FEATURE DRIVES, CONTROLS & MOTORS
IF YOU CAN’T TAKE THE HEAT... W
To prevent stepper motors
from overheating, it is essential that the products are properly sized for the load, as Eric Rice, Applied Motion Products, explains
hile it is completely normal for step motors to get warm – or even hot
– during operation, they do have a temperature limit. So, when specifying a motor for an application, it is essential that the motor is properly sized for the load and that it operates at a safe current setting and appropriate power supply. The thermal
weak point in a step motor is the insulation on the magnet wire composing the coils. Without insulation, each coil would operate as one big turn. Most step motors utilise Class B insulation, which is
StepSERVO Integrated Motors offer numerous benefits
limited to 130˚C, meaning that as long as the outside housing of the motor does not exceed 100˚C,
the inside of the motor will typically remain cool enough to prevent the
insulation from melting. There are, however, three causes of
heating in a step motor: Coil Resistance; Hysteresis; and Eddy Currents. Coil Resistance: The magnet wire has
DRIVING CRITICAL FOOD PROCESSING EQUIPMENT
Back in 2013, Tate & Lyle acquired Swedish company Biovelop, a manufacturer of oat beta glucan. Oat ingredients have been actively involved in the growing trend for specialty health and wellness products as they offer some key nutritional and functional benefits – oat contains beta glucan, for example, a soluble fibre that has been proven to lower cholesterol and reduce post prandial glycaemic response. To meet this growing demand for healthier foods, Tate & Lyle needed to boost the performance of the plant, so turned to ABB for the latest automation solutions. The process needed to be highly automated and capable of running 24 hours a day, seven days a week. So, ABB delivered 44 digital electro pneumatic positioners with hart protocol for the control valves, 85 VSDs with power ratings ranging from 0.37kW to 55kW, as well as MNS 3.0 MCCs and low voltage motors. The automation products drive critical food processing equipment including pumps and decanters, as well as the material handling machinery. To ensure the equipment worked optimally, ABB also added remote monitoring and fieldbus flexibility
into the drives. For Tate & Lyle, it was essential that engineers could digitally interact with the system, commission devices and diagnose faults from anywhere in the world. ABB designed the equipment so that the fieldbus responsible for device control is split from the
fieldbus used for asset management. As a result, any changes to the configuration of the devices can be made without the risk of the whole network going down. Following the changes to the factory, the company has seen a ten times increase in capacity with
the same number of shift operators compared to four years ago. “In our pump applications alone, we can use up to 50% less energy thanks to the VSDs, which have been
running non-stop for the last two years without a single failure,” said Leo Dijkstra, power & controls team leader Europe at Tate & Lyle. “What's more, ABB was so quick to deliver products compared to other suppliers we've worked with before, that we even had the first VSD delivered in just a few days.” Of additional benefit, despite the fact that 60 VSDs are mounted in one electrical room at any given time, the system runs so quietly that there is no need for staff to wear hearing protection.
ABB
www.abb.com
resistance, which generates heat proportional to the coil resistance and the square of the applied current. So, if you wanted to get twice as much torque by doubling the current, the motor would get four times as hot. Do not try this: step motors have a rated current specification for a reason. Hysteresis: When the rotor spins, it
causes the magnetic field in the stator steel to reverse each time a rotor tooth passes a stator tooth. That changing field warms the stator steel. Known as ‘hysteresis loss’, this happens even if the motor is producing no output power. If you disconnect a step motor from the driver and spin it with another motor, it will warm up. The faster the motor spins, the more hysteresis loss, regardless of applied current or voltage. Eddy Currents: To make a motor
produce output power, you must drive the stator coils with an alternating current. That current causes an alternating magnetic field in the stator steel which, in turn, induces an electric current in the steel itself - the eddy current. Since steel has electrical resistance, eddy currents generate heat which increases with motor speed and applied voltage. The chart illustrates how the sum of all
three types of motor losses is affected by speed and voltage – and this is at the rated current. Increase the voltage and current setting, and you live dangerously! This is why it is so important to
properly size the motor for the load. A stepper motor drive user manual will help you get it right first time.
Applied Motion Products
www.applied-motion.com
20 MAY 2018 | DESIGN SOLUTIONS
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