as industrial drives due to its low price, sturdy structure, low maintenance cost, high efficiency, easy operation, reliability and durability.

Choosing an Induction Motor

When buying an induction motor, the primary concerns are speed, voltage, torque and power. Other things you've got to take into consideration are working system, overload requirements, insulation class, protection class, moment of inertia, load, installation method, maintenance, ambient temperature, altitude, enclosure rating, outdoor environments, energy efficiency, etc. Now, ATO provides you with its reliable and economical induction motors and helps you make best choices in your purchases.

1-Phase Induction Motors

An induction motor using a single-phase AC power supply is called a 1-phase induction motor. Because single-phase induction motors only need single-phase alternating current, they are easy to use and widely used, and have the advantages of simple structure, low cost, low noise, and little interference to radio systems, so they are often used in household appliances and small power machinery with low power. Among them, such as electric fans, washing machines, refrigerators, air conditioners, range hoods, electric drills and medical equipment.

3-Phase Induction Motors

Three-phase induction motor is a kind of induction motor, which is powered by three-phase AC current. Compared with single-phase induction motors, three-phase induction motors have better running performance and can save various materials. Three-phase induction motor are mainly used in excavation, fluid transportation and other fields that need to provide power, such as machine tools, small and medium-sized steel rolling equipment, fans, pumps, light industrial machinery, metallurgy and mining machinery, etc.

Are you interested in learning more about Synchronous Motor Starting Methods? Contact us today to secure an expert consultation!

Explosion Proof Motor

Explosion-proof motor is a kind of motor that can be used in flammable and explosive places and does not produce electric sparks when running. Explosion-proof motors are mainly used in coal mines, oil and gas, petrochemical and chemical industries. In addition, in the textile, metallurgy, city gas, transportation, grain and oil processing, paper, medicine and other sectors are also widely used. As the main power equipment, explosion-proof motors are usually used to drive pumps, fans, compressors and other transmission machinery, etc.

Brake Motor

The brake motor is a fully enclosed self-fan-cooled squirrel cage type three-phase asynchronous motor with an additional disc type DC brake. It is composed of a DC disc electromagnetic brake motor attached between the rear end cover of the Y series motor and the fan. It is a derivative series of the Y series. It has the advantages of rapid braking, simple structure, high reliability and strong versatility. Brake motors are widely used in various mechanical equipment and transmission devices that require quick stop and accurate positioning.

Variable Speed Motor

A variable speed motor is a motor that changes the speed of the motor by changing the number of stages, voltage, current, frequency, etc. of the motor, so that the motor can achieve higher performance. Due to its excellent performance, variable speed motors have been widely used in steel, power stations, cables, chemicals, petroleum, cement, textiles, printing and dyeing, papermaking, machinery and other industrial sectors for stepless speed regulation of load machinery with constant torque or decreasing torque.

Stainless Steel Motor

The stainless steel motor is designed to avoid flats, cracks and crevices, preventing the accumulation of foreign objects and bacteria. Its housing and cables can withstand daily high pressure, high temperature and aggressive chemical washdowns. The robust design means the motor can be cleaned without the need for guards and covers to protect it. These features ensure faster cleaning, higher uptime, and higher overall line equipment efficiency.

Induction Motor Advantages and Disadvantages

Advantages:

• Various control methods: Since induction motor has no permanent magnet, it is easier to achieve different speeds and torques by weakening magnetic field or frequency conversion, etc. It can be used in industrial or domestic applications that do not require high torque density, and has the advantages of self-starting, economical, and reliable.
• Low cost: Induction motors are cheap compared to synchronous and DC motors. This is determined by the design of the induction motor. Therefore, induction motors are the first choice for fixed speed applications in industry, as well as for commercial and domestic applications where AC power can be easily connected.
• Low maintenance cost: The structure of the induction motor is simple, so the maintenance is also easy, which reduces the maintenance cost.
• Easy to operate: Induction motors are self-starting motors (runs on their own with alternating current). It operates easily since there is no electrical connector on the rotor.
• Speed variation: The speed variation of an induction motor is constant. Speeds vary by only a few percent from no load to rated load.
• High starting torque: Induction motors have a high starting torque, which allows a load to be applied before the motor starts. Three-phase induction motors have a different self-starting torque than synchronous motors. However, a single-phase induction motor has no self-starting torque and can only be rotated by using some auxiliary means.
• Durability: Another advantage of induction motors is durability, which allows the motor to operate for many years without any cost and maintenance.

Disadvantages:

Compared to permanent magnet motors, induction motors are less efficient because of the current heat loss caused by the induced current.

Contact us to discuss your requirements of How to Start Synchronous Motor. Our experienced sales team can help you identify the options that best suit your needs.