Why some loads are hard to start
Torque is the twisting force a motor applies to a shaft. Some machines ask for very little of it at the start and more as they pick up speed. Others ask for a great deal immediately. A loaded conveyor that stopped with material still on the belt, a crusher stalled with rock in the chamber, a mixer full of thickened product or a piston compressor starting against pressure all need a large push to begin moving. That initial demand is often called breakaway torque, and it can be noticeably higher than the torque needed once the machine is running.
Engineers usually sort loads into two broad groups. Variable-torque loads, such as centrifugal fans and pumps, need little torque at low speed. Constant-torque loads, such as conveyors, extruders, mixers and crushers, need roughly the same torque across the speed range. High-torque duty often means constant torque with an extra requirement at startup, and drives for it are chosen with that in mind.
What a drive does about it
A motor connected directly to the supply can produce high torque at start, but it takes a large inrush of current to do it, and the start is abrupt. That can shock couplings, gearboxes and belts, and it can sag the supply voltage for other equipment.
A drive changes the picture in several ways:
- Controlled ramping. The drive brings the motor up through the speed range gradually, which limits current and mechanical shock.
- Current limiting. The drive can hold output current at a set limit, so the motor produces as much torque as the drive and motor safely allow without tripping on every heavy start.
- Boost and torque control. Basic drives apply extra voltage at low frequency to help the motor keep its magnetic strength. More advanced drives use vector control to manage torque directly.
How torque is held at low speed
An induction motor makes torque by interaction between the stator’s rotating magnetic field and currents induced in the rotor. At very low frequency, the stator’s resistance takes up a larger share of the applied voltage, so the motor’s magnetic field weakens unless the drive compensates. Without compensation, torque falls away just when it is needed.
Approaches commonly used to hold torque down at low speed include:
- Voltage boost in simple drives. Adds voltage at low frequency, but it is a rough tool and may overheat the motor if overused.
- Sensorless vector control. The drive uses a mathematical model of the motor and measured currents to estimate what the rotor is doing, then controls the torque-producing current separately. It generally holds useful torque down to quite low speeds, though not necessarily to zero.
- Closed-loop vector control with feedback. An encoder on the motor shaft reports actual speed and position. This allows full torque at very low speed, and in many drives at zero speed, which matters for hoists or machines that must hold a load. Position encoders are covered separately in the motion-control topics.
- Permanent magnet or specialized motors. Some high-torque, low-speed duties use motors designed for that role, sometimes with the drive and motor engineered together.
Manufacturers publish how much torque a given drive and motor can hold at low speed and for how long. Those figures should be checked rather than assumed.
Sizing for demanding duty
Selection typically revolves around current, not just power. The questions below are a sensible checklist.
- What is the torque required at start and at running speed? Estimate or measure it. Starting torque requirements can be a multiple of the running value, and the multiple varies widely by machine.
- What overload rating does the drive have? Drives often carry a normal-duty and a heavy-duty rating. Heavy-duty ratings allow short periods of higher current, such as for a start, but the continuous current rating is lower for the same frame size. Choosing a heavy-duty rating is common for crushers and loaded conveyors.
- How long and how often are the peaks? One brief start per shift is different from frequent starts and stops, or from a process that sits in overload for minutes at a time.
- Can the motor cool itself at low speed? Most standard motors cool with a fan on the shaft. At low speed that airflow drops while torque, and therefore current, stays high. Heat builds in the windings. An auxiliary blower, a larger frame or a motor designed for inverter duty may be needed.
- Does the mechanical train suit the duty? Gearboxes, couplings and belts see the peak torque. Gear reducers are often paired with drives for these loads, and shock loads must be within the rated limits of each component.
- Is braking or holding needed? Stopping a heavy, high-inertia load can return energy to the drive. A braking resistor or a regenerative unit may be needed, and a mechanical brake is usually used to hold a load once stopped.
Typical examples
On a belt conveyor, the main risk is a restart under full load after a stoppage. A drive with current limiting and a ramp suited to the belt length helps avoid belt stretch and spillage. Plants dealing with long or inclined conveyors often discuss conveyor types and conveyor selection with the drive in mind, since belt tension and starting behavior are linked.
On a crusher, the load can be impulsive: rock arrives unevenly and torque demand spikes. The drive needs a suitable overload margin, and the system may include a flywheel that stores energy to help ride through the spikes. Crushers are also sensitive to jams, so the drive’s protection settings should be tuned to trip safely instead of letting the motor stall indefinitely.
On mixers and extruders, viscosity may change with temperature or product, so torque demand changes too. The drive can also serve as a measurement tool, since motor current gives a rough indication of load.
Safety and limits
Heavy starting duty places stress on both electrical and mechanical parts, so protective settings matter. Overload, stall and overtemperature protection should be configured according to the manufacturer’s instructions. Anyone working on the equipment should follow lockout tagout procedures, since stored energy in a loaded conveyor or crusher can move unexpectedly. Work should be done only by trained, qualified personnel and in line with local regulations. This article is general information and not an engineering design for any particular machine.