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Starter block: which motor starting strategy?

11 January 2015

Various techniques for reduced voltage during motor starting are available; here, Jerry Hodek explores which might be best for your application.

The simplest way to start a motor is ‘direct on line’ (DoL). The only equipment required is a contactor and an overload relay, but there is a disadvantage – initially, energisation of the motor’s internal magnets draws a large current from the mains. This is typically six to ten times the rated motor current and can cause an under-voltage in the mains. It also means that starting torque is high, leading to slipping belts, shock loads on bearings and mechanical drive components, and possible product damage and water hammer.

DoL can be acceptable for small motors, but the larger the motor the greater the potential problems. There are several ways these effects can be mitigated, including star-delta starters, softstarts, auto-transformers, resistance starters and variable speed drives. A star-delta starter consists of three contactors, an overload relay and a timer that switches the contactors from star configuration to delta after a set period of time. With star-delta both the motor current and the torque are two-thirds lower in ‘star’ than in ‘delta’ (because v3*v3=3).

An auto-transformer uses a transformer to reduce the voltage supplied by between 30 and 60 percent, so current and torque are reduced, but less drastically than with a star-delta starter. A timer is then used to switch to full speed, so providing a stepped-speed start.

A resistance starter can be used with slip-ring motors (but not squirrel cage motors) which have rotors that are wound for use with an external resistor. The resistor is used to increase the torque; this, in turn, reduces the starting speed and limits the in-rush current. Once the motor is running at the reduced speed the resistance is removed, usually by manually winding it out, so that the motor assumes its rated speed.

Soft starters and inverters both throttle current by reducing the voltage at start-up. Soft starters do not control speed - inverters do. During the start up phase they both behave in the same way. Switching the thyristors or transistors can generate voltage spikes and harmonics back into the mains and create electromagnetic pollution or noise, which may need to be filtered out.

Which starter, where?
When selecting a starting method for a given motor there are several factors to be considered. Motor suppliers are usually able to offer detailed and expert advice, but the basic ideas can be summed up as follows:

- Is the mains supply robust and reliable? If not, a gentle starting method is advisable. (This may seem an unnecessary question in Western Europe, but it can be an issue with big motors and elsewhere in the world – including North America – where the mains can be less dependable).
- The start-up torque, load, inertia, acceleration and speed range requirements of the application.
- The short-circuit capacity of the distribution system.
- Process considerations, such as the effects of shock loads and vibration on machinery or water hammer and pressure surge issues in pumping applications.
- Motor characteristics such as initial inrush currents, locked rotor currents and consequent torque values produced during operation.

DoL or full voltage starting can be used where the driven load can withstand the shock of sudden start-ups and where voltage drop and other disturbances to the mains supply can be tolerated. In general, only small- and medium-sized motors can be started so abruptly; larger motors and motors in sensitive applications need reduced voltage starting to diminish mechanical shock loads to equipment, limit inrush current in the electrical mains and circuits, and to reduce pressure waves in pipework.

It should be noted that voltage spikes can travel through the mains distribution systems and cause problems at a considerable distance from the motor where they originate. This is particularly true for larger motors, so suppliers have developed considerable expertise in this field.

For instance, the voltage spikes can burn out fuses, contactors and switchgear; cause lights to flicker, go out or bulbs to fail; cause problems with other equipment, such as control systems and office computers; cause false reading from sensors; disrupt radio, telephone and wi-fi signals. 

These problems may not be confined to the motor operator’s premises – neighbours may have grounds for compensation and supply utilities may take enforcement action if the mains network is compromised.

Every method of reduced voltage starting has a degree of cost penalty compared to DoL. Star-delta is by far the cheapest option for voltage reduction, so tends to be a popular choice.

Soft starts are more expensive, but offer many advantages in terms of finesse and reliability; they are particularly popular on water and waste-water pumping installations where motors are large and water hammer must be avoided at all costs.

Variable speed drives are more expensive yet, but if the motor’s output speed needs to be profiled, they are really the only solution.

Autotransformers and resistance starters are quite old-fashioned technologies and are only used rarely in contemporary installations.

Large motors are increasingly used in remote locations, or at sites where manpower has been reduced significantly over the years. Therefore, consideration has to be given to maintenance and servicing, for the – possibly long – expected service life of the installation.

While the simpler technologies would seem the natural choice here, it is notable that drives can be remotely interrogated and operated via telemetry, wi-fi or a hard wire communications network. As such, they open up a whole new dimension to the operators, who will be able to monitor remote equipment and reset it to meet new conditions or to optimise it against other parts of the system. 

Generally, we can say that the vast majority of low voltage motor starting options are star-delta, softstarts and variable speed drives and that the associated costs will reflect their level of sophistication. My company, and other reputable motor suppliers, are able to provide advice on an installation-by-installation basis.

Jerry Hodek is with Marathon Electric

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