One of my very first orders when I took over a territory in northern Ontario, Canada, was from an entrepreneur who was making insulation from newspaper and newsprint. It had a big shredder/pulverizer, which was run by a 250-hp motor.
In Ontario, there is an electrical component for running large machines called a demand charge. This charge was a separate line item on the bill, which related to the peak power used during any given time period.
Get your subscription to Control Design’s daily newsletter.
A large current draw, even for a short period of time, would result in an increased demand charge for the time period allotted. In order to reduce the impact of this charge, large motors utilized reduced voltage starters. Using this technology made sure the lights didn’t dim when the shredder was started up.
Applications such as conveyors with multi-motor sections would create a large power peak when the conveyor started up.
Automatic and manual reduced voltage starting was employed to minimize the effects of across-the-line starting, which would include mechanical torque effects from initial startup.
The rise of the VFD
As technology has moved forward, ac/dc drive control, while initially expensive, has proven to be the evolution of reduced voltage starting.
Using ac variable-frequency drives (VFDs) on even smaller motors has created reduced mechanical stress on the components used in the process. In across-the-line starting of motors, a flex rotational coupling would often be used to soften the blow to the system, which is not required when using a VFD.
Using this softened startup speed curve has many benefits for the process materials and components. Blowers and fans benefit from this approach in many applications. Pumps also benefit with a controlled startup of flow, which would also benefit the control valves, should they be present.
The use of ac motor and drive technology, even in low-speed applications, has allowed control-system designers to build more robust systems with a better maintenance record. Arguably, dc motor control is rarely used in modern system designs.
Energy savings is an unintended benefit of reduced starting speed applications. In fan/pump applications, a reduced percentage of speed results in horsepower reduction of greater proportions.
One of the most complex systems I was involved with, from afar, was a printing-press application. Synchronized drive systems allowed for the line to start up as one, and the drives had a common speed signal and tach/resolver following control to make sure the drives started up and ran at the same speed.
With 2025 drive technology, this can be done over a network; however, having a hardwired solution may be better served. Network non-deterministic characteristics may play havoc with motor speed timing.
Alternating-current drives can come in low-voltage to medium-voltage applications. The mining industry would have medium-voltage hoist drives for skip control and ball mills, for example.
There are varying technological drive properties. Volts/Hertz is a standard method of speed control; however, the precision is a bit in question. If a blower or fan is rotating at 900 rpm or 895 rpm, it isn’t a really big deal. A pump for flow, however, may be critical enough to employ a different control method.
This method of speed control could be scalar or vector. One of the advantages of this method of control is that multiple motors can be controlled with the same drive output to maintain a similar speed for all motors. Speed regulation methods should be employed in the case of multi-motor control, which is application-dependent.
PWM, IGBTs, SCRs and TRIACs
For a more precise speed control profile, most applications would choose a pulse-width-modulated (PWM) VFD. Using speed feedback for error compensation allows for very precise speed control, should the application require it.
Sometimes it comes down to motor selection, as well. Using an ac servo motor gives the application a better torque profile than a standard induction motor. Positioning applications would normally utilize a servo motor, while typical rotational applications can use a standard ac induction motor.
For high-torque or quick-acceleration movement profiles, a servo motor would better suit that application.
Vector-controlled motors use a VFD, which typically employs a closed-loop controller for precise speed control. This also means that a feedback device, such as an encoder or resolver, needs to be used to attain this tight control. Pulse-width modulation is used for speed control rather than V/Hz.
Over my career, spanning more than 45 years, I have witnessed the rise of VFDs for energy control, electricity bill reduction and creating a more precise manufacturing environment. The advent of high-powered insulated-gate bipolar transistor (IGBT) has allowed for VFD development in higher-current applications, as well as power conversion. Common silicon-controlled rectifiers (SCRs) and triodes for alternating current (TRIACs) have been phased out in favor of IGBTs.
